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2.1 2. FINDINGS 2.1 Survey for Current State of Practice The approved Task 2 survey was programmed into Survey Monkey format and submitted to NCHRP for distribution to the Panel. In addition, an email list of 130 agency personnel and practitioners was assembled for broad and comprehensive distribution of the survey. The survey was distributed by email in December 2014. To reach U.S. Army Corps of Engineers (USACE) districts, regional offices, and research facilities, Corps protocol required submittal of the survey to the Chief of the Civil Works Branch in Washington D.C. who would then consider forwarding the survey to approximately 50 recipients within the Corps. By using the Survey Monkey online service, summary statistics were immediately available upon survey closure. We have received a total of 42 responses. A review of the respondents' agency affiliations indicates that no responses from the Corps of Engineers were received; therefore, we can assume that the distribution within the Corps did not occur. A total of 138 surveys were distributed, and the response rate was 30%. The Survey Form is included as Appendix A. An evaluation of the survey results is presented in the following sections. 2.1.1 General Information â¢ Of the 42 responses received, 31 were from state government personnel (primarily DOTs). FHWA Resource Center and Federal Lands personnel contributed another 5 responses. The remainder of the responses included personnel from local government agencies (1), construction contractors (1), engineering/design consultants (3), and academia (1). â¢ 87.5% of respondents indicated that they "Always" or "Usually" require filters beneath countermeasures when underwater placement is necessary. Only 12.5% indicated that filter placement underwater was "Sometimes" or "Rarely" required. None indicated that a filter was "Never" required. â¢ Regarding areas of application, underwater filter placement was most often encountered for installations at: (1) bridge abutments; (2) bridge piers; (3) for lateral stability at the toe of bank slopes; and (4) for vertical stability (e.g., contraction scour or long-term degradation). All four of these categories received an average score of 6 or more on a scale of 1 to 10. â¢ Less common applications were for filters beneath rock spurs, dikes, and groins (score of 4.8), and at vertical walls or bulkheads (score of 4.0). â¢ One respondent noted, "In most cases, underwater placement is not allowed by the resource agencies." 2.1.2 Design-Related Issues â¢ Geotextiles were by far the most common filter type, with about 69% of the respondents indicating this preference. Granular filters were preferred by only 17% of the respondents. Significantly, about one third of the respondents indicated that they would consider either a geotextile, granular, or combination (hybrid) filter depending on site-specific conditions. This indicates that a good percentage of the respondents are well-versed in designing filters for compatibility with the subgrade material or site-specific requirements of other permitting agencies.
2.2 â¢ The design guidance document most often cited is FHWA Hydraulic Engineering Circular No. 23 (74% of respondents). The next most common design method is agency-specific guidance, with the following comments provided: - Mississippi Standards Specifications for Road and Bridge Construction - Federal Lands Highway Project Design and Delivery Manual (PDDM) - No design method, per se - fabric meets Idaho DOT standard spec. - Engineering fabric - NRCS National Engineering Handbook (NEH) 654 - Caltrans Standard Specification 88-1.02I - City of Austin Environmental Criteria Manual 1.4.6 D Permanent Structural Practices - Rock Riprap - Standard Specification 591S Rock Riprap for Slope Stabilization - Oregon DOT Hydraulic Design Manual - District of Columbia Department of the Environment Stormwater guidance - California Bank and Shore Protection Manual 2.1.3 Underwater Installation Issues â¢ Only 32 respondents answered the questions regarding the installation/placement of filters under water. â¢ Geotextile Filters: From the responses, geotextile placement techniques in current U.S. practice include a wide variety of means and methods. From a list of 8 techniques, the respondents were asked to indicate all methods that they use, and the responses indicate that many use multiple techniques. The three most common methods were: 1) Redirecting the current to decrease velocities in the vicinity of the work area (59%) 2) Floating the geotextile on the water and subsequently sinking it (44%) 3) Hand placing and pinning the geotextile by wading in shallow water (41%) The following comments were received: - Really depends on depth, velocity, and temperature of water - In most cases the work area has to be dewatered or completely separated from the channel with a barrier. - Dewater the site - Most applications where filter fabric is installed under riprap are done with dewatering or separated from flowing water with sandbags, rock berms, jersey barriers, or other cofferdam. - Virtually every scour countermeasure project done by MaineDOT has been done inside a cofferdam that is pumped dry. To maintain the same hydraulic opening the stream bed has to be excavated before placing any scour countermeasures. The cofferdam contains any sediments that may be generated during any excavation. â¢ Granular Filters: The method of choice for placing a granular filter is "Dumping from bank, bridge, or barge with a clamshell, bucket or front-end loader". The responses indicate that this placement technique is often used in combination with redirecting the current to decrease velocities in the vicinity of the work area. Less than 10% of the responses indicate the use of a tremie pipe or hose.
2.3 The comments received included the following: - I'm not sure we use granular material. - Not a typical usage. - Dewater the site. - I don't believe that we place granular filters under water, only in "new" construction where we might have portions dewatered. - We haven't used this filter method. - We have allowed the option of a granular filter on several projects, however, contractors have always opted for using geotextile. - We have not used granular filter layers under water. We do use granular material as a leveling course however. â¢ When asked if inspection of the underwater filter installation is required prior to placing the armor on top, 15.6% of the responses were "Yes," and 40.6% were "No." The remaining 43.8% of respondents answered "I don't know." â¢ The respondents were asked to rate, on a scale of 1 to 10, the importance of 7 factors with respect to the successful placement of a filter under water. All 7 factors received an average rating of 6 or higher. The factors as listed in the survey were: - Velocity of flow - Depth of water - Visibility under water (i.e., inability to place and inspect) - Construction access - Constructability issues - Environmental concerns/permitting - Lack of knowledge of importance of filter by construction personnel In addition, the following comments were received: - Grade preparation, including key trenches. - These are all ranked low because the effectiveness and necessity of a filter underwater is questionable in many cases for our type of work. - Sounding can be used if visibility is not good. â¢ Three respondents provided additional comments, observations and insights relevant to this project, as follows: - Inspection folder may contain information, if there are problems. - In many cases we simply avoid using a filter underwater, but when one is used the preference is for granular. I often question how effective the filter is or whether it is necessary for underwater applications when the slope or bed is submerged all of the time. - Std Specs, May 2006, Section 72-2.025.
2.4 2.1.4 Summary of Survey Results Despite the disappointing rate of return, some valuable information can be derived from the responses received: 1. Given the fact that almost 90% of respondents indicated that a filter is "Always" or "Usually" required for installations placed under water, only 16% of respondents indicated that the filter installation is inspected prior to the armor being placed on top. This led to the conclusion that QA/QC protocols should be developed and included as an important component of the guidance to be developed during Phase 2 of this research project. 2. Geotextile filters are overwhelmingly preferred over granular filters. 3. Many different placement techniques for geotextiles are being used in the U.S., and many respondents indicated that they use multiple techniques (presumably to accommodate different constructability issues such as hydraulic conditions and equipment access constraints). 4. When granular filters are placed, less than 10% of respondents indicated that they use a tremie pipe or hose. However, our synthesis of current practice (see Section 2.2) has revealed that the use of a flexible hose, through which a slurry of granular material can be pumped, is quite popular in other countries. This method is often used to fill geotextile bags or tubes under water, and in deep water it is deployed by divers. This method could easily be adopted for U.S. practice to place a uniform layer of granular material, without the need for a geotextile container, when flow velocities are less than the critical velocity of the particles. 5. When filter placement must occur under water, many factors must be considered and accommodated in order for the filter installation to be successful: a. Velocity of flow b. Depth of water c. Visibility under water d. Construction access e. Constructability issues f. Environmental concerns/permitting g. Lack of knowledge of importance of filter by construction personnel 6. One respondent noted that underwater subgrade preparation, including termination details such as toe-downs and key trenches are important as well. 2.2 Task 3 - Synthesis of Current State of Practice 2.2.1 Overview Based on the findings of Task 1, this section synthesizes the current state of practice for the, design, installation, inspection, and quality assurance of granular and geosynthetic filters for armoring and other erosion control countermeasures at stream and river banks, bridge piers, bridge abutments, and other locations requiring scour countermeasures. The focus is on the availability of guidance for placing filters underwater in riverine settings with various depths and velocities of flow. However, as noted in the NCHRP 24-42 Problem Statement, DOT construction and maintenance personnel along with general contractors who perform
2.5 countermeasure installations may not be aware of how countermeasures function and may not appreciate the value of the underlying filter. Consequently, this synthesis also includes an overview of the purpose, need, and function of the filter component of countermeasure armor systems for scour and other erosion control requirements. 2.2.2 Background and Approach The ongoing occurrence of stream channel migration and scour are often cited as the leading cause of bridge and other failures in the United States (Arneson et al. 2012). The growing need for techniques to control stream instability and scour occurrences have resulted in considerable research on the benefits of various types of hydraulic countermeasures and a number of publications have been written, including FHWA's HEC 23 "Bridge Scour and Stream Instability Countermeasures Experience, Selection, and Design Guidance" (Lagasse et al. 2009), that provide guidance on the applicability and design of different countermeasure types. A necessary component found in many countermeasure designs is the provision of a filter between the countermeasure and the underlying soil. While the countermeasure protects the soil from the shear stresses that erode the soil particles, a filter has been found to be necessary to prevent the removal of soil particles through the voids and cracks in the countermeasure structure. Installations that do not include a filter often delay the scour process, but ultimately the removal of supporting soil particles results in an undermining and failure of the countermeasure. Geotextiles have become the filter of choice for most designers, but granular or composite filters may be effective (or required) in some cases. The current technical guidance on countermeasure design includes recommendations for either a geotextile or a granular filter to be placed under the countermeasure. However, there is little guidance to construction personnel on actual installation techniques when installing a filter under water. FHWA program reviews and DOT technical assistance calls, as well as the survey of practitioners, indicate that few countermeasure installations in water actually include a filter as shown on the design plans and as recommended in the technical guidance publications. The most common reasons given for this omission of a filter have been related to constructability issues or environmental concerns. Without knowledge of the function and value of a properly installed filter, DOT construction and maintenance personnel along with their contractors have often eliminated underwater filters instead of developing creative techniques for the filter installation. Similarly, the designer may not recognize the importance of addressing constructability and environmental concerns when selecting a filter system. Since an appropriate filter is an essential component of countermeasure armoring systems, this practice must be corrected immediately if these systems are to function as designed. This synthesis of the current state of practice for the selection, design, and installation of geotextile and granular filters for armoring countermeasures is presented in the following sequence: â¢ General guidance on the purpose, need, functions, and design of filter systems is summarized with emphasis on current guidance in the U.S (Section 2.2.3). Supporting documentation from, primarily, European practice is also provided (Section 2.2.4). â¢ The development of guidance for installation of filter systems in Europe and specific techniques for placement underwater are summarized, to include a historical survey and current recommendations and practice (Section 2.2.5).
2.6 â¢ A historical survey and the current state of practice for placing filter systems underwater in the U.S. are presented. Current guidance documents and representative installation experience for U.S. practice are also summarized (Section 2.2.6). â¢ Additional applications of geotextiles for a variety of purposes, particularly in the coastal environment are referenced. For most of these applications, the geotextiles do not have, primarily, a filter function, but the methods of placement underwater provide additional insights on techniques that may have application to the problems of placing geotextile filters in situations involving deep water or high velocities and/or turbulence (Sections 2.2.7 and 2.2.8). â¢ Issues related to permitting of filter installations are introduced in Section 2.2.9. 2.2.3 Purpose, Need, Function and Design of the Filter Component of Countermeasure Armoring Systems Overview Before summarizing the state of current practice for underwater filter applications as outlined above, it is appropriate to consider a generalized overview of countermeasure armoring systems. A properly designed, installed, and maintained countermeasure system, as an integrated whole, has a functionality that is greater than the sum of its parts, i.e., successful performance depends on the system responding to hydraulic and environmental stresses as an integrated whole throughout its service life (Lagasse et al. 2006). This point of view provides context for the findings of this study and the guidance which follow. Erosion is a natural process resulting from water attacking stream and river banks. The erosion dislodges and removes material from one area, water transports the material and deposits it at some area downstream. Local scour can occur at structures located in a stream. Man-made changes to a river where streambank or bed soils have been disturbed or vegetation has been removed can induce or cause erosion or scour. Properly designed erosion control works can reduce or prevent natural and induced erosion and scour. Design of scour and other erosion control measures requires knowledge of: river bed, bank, and foundation material; flow conditions including velocity, depth and orientation; armor characteristics of size, density, durability, availability, and cost; location, orientation and dimensions of piers, abutments, guide banks, and spurs; and the type of interface material (filter) between the armor and underlying foundation which may be geotextile fabric or a filter of sand and/or gravel. Designing a scour or erosion control countermeasure as an integrated system requires a life- cycle approach to the design, production, transport, installation, inspection, and maintenance of the system. Failure of a countermeasure often is the result of poor construction techniques and poor quality control relating to weight, size, or placement of the armor or filter at the job site. Guidance and procedures for on-site inspection and monitoring must be developed providing reasonable limits and tolerances for materials and workmanship that meet construction industry standards. Constructability issues must be considered so as to accommodate site constraints, permit conditions, and environmental concerns. The physical characteristics of the system need to be considered for applications that call for placement under water versus in the dry, and for systems that must be installed below the (unscoured) bed level. Additionally, the placement of all system components, including filter and/or bedding requirements must be addressed for various applications. Practical matters of installation often dictate that suitable options be developed for these components, particularly when applications must address placement underwater or in flowing water.
2.7 Service life for a countermeasure installation can be considered a measure of the durability of the total, integrated bank, pier, abutment or countermeasure protection system. The durability of system components and how they function in the context of the overall design will determine the service life of an installation. The response of the system over time to typical stresses such as flow conditions (floods and droughts) or normal deterioration of system components (e.g., due to freeze-thaw and wet-dry cycles) must also be considered. Response to less typical (but plausible) stresses such as fire, vandalism, seismic activity or accidents may also affect service life. Finally, there may be opportunities for maintenance during the life cycle of a countermeasure installation and, where such work does not constitute total reconstruction or replacement, maintenance should not be considered as the end of service life for the system. In summary, design of a scour or erosion control countermeasure requires knowledge of: river bed, bank, and foundation material; flow conditions including velocity, depth and orientation; armor characteristics of size, density, durability, and availability; location, orientation and dimensions of piers, abutments, guide banks, and spurs; and the type of interface material between armor and underlying foundation which may be geotextile fabric or a filter of sand and/or gravel. Adequate "toe down" and termination details are essential to the performance of the system. Thus, a countermeasure should be considered an integrated system where successful performance of an installation depends on the response of each component of the system to hydraulic and environmental stresses throughout its service life. Purpose and Need for the Filter Component NCHRP Reports 568 and 593 (Lagasse et al. 2006, 2007) describe the importance of filters to the successful long-term performance of armoring-type countermeasures. Based on a survey of the existing state of practice, these reports indicate that filter design criteria has typically been the most overlooked aspect of countermeasure design, and recommend that more emphasis be given to understanding the purpose and need for a filter and ensuring compatibility between the filter and the underlying soil. Correct filter design reduces the loss of foundation material by limiting the passage of fine particles (suffusion), while simultaneously maintaining a permeable, free-flowing interface. Seepage flow and turbulence at the water-filter interface induce the migration of soil particles. The particle size distribution of the base soil underlying an armor layer must be determined to properly design a filter for particle retention. For example, when a filter with relatively large pores overlies a uniform fine-grained soil, suffusion of the fine particles may continue unabated, since there are no particles of large and intermediate sizes to prevent their migration. The presence of large and intermediate sized particles in the soil matrix prevents clogging from occurring at the soil-filter interface when filters with relatively small pores are used. Correct filter design reduces the effects of piping by limiting the loss of fines, while simultaneously maintaining a permeable, free-flowing interface. Figure 2.1a and b illustrate the basic difference between stable and unstable soil structures. Figure 2.1c through f illustrate several common filtering processes that can occur in stable and unstable base soils (modified from Geosyntec Consultants 1991). The large arrows indicate the direction of water flow in the base soil. In Figure 2.1c, the fine particles immediately adjacent to the filter are initially washed away (through the filter). The large and intermediate particles are retained by the filter; they in turn prevent any further loss of fines. This soil matrix will continue to remain stable over time.
2.8 Figure 2.1. Examples of soil and filter compatibility processes. a) Stable soil structure b) Unstable soil structure STABLE SOIL UNSTABLE SOIL Intermediate size particles Large particlesFines c) Filter with large openings over a stable soil Fines escaping Newly created voids Geotextile Weakened soil structure d) Filter with large openings over an unstable soil (piping) FinesLarge particles Geotextile Clogged zone f) Filter with small openings over an unstable soil (clogging) e) Filter with small openings over a stable soil
2.9 In Figure 2.1d, an unstable soil is covered by a filter with large pores. Suffusion of the fine particles will continue unabated, since there are no particles of intermediate size to prevent their movement by the forces of seepage flow and turbulence at the interface. In Figure 2.1e, a stable soil is covered by a filter with small pores. This filter will retain most of the fines, but the presence of intermediate sized particles prevents the continued migration of fines from lower in the matrix. Thus a clogging layer is prevented from forming to any significant extent. This is contrasted with the condition shown in Figure 2.1f, where no particles of intermediate size are present to mitigate the buildup of an impermeable barrier of plugged void spaces and clogging at the interface. Design of Granular and Geotextile Filters - Concepts and Definitions As noted, in addition to particle retention filters must be have sufficient hydraulic conductivity (sometimes referred to as "permeability") to allow unimpeded flow of water from the base soil through the filter material. This is necessary for two reasons: (1) regulating the particle migration process at the soil-filter interface, and (2) minimizing hydrostatic pressure buildup from seepage out of the channel bed and banks, typically caused by seasonal groundwater fluctuations or flood events. The hydraulic conductivity of the filter should never be less than the material below it (whether base soil or another filter layer). Figures 2.2 (a) through (c) illustrate the typical process that occurs during and after a flood event. Seepage forces can result in suffusion of the base soil through the armor layer. If a filter is less permeable than the base soil, an increase of hydrostatic pressure can build beneath the armor layer. A permeable filter material, properly designed, will alleviate problems associated with fluctuating water levels (Lagasse et al. 2009). Base Soil Properties. Base soil is defined here as the subgrade material upon which the filter and armor layer (riprap, for example) will be placed. Base soil can be native in-place material, or imported and recompacted fill. The following properties of the base soil should be obtained for proper design of the filter, whether using a geotextile or a granular filter. General Soil Classification. Soils are classified based on laboratory determinations of particle size characteristics and the physical effects of varying water content on soil consistency. Typically, soils are described as coarse-grained if more than 50% by weight of the particles is larger than a #200 sieve (0.075 mm mesh), and fine-grained if more than 50% by weight is smaller than this size. Sands and gravels are examples of coarse-grained soils, while silts and clays are examples of fine-grained soils. The fine-grained fraction of a soil is further described by changes in its consistency caused by varying water content and by the percentage of organic matter present. Soil classification procedures are described in ASTM D 2487 "Standard Practice for Classification of Soils for Engineering Purposes: Unified Soil Classification System" (ASTM 2014a).
2.10 Figure 2.2. Changes in water levels and seepage patterns during a flood. Particle Size Distribution. The single most important soil property for filter design is the range of particle sizes in the soil. Particle size is a simple and convenient way to assess soil properties. Also, particle size tends to be an indication of other properties such as hydraulic conductivity. Characterizing soil particle size involves determining the relative proportions of gravel, sand, silt, and clay in the soil. This characterization is usually done by sieve analysis for coarse-grained soils or sedimentation (hydrometer) analysis for fine-grained soils. ASTM D 422 "Standard Test Method for Particle-Size Analysis of Soils" describes the specific procedure (ASTM 2014a). Plasticity. Plasticity is defined as the property of a material that allows it to be deformed rapidly, without rupture, without elastic rebound, and without volume change. A standard measure of the plasticity of soil is the Plasticity Index (PI), which should be determined for soils with a significant percentage of clay. The results associated with plasticity testing are referred to as the Atterberg Limits. ASTM D 4318 "Standard Test Methods for Liquid Limit, Plastic Limit, and PI of Soils" defines the testing procedure (ASTM 2014a). Porosity: Porosity is that portion of a representative volume of soil that is interconnected void space. It is typically reported as a dimensionless fraction or a percentage. The porosity of soils is affected by the particle size distribution, the particle shape (e.g., round vs. angular), and degree of compaction and/or cementation. Seepage flow Normal water level a) Normal (baseflow) conditions Groundwater table Elevated groundwater after flood Seepage flow Normal water level Area of high seepage gradients and uplift pressure c) After flood recession Seepage flow Flood water level b) During flood peak
2.11 Hydraulic conductivity. Hydraulic conductivity (sometimes referred to as permeability) is a measure of the ability of soil to transmit water. ASTM provides two standard laboratory test methods for determining hydraulic conductivity. They are ASTM D 2434 "Standard Test Method for Permeability of Granular Soils (Constant Head)" and ASTM D 5084 "Standard Test Method for Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter" (ASTM 2014b). In these tests, the amount of water passing through a saturated soil sample is measured over a specified time interval, along with the sample's cross-sectional area and the hydraulic head at specific locations. The soil's hydraulic conductivity is then calculated from these measured values. Hydraulic conductivity is related more to particle size distribution than to porosity, as water moves through large and interconnected voids more easily than small or isolated voids. Various equations are available to estimate hydraulic conductivity based on the grain size distribution, and the practitioner is encouraged to consult with geotechnical and materials engineers on estimating this property. Table 2.1 lists typical values of porosity and hydraulic conductivity for alluvial soils. Table 2.1. Typical Porosity and Hydraulic Conductivity of Alluvial Soils (after McWhorter and Sunada 1977). Type of Material Porosity (vol/vol) Hydraulic conductivity (cm/s) Gravel, coarse 0.28 4 x 10-1 Gravel, fine 0.34 1 x 10-1 Sand, coarse 0.39 5 x 10-2 Sand, fine 0.43 3 x 10-3 Silt 0.46 3 x 10-5 Clay 0.42 9 x 10-8 Granular Filter Properties. Generally speaking, most required granular filter properties can be obtained from the particle size distribution curve for the material. Granular filters may be used alone or as a transitional layer between a predominantly fine-grained base soil and a geotextile. Particle Size Distribution. As a rule of thumb, the gradation curve of the granular filter material should be approximately parallel to that of the base soil. Parallel gradation curves minimize the migration of particles from the finer material into the coarser material. Heibaum (2004a) presents a summary of a procedure originally developed by Cistin and Ziems whereby the median grain size (d50) of the filter is selected based on the coefficients of uniformity (d60/d10) of both the base soil and the filter material (see Figure 2.3). With this method, the grain size distribution curves do not necessarily need to be approximately parallel. Hydraulic conductivity. Hydraulic conductivity of a granular filter material is determined by laboratory test, or estimated using relationships relating hydraulic conductivity to the particle size distribution. The hydraulic conductivity of a granular layer is used to select a geotextile when designing a composite filter. For countermeasure installations, the hydraulic conductivity of the filter should be at least 10 times the hydraulic conductivity of the underlying material. Porosity. Porosity is that portion of a representative volume of soil that is interconnected void space. It is typically reported as a dimensionless fraction or a percentage. The porosity of soils is affected by the particle size distribution, the particle shape (e.g., round vs. angular), and degree of compaction and/or cementation.
2.12 Thickness. Practical issues of placing a granular filter indicate that a typical minimum thickness of 6 to 8 inches should be specified. For placement under water, thickness should be increased by 50%. Quality and Durability. Aggregate used for a granular filter should be hard, dense, and durable. Geotextile Filter Properties. For compatibility with site-specific soils, geotextiles must exhibit the appropriate values of hydraulic conductivity, pore size (otherwise known as Apparent Opening Size, or AOS), and porosity (or percent open area). In addition, geotextiles must be sufficiently strong to withstand the stresses during installation. Values of these properties are available from manufacturers. Only woven monofilament or nonwoven needle-punched geotextiles should be considered for filter applications. Slit-film, spun-bonded, or other types of geotextiles are not suitable as filters. If a woven monofilament fabric is chosen, it should have a Percent Open Area (POA) greater than, or equal to 4%. If a nonwoven needle-punched fabric is chosen, it should have a porosity greater than, or equal to 30%, and a mass per unit area of at least 400 grams per square meter (12 ounces per square yard). The following list briefly describes the most relevant properties of geotextiles for filter applications. Hydraulic conductivity. The hydraulic conductivity, K, of a geotextile is a tested property of geotextiles that is reported by manufacturers for their products. The hydraulic conductivity is a measure of the ability of a geotextile to transmit water across its thickness. It is typically reported in units of centimeters per second (cm/s). This property is directly related to the filtration function that a geotextile must perform, where water flows perpendicularly through the geotextile into a crushed stone bedding layer, perforated pipe, or other more permeable medium. The geotextile must allow this flow to occur without being impeded. A value known as the permittivity, Ï, is used by the geotextile industry to more readily compare geotextiles of different thicknesses. Permittivity, Ï, is defined as K divided by the geotextile thickness, t, in centimeters; therefore, permittivity has a value of (s)-1. Hydraulic conductivity (and permittivity) are extremely important in filter design. Transmissivity. The transmissivity, Î¸, of a geotextile is a calculated value that indicates the ability of a geotextile to transmit water within the plane of the fabric. It is typically reported in units of cm2/s. This property is directly related to the drainage function, and is most often used for high-flow drainage nets and geocomposites, not geotextiles. Woven monofilament geotextiles have very little capacity to transmit water in the plane of the fabric, whereas non- woven needle punched fabric have a much greater capacity due to their 3-dimensional microstructure. Transmissivity is not particularly relevant to filter design. However, geotextiles (and geonet composites) with transmissive capacity have been used to relieve pore water beneath flat block revetments placed over the geosynthetic to minimize uplift and improve slope stability. Apparent Opening Size (AOS). Also known as Equivalent Opening Size, this measure is generally reported as O95, which represents the aperture size such that 95% of the openings are smaller. In similar fashion to a soil gradation curve, a geotextile hole distribution curve can be derived. The AOS is typically reported in millimeters, or in equivalent U.S. standard sieve size.
2.13 Porosity. Porosity is a comparison of the total volume of voids to the total volume of geotextile. This measure is applicable to non-woven geotextiles only. Porosity is used to estimate the potential for long term clogging, and is typically reported as a percentage. Percent Open Area (POA). POA is a comparison of the total open area to the total geotextile area. This measure is applicable to woven geotextiles only. POA is used to estimate the potential for long term clogging, and is typically reported as a percentage. Thickness. As mentioned above, thickness is used to calculate hydraulic conductivity. It is typically reported in millimeters or mils (thousandths of an inch). Grab Strength and Elongation. Force required to initiate a tear in the fabric when pulled in tension. Typically reported in Newtons or pounds as measured in a testing apparatus having standardized dimensions. The elongation measures the amount the material stretches before it tears, and is reported as a percent of its original (unstretched) length. Tear Strength. Force required to propagate a tear once initiated. Typically reported in Newtons or pounds. Puncture Strength. Force required to puncture a geotextile using a standard penetration apparatus. Typically reported in Newtons or pounds. Gradient Ratio. A performance test for measuring the soil-geotextile system clogging potential. There are many other tests to determine various characteristics of geotextiles; only those deemed most relevant to applications involving countermeasures have been discussed here. Geotextiles should be able to withstand the rigors of installation without suffering degradation of any kind. Long-term endurance to stresses such as ultraviolet solar radiation or continual abrasion are considered of secondary importance, because once the geotextile has been installed and covered by the armor layer, these stresses do not represent the long-term environment that the geotextile will experience. Table 2.2 provides recommended tests and allowable values for various geotextile properties (see AASHTO 2006). Granular Filter Design Procedure (FHWA HEC-23) Numerous texts and handbooks provide details on the well-known Terzaghi approach to designing a granular filter. That approach was developed for subsoils consisting of well-graded sands, and may not be widely applicable to other soil types. An alternative approach that is considered more robust in this regard is the Cistin â Ziems method (Lagasse et al. 2009). For additional background on the Cistin - Ziems method see Section 2.2.4. The suggested steps for proper design of a granular filter using this method are outlined below. Note that "ds" is used to represent the base (finer) soil, and "df" is used to represent the filter (coarser) layer. Step 1. Obtain Base Soil Information. Typically, the required base soil information consists simply of a grain size distribution curve, a measurement (or estimate) of hydraulic conductivity, and the PI is required only if the base soil is more than 20% clay). Step 2. Determine Key Indices for Base Soil. From the grain size information, determine the median grain size d50 and the Coefficient of Uniformity d60/d10 of the base soil. Due to the inherent variability of natural soils, these parameters should be determined for a number of samples and a representative value, or range of values, should be used for design based on engineering judgment.
2.14 Table 2.2. Recommended Tests and Allowable Values for Geotextile Properties. Test Designation Property Allowable value (1) Comments Elongation < 50%(2) Elongation > 50% (2) ASTM D 4632 Grab Strength > 315 lbs (Class 1) > 250 lbs (Class 2) > 180 lbs (Class 3) > 200 lbs (Class 1) > 160 lbs (Class 2) > 110 lbs (Class 3) From AASHTO M 288 ASTM D 4632 Sewn Seam Strength (3) > 270 lbs (Class 1) > 220 lbs (Class 2) > 160 lbs (Class 3) > 180 lbs (Class 1) > 140 lbs (Class 2) > 100 lbs (Class 3) From AASHTO M 288 ASTM D 4533 Tear Strength (4) > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) From AASHTO M 288 ASTM D 4833 Puncture Strength > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) > 110 lbs (Class 1) > 90 lbs (Class 2) > 70 lbs (Class 3) From AASHTO M 288 ASTM D 4751 Apparent Opening Size Per design criteria (See section 16.2) Maximum allowable value ASTM D 4491 Permittivity and Hydraulic Conductivity Per design criteria (See section 16.2) Minimum allowable value ASTM D 4355 Degradation by Ultraviolet Light > 50% strength retained after 500 hours of exposure Minimum allowable value ASTM D 4873 Guide for Identification, Storage, and Handling Provides information on identification, storage, and handling of geotextiles. ASTM D 4759 Practice for the Specification Conformance of Geosynthetics Provides information on procedures for ensuring that geotextiles at the jobsite meet the design specifications. ASTM D 5101 Gradient Ratio Test GR < 3.0 Values less than 1.0 may indicate a potential for piping. 1) Required geotextile class for permanent erosion control design is designated below for the indicated application. The severity of installation conditions generally dictates the required geotextile class. The following descriptions have been modified from AASHTO M 288: Class 1 is recommended for harsh or severe installation conditions where there is a greater potential for geotextile damage, including placement of riprap that must occur in multiple lifts, drop heights that may exceed 1 foot (0.3m) or when repeated vehicular traffic on the installation is anticipated. Class 2 is recommended for installation conditions where placement in regular, single lifts is expected and little or no vehicular traffic on the installation will occur, or when placing individual rocks by clamshell, orange peel grapple or specially-equipped hydraulic excavator with drop heights less than 1 foot. Class 3 is specified for the least severe installation environments, with drop heights less than 1 foot onto a bedding layer of select sand, gravel or other select imported material. 2) As measured in accordance with ASTM D 4632. 3) When seams are required. 4) The required Minimum Average Roll Value (MARV) tear strength for woven monofilament geotextiles is 55 pounds. The MARV corresponds to a statistical measure whereby 2.5% of the tested values are less than the mean value minus two standard deviations (Koerner 1998).
2.15 Step 3. Determine Key Indices for Granular Filter. One or more locally available aggregates should be identified as potential candidates for use as a filter material. The median grain size d50 and the Coefficient of Uniformity d60/d10 should be determined for each candidate material. Alternatively, candidate materials may be identified from standard aggregate specifications (e.g., AASHTO, ASTM, DOT, etc.). A range of values corresponding to the allowable gradation limits should be evaluated to determine an appropriate value for design. Step 4. Determine Maximum Allowable d50f for Filter. Enter the Cistin - Ziems design chart (Figure 2.3) with the Coefficient of Uniformity for the base soil on the x-axis. Find the curve that corresponds to the Coefficient of Uniformity for the filter in the body of the chart, and from that point determine the maximum allowable A50 from the y-axis. Compute the maximum allowable d50f of the filter using d50f(max) = A50max times d50s. Check to see if the candidate filter material conforms to this requirement. If it does not, continue checking alternate candidates until a suitable material is identified. Step 5. Determine Hydraulic Conductivity Criterion. Check to ensure that the hydraulic conductivity of the filter is at least 10 times greater than that of the base soil. Step 6. Check for Compatibility with Armor Layer. Repeat steps 1 through 4 above, considering that the filter material is now the "finer" soil and the particles comprising the armor are the "coarser" material. This check ensures that the particles of the granular filter will not be winnowed out through the voids of the armor layer. If the Cistin-Ziems criterion is not met, then multiple layers of granular filter materials should be considered. Step 7. Filter Layer Thickness. For practicality of placement, the nominal thickness of a single filter layer should not be less than 6 inches (15 cm). Single-layer thicknesses up to 15 inches (38 cm) may be warranted where large riprap particle sizes are used as armor. When multiple filter layers are required, each individual layer should range from 4 to 8 inches (10 to 20 cm) in thickness. Geotextile Filter Design Procedure (FHWA HEC-23) The suggested steps for proper design of a geotextile filter are outlined below (Lagasse et al. 2009): Step 1. Obtain Base Soil Information. Typically, the required base soil information consists simply of a grain size distribution curve, a measurement (or estimate) of hydraulic conductivity, and the PI is required only if the base soil is more than 20% clay). Step 2. Determine Particle Retention Criterion. A decision tree is provided as Figure 2.4 to assist in determining the appropriate soil retention criterion for the geotextile. The figure has been modified to include guidance when a granular transition layer (i.e., composite filter) is necessary. A composite filter is typically required when the base soil is greater than 30% clay having relatively low cohesion, or is predominantly fine-grained soil (more than 50% passing the #200 sieve). If a granular transition layer is required, the geotextile should be designed to be compatible with the properties of the granular layer.
2.16 Figure 2.3. Granular filter design chart according to Cistin and Ziems (Heibaum 2004a).
2.17 Figure 2.4. Geotextile selection for soil retention (modified from NCHRP Report 593). FROM SOIL PROPERTY TESTS MORE THAN 30% CLAY (d30 < 0.002 mm) LESS THAN 30% CLAY AND MORE THAN 50% FINES (d30 > 0.002 mm, AND d50 < 0.075 mm) LESS THAN 50% FINES AND LESS THAN 90% GRAVEL (d50 > 0.075 mm, AND d10 < 4.8 mm) MORE THAN 90% GRAVEL (d10 > 4.8 mm) USE CISTIN â ZIEMS METHOD TO DESIGN A GRANULAR TRANSITION LAYER, THEN DESIGN GEOTEXTILE AS A FILTER FOR THE GRANULAR LAYER O95 < d50 WIDELY GRADED (CU > 5) O95 of the geotextile must be less than 2.5d50 of the base soil, and also less than d90 of the base soil UNIFORMLY GRADED (CU â¤5) d50 < O95 < d90 WAVE ATTACKOPEN CHANNEL FLOW Definition of Terms dx = particle size for which x percent is smaller K = hydraulic conductivity of the base soil c = undrained shear strength of the base soil PI = plasticity index of the base soil Cu = Coefficient of Uniformity, d60/d10 O95 = the AOS of the geotextile Notes: 1) If the required O95 is smaller than that of available geotextiles, then a granular transition layer is needed. 2) Hydraulic conductivity of the geotextile should be at least 10 times greater than that of the soil. O95 â¤ #70 SIEVE (0.2 mm) YES NO PI > 5 ? YES NO K < 10-7 cm/s, and c > 10 kPa, and PI > 15 ?
2.18 Note: If the required AOS is smaller than that of available geotextiles, then a granular transition layer is required, even if the base soil is not clay. However, this requirement can be waived if the base soil exhibits the following conditions for hydraulic conductivity (K), plasticity index (PI), and undrained shear strength (c): K < 1 x 10-7 cm/s PI > 15 c > 10 kPa Under these soil conditions there is sufficient cohesion to prevent soil loss through the geotextile. A geotextile with an AOS less than a #70 sieve (approximately 0.2 mm) can be used with soils meeting these conditions, and essentially functions more as a separation layer than a filter. Step 3. Determine Geotextile Hydraulic Conductivity Criterion. The hydraulic conductivity criterion requires that the filter exhibit a hydraulic conductivity at least 4 times greater than that of the base soil (Koerner 1998) and for critical or severe applications, up to 10 times greater (Holtz et al. 1995). In riverine or coastal revetment works where floods or wave attack can create high seepage gradients, the application is considered severe and a minimum hydraulic conductivity ratio of 10 is adopted for filter design. Generally speaking, if the hydraulic conductivity of the base soil or granular filter has been determined from laboratory testing, that value should be used. If lab testing was not conducted, then an estimate of hydraulic conductivity based on the particle size distribution should be used. To obtain the hydraulic conductivity of a geotextile in cm/s, multiply the thickness of the geotextile in cm by its permittivity in s-1. The designer may need to contact the geotextile manufacturer to obtain values of permittivity and thickness. Alternatively, the AASHTO National Transportation Product Evaluation Program data base may be consulted at: www.ntpep.org/Pages/GeosyntheticsDocuments.aspx. Step 4. Minimize Long-Term Clogging Potential. When a woven geotextile is used, its percent open area (POA) should be greater than, or equal to, 4% by area. If a non-woven geotextile is used, its porosity should be greater than, or equal to, 30% by volume. A good rule of thumb suggests that the geotextile having the largest AOS that satisfies the particle retention criteria should be used (provided of course that all other minimum allowable values described in this section are met as well). For critical applications where clogging is of concern (i.e., soils containing significant levels of non-plastic fines, a Gradient Ratio test could be performed as recommended in the FHWA/NHI guidance found in Appendix B of this report. Step 5. Select a Geotextile that Meets the Required Strength Criteria. Strength and durability requirements depend on the installation environment and the construction equipment that is being used. AASHTO M-288, "Geotextile Specification for Highway Construction" provides guidance on allowable strength and elongation values for three categories of installation severity. These criteria are reflected in Table 2.2, presented previously. For additional guidance regarding the selection of durability test methods, refer to ASTM D 5819, "Standard Guide for Selecting Test Methods for Experimental Evaluation of Geosynthetic Durability" (ASTM 2014b). Step 6. Check for Compatibility with Armor Layer. For checking stability on slopes, the friction between the armor and the geotextile should be considered in order to prevent the armor from sliding. In addition, the permeability of the geotextile to prevent uplift should be checked against the area through the armor that is available for flow (see FHWA/NHI guidance in Appendix B).
2.19 Note: Alternative Filter Design Procedures from U.S. Practice (1) FHWA/NHI (Holtz et al. 2008) and (2) USACE (2011) are presented in Appendix B. 2.2.4 Documentation from European Practice for Filter Design Much of the guidance for filter design in HEC-23 (Lagasse et al. 2009), particularly the design of granular filters, is a direct result of a 1998 Scanning Review of European Practice for Bridge Scour and Stream Instability Countermeasures sponsored by TRB/NCHRP and FHWA (Lagasse 1999 and Bryson et al. 2000). Regarding filter design, the findings of this Scanning Review included: â¢ The use of riprap (i.e., armor stone in combination with a geotextile or granular filter) is by far the most common scour and stream instability countermeasure in all countries visited in Europe. â¢ During the scanning review, it was apparent that European counterparts in the countries visited consider riprap as a permanent pier scour countermeasure. Current policy in the United States considers riprap placed at bridge piers to be only a temporary countermeasure against pier scour, and guidance dictates that riprap placed at bridge piers must be monitored by periodic inspection or with fixed instruments. â¢ The confidence that European hydraulic engineers have in the use of riprap as a permanent local scour countermeasure is based in part on their use of innovative techniques for placing an effective filter beneath the riprap in flowing or deep water (see Section 2.2.5). In Germany and the Netherlands, a significant investment has been made in the development and testing of geosynthetic materials, and innovative installation techniques have been developed. For example, the Dutch have investigated the use of granular filters with large ratios for top layer and filter/base material instead of geometric tightness. Design rules for these "hydrodynamically sandtight" filters or geometrically open filters are presented by Bakker et al. (1994). The concept can be applied to geotextile filters and design rules for hydrodynamically sandtight geotextiles were developed at Delft Hydraulics. Erosion control by hydrodynamically sandtight geotextiles is discussed by Klein and Verheij (1990). At the Federal Waterways Engineering and Research Institute (BAW) in Karlsruhe, Germany, a highly specialized laboratory is available for testing a wide range of geotextile characteristics, including the following (1) impact testing performed to determine punching resistance (e.g., when large stone is dropped on the geotextile); (2) abrasion test; (3) permeability, clay clogging, and sand clogging tests; and (4) tests of material characteristics such as elongation and strength. Testing apparatus has been devised to test performance under typical conditions that might lead to failure when geotextiles are used with scour and erosion control countermeasures. Through this testing program, geotextile materials have been developed for use in innovative approaches to filter placement for riprap and other armoring countermeasures (see Section 2.2.5). The BAW Code of Practice for the use of geotextile filters on waterways (Federal Waterways Engineering and Research Institute 1993) covers various filter applications. Other relevant publications are DVWK Guideline 306 (1993) for Application of Geotextiles in Hydraulic Engineering and several Permanent International Association of Navigation Congresses (PIANC) guidance (1987, 1992). Many of the specialized filter design and installation techniques are summarized in a 1996 paper by BAW staff on installation of geosynthetics in waterways (Abromeit and Heibaum 1996). Additional discussion is presented by Pilarczyk (1998) and Kohlhase (1998).
2.20 The results of more than 30 yearsâ experience with research, testing, construction observation, field installation, monitoring and maintenance on Germany's extensive inland waterway system are summarized in a series of papers by M.A. Heibaum of the BAW Institute: Heibaum (1999, 2000, 2002a, 2002b, 2004a, 2004b, 2008) and Heibaum et al. 2006. The recommended approach for granular filter design by FHWA (Section 2.2.3) is based on the BAW research and experience. As Heibaum (2004a) notes, to design an armor system properly, one must take into account that the hydraulic load differs somewhat from other filter applications such as, for example, drains. It is an important requirement that filter criteria both at the subsoil-filter interface and at the filter-armor interface are satisfied. Most of the design rules for filters are developed for gravel or smaller grains and for unidirectional and laminar flow. Filters used in scour and erosion protection measures are loaded by unsteady, turbulent, pulsating, and reversing flow. Consequently, the rules for filters must be checked to ensure they are reliable in the case of such dynamic hydraulic loading. Additionally, the development of excess pore water pressure in the subsoil due to rapid changes in the hydraulic head may complicate the design of the appropriate filter (Kohler 1993). To design a filter, two criteria have to be fulfilled (1) the finer material has to be retained (a marginal loss may be allowed) and 2) permeability must not decrease significantly in order to avoid a build-up of pore water pressure (Heibaum 2004a). These criteria conflict. Therefore, any design has to be carried out within narrow margins. On the other hand, the design should be kept as simple as possible. The approaches are usually grouped into "geometrical" and "hydraulic" criteria. Geometrical criteria define limit values for grain and/or void diameters to hinder the transport of finer particles through the voids of the coarser material. Hydraulic criteria define a limit value for the hydraulic gradient at which the transport of particles begins. The grain (or void) size distribution has to be known when geometrical criteria are used while the application of hydraulic criteria requires data on the flow velocity and/or the hydraulic gradient at the sublayer-filter interface. Both grain size distribution and/or hydraulic gradient often vary greatly. Since the grain size distribution is much easier to obtain than hydraulic gradients, geometrical criteria are better known and used more often. With geometrical criteria the main parameter is a geometrical value, the grain diameter dx, i.e. the grain size at which x% by weight of the total soil particles are smaller, also called "x% passing size." The usual parameter is the ratio of the grain size of the filter to that of the base, i.e., d15,F/d85,B (grain size at 15% passing of the filter and at 85% passing of the base soil). Most geometrical criteria are results obtained from numerous tests and are therefore empirical. They are often restricted as to the coefficients of uniformity of filter and subsoil or the grain size (which is often forgotten) (Heibaum 2004a). Since a natural grain size distribution incorporates a range of grain diameters, Bertram (1940) introduced the ratios d15,F / d85,B â 6 and d15,F / d15,B â 9 as empirical filter designs taking into consideration the grain size distribution. The first criterion was given to ensure that the soil particles do not pass through the voids of the filter (retention or stability criterion) and the second to avoid clogging of the filter, i.e., the filter voids should not be filled with too many fine particles (permeability criterion). Terzaghi & Peck (1948) specified more precisely: â¢ d15,F / d85,B â¤ 4 â¢ d15,F / d15,B â¥ 4
2.21 based on the tests conducted by Bertram (1940). However, these criteria were limited to a maximum coefficient of uniformity of CU = 2, and generally considered unidirectional or laminar flow (Heibaum 2004a). In the U.S. most filter criteria have been of the form of the classic Terzaghi ratios (USBR 1977, USACE 1995, and FHWA's HEC-11 (Brown and Clyde 1989)). The Terzaghi approach was also used in Central Europe (with the limits of application frequently being neglected) (Heibaum 2004a). In Eastern Europe, criteria proposed by Cistin (1968) became very popular. Further tests were added by Ziems (1968). This approach became known as the 'Cistin/Ziems approach.' A design chart was developed that enabled filters to be designed for a very wide spectrum of grain size distributions of filter and base soil (see Figure 2.3). Heibaum (2004a) notes that the Terzaghi approach was developed for subsoils consisting of uniformly-graded sands, under unidirectional flow, and may not be applicable to other soil types or flow conditions. BAW Code of Practice (Federal Waterways Engineering and Research Institute 1993) currently recommends the Cistin/Ziems approach for granular filter design for the unsteady, turbulent, pulsating, and reversing flow encountered on Germany's inland waterways. Another area of intense research interest in Europe has been the macro- and micro-scale role of pore water pressure in the design and stability of armor/filter systems (Heibaum 2002b, 2008, Kohler 1993, Heibaum and Kohler 2000). Natural surface water and pore water in the subsoil are not an ideal, incompressible fluid. Small microscopic air (more generally: gas) bubbles are dispersed in the water, so the fluid shows a certain compressibility. Compressible pore water causes a delayed reaction of the pore water pressure on any pressure change at the boundaries if the hydraulic conductivity of the subsoil is lower than the velocity of that pressure change. An example is the rapid lowering of the surface water level, resulting in an excess pore water in the adjacent soil (see Figure 2.2). Due to this phenomenon, bank stability and the stability of an armor/filter countermeasure are affected by the interaction of surface water and pore water. Heibaum (2008) provides a simplified formula for the pore pressure parameter which is dependent on soil permeability and the compressibility of the pore water. Schulz and Kohler (1999) have developed an equation for the excess pore water pressure with depth and, for many practical applications, a design chart has been developed considering the soil permeability and time of pressure drop (Bezuijen and Kohler 1996). Additional information of the influence of pore water pressure on bank stability and filter design can be found in Heibaum and Kohler 2000. To account for this phenomena in current U.S. practice the HEC- 23 granular and geotextile design procedures recommend a minimum hydraulic conductivity ratio of 10 (filter to base soil) since revetment design for riverine or coastal applications is considered a "severe" condition with potentially high seepage gradients (see Section 2.2.3, Granular Filter Design Procedure, Step 5 and Geotextile Filter Design Procedure, Step 3). Heibaum (2008) notes that countermeasures to prevent scouring may be permeable or impermeable. Impermeable systems can be acceptable only when development of excess pore water pressure below the countermeasure is ruled out. Excess pore water pressure or uplift pressure often is one of the reasons for scour and erosion developing. A steady excess pore water pressure may be caused by a groundwater table that is high compared with the level of the surface water (uplift pressure). An unsteady excess pore water pressure below the countermeasure develops when there is a rapid drop of the surface water pressure due to waves or, as noted, a rapid drawdown. Consequently, even in the subsoil below permeable cover layers an excess pore water pressure may develop. For nearly all scour countermeasures a permeable armor layer is recommended even though permeable armor
2.22 layers may require more effort and cost than an impermeable one. A filter is always needed for a permeable system and placement of the top layer material has to be done much more accurately, but in the end such a system is, in most cases, more successful (Heibaum 2008). 2.2.5 European Practice for Installation of Filter Systems Underwater Traditional Techniques and Introduction of Geotextiles During the 1998 scanning review of European practice for scour and erosion countermeasures, it was found that three countries (Germany, the Netherlands, and the United Kingdom) still consider fascine mats, a very old, traditional approach for scour protection as an effective means of placing a geotextile filter in deep water. The fascines consist of a matrix of willow or other natural material woven in long bundles (15 to 20 cm in diameter) to form a matrix which is assembled over a layer of woven geotextile (Figure 2.5). The geotextile has ties which permit fastening it to the fascine mat. The fascine mattress or "sinker mat" is floated into position and sunk into place by dropping riprap-size stone on it from a barge (Lagasse 1999). Figure 2.5. Use of a fascine mattress for placing a countermeasure with filter under water. Similarly, Heibaum (2008) notes that the oldest form of a mattress is the fascine mattress, (willow bundles with a diameter of 10 to 40 cm tied together). In the beginning only fascines and brushwood were combined to a mattress. Later a woven geotextile was used as the base with the fascine bundles tied on it, if necessary a brushwood layer was added (Figure 2.6). Fascine mattresses are prefabricated according to the desired geometry on land then they are pulled to the desired position and sunk by dumping the armor material on the mat. However, all mattresses require very precise placement, which may be difficult at greater depths. It is very difficult to place mattresses without gaps so either a certain overlap is necessary or the gap is filled with an erosion resistant material. With a geotextile filter beneath the mattress, narrow gaps may be acceptable.
2.23 Figure 2.6. Preparing a fascine mattress (including additional brushwood layer)(Heibaum 2008). The use of fascines to place an armor countermeasure and/or filter underwater is considered in detail in the Netherlands Centre CUR Report 169 (1995). Section 22.214.171.124 of the CUR report provides criteria and specifications for constructing fascine mattresses underlain by a geotextile. A paper by Schulz of the BAW Institute in Germany summarizes the evolution and problems encountered in developing techniques for placing filters under water when Germany's inland waterways system was expanded to meet PIANC - Class IV standard in the 1970s and 80s (Schulz 1995). The following paragraphs are extracted with only minor editing from this paper. When reconfiguring and enlarging sections of the waterway, it was found that the accuracy of the profile, which forms the subgrade, is an important precondition for producing a good filter. When geotextile filters were first introduced, it was thought that the problems with the placement of grain filters regarding the positional accuracy, the danger of segregation and the constant layer thickness had been solved. However, the installation technique at the time was not at all suitable for laying down a geotextile without folds and for covering the subgrade completely. Further problems arose with placing geotextiles when lowering them down to the bed of the waterway from the pontoons and, subsequently, covering them with riprap, because the geotextiles act as a 4 m high sail in the water and the hydrodynamic forces from the return current of passing ships had considerable consequences [emphasis added]. Pontoons used to place the geotextile could move by several meters when other ships passed, and both the geotextile and the seams had to tolerate considerable tensile forces. Furthermore, the laying was not very exact and as the riprap is dumped, any movement of the loosely laid geotextile could not be controlled. The findings of divers in the 1980s also confirmed that considerable transverse shortening occurred and the specified overlapping of 1 m had not been maintained. The edges in the overlapping areas, where the geotextile was not covered by riprap, had become folded over; and the design objective had not been achieved at all. In the second half of the 1980s new ideas came up and a system of placing the geotextiles with a big motor driven roller which moves on the subgrade was introduced (Figure 2.7). Today practically no geotextile is placed without using a roller (Schulz 1995) (Also see Abromeit and Heibaum (1996)).
2.24 Figure 2.7. Roller for placing geotextiles onto the subgrade. After the "initial euphoria" regarding the use of geotextile filters, negative experiences and problems with the application of geotextiles caused more attention to be paid to granular filter construction methods and the problems regarding granular filters under water were examined more closely. In the mid-1980s, fall tests in West German canals showed that grain filters which are placed correctly from the water surface are found to meet specifications even at a depth of 4 m. Tests with large crates anchored at various points on the subgrade confirmed this (Schulz 1995). More recent investigations in the Mittelland Canal with a lowerable feeding structure [large diameter tremie pipe or (tube)], whereby the feeding tube is not opened until the structure is immediately above the subgrade, have shown that by shortening the free fall distance of the filter material it can be distributed more evenly (Schulz 1995). It was found that where the exactness of the subgrade preparation does not come up to standard, it is difficult to guarantee the regularity of the filter construction. The difficulty in controlling the complete process from a technical point of view played a very large role. Schulz (1995) concluded that up to 1995, little had been invested in the sector of quality control. A very wide gap existed between the design, tender and contract stages and the monitoring and controlling of the work carried out on the construction site. His final conclusion was that this gap must be closed, in order to obtain more stable and durable revetments.
2.25 In the 1970s - 1980s time frame countries in Europe (in addition to Germany) were developing procedures for and encountering problems with incorporating geotextiles in revetment construction, including Yugoslavia (Bozinovic et al. 1984), Austria (Wewerka 1984), the U.K. (Wise 1984), and the Netherlands (CUR 1995). The experience and conclusions related to the use of geotextiles as a filter for waterway revetments from the U.K. and the Netherlands are representative and are summarized in the following sections. The United Kingdom. Wise (1984) is concerned principally with the development and behavior of revetment systems provided by an inter-connected, non-rigid, single layer of armor units, overlying geotextile filters in alternating flow situations. Wise notes that components of non- rigid revetments may be pre-formed and assembled under carefully controlled production conditions, and with the close degree of supervision inherent in factory processes. This manufacturing characteristic is one which, in effect, extends from the screening of a specified rock size, through gabion or mattress construction, and to the machine casting of concrete blocks or other armoring units which have been devised as alternatives to rock. A schedule of the components of an armored flexible revetment, including its handling and installation equipment, is a quite modest one amounting to no more than: a) Precast concrete armor units b) Anti-abrasive liners c) Cables, having anti-abrasion sheaths d) Cable connectors and associated tools e) Geotextile filters f) Handling equipment g) Anchors Wise (1984) summarizes the advantages of non-rigid armoring systems as follows: a) Production standardization of armoring units under controlled conditions. b) The simplicity of their bulk handling and/or assembly into easily transportable loads c) A variety of available installation techniques to suit differing site conditions d) Accurate underwater laying and anchoring of pre-fabricated assemblies [emphasis added]. e) The ability to retrieve and to re-use non-rigid panels in the event of accidental damage to the protected earth mass f) The ease with which geotextiles may be laid in association with non-rigid armor, particularly in underwater situations [emphasis added]. Regarding geotextiles, Wise (1984) states: "In calm-weather conditions the handling and laying of geotextiles above water, well in advance of covering armor, presents no particular problem. However, because of low specific gravity, the fabrics usually require either pre- weighting or similar restraint when laid, alone, below tidal ranges; and particularly so when wave or current conditions prevail." This may entail added expensive diving effort and it has been found advantageous in several underwater revetment projects to pre-attach the geotextile underlay, having a leading valence or skirt, to the underside of a flexible armor panel with the skirt temporarily secured to its upper surface, and to thereafter install the combined revetment unit in a single operation. Following the installation the leading edge of the skirt may be automatically detached from the upper surface of the panel and be temporarily restrained on the earth bed pending the placing of the following revetment unit. Figure 2.8 illustrates one such assembly suspended from a spreader beam during installation.
2.26 Figure 2.8. Flexible revetment with integrated geotextile filter ready to be placed under water. Wise (1984) concludes his observations on integrated flexible revetment systems with the following points: a) Single-layer flexible armor integration will result in improved structural stability with increased economic benefits. b) Much experimental and observational effort remained (circa 1984) to be undertaken in order to prove recent development in flexible revetment techniques. The Netherlands. Netherlands Centre for Civil Engineering Research and Codes (CUR), published their Manual on the use of rock in hydraulic engineering (Netherlands Centre CUR Report 169), in 1995. This Manual was produced to provide practical guidance on the use of rock in hydraulic engineering. It reflects Dutch and British national and international experience in applications where protection against the action of wind-generated waves (marine structures) and currents (river structures, closure dams) are the dominant design considerations. In a section on construction methods for bed and bank protection, the CUR manual notes that underwater construction of protection works is extensively discussed by Veldhuijzen van Zanten (1986) and much of the discussion on this topic "is almost literally obtained from this reference" for this section of the Manual. The work by van Zanten in 1986 can be considered representative of Dutch practice for underwater installation of revetment and filters in the 1980s time frame. Since the CUR Manual relies on this material for reporting the state of practice in the U.K. and the Netherlands in the 1995 time frame, it is reasonable to infer that these techniques were still considered relevant in the 1990s.
2.27 CUR Report 169 (CUR 1995) includes a detailed section on construction methods for bed and bank protection. Bed protection works are applied as (1) a foundation layer of main structures such as a breakwater or sea wall, (2) to secure the stability of a closure gap, or (3) to prevent erosion of a river bed and river or canal bank. Offshore, bed or scour protection provides for stable foundations of offshore structures by preventing or controlling scour (similar functions as (1) and (3) above). Bank protection may, for example, be applied in river embankments or in sea walls, with a function (as 3 above) to protect the slope or a river or canal. The bed and slope protection works may be composed of various stone layers: a top or armor layer to withstand the erosive action of waves and currents and subsequent filter layers. The gradation of these layers should be such that migration of material from one layer to the other, and eventually out of the structure, is prevented. At the bed this filter function is often served by a geotextile which reduces the number of filters required. Depending on local circumstances and structure requirements, this geotextile may be attached to a fascine mattress which is sunk to the bed or placed directly on the bed (van Zanten 1986). Basically two types of bed protection are applied, loose stone or mattresses. Loose stone is used as single or multi-layered structures. Mattresses are usually applied as filter layers and contain or consist entirely of geotextiles and/or fascine mattresses. Associated with these types of structures are two basic construction methods: â¢ Stone filter constructed directly on the bed â¢ Prefabricated filter mattresses placed directly on the bed with two options: - Placement from the water surface (sinking/ballasting as with a fascine) - Direct placement under water (by divers or from a pontoon) The subsequent layers are constructed by controlled placing preferably using a side stone dumping vessel. Because the filter material at the bed will be rather fine, this type of structure may only be applied under conditions of minimum current and wave action. To avoid erosion of filter layers during construction, these layers should be covered rapidly by the coarser top layer(s). Direct placement of a filter can be done either underwater on the bed or from a pontoon at the water surface. When placing directly underwater, a geotextile fabric is rolled out on the (river) bed. Initial stabilization is obtained by placing stone or sand bags by divers. This method may be applied when current and wave action is negligible and for smaller sized protection works at shallow depth. Another method of direct placement is done with a pre-fabricated filter mattress, which is rolled out on to the river bed from a pontoon. This method requires special equipment and is usually applied on large scale projects and when rather strict filter requirements have to be met. The geotextile must be sufficiently strong to cope with the impact of falling stones. In practice this means that the gradation of the first filter layer should not exceed 10 to 60 kg. Subsequent stone layers are placed on the mattress or geotextile either by dumping stone from side dumping barges or by a crane operating from a pontoon.
2.28 Bank protection works are carried out on underwater slopes and usually extend to the water level. Therefore, waterborne operations close to the structure are limited by the draft of the equipment. When applicable, use can be made of the tide when working at the top end of the slope. The operations are either waterborne or land-based or a combination of both. The structure usually includes placement of (1) a geotextile fabric, attached to a fascine mattress, (2) only a geotextile fabric, or (3) a prefabricated geotextile filter mattress. The working method is a combination of the placement of a geotextile mattress and dumping stone. For the placement of the mattress, the following methods can be used: (1) Sinking method (2) Direct placement (3) Placement from a pontoon (4) Placement by a land-based crane The sinking method is still in use in Europe for placing a filter underwater. This method employs the traditional technique of a specifically constructed fascine mattress (see Figure 2.5) which is floated in position above the required location. Subsequently the mattress is anchored at the top end of the slope after which the mattress is stabilized by placing stone (preferably) from the toe of the slope in an upward direction. Anchoring and ballasting may have to be done during a high water level to accommodate barge-based equipment. For direct placement the geotextile fabric is unwound downward from the top of the slope. After unwinding, the geotextile is fixed at the lower end of the slope by divers and initial stabilization is obtained by placing stone from the lower end of the slope in an upward direction. In this way, steeper slopes may be constructed than when placing stone on the slope in a downward direction as described above. For placement from a pontoon a pre-fabricated geotextile filter mattress may be unwound from a pontoon and placed on a submerged slope in a downward direction. This method requires special designed equipment, both for the placement of the mattress as well as for the fabrication and is usually only feasible for larger sized bank protection works (see Figure 2.7). An example of placement of a pre-fabricated filter mattress by crane positioned on the shore is shown in Figure 2.8. Depending on the situation (current, tide etc.) the slope protection may also be constructed on site. Depending on the length of the slope, water depth and available equipment, placement of subsequent stone layers on top of the mattress may either be carried out from land or from water. Current Technology and Applications in Europe Overview. As noted in the previous section, CUR Manual 169 (Netherlands Centre CUR 1995) reports the state of practice in the U.K. and the Netherlands in the 1995 time frame. Many of these techniques are in use today and are representative of current technology in Europe. However, an update and overview of the current state of practice for placing filters underwater in Germany was obtained in 2001 during field trips and site visits to the BAW laboratories and construction sites on Germany's inland waterways and North Sea Coast during NCHRP Project 24-07(2) (Lagasse et al. 2007). These field trips and site visits were guided by Dr. Michael Heibaum of the German Federal Waterways Engineering and Research Institute (Bundesanstalt fÎ·r Wasserbau or BAW) and Mr. Justus Trentmann of Gewatech-Soil and Hydraulic Engineering (Gewatech Grund- und Wasserbau GmbH Co. KG) who is a leading expert for placement techniques, equipment, and specifications for cement grout as used in the construction industry in Germany (e.g., partially grouted riprap) and installation techniques for placing geocontainers and other filters in flowing water.
2.29 The following paragraphs provide an overview of current practice in Germany for placing filters underwater based on the 2001 field visits. This overview is followed by examples and specific citations that further document current technology in Europe for placing filters underwater. Placing geotextiles under water is problematic for a number of reasons. Most geotextiles that are used as filters beneath riprap are made of polyethylene or polypropylene. These materials have specific gravities ranging from 0.90 to 0.96, meaning that they will float unless weighted down or otherwise anchored to the subgrade prior to placement of the armor layer (Koerner 1998). In addition, unless the work area is isolated from river currents by a cofferdam, flow velocities greater than about 1.0 ft/s (0.3 m/s) create large forces on the geotextile. These forces cause the geotextile to act like a sail, often resulting in wavelike undulations of the fabric (a condition that contractors refer to as "galloping") that are extremely difficult to control. In mild currents, geotextiles (precut to length) have been placed using a roller assembly, with sandbags to hold the fabric temporarily. To overcome these problems, engineers in Germany have developed a product known as a sandmat. This blanket-like product consists of two nonwoven needle-punched geotextiles (or a woven and a nonwoven) with sand in between. The layers are stitch-bonded or sewn together to form a heavy, filtering geocomposite. The composite blanket exhibits an overall specific gravity ranging from approximately 1.5 to 2.0, so it sinks readily. According to Heibaum (2002a), this composite geotextile has sufficient stability to be handled even when loaded by currents up to approximately 3.3 ft/s (1 m/s). At the geotextile-base soil interface, a nonwoven fabric should be used because of the higher angle of friction compared to woven geotextiles. Figure 2.9 shows a close-up photo of the sandmat material. Figure 2.10 shows the sandmat blanket being rolled out using conventional geotextile placement equipment. In deep water or in currents greater than 3.3 ft/s (1 m/s), German practice calls for the use of sand-filled geotextile containers. For specific project conditions, geotextile containers can be chosen that combine the resistance against hydraulic loads with the filtration capacity demanded by the application. Geotextile containers have proven to give sufficient stability against erosive forces in many applications, including wave-attack environments. The size of the geotextile container must be chosen such that the expected hydraulic load will not transport the container during placement (Heibaum 2002a). Once placed, the geotextile containers are overlaid with the final armoring material (e.g., riprap or partially grouted riprap) as shown in Figure 2.11 in a pier scour countermeasure application. Figure 2.12 shows a geotextile container being filled with sand. Figure 2.13 shows the sand- filled geotextile container being handled with an articulated-arm clam grapple. The filled geotextile container in the photograph is a nominal 1-metric-tonne (1,000 kg or 2,200 lb) unit. The preferred geotextile for these applications is always a non-woven needle punched fabric, with a minimum mass per unit area of 500 g/m2 (15 oz/yd2). Smaller geotextile containers can be fabricated and handled by one or two people for smaller-sized applications. As a practical minimum, a 200-lb (91 kg) geotextile container covering a surface area of about 6 to 8 square ft (0.56 to 0.74 m2) can be fashioned from nonwoven needle punched geotextile having a minimum mass per unit area of 200 grams per square meter, filled at the job site and field- stitched with a hand-held machine.
2.30 Figure 2.9. Close-up photo of a geocomposite blanket (photo from NCHRP Project 24-07(2), courtesy Colcrete â Von Essen Inc.) (Lagasse et al. 2007). Figure 2.10. A geocomposite blanket being unrolled (photo from NCHRP Project 24-07(2), courtesy Colcrete â Von Essen Inc.) (Lagasse et al. 2007).
2.31 Figure 2.11. Sand-filled geotextile containers (Lagasse et al. 2007). Figure 2.12. Filling 1.0 metric tonne geotextile container with sand (photo from NCHRP Project 24-07(2), courtesy Colcrete â Von Essen Inc.) (Lagasse et al. 2007). FLOW Geotextile containers filled with sand Rock riprap placed flush with channel bed Pier
2.32 Figure 2.13. Handling a 1.0 metric tonne sand-filled geotextile container (photo from NCHRP Project 24-07(2), courtesy Colcrete â Von Essen Inc.) (Lagasse et al. 2007). BAW Guidance. The Federal Waterways Engineering and Research Institute (BAW) in Germany published a Code of Practice for the use of geotextile filters on waterways in 1993 (Federal Waterways Engineering and Research Institute 1993). This Code of Practice (MAG) recommends the use of the Cistin/Ziems approach for the design of granular filters [see Section 2.2.3 Granular Filter Design Procedure (FHWA HEC-23)] and provides detailed examples of many types of filter construction and installation guidance as well as testing and specification guidance. The Code of Practice also provides recommendations on the installation of geotextile filters, both in the dry and underwater. The following paragraphs provide an extract of this guidance with only minor editorial changes. MAG notes that installation of geotextiles in the dry is generally not problematical. Nevertheless it must be ensured that measures to secure the geotextile against displacements caused by wind, waves (tidal zone) etc. do not lead to perforation of the filter and that they do not result in uncontrollable tensile strengths when placing the protective layer. The geotextile must not be allowed to slide on the subgrade. Driving on geotextiles without a sufficient protective layer must be avoided. Placing a geotextile unit underwater in the planned position is only possible with technical assistance or by use of a structural addition since geotextiles do not sink to the subgrade without surcharge because of their low unit weight and entrapped air bubbles. The installation method must meet the following requirements (does not apply to sinker mattresses, i.e., fascine mattresses): (a) The geotextile should be in contact with the subgrade if at all possible when cover layer aggregate is placed on it, or it should be held only a small distance above the subgrade (<0.50 m), with only moderate prestressing. It is not possible to sink a floating geotextile unit in the planned position and without folds simply by placing aggregate on it. In
2.33 addition, coarse aggregate (stones) may get beneath the geotextile leading to risk of perforation, reduction of mechanical stability of the top layer on slopes, and increased risk of abrasion damages. The design position of the geotextile must be checked by a diver in each case before placing the protective layer if this is not guaranteed by the installation method or by the additional use of structural measures. (b) The area of overlaps must be checked by a diver immediately before installing the adjacent geotextile unit to ensure full-area coverage and freedom from stones if this is not guaranteed by the structural measures used and by the placing method. A geotextile protruding from an impermeable top layer (lining) may lead to uncontrollable water losses depending on the geotextile transmissivity. (c) Fixing of a geotextile that may cause damage of the filter material (e.g., pinning) is inadmissible. (d) All edges over which the geotextile is turned must be rounded off to minimize chafing loads due to movements of the installation equipment or of the geotextile itself. MAG also provides instructions for placing a granular leveling sublayer as a bedding layer for the geotextile. In a series of conference papers from 2000 to 2008 (Heibaum 2000, 2002a, 2002b, 2004a, 2004b, 2008), Dr. M.A. Heibaum of the BAW Institute provides additional guidance (based primarily on research at the BAW laboratories and experience gained on Germany's inland waterway system) for underwater placement of filters using advanced techniques such as geotextile containers (see Figures 2.12 and 2.13) and specialty geotextile products such as sandmats (see Figures 2.9 and 2.10). The guidance, recommendations, and observations on using these approaches to placing a filter underwater are summarized in the following paragraphs. Again, this material is extracted from the Heibaum papers with only minor editing. Traditionally, a layer of fascines was used as a filter when no equipment was available to place a granular filter correctly and before geotextile filters were invented (Heibaum 2000). But fascines alone will not function as a filter. Only coarse soil may be retained efficiently by fascines. Erosion may be slowed down because of damping of the erosive effect of the current, but it will not be stopped. An important step forward was made when fascines and geotextiles were combined into a fascine mattress [emphasis added]. Modern fascine mattresses usually comprise a base woven geotextile with willow bundles fastened on top of it (see Figures 2.5 and 2.6). The fascines ensure spreading of the geotextile and floating of the mattress while the geotextile acts as a filter. Mattresses usually are assembled in dry conditions and placed by special cranes (see Figure 2.8). Only geosynthetic grout-filled mattresses are filled in place, but, to place the geotextile cover, the same difficulties arise as with geotextile filters. Fascine mattresses are prefabricated in the dry. They need an extra armor layer--usually riprap or concrete elements. A part of that cover is dumped on the mattress to sink the mattress and to keep it in place until the whole area is covered, and then the armor layer is completed. For all mattresses incorporating geotextiles, one has to consider the question of friction and sliding if they are placed on steep slopes (as discussed below regarding geotextile filters).
2.34 Granular filters may be designed for almost any purpose according to one of the well-tried filter rules. However, in rivers granular filters can be used only if there is no current or low current. Otherwise, at the very least the layers with fine grains will be eroded instantly. Such difficulties were the reason that previously no filter at all or a badly dimensioned filter was placed. Thus the hydrodynamic impact was only reduced by coarse material such as riprap and blocks, as with the fascines, but erosion was not stopped [emphasis added]. Any granular material, stones as well as granular filters, may be placed underwater by an excavator or by dumping. The advantage of dumping is that a large amount of fill can be placed in a short time. But a disadvantage is that only narrowly graded grain size distributions may be used, because otherwise there will be segregation while grains are falling through the water. The coarse material will reach the bottom first and the fines will be on top--just the opposite of what is desired. To achieve layers with a constant thickness, special dumping devices are necessary. Using excavators, dumping vessels, or split barges will result in varying thickness. Excavators with a clamshell bucket are used to place a small amount of fill very exactly. For underwater applications, granular filters nearly always have to be placed in more than one layer. Each layer needs a minimum thickness to guarantee the necessary filtration length and to avoid bare spots caused by the irregular surface of the subsoil. The risk of segregation is obvious. If the scour hole has steep banks, it has to be verified that the filter material will not slide. Often the current velocity precludes the placement of a granular filter. Geotextile filters have to be placed underwater with special devices. The placement depth is limited to about 20 m (65 ft). In greater depths fascine mattresses may be used. As with grain filters, any current will hinder proper placement of a geotextile filter. Generally, special equipment is needed--for example, steel chains connected to the fabric or a sand fill in between two geosynthetic cloths (sandmat). The two layers are sewn or needlepunched to keep sand in place (see for example Figure 2.9). In tests, such sandmats filled with sand (5 kg/m2) (9 lb/yd2) have remained in place loaded by currents up to 1.0 m/s (3 ft/s) (Heibaum 2000, 2002a). The maximum fill available today is 9 kg/m2 (16 lb/yd2). Steep banks of a scour hole require high friction of geotextile and subsoil. Nonwoven fabrics exhibit a higher angle of friction than woven fabrics, so the fabric can be selected to meet site-specific requirements. Anchoring of the geotextile as is done in dry conditions is seldom possible underwater. In addition to their main function as a filter, geosynthetics in scour countermeasures can be used as containment or as reinforcement (combining several attributes). This is done when geocontainers are used as a special form of geotextile filters. Usually the geotextile container is intended to act as a filter itself. In a few cases it is designed only as a casing for granular filter material. A double line of defense is provided when putting granular filter material into a container that is designed as a geotextile filter [emphasis added]. With an appropriate size and fill, geocontainers may be placed even in high currents. Model tests in a flume with stacked geocontainers (three layers to a height of 1.8 m) proved stable up to an overtopping flow velocity of about 4 m/s (13 ft/s) and a mean velocity of 1.5 to 2 m/s (5 to 6.5 ft/s). The stability of geocontainers in a scour hole will be even better. In 1997 in Germany, geocontainers of 1-m3 (1.3 yd3) volume were used for scour protection at bridge piers in an estuary. Placement was successful in spite of the maximum flow velocity of 2 m/s (6.5 ft/s). Around the bridge piers, the geocontainers were covered by an armor layer, but around some dolphins near the bridge, the geocontainers were left without armor. Even today no erosion has been detected (Heibaum 2000, 2002a, 2002b, 2008).
2.35 To provide a flawless filter layer with geocontainers, it is essential that there are no gaps between the elements, so two layers usually are required. As with geotextile filters, nonwoven fabrics cover a larger spectrum of grain size distributions than woven fabrics. To obtain a double line of defense concerning the filter, the fill of a geocontainer should be graded according to grain filter rules. Such a grain filter in a geocontainer may even be of a widely graded grain size distribution, because no segregation will take place when material is dumped in a geocontainer [emphasis added]. For safe placement, high serviceability and sufficient long term resistance, the container material has to be chosen such that it will resist all mechanical loads. Usually there is a choice of woven and nonwoven fabric. The first has the advantage of high tensile strength, the second the advantage of large straining capacity. If a casing material is damaged, a woven cloth might be more susceptible to crack propagation (the zip effect) than a nonwoven fabric. Nonwoven fabric usually has a high straining capacity, so the tensile strength may be less to provide a similar resistance against mechanical impact. By allowing large deformations nonwoven fabric will be able to withstand the impact load when hitting the ground as well as when stones are dumped upon it. A minimum mass per unit area of 500 g/m2 (21 oz/yd2) and a minimum tensile strength of 25 kN/m (1,700 lb/ft) are recommended, the strain at rupture should be larger than 50%. Since the container has to sustain abrasive forces due to rocking armor stones or bedload transport, any geosynthetic material used for containers needs a high resistance to abrasion. The elements of any cover layer will rock and/or move in the immediate period after installation due to dynamic hydraulic loads and deformation of the subsoil caused by the new load (Heibaum 2002a). Very large containers are generally filled in a split barge, so the filling process is not a critical load during installation. Tubes are filled in situ; therefore, the filling process usually will create less stress than the service state. Smaller containers (â 1 m3) are filled before being placed. To avoid high stress or strain when filling, the container should not hang under a funnel but should touch the ground when being filled. The container should not be stretched by the filling process (see Figure 2.12). Special care has to be taken with the seams of the container. On three sides, the seam is prefabricated by the manufacturer. Usually a strength approximately as high as the geotextile itself is guaranteed. The container usually is closed by sewing double chain stitch seams at the top. One seam is straight and the second one is curved to allow for straining of the geotextile if the first seam is broken. A new development is closing the container with VelcroTM. In 2001 the first promising tests with VelcroTM closures were made (Heibaum 2002a). Large single elements like geocontainers usually are placed by an excavator. Numerous geocontainers also may be dumped by side-dumping vessels or split barges. For large containers dumped from a split barge, the installation process may be the highest load for the element during its lifetime. Design calculations are given by Pilarczyk (2000). Care has to be taken that the area to be protected is covered completely. Special equipment allows for very precise placement of geocontainers--for example, the framework girder device shown in Figure 2.14 that enables precise installation to a depth up to 25 m (82 ft). When geocontainers are used, the amount of fill should not exceed 80% of the theoretical volume, because tightly filled geocontainers will not adjust themselves to the subsoil, to structures, or to the neighboring geocontainers. With nonwoven geocontainers even a rather steep slope may be covered because of the high friction angle. The choice of a nonwoven geotextile for geocontainers will also minimize the risk of damage during placement because of its high strain capacity. By allowing large deformations the material will be able to withstand the impact load when it hits the ground as well as when the stones are dumped upon it. A minimum mass per unit area of 500 g/m2 (21 oz/yd2) and a minimum tensile strength of 25 kN/m (1,700 lb/ft) are recommended (Heibaum 2000).
2.36 Figure 2.14. Placing geosynthetic containers (sketch courtesy of Colcrete/von Essen-Bau (Heibaum 2000, 2000a). The advantages of combining underwater placement techniques developed by BAW including fascine mats, granular filters, and geocontainers can be judged from the stabilization of possibly the largest scour hole on the German coast (Figure 2.15). Large geotextile containers (approximately 1.25 m3 (1.6 yd3) in volume) were placed at Eidersperrwerk storm surge barrier on the Eider estuary using side-dump pontoon barges and divers to ensure the filter attained the desire coverage and thickness. The elongation capabilities of the fabric and partial filling with sand allowed the containers to adjust to irregularities of the substrate at the site. For the Eider Estuary project, more than 48,000 geotextile containers were used to repair a 30 meter (100 ft) deep scour hole at the barrier. An armor layer of 1 to 6-ton stone dumped through more than 20 meters of water and a fascine sinker mat with smaller stones to stabilize the toe completed the installation (Lagasse 1999). In the upper part the slope was as steep as 1:1. The tidal current had a flow velocity up to 2.5 m/s (8 ft/s), and the final revetment had to sustain flow velocities up to 5 m/s (16 ft/s) (also see NAUE Faestechnik GmbH (undated) "NAUE Sand Containers" for specifications and installation details). Figure 2.15. Eidersperrwerk on the Eiden estuary, Germany. Geotextile containers used to repair 30 m (100 ft) deep scour hole and fascine sinker mat to stabilize the toe.
2.37 Documentation From Other Sources. In addition to the detailed guidance on underwater placement of filter systems provided by the BAW experience in Germany, several other sources of documentation of European practice are available. In particular, Pilarczyk (2000) contains extensive coverage of the use of geosynthetics and geosystems in hydraulic and coastal engineering. The aims of this book are "to review the pros and cons for the use of geotextiles/geosynthetics in various geosystems with applications in hydraulic and coastal engineering, to present relevant data gained from various studies, and to record data from projects where geotextiles and geosystems were installed." For example, Pilarczyk (2000) provides additional detail on the "sandmat" approach introduced by BAW (see Figure 2.9). An example of such a mat is the sandmat shown in Figure 2.16. The mat consists of three layers and is mechanically bonded, needle-punched nonwoven filter filled with sand. Based on tests at the BAW Institute in Germany, the filter is stable for a wide range of soil types. The quartz sand filling (> 5,000g/m2) (150 oz/yd2) not only improves the filter properties, it is often also helpful at installation because of the high weight per unit area that prevents floating of the mat. Thus, underwater installation can be carried out by simple means (unrolling of mat by spreader bar) and the mat then can be covered with an armor layer. Figure 2.16. Cross section of a typical sandmat (after Pilarczyk 2000). For the placement of geocontainers Pilarczyk (2000) provides a summary of the dumping process and detailed design rules for stresses on the geotextile containers at each stage of installation. Pilarczyk notes that when applying geocontainers, the major design considerations/problems are related to the integrity of the units during release and impact (impact resistance, seam strength, burst, abrasion, durability, etc.), the accuracy of placement on the bottom (especially at large depths), and the stability of the containers, particularly if they are not to be covered with an armor such as riprap. Pilarczyk's (2000) discussion of the complex issues involved in the selection and design of geocontainers provides an excellent example of the level of technical sophistication and extent of the supporting research that are commonly used in Europe in selecting and designing geocontainers as a structural component or as a means of placing filters underwater. Pilarczyk summarizes this sub-section of his book with the following observations. â¢ It should be stated that the theoretical design models still are rough schematizations of the reality. These models can be used to give an indicative prediction only. They are merely a first step towards a more sophisticated model. Therefore, it is still necessary to gain more insight into the physics of the dumping process and to perform more tests in order to assess the validity of the model. However, the experimentally derived tensile strength design guides can be of use for verification of design.
2.38 â¢ Prototype experiments indicate that geocontainers with a volume of more than 200 m3 (260 yd3) and dumped in water of which the depth exceeds 10 m (33 ft), have frequently been damaged. Failure was caused by a collapse of the seams when a geotextile with a tensile strength of less than 75 kN/m (5,000 lb/ft) was used, whereas hardly any damage was observed with geocontainers up to 1000 m3 (1,300 yd3) when a geotextile with a tensile strength of 175 kN/m (12,000 lb/ft) or more. This information can be of use for the initial selection of geocontainers for a specific project. â¢ Based on past experience, the design concepts for geocontainers such as material tensile strength and seaming requirements, creep, abrasion, ultraviolet protection, tear and puncture properties are now much better understood and documented. This information can be provided by a manufacturer for specific project conditions. The manufacturer and/or experienced contractor may also provide additional details on the fabrication, filling, and installation of prefabricated geocontainers. From a less academic point of view, GEOfabrics (2011) of the United Kingdom provides installation guidance as recommended by a geotextile manufacturer for installing geotextiles in coastal and river applications. These suggestions are presented here not as an endorsement of any particular product or manufacturer, but as representative of the guidance for underwater installation provided by a representative manufacturer. Regarding underwater installation, GEOfabrics (2011) notes that all needle-punched geotextiles will float in seawater and therefore require some form of ballast if they are to be successfully placed below the low-water line. There are a number of ways of achieving this, and many contractors will have developed their own procedures. In shallow water, where it is possible for a machine to reach the full extent of the site, the geotextile can be rolled onto a steel pole with a buoy attached at one end. The leading edge is anchored beneath the tracks of an excavator and the roll can then be lowered into place. The pole can be retrieved once the fabric has been weighted with a quantity of stone (Figure 2.17). Figure 2.17. GEOfabrics suggested underwater installation techniques (2011). For speed of installation on larger projects, it is possible to join two widths of geotextile using a prayer seam formed with a portable, sack-closing, sewing machine. Widths up to 12m (40 ft) can be prefabricated prior to installation. Joints with 60% of the geotextile's strength can be fabricated using grades up to 6mm thick. Scrap rebar can be used as sacrificial ballast and lengths can be attached to the geotextile at intervals along its length using cable ties, tying wire or tape. The installation starts as follows: (1) unroll a suitable length of geotextile on level ground away from the installation area, (2) attach one end of a geotextile length to a suitable steel core, (3) attach two lengths of rope to the core and lay the lengths along the fabric, (4) roll the fabric, rebar and ropes onto the core and transport to the installation area.
2.39 The installation continues on site with: (5) use suitable methods to locate the exact position of the previous geotextile length to be placed. Divers may be required or it may be possible, in shallow water, to attach floats to the edges of the geotextile. White lines spray painted onto the fabric to identify the correct overlap position may be suitable in some waters. Finally: (6) the geotextile and core can now be lowered into position by unwinding the ropes. The steel core can be recovered for future use. On long slopes, it may be more effective to place the roll on the slope shoulder and have the ropes hauled from on board a barge (Figure 2.18). An initial layer of rock should be placed on the fabric immediately to ballast it. Figure 2.18. GEOfabrics installation techniques (2011). For large projects, where sacrificial ballast would be expensive, it is possible to use a steel- wire net laid out on top of the geotextile in place of the rebar, rope, and core. This is wound up inside the fabric and can be fully recovered once an initial layer of ballast (smaller than the steel-wire net apertures) has been placed to ballast the fabric. Questions were raised by the NCHRP 24-42 Panel regarding the potential environmental impacts of placing geotextiles underwater (e.g., long-term degradation, deterioration, toxicity, etc.). Again using the GEOfabrics installation document as representative, not as an endorsement, the following points are made: â¢ Typically, geotextiles are manufactured from fibers, filaments, and yarns formed by the extrusion of polypropylene or polyethylene, individually or in combination. The handling and storage of these products presents little or no health hazard. Raw Materials â¢ The polymers used to manufacture the fibers - polypropylene and polyethylene - are polyolefins derived from oil and are regarded as chemically and biologically inert. â¢ A lubricant is applied to the fibers during their manufacture to aid the subsequent needle punching process. This lubricant, a blend of fatty acid esters and diethanolamide, is added in extremely small quantities - 0.4% by weight. The ecological data from the lubricant supplier refers to the lubricant in concentrated form and even then, it is only considered to be moderately toxic to aquatic organisms. â¢ In some situations a foaming effect may appear on the surface of a geotextile. This is a physical interaction between water and this lubricant. It is a transient effect and has no harmful effects on the environment.
2.40 Typically, geotextile products do not contain: - Chlorofluoro carbons (CFC) - Pentachloro phenols (PC) - Urea formaldehyde or derivatives - Any product capable of forming dioxin - Any toxic substance Potential Hazards Toxicity: the products are regarded as chemically and biologically inert. Inhalation: the products do not release any toxic or obnoxious fumes at ambient temperatures. The fibers are long, greater than 50 mm. They cannot normally be inhaled. Ingestion: the fibers used are inert and regarded as harmless. Certain additives and lubricants may be harmful if ingested in significant quantities. Typically products do not contain quantities of these materials considered to be significant. Skin contact: the products will not cause skin irritation under normal conditions. However, precautionary measures must be taken and employees who have a history of skin disease or allergy should receive medical clearance prior to direct contact. Eye contact: the products are unlikely to come into contact with eye. Loose fibers are not normally released from the products. Flammability: the products will not ignite easily. Melting will occur when heated in air at 165 - 1700C and decomposition will commence at about 3000C with the release of volatile, lower molecular weight hydrocarbons; carbon monoxide, carbon dioxide, water and carbon. In addition, very low concentrations of oxidation and breakdown products, associated with the additives and lubricants, may be released but are regarded as virtually insignificant. Combustion of GEOfabrics' products is similar to most organic materials e.g., wood, paper, and cellulose, thus requiring similar precautions in the event of fire in particular in relation to the carbon monoxide. Explosion: the products do not present an explosion hazard. Preventative Measures Handling precaution: Operatives involved in normal handling and laying of GEOfabrics' products do not require special protective clothing or equipment. Operatives with sensitive skin or allergies are advised to wear gloves and seek medical advice. Standard roll weights range from 235 to 650 kg (520 to 1,400 lb). Mechanical handling and lifting should be used. Storage: The products may be stored inside or outside without special precaution. No environmental impairment will be caused.
2.41 Emergency Action Fire: Toxic fumes are not produced but breathing apparatus may be required to combat smoke and carbon monoxide particularly in confined spaces. Molten burning droplets require resistant clothing and footwear. 2.2.6 U.S. Practice for Installation of Filter Systems Underwater Introduction of Geotextiles in the U.S. As part of early efforts to stabilize streambanks with riprap, observations by the U.S. Army Corps of Engineers (USACE) indicated that placement of a granular filter (sand, gravel, crushed rock) between a riprap blanket and the prepared bank surface resulted in a measurable improvement of revetment stability at sites where the soil material was erodible or in a high energy environment (wave action, eddy currents, prop wash, etc.). A properly designed granular filter effectively reduced the amount of soil being eroded through the riprap blanket, provided a bedding layer for the riprap, and still allowed for natural drainage from the streambank. Without the filter, the integrity of the structure could have been seriously compromised as more and more material was removed from the bank slope through the riprap. Calhoun (1972) contained the first design guidance for geotextile filters for erosion control applications in the U.S. This was followed by a specification in 1977 (USACE 1977) published by the U.S. Army Corps of Engineers. Christopher (1983) provided an evaluation of the long term (10 year) performance of two projects constructed with geotextile filters by the Florida DOT in cooperation with the Corps of Engineers in 1969. Other important early contributions were the 1st International Conference on Geotextiles in Paris in 1977 and 2nd International Conference on Geotextiles in Las Vegas in 1982, both of which had sections on erosion control and papers from both the U.S. and Europe. Also, the FHWA Manual on Geotextile Engineering was published in 1985 and contained an extensive section on design and construction guidance for geotextiles in erosion control applications and formed the basis for the current design method in the FHWA/NHI design guidance (Christopher and Holtz 1985). Keown and Oswalt (1984) note that during the 1960s fabric materials were introduced as a revetment filter for USACE projects where suitable granular materials were not readily available or were not cost-effective due to transportation, quality control, or manpower constraints. Although use of granular filters was still considered as part of the "traditional" approach for revetment design and construction, filter fabric was being used for many projects. The initial use of filter fabric for hydraulic applications can be traced to projects placed by Dutch engineers in 1956. However, in the ensuing years, filter fabric did not find widespread acceptance in the U.S. engineering community. As late as 1967, there were only two domestic sources of fabric, although the use of fabric as a filter under an interlocking block revetment had been reported as early as 1958 in Florida. As the utility of filter fabric became apparent, the U.S. Army Engineer Waterways Experiment Station (WES) conducted a study to determine the extent and diversity of use of this material by USACE Divisions and Districts.
2.42 The findings of the study indicated that although there was wide and varied use of filter fabrics by Corps Districts, a test program was needed to define the engineering properties of the fabrics when used for filter and drainage applications. As part of this program (1967-1972) several filter fabrics (six woven and one non-woven) were evaluated by chemical, physical, and filtration testing. From the results of the WES program and project experience, USACE guide specifications were developed for field use of filter fabric. Prior to fabric placement, the streambank soil surface should be graded to a relatively smooth plane, free of obstructions, depressions, and soft pockets of material. Depressions or holes in the soil should be filled before the fabric is spread since the fabric could bridge such depressions and be torn when the revetment materials are placed. The revetment and fabric should extend below mean low water to minimize erosion at the toe. When the revetment materials and fabric are subject to wave attack, the customary construction practice is to place the fabric strips vertically down the slope of the bank. The upper vertical strip should overlap the lower strip. The fabric usually needs to be keyed at the toe to prevent uplift or undermining. In March 1982 a survey was distributed to USACE users of filter fabric requesting descriptive information on those streambank protection projects where either failure or less than satisfactory performance had occurred that could be specifically attributed to the use of filter fabric. The survey responses not only identified problem areas, but also provided timely guidance, in the form of recommendations and cautions, for dealing with these problems. The problem areas that were identified, as discussed below, reflect the spectrum of USACE experience with filter fabric in the 1980 time frame (Keown and Oswalt 1984). 1. Erosion under the fabric. Small voids and loose fill areas are generally bridged by filter fabric, providing a site for potential erosion. As surface runoff moves downslope between the fabric and bank material, soil loss may occur. Silt, silty sand, and sand banks are particularly susceptible to this problem. 2. Slope failures. This was a widely reported problem. A typical sign that a failure has occurred is a bulge in the fabric near the bank toe and a depression upslope above the bulge. Although not a slope failure, a similar phenomenon may result when erosion occurs under the fabric and material is transported to the toe of the bank which in turn clogs the fabric. Some survey responses indicated that failures could also occur on a saturated slope during rapid drawdown or due to the inability of the fabric to pass flow quickly enough to relieve pore pressure from groundwater flow. 3. Tearing/puncture of the fabric. This problem may lead to entry of large volumes of water or exit of eroded soil. 4. Slippage of revetment material. This type of failure occurs primarily due to poor support at the bank toe or placement of the fabric on a steep slope (greater than IV on 3H). 5. Ultraviolet light. Fabric exposed to sunlight for long periods during storage, construction, or maintenance can suffer a significant loss of strength. 6. Vandalism. Fire can destroy fabric. Designers should be aware of this problem, especially when placing revetments in recreation areas.
2.43 An International Conference in London on flexible armored revetments incorporating geotextiles (Institution of Civil Engineers 1984) provides a "snap shot" (circa 1984) of the state of practice in using geotextile filters (installed both in the dry and underwater). Papers at this conference from practitioners in the U.K. and the Netherlands are summarized in Section 2.2.5 to illustrate initial experience in Europe related to the use of geotextiles as a filter for waterway revetments. The Keown and Oswalt (1984) summary of the introduction of geotextile use as a revetment filter in the U.S. and the Corps' guidance in the 1980 time frame (reference above) was presented at the 1984 London Conference. A second paper by USACE authors at the 1984 London conference reported on case histories involving the use of filter fabric underneath revetments in lower Louisiana, including projects along the Lower Mississippi River and its Gulf outlets. In this paper Dement and Fowler (1984) conclude that flexible articulated mattresses used in conjunction with geotextiles or filter fabrics are a viable method of erosion protection. However, several case histories in Louisiana indicated that selection or proper specifications of filter fabric along with adequate weight of armor unit are essential factors in achieving an acceptable design. Dement and Fowler (1984) note that there are many case histories where "filter cloth" has been shown to be a valuable component (of a revetment armoring system). Private interests, state agencies, and the New Orleans District of the Corps of Engineers have used filter fabric on many projects and on occasion they have built isolated test sections. Cost estimates show that filter fabric is far cheaper than conventional rock filter layers. Control of filter installations, both underwater and in the dry, is more efficient and cost effective with filter fabric and it assures positive coverage. Filters or filter layers should be considered as integral parts of a typical dike, breakwater, jetty, or revetment where the dynamic forces of water such as wind- waves, currents, and ship forces interact on all components of the structure. Dement and Fowler conclude that engineers should continue to investigate the advantages of using certain types of filter fabrics and should challenge the industry to provide better fabrics that will improve the performance of structures. Current Technology and Applications in the U.S. Laboratory Studies and Results. As reported in Section 2.2.4, recent efforts in the U.S. to improve the state of practice for filter design and placement can be traced to the 1998 Scanning Review of European Practice for Bridge Scour and Stream Instability Countermeasures sponsored by TRB/NCHRP and FHWA (Lagasse 1999 and Bryson et al. 2000). The Scanning Review found that: (1) In Germany and the Netherlands, a significant investment has been made in the development and testing of geosynthetic materials, and innovative installation techniques have been developed, and (2) The confidence that European hydraulic engineers have in the use of riprap as a permanent local scour countermeasure is based in part on their use of innovative techniques for placing an effective filter beneath the riprap in flowing or deep water. Concurrent with and following the Scanning Review, efforts to investigate and improve guidance for the installation of pier scour countermeasures included NCHRP Project 24-07(1) (Parker et al. 1998) and the extension of this project with NCHRP Project 24-07(2) (Lagasse et al. 2007). Based on laboratory work completed at CSU (sponsored by Pennsylvania Department of Transportation and the FHWA), Ruff and Fotherby (1995) also touched on installation techniques for placing a geotextile filter beneath prefabricated armor units for pier scour protection. The following sections summarize results of these laboratory investigations and the resulting recommendations for placing filters underwater.
2.44 NCHRP 24-07(1). NCHRP Project 24-07(1) "Countermeasures to Protect Bridge Piers from Scour" was implemented by the University of Minnesota, St. Anthony Falls Laboratory supported by additional laboratory work at the University of Auckland, New Zealand. The focus of this project was to investigate alternatives to standard riprap installations as pier scour countermeasures. One significant finding was that under flood conditions in sand bed streams, riprap placed in the absence of a geotextile or granular filter layer gradually settled and lost effectiveness over time even under conditions in which the riprap was never directly mobilized by the flow. This settling is due to deformation and leaching of sand associated with the passage of bedforms. Riprap performance can be considerably improved with the use of a geotextile. Another countermeasure that provides excellent protection is a mattress of cable tied blocks (articulating concrete blocks) underlain by a geotextile tied to the pier (Parker et al. 1998). As a task under this project during 1996, the St. Anthony Falls research team conducted an extensive field survey of standard practice and problems. Field trips were selected based upon the need to effectively review the performance of alternative countermeasures under as large a variation of the many controlling parameters as possible. Trips covered all four corners of the contiguous United States and included visits to five to ten bridge sites in each of 18 states. Inspections focused on bridge pier scour countermeasures and the use of granular and geotextile filters, but also included other installations (such as abutments) from which valuable performance information could be gleaned. It was found that the vast majority of riprap field installations were of the dumped variety. Riprap performance was effective in many situations but varied significantly depending on placement technique. The method of placement ranged from simply dumping from a truck to hand placement of each stone. Geotextiles were used in many newer installations with mixed success. Geotextile performance was highly dependent upon the material's ability to resist rupture, as tearing of the fabric reduces its effectiveness greatly. Similarly, the installation of the geotextile edge is critical to its effectiveness. Numerous installations were observed where the edge of the fabric had peeled back and was flapping in the flow. Key findings from this field survey included: â¢ Two primary methods of failure were noted for properly sized riprap: 1) Instability of the river bed. 2) Failure caused by an inadequate filter. â¢ Countermeasure failure due to stream instability was consistently reported by the host engineers in most states. Many designers and nearly all maintenance personnel simply do not have the tools to effectively address stream stability issues. â¢ Geotextiles must be placed so that no gaps are present, or can form, between the geotextile and any structure it is protecting. Based on the findings of the bridge site visits and laboratory testing, Parker et al. (1998) developed a set of implementation notes for geotextile filters and granular filter layers when used in conjunction with countermeasures. The research indicated that under flood conditions in sand bed streams with developed bedforms the leaching of sand from the interstices of any armoring countermeasure may ultimately result in failure of the countermeasure. With this in mind, and in light of the positive results of experimental testing, it was recommended that any armoring countermeasure in a sand bed stream be underlain by an appropriately selected geotextile filter. The following additional recommendations were made: â¢ The areal cover of the geotextile filter should be less than that of the armoring countermeasure in order to allow for anchoring of the edges of the geotextile filter.
2.45 â¢ The porosity of the geotextile filter should be sufficient to allow release of pore pressures without causing uplift of the fabric under flood conditions. The selection of a relatively open fabric may be advantageous. Such a selection may encourage the formation of a natural granular filter layer below the geotextile filter. Failure to properly release the buildup of pore pressure may lead to uplift and catastrophic failure of the geotextile filter and overlying armoring countermeasure. â¢ The geotextile filter should be resistant to tearing or puncturing during countermeasure placement or settling. Testing indicates that even gaps as small as 0.25 in. can allow the leaching of a significant quantity of bed material. â¢ The geotextile filter should have a lifetime of at least 20 years without decay when placed on the bed of a natural river in the vicinity of a bridge pier. â¢ The geotextile filter should be fabricated from ultraviolet light resistant materials. â¢ Geotextile filters are not recommended for gravel bed streams both due to the abrasive nature of gravel and its low potential for leaching. Granular filter layers were not tested experimentally in this study. Information collected in the course of the field site visits and in consultation with other engineers indicated that the underwater installation of granular filter layers can be difficult, and is often omitted even though recommended by e.g., Neill (1973). In addition, granular filter layers may be subject to reworking by river bedforms such as dunes, and thus may fail where a geotextile filter would not. This notwithstanding, the overall accumulation of experience with granular filter layers indicates that they are recommended for use in place of a geotextile filter in the event that a geotextile filter cannot be installed. NCHRP 24-07(2). As noted above, NCHRP Project 24-07(1), completed in October 1998 by the University of Minnesota, was undertaken to investigate the performance of various countermeasures for pier protection. Under NCHRP Project 24-07(2), Ayres Associates Inc was contracted to extend the results and applicability of the earlier project by developing and recommending practical selection criteria for bridge pier scour countermeasures; guidance and specifications for design and construction of the suitable countermeasures; and guidance for inspection, maintenance, and performance evaluation of the countermeasures. The countermeasures addressed include riprap, partially grouted riprap, articulating concrete block systems, gabions, grout-filled mattresses, and geotextile sand containers (used as a filter). Laboratory testing was conducted at the Colorado State University Hydraulics Laboratory on 8 in. (200 mm) square piers for a variety of countermeasures, and, for partially grouted riprap and the use of geotextile sand containers as a filter, at a prototype scale pier. As noted, this project included a site visit to Germany in 2001 by the research team to update information gained in the 1998 scanning review of European practice, and to investigate techniques for placing partially grouted riprap and installation of filters underwater using, primarily, geotextile containers (see Figures 2.9 through 2.13). NCHRP Project 24-07(1) (Parker et al. 1998) determined that placing a geotextile under a riprap layer with the same areal coverage as the riprap layer resulted in a relatively poor performance of the riprap. Parker et al. suggested that extending the geotextile from the pier to about 2/3 of the way to the periphery of the riprap would result in better performance. Additional test results for NCHRP Project 24-07(2) (Lagasse et al. 2007) confirmed that riprap performance was best when a geotextile filter extended 2/3 the distance to the periphery of the riprap (Lagasse et al. 2007). Figure 2.19 shows two piers after testing, one pier had a geotextile filter that extended 2/3 the distance from the pier face to the periphery of the riprap (Figure 2.19a) and the other pier had a granular filter that extended the full distance from the
2.46 pier face to the periphery of the riprap (Figure 2.19b). The enhanced performance of the 2/3 extent geotextile filter is obvious. a. Test 5d, riprap with 2/3 extent geotextile filter. b. Test 5d, riprap with full extent granular filter, note displacement of riprap. Figure 2.19. Testing of granular and geotextile filters. Based on laboratory testing, it was found that granular filters performed poorly in the case where bed forms are present. Specifically, during the passage of dune troughs past the pier that are deeper than the riprap armor, the underlying finer particles of a granular filter are rapidly swept away. The result is that the entire installation became progressively destabilized beginning at the periphery and working toward the pier. Thus, in cases where dune-type bed forms may be present, it was strongly recommended that only a geotextile filter be considered. For NCHRP Project 24-07(2) the use of sand filled geotextile containers as a filter under riprap was tested at a prototype scale pier (Lagasse et al. 2007). A test section was created that was 30.7 ft (9 m) long and spanned the width of the flume. It was filled with sand level with the approach section. Upstream and downstream of the test section the flume bed consisted of smooth concrete floors. A rectangular pier measuring 1.5 ft (0.5 m) by 4.5 ft (1.5 m) was installed in the center of the test section. Figure 2.20 is a layout diagram for the prototype testing program. Surrounding the pier, a scour hole measuring 12 ft by 16 ft (4m x 5 m) was pre-formed into the sand bed to a maximum depth of 3 ft (0.4 m) as shown in Figure 2.20. For the geotextile containers the test at prototype scale was, primarily, to demonstrate constructability in flowing water and performance in high velocity flow conditions. Sand filled geotextile containers were constructed using a geotextile fabric with the characteristics presented in Table 2.3. The geotextile containers measured 4 ft x 1.5 ft x 0.33 ft (1.2 m x 0.5 m x 0.1 m) with a typical volume of 2 ft3 (0.6 m3). Approximately 220 lbs (100 kg) of sand was placed in each bag. Commercial concrete sand meeting appropriate filter criteria was used to fill the geotextile container. Figure 2.21 shows the geotextile containers before being placed around the pier.
2.47 Figure 2.20. Schematic layout for sand filled geotextile containers and riprap tests (dimensions approximate). Table 2.3. Characteristics of Geotextile used for NCHRP Project 24-07(2). Trade Name Mass per Unit Area AOS Permeability Geotextile Type Kg/Ks MirafiÂ® 180 N 278 g/m2 (8 oz/yd2) 0.18 mm 0.21 cm/s Nonwoven needle punched 5.25 Figure 2.21. Geotextile containers before installation around the pier. An approach flow 1 ft (0.305 m) deep at approximately 1.5 ft/s (0.5 m/s) was established. A total of 32 geotextile containers were placed around the pier by dropping from a height of about 5 ft (1.5 m) above the water surface. Installation was facilitated by a backhoe fitted with a special grapple attached to the bucket, which enabled the backhoe to pick up the geotextile container, position it around the pier to a specified location, and release the container. Figure 2.22 is a photograph of a geotextile container being dropped near the pier; note the grapple plate attachment to the backhoe. Figure 2.23 shows the geotextile containers after installation in flowing water. FLOW 9 m PLAN VIEW 6 m 1.5m 0.5m Pier PROFILE Pier Riprap Sand-filled geotextile containers 0.4m 0.5m/s Sand Concrete Concrete
2.48 Figure 2.22. Installation of geotextile containers, pier is on the left. Next, riprap was positioned on top of the geotextile containers using the backhoe with the grapple removed. Figure 2.24 shows riprap being dropped near the pier and Figure 2.25 shows the riprap after installation. Riprap was placed so that the riprap/filter relationship followed the 2/3 extent rule. These tests confirmed that geotextile containers can be fabricated locally and that the containers and riprap can be placed in flowing water with standard commercially available equipment. Figure 2.23. Geotextile containers after installation.
2.49 Figure 2.24. Installation of riprap around pier. Figure 2.25. Riprap armor over geotextile containers. A review of the conclusions and recommendations outlined in NCHRP Report 593 (Lagasse et al. 2007) indicates that for each countermeasure type tested a range of commonalities and contrasts can be identified for these systems. In most cases a filter layer is essential for successful performance of all (pier) scour protection. However for the countermeasures that incorporate rock particles, including gabions, the filter should extend only two-thirds of the distance from the pier to the perimeter of the armor. In contrast, Articulating Concrete Block (ACB) mats and grout-filled mattresses should have a filter underlying the full extent of the armor layer. In all cases, a granular filter should not be used when dune-type bed forms are expected in sand channels (i.e., under live-bed conditions). For the ACB system, granular filters are not recommended under most conditions. During testing, geotextile filters generally performed well for all countermeasure types when all components of the countermeasure system were properly designed and installed.
2.50 Geotextile sand containers are strongly recommended as a practical, proven, and effective technique for placing a filter under water for riprap or partially grouted riprap, and gabion and grout-filled mattresses. For the partially grouted riprap/geotextile container filter system described above, the flume was reconfigured to achieve a maximum velocity at the pier with the flow discharge available (Figure 2.26). The high-velocity test ran for 4 hours, during which time the discharge was steadily increased to the full flow capacity (160 cfs (4.5 m3/s)). At maximum discharge, the approach velocity upstream of the pier reached a maximum of 6.4 ft/s (2 m/s). The estimated local velocity at the pier was approximately 11 ft/s (3.4 m/s). The partially grouted riprap armor with a geotextile container filter remained essentially undisturbed during this test run (Lagasse et al. 2007). For the ACB systems, a conventional geotextile filter should be used because placement and grading tolerances would be difficult to meet if geotextile containers were used as a filter. For scour countermeasures consisting of a thin veneer of armor (ACBs and the mattresses), termination details and, where necessary, anchor systems play a significant role in successful performance. Figure 2.26. High velocity test of geotextile container filter with riprap armor. These recommendations and detailed design guidance resulting from NCHRP Project 24- 07(2) were incorporated into the 2009 update of FHWA's Hydraulic Engineering Circular 23 (Lagasse et al. 2009). Design Guideline 11 "Rock Riprap at Bridge Piers" incorporates the results and recommendations from prototype scale testing of geotextile containers outlined above and Design Guideline 16 "Filter Design" includes the detailed design guidance summarized in Section 2.2.3 (above) for design of granular and geotextile filters. Bridge Scour Protection System Using Toskanes. Based on laboratory work completed at CSU (sponsored by Pennsylvania Department of Transportation and the FHWA), Ruff and Fotherby (1995) briefly considered installation techniques for placing a geotextile filter beneath prefabricated armor units for pier scour protection. The following paragraphs summarize the results of this research.
2.51 A bridge scour countermeasure was investigated during this study. A non-proprietary concrete hammer head prefabricated block called a Toskane was developed and tested as a hydraulic model (Figure 2.27). Over 400 test runs were conducted. These tests included pier and abutment scour without Toskanes, random and pattern placement of Toskanes tested to failure around piers and abutments, riprap tested to failure, protective pad radius determination, pad height comparing level with surface installations, and comparison of gravel and geotextile filters. Figure 2.27. Colorado State University Toskane units (Ruff and Fotherby 1995). The study report Ruff and Fotherby (1995) includes recommendations for placing either a geotextile fabric filter or a granular material filter under the pad. Design examples are included for installing Toskanes at five existing Pennsylvania bridges. Cost estimates for installation and techniques for installing are also presented. Four sets of tests were conducted to look at the effects of pattern placement, and also, the effects of a filter placed under the Toskanes. Toskanes can be randomly dumped into position, or they can be individually placed in a consistent pattern. In several tests, a geotextile filter was added to a 1-2 layer pattern placement, and to a randomly placed pad of Toskanes. The geotextile was a non-woven polypropylene fabric. A circle was cut in the center of the geotextile to fit snugly around the pier. The outer edge of the cloth was trimmed to a circle slightly larger than the Toskane pad. The edges were periodically slit so the cloth could be folded down at about 45Â° to 60Â°from the horizontal, and anchored into the sand. The Toskanes were placed directly on top of the filter cloth. As a result of these tests, the following recommendations were made (Ruff and Fotherby 1995): â¢ Toskanes are a viable alternative for rock riprap in many cases. At some sites, rock with the proper dimensions, weight, and freeze-thaw durability may not be available locally and/or may be costly. Riprap, even when properly specified is difficult to inspect and evaluate at the quarry and especially on site. â¢ Toskanes should be placed on a granular filter or a geotextile filter that meets state DOT specifications for riprap filters, if construction is performed in the "dry." For construction in water, a geotextile filter cloth is recommended. The geotextile cloth can be attached to a rebar frame and lowered into the water. A detail of the frame is shown in Figure 2.28. If the framework is composed of several sectional frames, installation should begin at the
2.52 furthest downstream section to be covered. The upstream frame and cloth should overlap the downstream frame and cloth at least 0.5 m. Placement of Toskanes on top of the filter cloth can proceed immediately, without interfering with the placement of the adjacent framework and filter cloth. Figure 2.28. Rebar frame for placing geotextile filter underwater (Ruff and Fotherby 1995). Federal Agency Guidance. U.S. Federal Agency technical reports and memoranda were reviewed for specific guidance on filter placement (either in the dry or underwater). Based on the paucity of specific guidance located in the literature (other than from FHWA), a follow-up series of consultations were conducted with knowledgeable individuals in the U.S. Army Corps of Engineers (Engineering Research and Development Laboratory), U.S. Bureau of Reclamation (Denver), and the USDA NRCS. The very limited findings found in the literature supplemented by personal communications from various agency aspects are summarized in the following paragraphs. FHWA. Of the U.S. Federal agencies, the FHWA currently provides the most comprehensive guidance for installing filters underwater. This guidance is based, primarily, on the recommendations and detailed design guidance resulting from NCHRP Project 24-07(2) which were incorporated into the 2009 update of FHWA's Hydraulic Engineering Circular 23 (Lagasse et al. 2009). Design Guideline 11 "Rock Riprap at Bridge Piers" incorporates the results and recommendations from prototype scale testing of geotextile containers outlined in the previous section (see text associated with Figures 2.21 through 2.26). HEC-23 Design Guideline 16 "Filter Design" includes the detailed design guidance summarized in Section 2.2.3 (above) for design of granular and geotextile filters. Arneson et al. (2010) in an article for Geo-Strata summarize FHWA's current guidance on granular filter design and reference HEC-23 for greater details regarding the design of scour and erosion countermeasures. Arneson (formerly FHWA National Hydraulics Team Senior Hydraulic Engineer) notes that the Cistin-Ziems methodology, adapted from European practice and now recommended by FHWA, has proven to be a more robust design method for the filtering design of bridge scour countermeasures. But the success of a well-performing countermeasure is only partly due to filtering, so interested readers should refer to NCHRP Report 593 "Countermeasures to Protect Bridge Piers from Scour" (http://onlinepubs.trb.org/ onlinepubs/nchrp/nchrp_rpt_593.pdf) and FHWA Hydraulic Engineering Circular No. 23 "Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design
2.53 Guidance" - Third Edition (http://www.fhwa.dot.gov/engineering/ hydraulics/ pubs/ 09111/091216.cfm). Although the Arneson et al (2010) Geo-Strata article does not provide specific filter installation techniques (other than by reference to the publications cited), it does provide a sketch of a typical granular filter under a riprap revetment (Figure 2.29). The article concludes with the observation that as these new tools find their way into practice, the prevalence of scour as the main cause of bridge failure in the U.S. is anticipated to significantly diminish. Figure 2.29. Typical filter detail beneath streambank armor (Arneson et al. 2010). For many years, the FHWA NHI has included in its curriculum a training course on Geosynthetic Design and Construction Guidelines (NHI Course No. 132013). See Appendix B for FHWA/NHI filter design guidance. The latest edition of the course Reference Manual (Holtz et al. 2008) acknowledges issues related to geotextile placement under wave conditions and for stream bank and scour protection underwater (Section 3.8 Geotextile Installation Procedures). This Reference Manual notes that construction requirements will depend on specific application and site conditions and provides photographs of a range of erosion control installations, including: wave protection, river shoreline protection, and stream applications (Figure 2.30). General guidance in the Manual includes: â¢ Place geotextile loosely, laid with machine direction in the direction of anticipated water flow or movement. â¢ For streambank and wave action applications, the geotextile must be keyed in at the bottom of the slope. If the riprap-geotextile system cannot be extended a few yards above the anticipated maximum high water level, the geotextile should be keyed in at the crest of the slope. The geotextile should not be keyed in at the crest until after placement of the riprap. Alternative key details are shown in Figure 2.31. Holtz et al. (2008) observed that construction requirements will depend on specific application and site conditions. In some cases, geotextile selection is affected by construction procedures. For example, if the system will be placed below water, a geotextile that facilitates such placement must be chosen. The geotextile may also affect the construction procedures. For example, the geotextile must be completely covered with riprap for protection from long- term exposure to ultraviolet radiation. Sufficient anchorage must also be provided by the riprap for weighting the geotextile in below-water applications. Other requirements related to specific applications are depicted in Figure 2.32.
2.54 Figure 2.30. Erosion control installations: (a) installation in wave protection revetment; (b) river shoreline application; and (c) stream application (Holtz et al. 2008).
2.55 Figure 2.31. Construction of hard armor erosion control systems (a, b, after Keown and Dardeau 1980; c after Dunham and Barrett 1974) (Holtz et al. 2008).
2.56 Figure 2.32. Special construction requirements related to specific hard armor erosion control applications (Holtz et al. 2008).
2.57 For streambank protection, selecting a geotextile with appropriate clogging resistance to protect the natural soil and meet the expected hydraulic conditions is extremely important. Should clogging occur, excess hydrostatic pressures in the streambank could result in slope stability problems. Do not solve a surface erosion problem by causing a slope stability problem. Detailed data on geotextile installation procedures and relevant case histories for streambank protection applications are given by Keown and Dardeau (1980). The geotextile should be placed on the prepared streambank with the machine direction placed parallel to the bank (and parallel to the direction of stream flow). Adjacent rolls of geotextile should be seamed, sewed, or overlapped; if overlapped, secure the overlap with pins or staples. A 1 ft (0.3 m) overlap is recommended for adjacent roll edges, with the upstream roll edge placed over the downstream roll edge. The upslope roll should overlap the downslope roll (Holtz et al. 2008). Roll ends should be overlapped 3 ft (1 m) and offset as shown in Figure 3.42a. If strong water movements are expected, the geotextile must be keyed in at the top and toed in at the bottom of the embankment. The riprap or filled geocells should be extended beyond the geotextile 18 in. (0.5 m) or more at the toe and the crest of the slope. If scour occurs at the toe and the surface armor beyond the geotextile is undermined , it will in effect toe into the geotextile. The whole unit thus drops, until the toed-in section is stabilized. However, if the geotextile extends beyond the stone and scour occurs, the geotextile will flap in the water action and tend to accelerate the formation of a scour hole or trench at the toe. Alternative toe treatments are shown in Figure 2.31. Because of cyclic flow conditions, geotextiles used for wave protection systems in most cases should be selected on the basis of severe criteria (Holtz et al. (2008). The geotextile is unrolled and loosely laid on the smooth graded slope. The machine direction of the geotextile should be placed parallel to the slope direction rather than perpendicular to the slope, as was recommended in streambank protection. Thus, the long axis of the geotextile strips will be parallel to anticipated wave action. Sewing of adjacent rolls or overlapping rolls and roll ends should follow standard construction requirements except that a 3 ft (1 m) overlap distance is recommended by the USACE for underwater placement. Again, securing pins should be used to hold the geotextile in place. If a large part of the geotextile is to be placed below the existing tidal level, special fabrication and placement techniques may be required. It may be advantageous to pre-sew the geotextile into relatively large panels and pull the prefabricated panels downslope, anchoring them below the waterline Depending upon the placement scheme used, selection of a floating or nonfloating geotextile may be advantageous. In some cases with very strong storm waves, composite mats made of geotextiles, fascines, and other bedding materials are constructed on land, rolled up, and then unrolled off of an offshore barge with divers and weights facilitating underwater placement. Because of potential wave action undermining, the geotextile must be securely toed-in using one of the schemes shown in Figure 2.31. Also, a key trench should be placed at the top of the bank, as shown in Figure 2.31a, to prevent revetment stripping should the embankment be overtopped by wave action during high-level storm events. Riprap or cover stone should be placed on the geotextile from downslope to upslope, and stone placement techniques should be designed to prevent puncturing or tearing of the geotextile. Drop heights should follow standard construction criteria. Riprap or cover stone can also be placed underwater by cranes or bottom dump barges.
2.58 Scour caused by high flows around or adjacent to structures in rivers or coastal areas generally requires scour protection for structures. Scour protection systems generally fall under the critical and/or severe design criteria (see Appendix B) for geotextile selection (Holtz et al. 2008). An extremely wide variety of transportation-associated structures are possible and, thus, numerous ways exist to protect such structures with riprap/geotextile systems. In all instances, the geotextile is placed on a smoothly graded surface following general construction requirements. Such site preparation may be difficult if the geotextile will be placed underwater, but normal stream action may provide a fairly smooth streambed. In bridge pier protection or culvert approach and discharge channel protection applications, previous high-velocity stream flow may have scoured a depression around the structure. Depressions should be filled with granular cohensionless material. It is usually desirable to place the geotextile and riprap in a shallow depression around bridge piers to prevent unnecessary constriction of the stream channel. Securing the geotextile in the proper position may be of extreme importance in bridge pier scour protection. However, under high-flow velocities or in deep water it will be difficult, if not impossible, to secure the geotextile with steel pins alone. Underwater securing methods must then be developed, and they will be unique for each project. Alternative methods include floating the geotextile into place, then filling from the center outward with stones, building a frame to which the geotextile can be sewn; using a heavy frame to submerge and anchor the geotextile; or constructing a light frame, then floating the geotextile and sinking it with riprap. In any case, it may be desirable to specify a geotextile which will either float or sink, depending upon the construction methods chosen. In general, geotextiles with a bulk density greater than 1 g/cm3 will sink provided the air contained in the geotextile can be readily removed by submersion) while those less than 1 g/cm3 will float (Holtz et al. 2008). Other Federal Agency Guidance. The U.S. Army Corps of Engineers (USACE) EM 1110-2- 1601 (1991) (Change 1 dated June 1994) was reviewed and found to contain no specific guidance on filter installation/placement. Under a subsection on gradation (of riprap revetment stones) EM 1601 provides the following cross reference: "Filters may be required under riprap revetments. Guidance for filter requirements is given in EM 1110-2-1901. Filter design is the responsibility of the Geotechnical Branch in each District." In EM 1901 (USACE 1986), Appendix D addresses filter design from a seepage control point of view and states that the objective of filters and drains used as seepage control measures for embankments is to efficiently control the movement of water within and about the embankment. In order to meet this objective, filters and drains must, for the project life and with minimum maintenance, retain the protected materials, allow relatively free movement of water, and have sufficient discharge capacity. For design, these three necessities are termed, respectively, piping or stability requirement, permeability requirement, and discharge capacity. Under a section on construction guidance, EM 1901 refers to EM 1911 and EM 2300 and highlights the major concerns as: â¢ Prevention of segregation, particularly well-graded filters, during handling and placement. â¢ Gradation should be monitored closely so that designed filter criteria are met.
2.59 â¢ Thickness of layers should be monitored to ensure designed discharge capacity and continuity of the filter. â¢ Quality control/assurance is very important during filter construction because of the critical function of this relatively small part of the embankment An Army/Air Force Technical Manual on "Engineering Use of Geotextiles" was released in July 1995 (Departments of the Army and the Air Force 1995). Again, only a passing reference to filter placement is made. Chapter 6 "Erosion and Sediment Control" of this document notes that for bank erosion there are a number of special design considerations, including: â¢ Durability - consider chemical, biological, thermal, and ultraviolet (UV) stability. â¢ Strength and abrasion resistance - consider the size, weight, and shape of the armor stone, the handling placement techniques (drop height), and the severity of the conditions (stream velocity, wave height, rapid changes of water level, etc.). Abrasion can result from movement of the cover material as a result of wave action or current. â¢ Cover material - provide a protective covering over the geotextile that minimizes or dissipates the hydraulic forces, protects the geotextile from extended exposure to UV radiation, and keeps it in intimate contact with the soil. â¢ Anchorage - if strong water movements are expected, the geotextile needs to be anchored at the crest and toe of the streambank. â¢ Underwater placement - if the geotextile must be placed below low water, a material of a density greater than that of water should be selected. Construction considerations in the Army/Air Force Manual include: â¢ Site preparation â¢ Placement of geotextiles (currents parallel to the bank) - when used for streambank protection, where currents acting parallel to the bank are the principal erosion forces, the geotextile should be placed with the longer dimension (machine direction) in the direction of anticipated water flow. The upper strips of the geotextile should overlap the lower strips (Figure 2.33). â¢ Placement of geotextiles (wave attack) - when used for wave attack or cut and fill slope protection, the geotextile should be placed vertically down the slope (Figure 2.33) and the upslope strips should cover the downslope strips. â¢ Overlaps, seams, securing pins - adjacent geotextile strips should have a minimum overlap of 12 inches along the edges and at the end of rolls. For underwater placement, minimum overlap should be 3 feet (see Figure 2.33). â¢ Placement of cover material on geotextile - for sloped surfaces, placement of the cover stone or riprap should start from the base of the slope moving upward and preferably from the center outward to limit any partial movement of soil because of sliding. In no case should drop heights which damage the geotextile be permitted. Testing may be necessary to establish an acceptable drop height.
2.60 Figure 2.33. Geotextile placement for currents acting parallel to bank or for wave attack on the bank (Departments of the Army and Air Force 1995). To validate that the major federal agencies involved in streambank protection, revetment design, and shore protection provide no additional guidance (internal memoranda, unpublished procedural guidance, etc.) personal contact was made with several past and present agency experts on riprap, revetment design, and filter requirements. The consensus of the experts consulted is that agency guidance on the specific topic of underwater placement of granular and geotextile filters is scarce. Hughes (2006) from the USACE Engineer Research and Development Center (ERDC) provides guidance on the use of geogrid mattresses filled with small stones as erosion protection or as filters underneath larger stone in marine environments. Geogrid panels are laced together to form mattress-shaped baskets that are filled with small stones similar to construction of gabions. Mattresses may be placed over geotextile by attaching the geotextile to the underside of the mattress before placement. If the mattress is to be placed on top of a geotextile filter cloth, it may be feasible to preattach the filter cloth to the bottom of the mattress with provision for sufficient overlap of geotextile to assure complete coverage at placement. Hughes (2006) notes that underwater operations may require guidance by divers to ensure placements are accurate. Although not related to revetment design or erosion control directly, additional documents containing relevant information include a Corps report (in support of the Environmental Protection Agency) describing use of geotextiles when placing riprap caps on top of contaminated bottom sediments to prevent scour and erosion (Palermo et al. 1998). The contaminated sediments (either in-situ or relocated by dredging) may be very soft, so the geotextile layer prevents the riprap from simply flushing the contaminated sediments away rather than isolating them in place (Figure 2.34). In some cases, sand instead of riprap is used to cap very soft sediments, and the geotextile layer serves to increase the stability of the resulting mound. Several techniques for placing sand caps are described including
2.61 dumping from barges, pumping through tremies or diffusers, and placement from land- based equipment (see schematics in Figure 2.35). Figure 2.34. Schematic of underwater mound of contaminated dredged material protected by layer of geotextile and stone (Maynord 1998). Maynord (1998) prepared an appendix (Armor Layer Design) to the Palermo et al. 1998 report that provides design guidance for granular filters. This appendix has limited, but relevant, comments about construction. For example, "Geotextile filters are less expensive but have not been around long enough to completely evaluate the potential for clogging of the geotextile over long time periods. Problems can occur with geotextiles if the permeability factor is too low. Gas and advective ground water may displace a cap that has too low a permeability. Uncertainty in design should err on the side of providing too large a permeability. A sand layer on top of fine-grained sediments may be required prior to placement of either a granular or geotextile filter. A bedding layer of granular material (sand or gravel) may be placed on top of the geotextile to prevent damage during placement of the riprap." and: "Underwater placement presents uncertainties with even coverage of stone, and a 50 percent increase in granular filter and riprap volume is required. Placement of geotextiles in shallow depths and low velocity can be accomplished as described in the Appendix C case studies, by the method shown in the main body of this report or by attaching the fabric to a framework and lowering the framework into position prior to stone placement. Underwater placement in moderate to high velocity (> 2 ft/sec) would present significant problems with geotextile placement. With a granular filter, a diver may be required to insure adequate coverage in deep placement conditions [emphasis added]. The reference to Appendix C in the quote above is to Ling and Leshchinsky (1998) (Department of Civil Engineering, University of Delaware) who developed a set of case studies for the Palermo et al. 1998 EPA report. This appendix features the use of geotextiles and sand to cap contaminated sediments; however, little is said about installation.
2.62 Figure 2.35. Techniques for placing sand caps on contaminated sediments underwater (Palermo et al. 1998).
2.63 2.2.7 Coastal and Offshore Applications There are additional applications of geotextiles for a variety of purposes (e.g., geotubes used as a construction element), particularly in the coastal environment. For most of these applications, the geotextiles do not have, primarily, a filter function, but the methods of placement under water provide additional insights on techniques that may have application to the problems of placing geotextile filters in situations involving deep water or high velocities and/or turbulence. The following paragraphs extend this survey of current practice with an overview of these other applications. Coastal and Near Shore Applications A recent paper by Bezuijen and Pilarczyk (2012) at the 5th European Geosynthetics Congress on geosynthetics in coastal engineering deals with two applications of geotextiles in coastal and hydraulic engineering: geotextile in filters and revetments, and geotextiles in sand filled structures. The discussion of geotextiles in filters and revetments centers on granular filter design including grain size distribution and filter criteria. They point out that, today, geotextiles often replace granular filters in revetment applications; and note that in applications such as coastal engineering, the filter properties of geotextiles are only secondary. For example, in geotextile tubes and containers the geotextile is used as 'wrapping material' to create large units that will not erode during wave attack. The structures with geotextile tubes and containers serve as an alternative for rock based structures. The first of these structures were "more or less constructed by trial and error," but research on the shape of the structures, the stability under wave attack, and the durability of the material used has made it possible to use design tools for these structures. The morphological aspects of these structures have also been investigated. This is important because structures with geotextile tubes fail regularly due to insufficient toe protection against the scour hole that develops in front of the structure, leading to undermining of the structure. Recent research has also led to better understanding what mechanisms determine the stability under wave attack, including the degree of filling and the position of the water level with respect to the tube (Bezuijen and Pilarczyk (2012). With regard to the use of geotextiles as bags or tubes Bezuijen and Pilarczyk (2012) provide an overview of applications in the coastal zone and compare and contrast the use of geocontainers and geotubes. They observe that in principle sand filled synthetic tubes and sand filled geosynthetic containers are quite comparable. Geotextile is wrapped around sand or slurry to make a large structure that is stable in current and wave attack. Tubes are used on shore or in shallow water where a large geotextile tube is filled with sand by means of hydraulic fill. Dimensions can be up to 4 m high. In contrast, geosynthetic containers are used in deeper water. They are constructed in a split barge and sailed to the desired position. There, the split barge opens and the geosynthetic container is dumped in position (Pilarczyk 2000). With respect to the water depth, sand filled geosynthetic tubes and geosynthetic containers are complementary. Tubes need rather shallow water or construction on shore. Containers need deeper water, because the split barge must be able to sail over the dumping location. Geosynthetic tubes are mostly used for coast or shore protection. Geosynthetic containers are used to make steep slopes on a sand dike, hanging beaches and also to dump polluted slurry in a way that it does not pollute large areas of the seabed.
2.64 Bezuijen and Pilarczyk (2012) provide the following observations: â¢ Geosynthetics and geosystems constitute potential alternatives for more conventional materials and systems. They deserve to be applied on a larger scale. However, doubts among specifying authorities and design engineers about the quality of the design criteria for some of the products, and their long-term performance, are still limiting factors in the increased use. â¢ There are still many uncertainties in the existing design methods. Therefore, further improvement of design methods and more practical experience under various loading conditions is still needed. Several investigators have studied the stability of geobags as a slope-protection unit in coastal applications (Ray 1977; Jacobs and Kobayashi 1983; Gadd 1988; Pilarczyk 1998). Pilarczyk (1990) provided an empirical equation for stability of revetment material under flow attack. His formula can be used for different materials--such as riprap, geobags, geomattresses, gabions, and block or block mats--using different coefficients provided for each material. As a follow on to the preceding overview, Bezuijen and Vastenburg (2012) provide a text on design rules and applications for geosystems. In "Geosystems Design Rules and Applications" four types of geotextile sand elements are distinguished, each with specific properties: geo-bags, geo-mattresses, geotextile tubes, and geotextile containers. The focus is on the use of geosystems filled with sand for construction in river and coastal engineering. They define geotextile encapsulated sand elements as three-dimensional systems manufactured from textile materials, non-woven materials or combinations of textile and non- woven materials that are filled with sand on-site. These systems are relatively new and the number of applications is growing in river and coastal engineering. Quite often geosystems are mentioned as a possible solution, but planners, designers, and contractors feel rather hesitant about the application of geotextile encapsulated sand elements due to a lack of experience and adequate design rules [emphasis added]. The use of geosystems has the advantage that local material can be applied and that no (expensive) quarry stone needs to be extracted and transported from the mountains to the site. Compared to traditional construction methods (with quarry stone) the application of geotextile sand filled elements may add considerable operational advantages to the execution of marine works and may offer attractive financial opportunities. This text does not appear to provide guidance on underwater installation and was not reviewed beyond the abstract. Restall et al. (2002) summarize case studies showing the development and use of geotextile sand containers in Australia. The authors have been involved in the manufacture and installation of geotextile containers in a variety of forms since early 1984 in Australia (17 years experience in the field). This paper outlines the historical development of the types of materials used for geotextile containers and the diversity of applications in which these containers are being used. The type of geotextile used for the containers varies depending on installation conditions. This information has been compiled from years of experience. The range of application for these products is extensive and covers areas such as scour protection, groines, berms, artificial reefs and containment of hazardous materials. This form of coastal protection has developed to such a stage that in many cases it is no longer regarded as an alternative construction method but rather the desired solution; however, there is a great deal still to be learned from this type of protection. Initially the main emphasis was on hydraulically filled geotextile tubes (typically 1.2 m3) used mainly as groines to protect beaches. With time this focus has changed to individual containers used in coastline protection and marine structures (reefs). As papers in Geotextiles and Geomembranes are not available in the open literature, this document was not reviewed further; however, it does serve to document the growing use and continued interest in the use of geotextile containers and tubes in coastal applications internationally. The following paper references Restall et al. (2002) and, presumably, contains some of the same material.
2.65 Restall et al. (2004) report on Australian and German experiences with geotextile containers for coastal protection. They note that encapsulating or wrapping soil materials into geotextiles provides a variety of flexible, economical, and ecological applications in the field of hydraulic engineering. Geotextile containers and tubes are used for dam and dike flood emergency protection and also as construction elements for erosion control, scour fill, reefs, groines, dams, breakwaters, and dune revetments. New shore protection structures, especially at sandy coasts, are increasingly required which have less ecological and visual impacts than conventional structures. Furthermore, these reinforcement/protection solutions are more cost effective, which implies the use of local material without any heavy equipment, especially when the required infrastructure is not available. Geotextiles used for sand filled containers are subjected to significantly different forces than geotextiles used in the conventional drainage and separation applications. These differences must be taken into account when designing these structures. Restall et al. (2004) describe the following issues which should be considered when designing a sand filled geotextile container. â¢ UV resistance â¢ Abrasion resistance â¢ Damage resistance â¢ Fines retention â¢ Permeability â¢ Interface friction â¢ Elongation Regarding permeability and elongation, this paper notes that the geotextiles should be dimensioned (specified) as a filter or alternatively have a minimum permeability of 10 higher compared to the fill material. In addition, a high elongation geotextile allows the containers to mold itself in with the existing features and also allows a certain degree of self healing of the structure (see Figure 2.36). An ultimate elongation of greater than 50% is recommended to limit installation damage and allow flexibility of the structure. Figure 2.36. Self-healing characteristic of high elongation containers (Restall et al. 2004).
2.66 In a series of case studies of applications in Australia Restall et al. (2004) make the following points: â¢ In one case the use of a small dredge was used to install hydraulically filled 1.2 m diameter geotextile tubes and nourish a beach in a series of progressive "working bees." Although the wave climate was such that some displacement of the tubes occurred, the inherent flexibility of the nonwoven needle punched material utilized enabled realignment and settlement to follow scour contours and continue to provide stabilizing protection. Ultimately a 250 m long sand filled geotextile groine was constructed, which has withstood extreme UV and abrasion for over 10 years. â¢ For another application, beach revetment was designed as a short term solution; however, this project demonstrated that geotextile sand containers can provide longer term protection to important structures. Here, the geotextile sand container option was chosen because the structure provided an economical and user friendly solution. The structure consisted of in total 480 nonwoven geotextile containers each with a fill volume of 0.75 m3. The structure design included an encapsulated self-healing toe. â¢ One of the most innovative and complex geotextile sand container structures ever built was a 200 m x 400 m submerged reef installed as an integral part of the Northern Gold Coast Beach Protection Strategy. The aim of this project was to widen and protect the northern beaches as well as enhance the surfing amenity. The reef provides a low profile near shore control point to retain approximately 80,000 m3 of the 500,000 m3 of sand transported each year to the north along this shoreline. Nearly 400 heavy-duty polyester non-woven geotextile containers varying from 3.0 meters to 4.6 meters in diameter were placed using a split hulled, trailing suction hopper dredge fitted with computer interfaced DGPS. The containers were accurately filled utilizing a calibrated flow density meter, ensuring repeatability and consistency of the construction. Containers were dropped in depths of water ranging from 3 m to 10 m, onto a sandy seabed. In addition, this paper (Restall et al. 2004) reports on large scale model studies conducted as part of an applied research program in Germany. These studies focused on the hydraulic stability of nonwoven geotextile containers used as dune protection. These studies were followed by the formation of a Working Group on geotextile container technology. The main objectives of the Working Group were directed to practical planners and users in the field of hydraulic and coastal engineering applications. The aim was to provide technical information and recommendations for geotextile container solutions (geotextile sand bags, large bags, containers, tubes, mattresses and double layered elements) as well to give details relating tendering, contracting, and quality assurance. A focus set of case studies relating experience with geotextile container applications was also developed. A summary of geotextile container projects in Germany is presented in Saathoff (2001). The technical recommendations from this Working Group include the following topics related to principles in geotextile container technology. â¢ Principles in construction methods and installation possibilities â¢ Installation with in situ fill methods at the final geotextile container position (fill materials, filling with suction excavator, filling with mobile excavator pump or solid and thick-matter pumps, filling with dredge or a simple hopper fill and pneumatical filling) â¢ Installation with pre-filled geotextile containers (fill methods for small sandbags, large geotextile containers up to 1 m3 fill volume, loading and transportation facilities, installation methods for previous filled and for very large geotextile containers.
2.67 Offshore Applications Two publications from the International Conference on Scour and Erosion (ICSE6) in Paris, France in 2012 provide insights on scour and erosion problems with offshore structures and filter design and installation approaches in a deep water environment. Both papers deal with protection of the foundations of offshore wind turbines (OWT). Observations on design and installation of filters in this deep water application are summarized in the following paragraphs. Schurenkamp et al. (2012) in a paper on granular filter design for scour protection at offshore structures observe that a slender pile is commonly used as a monopile foundation for OWT. A horse shoe vortex system around the pile induced by wave and currents results in a highly complex loading of the sea bed. In order to guarantee the geotechnical safety of the foundation and the serviceability of the wind turbine, scour protection is generally required. For this purpose, large rock units are commonly used as an armor which has to be stable against the design waves and currents. To prevent the finer material of the sea bed from being washed out through the pore of the armor and the large stones from sinking into the sea bed, a granular filter is used which generally consists of different layers. The major requirements for the design and construction of a scour protection are: â¢ Hydraulic stability during and after construction â¢ Constructability by considering different aspects such as the segregation, accuracies, and tolerances during the installation of the granular material on the sea bed â¢ Cost effectiveness (considering construction, monitoring, and maintenance for the entire life time). The main objective of the design is to achieve a hydraulically stable and cost effective scour protection which can easily be constructed, monitored, and maintained. The failure of marine structures is often due to the failure of their granular filters, as reliable filter design formulae are still lacking (Oumeraci 1996). Based on a critical review of the literature of the present knowledge Schurenkamp et al. (2012) show that the knowledge associated with the hydro-geotechnical processes involved in the flow-filter-interaction is still not sufficient for the development of a definitive formulae for granular filter design under these conditions. Schurenkamp et al. 2012 observe that depending on the design approach selected, different filter criteria must be fulfilled. These include: â¢ Retention criterion to ensure stability against contact erosion and thus prevent leaching of finer fractions through the adjacent layer â¢ Internal stability criterion to prevent suffusion â¢ Permeability criterion to minimize the hydraulic pressure gradient through the layer â¢ Filter thickness criterion to damp severe hydrodynamic loading Further considerations such as sea bottom irregularities, compensation of differential settlements, construction method and tolerances, exposure during construction, construction in deeper water may dictate much thicker filters than required from the hydraulic stability criteria alone. Moreover, stability against segregation during construction may also be an important issue depending on the water depth and the construction methods. For a filter construction under water, a narrowly graded and possibly coarser filter material with a uniformity coefficient Cu â¤ 5 is recommended by the BAW (1989).
2.68 Schurenkamp et al. (2012) conclude that at present both basic and applied research are needed to enhance the understanding of physical processes involved and based on this improved understanding to develop design formulae and guidance for engineering practice. Important topics include: â¢ Flow velocity and shear stress distribution in both filter and armor layer â¢ Pore pressure distribution in the base material (hydraulic gradients) â¢ Incipient motion/transport of granular material under combined waves and currents for different filter and cover layer configurations, including internal erosion â¢ Dynamic design concepts/formulae for semi-stable filters At ICSE6 Peters and Werth (2012) also noted that the continuous development and installation of OWT in water depths up to 40 meter in the North Sea raise questions concerning long-term stability of the support structures. Like every offshore structure installed at the seabed, an OWT influences the local flow regime, the seabed, and the dynamic equilibrium between external hydrodynamic impacts and the movable sediment bottom. In the North Sea, the seabed mainly consists of sand and fine sand. Because of interaction between the foundation structure and the seabed, more or less distinctive effects of erosion and sedimentation processes in the near-field are expected, caused by currents and waves, which may negatively influence the operation, usability, and stability of the wind turbine. These time- dependent erosion processes depend on the type of structure, hydrodynamic impact forces, and seabed properties. After the construction and commissioning of twelve OWTs with a tripod foundation type and one transformer station with a jacket foundation in 2009 in the southern North Sea at a water depth of approximately 30 m, the first results regarding local changes at the bottom were published. After just one relatively calm winter period without extraordinary storms, scour depths of up to 6 m had formed in the center of the tripod foundation units, whereas scouring depths of up to 4.5 m were measured at the foundation piles. On the foundation piles of the jacket, scour depths of up to 4.5 m had been developed by 2010. Recently, scour depths up to 7 m have been measured. Rather than suggesting a granular filter approach when installing scour protection, Peters and Werth (2012) propose scour protection using geotextile sand- filled containers (GSC). Peters and Werth (2012) note that protection schemes against scour are well developed for bank and bottom protection, as well as for waterfront structures (typically, a filter layer covered by an armor layer). However, simply transferring the conventional systems to offshore conditions with fine bottom soils, large water depths and strong currents or waves seldom achieves practical success. No scour protection system works properly if a filter layer between the armor layer and fine bottom soil is neglected, because the armor layer will sink into the bed/bottom. Many scour protection designs with beautifully drawn gravel filter layers have been executed. But a closer inspection shows that execution phases and final state is far from the original design concept. Typical granular as well as geotextile cross-section designs for scour protection solutions for OWTs are shown in Figure 2.37. In contrast to granular solutions, the simplified geocontainer system includes two layers of robust nonwoven GSC. Based on long-term experience gained from installation of scour protection and stabilization measures the use of robust GSC, manufactured from thick needle-punched nonwoven filter fabric (NWGSC) provide significant advantages as scour protection for offshore wind energy turbines:
2.69 â¢ If installed in two layers and properly designed there is no need for any additional granular filter or cover layers. The complete scour protection is given by two layers of GSC (Figure 2.37). The installation process is simplified and no heavy armor layers may damage expensive turbine foundation equipment during the dumping process. â¢ The size of GSC should be as small as possible and as large as necessary to withstand hydrodynamic loads and to be stable against displacements. Up to a fill volume from approximately V = 1 m3 up to 1.5 m3 (1.3 to 2.0 yd3), the use of mechanically bonded (needle-punched) staple fiber nonwoven fabric with a mass per unit area of 600 g/m2 (18 oz/yd2) has proved to be effective, i.e., a flexible scour protection system is created and supported by interlocking effects between GSC. â¢ The geotextile material needs to be proofed in special performance tests on robustness, abrasion resistance, and filter effectiveness which should focus on hydraulic applications for geotextiles. The guidance document from the Federal Waterways Engineering and Research Institute (RPG 1994) for testing geotextiles is recommended. Various filling and operation methods can be developed. Examples are shown in Figures 2.38 to 2.40. â¢ Scouring may develop concurrent with or shortly after pile installation. Therefore, protection of the bottom should be achieved before pile driving begins. By the use of NWGSC installed on the full bottom area prior to pile driving an intact scour protection is ensured before and after pile driving. No additional bed protection works are necessary. This is contrary to conventional methods, where pile driving processes cannot be done through the armor layers but can be done through the filter layer. This means that the granular filter layer is exposed and unprotected often for months and therefore not proof against erosion. Peters and Werth (2012) conclude that geosynthetics are accepted worldwide and are essential construction materials for use in hydraulic engineering. Geotextile filter layers are an effective and erosion-stable alternative to granular filters if made from thick and robust needle- punched nonwoven geotextiles. The dynamic puncture resistance against stone dumping is outstanding given the consideration that geotextiles normally are known as being sensitive to mechanical loads. The reason behind this is due to the elongation capacity of the needle- punched nonwoven fabric whereby critical stresses in the material during dynamic impacts are prevented. Offshore wind turbine foundation structures need to be protected by simple and easy systems due to water depths of 40 m and very harsh wind and wave climate conditions. It is expected that, by using nonwoven geosynthetic sand containers for scour protection, a cost reduction for the growing offshore wind energy market can be achieved and the likelihood of permanent pile stability will be increased. 2.2.8 Special Applications The Geosynthetic Research Institute (GRI) (2012) provides a Standard Practice document for installation of geotextile tubes used for coastal and riverine structures. This practice provides guidance for installation, "but is not to be considered as all-encompassing since each material and site specific condition usually presents its own challenges and special issues." The practice includes installation of the main geotextile tube, its scour apron(s) and the filling procedure, but presumes that the proper geotextile tubes and ancillary materials have been chosen and fabricated for the site specific conditions per the plans and specifications.
2.70 Figure 2.37. Scour protection solutions for OWT foundations. Above: Granular solution and below: solution with geotextile containers (Peters and Werth 2012), Figure 2.38. Handling, filling and, closing of geotextile containers at the Eidersperrwerk (Peters and Werth 2012) (Also see Figure 2.15).
2.71 Figure 2.39. Installation of scour stabilization by the use of a stone dumping barge/vessel at the Eidersperrwerk (Peters and Werth 2012) (Also see Figure 2.15). Figure 2.40. Installation of scour stabilization by the use of a jib-crane in List Harbour Island Sylt (left) and right: scour protection installation for an offshore wind energy pile in Ireland (2011) (Peters and Werth 2012). The GRI Standard Practice document provides the following definitions/terminology (Geosynthetic Research Institute 2012): â¢ Geotextile Tube - A large tube [greater than 7.5 feet (2.3 m) in circumference] fabricated from high strength, woven geotextile, in lengths greater than 20 linear feet (6.1 m) (see Figure 2.41. Geotextile tubes used in coastal and riverine applications are most often filled hydraulically with a slurry of sand and water, although many other fill materials have been used. Tubes can also be filled by a combination mechanical/hydraulic method. â¢ Scour Apron - An apron of geotextile designed to protect the foundation of the main geotextile tube from the undermining effects of scour. In coastal and riverine applications, scour can be present at the base of the tube due to wave and current action. There may be aprons on both sides of the main tube, or only on one side. Scour aprons also reduce local erosion and scour caused during the hydraulic filling process of the main tube. Scour aprons are typically anchored by a small tube at the water's edge or by sandbags attached to the apron. â¢ Fill Port - Also called a fill spout or fill nozzle, fill ports are sleeves sewn into the top of the geotextile tube into which the pump discharge pipe is inserted (see Figure 2.41). Ports are typically 12 to 18 inches (300 to 450 mm) in diameter and 3 to 5 feet (0.9 to 1.5 m) in length. Fill ports are fabricated from the same geotextile as the main tube. Ports are spaced along the top of the tube to provide access to the contractor. Spacing is usually no closer than 25 feet (7.6 m) to accommodate sand slurry but can be as far apart as 100 feet (30 m) for some viscous fill materials. After pumping, ports are to be closed by tying, sewing, or gluing.
2.72 Figure 2.41. Geotextile tube and fill ports (courtesy Maccaferri). â¢ Installation - The use of geotextile tubes for coastal and riverine structures is a relatively new technology. While a few contractors who have followed the technology are well versed in proper installation practices, many are not. It is to this latter group of relatively inexperienced contractors and installers that this standard is focused [emphasis added]. This standard practice is focused on proper installation of the major facets of geotextile tubes, i.e., the main tube, its scour apron(s), and the filling sequence. There are many additional (and generally unique) situations which can, and do, arise which are beyond the scope of this practice and must be handled on a site specific basis. The GRI Standard Practice document (2012) provides the following installation guidance: â¢ Fill Material - Material for filling the geotextile tubes for coastal and riverine applications will normally consist of fine sand dredged from a designated borrow site (see Figure 2.42). Suitable material for filling tubes will contain not more than 15 percent fines (percent by weight passing the No. 200 sieve) to minimize subsidence of the tubes after filling. If excessive fines are observed during the filling process, the contractor should divert the flow until more suitable borrow material can be located. In addition, if the fill material is known to be primarily organic and/or fine-grained material, repeated fillings may be required to reach the design elevation of the tube. Also, considerable care must be taken to avoid overstressing the geotextile and inducing creep strains and excessive distortion. This type of fill material is not suitable for designs where the primary objective is a specified elevation.
2.73 I Figure 2.42. Equipment and filling process for geotextile tubes (courtesy Maccaferri). â¢ Fill Gradation - Gradation testing of hydraulic fill materials shall be conducted in accordance with ASTM D 422. Samples shall be obtained from the dredge discharge pipe immediately before inserting the pipe into the fill port. At a minimum, one gradation test shall be performed for each 1,000 linear feet (300 m) of fill tube. Extremely large tubes may require more frequent testing. Also, additional testing may be warranted at any time that visual inspection of the sand fill materials indicate that the percentage of fines may exceed the requirements presented herein. â¢ Tube Foundation - The foundation for the placement of the geotextile tube and its scour apron(s) shall be smooth and free of protrusions which could damage the geotextile. Remnant timber piles, piers, footings, underground utilities, etc., at or below grade, shall be removed if located within 20 feet (6.0 m) of the project site. Weak or unsuitable foundation material shall be removed or stabilized. â¢ Tube Filling - After completing the deployment and anchorage of the geotextile tube, filling with sand from the borrow area shall be accomplished in accordance with the approved Plan of Construction. The discharge line of the dredge shall be fitted with a "Y-valve" to allow control of the rate of filling. The Y-valve system must be fitted with an internal mechanism such as a gate, butterfly valve, ball valve, or pinch valve to allow the contractor to regulate discharge into the geotextile tube (see Figure 2.42). Any excess discharge shall be directed away from the tubes toward the borrow area. The discharge pipe shall also be fitted with a pressure gage as an aid to monitor pressure within the tube. In addition, it should be noted that internal pressure and stress on the tube fabric can vary along the length of the tube, therefore stress failure of seams and fill ports is not precluded by simply monitoring discharge fill pressure.
2.74 â¢ Discharge Pressures - Discharge pressures at the tube fill port shall not exceed 5 psi (35 kPa). As a rule of thumb, dredged discharge pipes should be limited to 10 inches (250 mm) diameter and smaller. This is due to the fact that as dredge discharge size increases, the flow rate being delivered by the pump increases greatly, increasing the potential for overstressing the tube. Dredge discharge pipes below 6 inches (150 mm) are often too small to adequately fill the tube to the proper height. â¢ Additional Precautions - The dredge discharge pipe shall be free of protrusions that could tear the fill port. It is generally accepted practice to support the dredge discharge pipe above the fill port in a manner which reduces stress on the fill port seams. Excessive movement of the dredge discharge pipe during filling can result in damage to the fill port. If a diffuser is used at the end of discharge pipe, it should not extend beyond the outer diameter of the discharge pipe. It is good practice to fill long tubes from multiple ports along the length of the tube. This reduces stress on the fill port and reduces the risk of sand bridging which can cause local stress on the fabric After filling the tube, the port sleeves shall be closed and attached to the main tube in a manner sufficient to prevent movement of the sleeve by subsequent wave action or other disturbances. â¢ Tube Alignment - Tubes used in coastal and riverine applications normally require alignment within Â± 2.0 feet (600 mm) of the baseline. The alignment can be facilitated by a number of methods, e.g., earthen cradles, tie-down straps, or physical buttressing. The filled tubes shall have an effective height of Â± 0.5 feet (150 mm) of the specified elevation. Effective height is defined as the height from the existing tube foundation to the average top of the filled tube measured every 25 feet (7.0 m) along the length of the tube between fill ports. Any subsidence of the top elevation of the tube below the specified height shall be corrected by supplemental filling or, if the tube has been damaged, replacement of the tube. Filling tubes higher than the manufacture's recommended height can lead to failure during construction. Also at no time shall construction equipment be operated directly on the geotextile tube or its ancillary materials. Filled geotextile tubes and scour aprons can be traversed if a 1 foot (300 mm) minimum of soil is covering the geotextile. No hooks, tongs, or other sharp instruments shall be used for handling. The geotextile tube or scour apron shall not be dragged along the ground. â¢ Tube Anchorage - The main geotextile tube and scour apron shall be deployed along the alignment and secured in place as necessary to assure proper alignment after filling (see Figure 2.42. No portion of the tube shall be filled until the entire tube segment has been fully anchored to the foundation along the correct alignment and pulled taut. â¢ Tube Overlaps - Tubes shall be overlapped at end joints or butted together so that there are no gaps unless permitted otherwise in the Plan of Construction. Beneath the geotextile tube, the ends of each geotextile scour apron shall be overlapped a minimum of 5 feet (1.5 m). The effective height of the tube structure at the overlap is typically 80% of the specified height. This equates to a 1-foot (300 mm) drop in effective height at the overlap for a 6-foot (1.8 m) high structure. The height to width ratio of the fully deployed tube shall not exceed a value of 0.5. The height to width ratio is an indicator of the stability of the tube in coastal and riverine applications. The design engineer should evaluate stability with respect to sliding, overturning, bearing, global stability, and overtopping of waves and associated wave forces.
2.75 2.2.9 Permitting of Filter Installations In general, permitting requirements for underwater installation of filter systems for scour and other erosion control measures will be subject to the same restrictions and requirements as the installation of the overlying armor system. Where required, permitting should consider the total proposed countermeasure, armor and filter, where the granular or geotextile filter is an essential component of the system. The NRCS provides general and agency-specific guidance for permitting stream restoration and design activities in their National Engineering Handbook, Part 654 (NRCS 2007). Chapter 17 of this document provides an overview of the regulatory authorities and programs that may be applicable to permitting stream design work. The focus of this chapter of the Handbook is to provide "an awareness-level understanding of this important issue and to list sources where current information can be obtained." NRCS notes that every stream design or restoration effort is subject to regulatory requirements and that designers should be aware of project permitting requirements and develop a project plan and budget identifying resources and project approaches that meet permit conditions. Depending on the type of project and its location, these can range from minimal to a full set of required Federal, state, and local permits. Chapter 17 of the Handbook lists 13 applicable programs/permits that could be considered and provides some detail on the processes and requirements of each of these programs/permits. General guidance on the permitting process includes the recommendation that permitting agencies should be approached as soon as conceptual plans for a project are developed, and in regulatory-intensive areas, or areas of high environmental risk, consultation with appropriate permitting agencies should be initiated in the early planning stages. The Handbook recommends that designers and planners should provide at least the following items to the permitting agency: â¢ Site map â¢ Description of existing environmental conditions (written and maps, photos, drawings) â¢ Description of the proposed work (written and drawings) â¢ Property ownership â¢ Access and staging information â¢ Preferred times of implementation Introductory guidance in the Handbook includes a tabular summary example of requirements for a typical restoration project. With this general guidance on the permitting process as background, issues specific to the underwater installation of granular and geotextile filters are addressed in Chapter 4 "Installation Guidance and Appraisal of Research Results" of this Final Report (see Sections 4.4.7 and 4.5.7 for discussion of environmental and permitting considerations for granular and geotextile filters, respectively). 2.2.10 Summary and Additional Observations At the Third European Geosynthetics Conference (Euro Geo3) in 2004, Dr. Heibaum presented a paper on cost effective construction methods using geosynthetic containers. This presentation concentrated on applications for hydraulic structures and coastal protection works. Consequently, it provides an excellent summary of the state of practice discussion presented in the foregoing synthesis of current practice, while at the same time providing
2.76 additional insights, observations, and guidance on several topics. The following paragraphs are extracted from Heibaum (2004b) with minor editorial changes, some topical rearrangement of the Heibaum discussion, and emphasis added, where appropriate. The term "geosynthetic containers" encompasses all elements that use a geosynthetic fabric as the material to enclose materials such as sand, concrete, waste material, etc. Such elements include bags of all sizes, tubes, mattresses of different shapes, gabions and more. Many applications have been developed to extend traditional construction methods. The use of construction methods incorporating geosynthetic containers has often resulted either in (1) better structures with increased resistance, (2) extended lifetime or shorter construction time, (3) lower costs of the same quality, or even all three simultaneously. The following paragraphs present an overview of elements with geosynthetic casing and their possible applications. Examples of cost effective applications are included comparing traditional and alternative construction methods. Examples in coastal protection show that sometimes only geosynthetic containers allow for protection measures at reasonable costs (Heibaum 2004b). Definitions Geosynthetic containers, also called "geosystems," (Pilarczyk 2000) are multi-purpose elements that can be manufactured according to almost any requirement. A wide range of forms, sizes, and materials are used to meet the specific requirements of an individual task. The containers are prefabricated, thus providing a constant quality, and filled on or near the site. Today most containers are made from woven or nonwoven geosynthetic fabric, but natural material such as jute or coir can also be used in some applications. Container Classes. The list of the use of containers, primarily in structures involved in soil- water interaction, is very long. The elements may be filled with air, water, sand, mortar, or waste. Often additional functions are provided: for example, the casing may be designed as a filter, or it may be used as reinforcement. While there are a large variety of containers, they generally can be grouped into several classes: bags, tubes and sheets (or mats), i.e., voluminous elements, long elements, and flat and/or thin elements. Voluminous elements or bags are those geosynthetic casings that might be best associated with the word "container," i.e., elements of limited length, limited width, and limited height but of a wide range of sizes from very small (decimetres) to very big (decametres). The origin of the development of all geosynthetic containers are sandbags. They have been used traditionally for immediate scour repair of dikes or as protection against rising water during floods. Bags up to 1 m3 (1.3 yd3) can be used as a filter under a protective armor layer (see Figures 2.12 and 2.13). Larger bags up to 2.5 m3 (3.3 yd3) are used for hydraulic structures and protection work (see Figure 2.36). Geosynthetic tubes are long elements installed in bank protection works in rivers and at the coast, as dikes for land reclamation, as the core of groines and longitudinal dikes, etc. (see Figures 2.41 and 2.42 ). They are also used as casings to be filled with sludge or slurry to dewater those materials for easier final disposition. With a watertight casing, tubes can serve as liquid tanks or flood protection, similar to rubber weirs.
2.77 Thin elements like sheets containing a fill material in between two geotextiles also belong to the container family even though they are rather thin. Examples are the sandmat and the geosynthetic clay liner. The sandmat provides immediate protection and hinders floating of a geotextile filter (see Figures 2.9 and 2.16); the geosynthetic clay liner is used as a flexible impervious lining. Comparing Traditional and Alternative Construction Methods Placing a Filter Under Water. A filter on the sea floor or on a river or canal bank beneath an armor layer could either be a granular or a geosynthetic filter. Placement underwater creates challenges in both cases, mostly due to currents and waves, but omitting the filter and only covering the subsoil with an armor layer means wasting money. Without a filter, the armor material will sink into the subsoil due to dynamic hydraulic actions and the missing cover will have to be replenished until sufficient armor material is added that the hydraulic load at the interface of the subsoil/armor is so low that no further erosion or liquefaction takes place. Granular Filters. Regarding granular filters, it would seem to be easy to place a mineral filter layer by just dumping it on the surface to be protected. But quite often placement underwater proves to be difficult. Two significant difficulties have to be considered: â¢ The first problem is the gradation of the filter material. Only very narrowly graded material can be dumped through water. When using a broadly graded filter material, the finer fraction takes longer to reach the bottom than the coarser fraction, causing material segregation. Thus a "reverse filter" will be created; with the fine material on top (which provides no filter function). If a narrowly graded material is used, dumping is possible but several layers with increasing grain diameter are needed to create a reliable filter between the subsoil and armor, increasing the costs significantly. â¢ The second problem is the erodibility. At least the first filter layer will consist of grain sizes not significantly larger than the subsoil, so the material may be eroded nearly as easily as the subsoil. As a result, a granular filter can be placed reliably underwater only in lakes or canals with no (or nearly no) flow. Geotextile Filters. A geotextile filter can be placed under water either by sliding it from a pontoon, holding it near the bottom by a guide-bar and ballasting it immediately or by rolling it up above the water, submerging the roll and unrolling the sheet under water (see Figures 2.17 and 2.18). These procedures require special equipment. Without these, the geotextile filter will always float, since polypropylene is lighter than water. But even polyester, a material with a unit weight higher than water, will float due to the air bubbles trapped in the fabric when sinking the sheet. Another problem can be insufficient friction at the geotextile/subsoil interface on a slope. This has been experienced when using the traditional type of bottom protection in riverine and coastal protection works, i.e., the fascine mattress (see Section 2.2.5 for a discussion of fascine mattresses). For such mattresses, usually a woven geotextile is chosen as a base and a filter with fascines (i.e., willow bundles) tied on it. With increasing steepness, the danger of a mattress sliding increases. For such applications a geocomposite fabric is available: a nonwoven fabric for filtration and friction combined with woven material for tensile strength.
2.78 To overcome the problems related to geotextile filters, stability can be increased by attaching heavy iron chains at the edges of the filter cloth; however, such a measure is rather time and cost intensive. To facilitate placement of a filter underwater, the "sandmat" was invented (see Figures 2.9 and 2.10). This is a geocomposite that might be called a thin geotextile container. Sand or another mineral fill, (e.g., granulated metal slag with a high specific weight), is confined between two geosynthetic sheets. The two geosynthetic sheets are either needle- punched or sewn in closely spaced rows to keep the fill in place. Such sandmats filled with 5 kg/m2 (9 lb/yd2) sand have proven to remain in place when loaded by currents up to 0.6 m/s (2.2 ft/s). The maximum fill weight available today is 9 kg/m2 (16.5 lb/yd2), increasing the resistance to 1 m/s (3.3 ft/s). The sandmat can be placed with the usual placing equipment for geotextile filters, thus saving costs, but the equipment has to be sufficiently strong to carry the higher load/weight of the mat. When neither a geotextile filter (even when placed as a sandmat) or a granular filter can be installed properly underwater because of the fabric being shifted, folded, turned, or eroded before it is covered by an armor layer, elements are needed that will be stable against the current, that do not undergo any alteration during the placement process, and that provide the filtration capacity needed. Geotextile containers with a casing designed as a geotextile filter and filled with sufficiently permeable material provide these functions. When using geosynthetic filtering containers, reliable and cost effective systems can be built. Hydraulic loads like waves and currents will not impede or even hinder the installation of an effective filter underwater using geocontainers. Scour Protection Using Geosynthetic Containers Instead of Fascines. As noted above, fascines are traditional large single elements for scour protection and repair. Large willow bundles are assembled with a core of rubble or riprap. Due to their size and weight they provide a sufficient resistance against the current to facilitate underwater installation of a scour countermeasure with a filter. Figures 2.5 and 2.6 show fascine mattresses with an underlying geotextile filter being constructed on a launching pad in Germany. Figure 2.43 shows a large fascine mattress being floated into place. With a long utilization history, there are many procedures for fabricating and placing fascines, but there is always the problem of ensuring a constant quality of material and fabric. In particular, the requirement for providing a filter to prevent erosion of the bottom soil of a river or in coastal environments cannot always be achieved in the desired manner. Fascines without an underlying geotextile will not function as a filter, since only coarse soil may be retained. Erosion may be slowed down due to the damping of the erosive effect of the current, but it will not be stopped. Therefore, elements are needed that combine the required filter capacity with sufficient weight to resist the hydraulic load. Geosynthetic tubes and containers can meet these requirements. Geosynthetic containers can be prepared with the same size and weight as fascines, but with a geosynthetic casing that is designed as a filter. The only requirement as to the fill material is sufficient permeability to allow the water to drain. To improve the filter reliability, the fill can also be designed as a granular filter. Often tests are considered the best way to find the appropriate filter in cases of turbulent and reversing flow (Federal Waterways Engineering and Research Institute 1993). Filtering geosynthetic containers are generally covered by an armor layer, but as long as there is not heavy bed load transport containers can be left without armor. In some cases, groines have been built successfully with extra resistance against UV degradation but without an extra armor layer.
2.79 Figure 2.43. Large fascine mattress being floated into place. (courtesy Colcrete - Von Essen Inc.) Design Considerations and Installation of Geocontainers Materials. For safe placement, high serviceability, and sufficient long-term resistance, the container material has to be chosen such that it will resist all loads that might occur. The raw materials used most often are polypropylene and polyester - other material can be used when special attributes are required, e.g., very high strength or chemical resistance. Natural material is used if degradation of the casing over time does no harm or is even desired. Fabric. As casing for geosynthetic containers, both woven and nonwoven materials can be selected. Woven geotextiles have the advantage of high tensile strength, while nonwovens have the advantage of large straining capacity. If the casing material is damaged, a woven cloth might be more susceptible to crack propagation (the zip effect) than a nonwoven. By allowing large deformations nonwovens are able to withstand the impact load when hitting the ground or when stones are dumped upon them. Wovens need rather high strength to achieve the same resistance. The nonwoven fabric exhibits a higher angle of friction than the woven and should be chosen if stability against sliding of the geocontainer has to be guaranteed. In addition, when a container has to sustain abrasive forces due to rocking armor stones or due to bed load transport, the casing needs appropriate resistance against this type of loading. Containers are often used in such a way that they are exposed to sunlight at least some of the time. As a result, the fabric has to exhibit sufficient resistance against weathering, in general, and against UV radiation, in particular.
2.80 Seams. Special care has to be taken concerning the seams of the container. The seam is prefabricated on three sides by the manufacturer. Therefore, a strength approximately as high as the geotextile itself is guaranteed. The container is usually closed on site by sewing double chain stitch seams at the top. The reliability of the seams is improved, if one seam is straight and the second one is curved to allow for straining of the geotextile if the first seam is broken. A new development in closing nonwoven containers is the use of Velcro; however, such seams can only be opened by destroying the fabric. Filling the Container. One of the advantages of utilizing geosynthetic containers is the possibility of using locally available fill material, thus keeping the costs low. However, it is necessary to develop an effective fill procedure for the intended application that is fast but does not overstress the fabric, especially when large units are used. However, with some experience, such filling operations can be rather simple but very effective. When the container is filled, the bottom should remain on the ground (or in a mold, and it may be appropriate (e.g., when containers with more than 1 m3 (1.3 yd3) of fill are used) for the container to be laterally restrained. Otherwise excessive stressing and straining during the filling process may occur. If bags or containers are to be handled with clam shells or similar tools, coarse fill material (e.g., gravel only) will cause damage to the fabric more easily than finer fill. Generally, coarse material should be mixed with finer grains to reduce the stress on the fabric. Tubes and large containers are often filled hydraulically. Even though this procedure is basically simple, care must be exercised and standard practice procedures have been developed (GRI 2012). For special applications, very large containers of 200 m3 (260 yd3) fill and more are used. These containers are placed by split barges. Either the ship's hold is lined by one large geotextile sheet, filled with an excavator, closed by sewing and then dumped, or the container is hydraulically filled. The latter method has the advantage that no long seams have to be closed on site, but only one small inlet and one small outlet opening. When geocontainers are used as the core of a hydraulic structure that will be protected by an armor layer, the amount of fill should not exceed 80% of the theoretical volume, to allow a proper adjustment to the subsoil, to structures, or to the neighboring geocontainers. However, when a partially filled container is placed without an armor layer, agitation by flow or waves may cause flapping of the fabric. Permanent flapping may result in fatigue failure of the casing fabric. Therefore, geosynthetic containers without an armor and exposed to cyclic loading should be filled completely to avoid flapping of the geotextile. Placement. The placement procedure is often the most severe mechanical load of a geosynthetic product during its lifetime (Heibaum 1999). During installation, it is often not possible to handle containers with special care, so it is essential that an appropriate casing material that will withstand the impact and handling loads be chosen. On the other hand, the demand of high strength during placement should not be cost prohibitive, so the installation procedure has to be chosen carefully. Single geosynthetic containers are generally placed by a hydraulic excavator. For elements up to 1.5 m3 (2.0 yd3) clam shells with soft edges are appropriate (see Figure 2.13) while for larger elements special buckets are used. To place such containers precisely in deep water, special equipment is necessary and available (see Figures 2.14, 2.38, 2.39, and 2.40).
2.81 The placement of numerous containers is generally not done element by element. Small containers can be placed using a belt conveyor. In large rivers and in the sea, a stone dumping vessel might be appropriate, but then a conveyor belt type should be used to minimize the abrasive forces acting on the fabric. If only a push-type barge is available, a sand layer under the containers is necessary to reduce the abrasion impact (see Figure 2.39). Placing mattresses that are filled on site creates extra problems. The fabric will float before it is filled. These mattresses do have the advantage of being installed "endlessly," i.e., no overlaps or open joints in the area to be covered, so special equipment is needed (see Figure 2.42). Up to a limited size, mattresses can be prefabricated or assembled in the dry. Such mattresses are placed by special cranes and need overlaps or special measures to bridge the joint between the mattresses (see Figure 2.14). Effective Scour Protection Using Geosynthetic Containers After severe scouring at the Eider storm surge barrier in Germany (with a scour hole progressing towards the structure) geosynthetic nonwoven containers proved to be the only applicable solution to stop scouring and provide protection for the long term (Heibaum 1999) (see Figure 2.15 and associated discussion). Operationally, the Eider barrier is opened to allow low and high tide to pass except during storm surges. During a long period of repair work the southern opening of the barrier was closed which caused severe scouring due to the increased velocity and the added redistribution of the tidal flow. A scour hole with a maximum depth of 31 m (100 ft) below Mean Sea Level developed (i.e., a "hole"' of approximately 25 m (80 ft) below the original sea floor). Progressive erosion at the barrier-side slope of the scour hole had caused failure of the edge of the bottom protection. An urgent stabilization measure was needed. To fill the scour hole with seabed material dredged from further out at sea would have been very economical. However, this was considered impossible since the barrier could not be closed long enough to refill and cover the scour hole. It was therefore decided not to refill the hole but to protect the barrier-side slope of the scour hole, even though it had a slope of 1:1 in the upper part (due to some "reinforcing" clay layers in the sandy subsoil). The main problem was the need to place a filter under the armor layer. A geosynthetic cloth could not be placed properly and a granular filter would be eroded before it could be covered. Fascine mattresses could not be used due to the risk of sliding on the steep slope. Placing only armor material, large enough to withstand the erosive forces of the tidal flow, would not guarantee long-term stability of the protection measure, since the fine sand would be eroded through the armor stones. The only solution possible was to combine the resistance of larger elements against hydraulic loads with the filtration capacity demanded. Geotextile nonwoven containers (volume â¥1 m3 (1.3 yd3)) filled with granular material were dumped from bottom to top on the slope of the scour hole and covered with fill and armor (Figure 2.44). This layer of geosynthetic containers had sufficient resistance to be stable against the hydraulic loads until the final armor system was installed. The nonwoven fabric was designed as a filter in relation to the subsoil and provided a high angle of friction of geotextile and subsoil to prevent sliding, making a geocontainer filter the most effective solution to this problem. Special handling and installation techniques and equipment were used for the preparation and placement of 48,000 geotextile containers at the Eider storm surge barrier (see Figures 2.38 and 2.39).
2.82 Figure 2.44. Scour-hole at Eider storm surge barrier-cross section of scour and barrier (top) slope protection of scour hole (bottom) (Heibaum 2004b). While the use of geosynthetic containers is still a relatively new science, a number of approaches for planning and design are available (e.g., Pilarczyk 2000). Increasingly, geosynthetic containers can replace other elements in many structures, especially in hydraulic works. They can be adapted to the individual application in form, strength, and permeability. Often geosynthetic containers result in lower costs compared to traditional construction methods. They allow easy adaptation to local conditions in regard to preparation, filling, transport, and installation. In some cases as with the Eider Storm Surge barrier discussed above, geotextile containers (either as a bag, blanket, or tube) may offer the only practical solution to the continuing problem of placing a filter under water.