Guidance for Underwater Installation of Filter Systems (2018) / Chapter Skim
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4 2.1 Survey on Current State of Practice An NCHRP-approved survey of practitioners was distributed to 130 agency personnel and practitioners. The survey was distributed by email in December 2014.
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Findings 5 2.1.2 Design-Related Issues The following describes design-related issues: • 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.
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6 Guidance for Underwater Installation of Filter Systems • Virtually every scour countermeasure project done by Maine DOT has been done inside a coffer dam that is pumped dry. To maintain the same hydraulic opening, the streambed has to be excavated before placing any scour countermeasures.
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Findings 7 2.1.4 Summary of Survey Results Despite the disappointing rate of return for the survey, some valuable information can be derived from the responses received: 1. Almost 90% of respondents indicated that a filter is "Always" or "Usually" required for installations placed under water, but only 16% of respondents indicated that the filter installation is inspected prior to the armor being placed on top.
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8 Guidance for Underwater Installation of Filter Systems research on the benefits of various types of hydraulic countermeasures, and a number of publications have been written, including FHWA's HEC-23 (Lagasse et al.
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Findings 9 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.
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10 Guidance for Underwater Installation of Filter Systems Purpose and Need for the Filter Component NCHRP Report 568 and NCHRP Report 593 (Lagasse et al.
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Findings 11 the basic difference between stable and unstable soil structures; (c) through (f)
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12 Guidance for Underwater Installation of Filter Systems of gravel, sand, silt, and clay in the soil. This characterization is usually done by sieve analysis for coarse-grained soils or sedimentation (hydrometer)
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Findings 13 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.
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14 Guidance for Underwater Installation of Filter Systems 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)
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Findings 15 geotextiles have very little capacity to transmit water in the plane of the fabric, whereas nonwoven needle-punched fabric has a much greater capacity due to its three-dimensional microstructure. Transmissivity is not particularly relevant to filter design.
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16 Guidance for Underwater Installation of Filter Systems 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.
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Findings 17 • Step 4. Determine maximum allowable d50f for filter.
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18 Guidance for Underwater Installation of Filter Systems FROM SOIL PROPERTY TESTS MORE THAN 30% CLAY (d30 < 0.002 mm)
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Findings 19 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.
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20 Guidance for Underwater Installation of Filter Systems 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.
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Findings 21 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.
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22 Guidance for Underwater Installation of Filter Systems have developed an equation for the excess pore water pressure with depth and, for many practical applications, a design chart has been developed that considers the soil permeability and time of pressure drop (Bezuijen and Kohler 1996)
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Findings 23 Schulz (1995) summarizes the evolution and problems encountered in developing techniques for placing filters under water when Germany's inland waterways system was expanded to meet the PIANC-Class IV standard in the 1970s and 1980s.
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24 Guidance for Underwater Installation of Filter Systems current of passing ships had considerable consequences [emphasis added]
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Findings 25 the United Kingdom (UK)
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26 Guidance for Underwater Installation of Filter Systems The Netherlands The Netherlands CUR published Manual on the Use of Rock in Hydraulic Engineering (CUR Report 169)
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Findings 27 Basically two types of bed protection are applied, loose stone or mattresses. Loose stone is used as single or multi-layered structures.
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28 Guidance for Underwater Installation of Filter Systems For placement from a pontoon, a prefabricated geotextile filter mattress may be unwound from a pontoon and placed on a submerged slope in a downward direction. This method requires specially 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)
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Findings 29 Figure 2.10 shows a 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)
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30 Guidance for Underwater Installation of Filter Systems Figure 2.12 shows a geotextile container being filled with sand. Figure 2.13 shows the sandfilled geotextile container being handled with an articulated-arm clam grapple.
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Findings 31 can be fashioned from a 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 handheld machine. BAW Guidance BAW in Germany published MAG, a code of practice addressing the use of geotextile filters on waterways, in 1993 (Federal Waterways Engineering and Research Institute 1993)
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32 Guidance for Underwater Installation of Filter Systems 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.
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Findings 33 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.
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34 Guidance for Underwater Installation of Filter Systems filled with sand. Based on tests at the BAW Institute in Germany, the filter is stable for a wide range of soil types.
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Findings 35 From a less academic point of view, GEOfabrics (2011) of the UK provides installation guidance as recommended by a geotextile manufacturer for installing geotextiles in coastal and river applications.
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36 Guidance for Underwater Installation of Filter Systems 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 (see Figure 2.18)
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Findings 37 – 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.
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38 Guidance for Underwater Installation of Filter Systems or were not cost-effective due to transportation, quality control, or human resource constraints. Although use of granular filters was still considered part of the "traditional" approach to revetment design and construction, filter fabric was being used for many projects.
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Findings 39 5. Ultraviolet light.
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40 Guidance for Underwater Installation of Filter Systems Pennsylvania DOT and FHWA) , Ruff and Fotherby (1995)
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Findings 41 • 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.
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42 Guidance for Underwater Installation of Filter Systems Laboratory testing showed that granular filters performed poorly in cases where bedforms 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.
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Findings 43 Surrounding the pier, a scour hole measuring 12 ft by 16 ft (4 m × 5 m) was pre-formed in the sand bed to a maximum depth of 3 ft (0.4 m)
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44 Guidance for Underwater Installation of Filter Systems contrasts can be identified for these systems. In most cases, a filter layer is essential for successful performance of all (pier)
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Findings 45 Figure 2.24. Installation of riprap around pier.
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46 Guidance for Underwater Installation of Filter Systems Geotextile sand containers are strongly recommended as a practical, proven, and effective technique for placing a filter underwater 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 (see Figure 2.26)
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Findings 47 around piers and abutments, riprap tested to failure, protective pad radius determination, pad height comparing flush vs. mounded installations, and comparison of gravel and geotextile filters.
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48 Guidance for Underwater Installation of Filter Systems of Agriculture Natural Resources Conservation Service (NRCS)
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Findings 49 For many years, the FHWA National Highway Institute (NHI) has included in its curriculum a training course called "Geosynthetic Design and Construction Guidelines" (NHI Course No.
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50 Guidance for Underwater Installation of Filter Systems (a)
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Findings 51 and tend to accelerate the formation of a scour hole or trench at the toe. Alternative toe treatments are shown in Figure 2.31.
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52 Guidance for Underwater Installation of Filter Systems recommended in streambank protection. Thus, the long axis of the geotextile strips will be parallel to anticipated wave action.
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Findings 53 Because of potential undermining by wave action, 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.
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54 Guidance for Underwater Installation of Filter Systems • Thickness of layers should be monitored to ensure designed discharge capacity and continuity of the filter. • Quality control/assurance is very important during filter construction because of the critical function of this relatively small part of the embankment.
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From page 55... ...
Findings 55 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.
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56 Guidance for Underwater Installation of Filter Systems cap that has too low a permeability. Uncertainty in design should err on the side of providing too large a permeability.
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Findings 57 Figure 2.35. Techniques for placing sand caps on contaminated sediments underwater (Palermo et al.
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58 Guidance for Underwater Installation of Filter Systems 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.
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Findings 59 The following installation guidance for fill material, fill gradation, tube foundation, tube filling, discharge pressures, additional precautions, tube alignment, tube anchorage, and tube overlaps is extracted from the GRI standard practice document (2012) : • 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 2.37]
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60 Guidance for Underwater Installation of Filter Systems of the sand fill materials indicates that the percentage of fines may exceed the requirements presented herein. • Tube foundation -- the foundation for the placement of the geotextile tube and its scour apron(s)
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Findings 61 This equates to a 1 ft (300 mm) drop in effective height at the overlap for a 6 ft (1.8 m)
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62 Guidance for Underwater Installation of Filter Systems This presentation concentrated on applications for hydraulic structures and coastal protection works. It provides an excellent summary of the state of practice and at the same time provides additional insights, observations, and guidance on several topics relevant to the installation of geotextile containers.
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From page 63... ...
Findings 63 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]
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64 Guidance for Underwater Installation of Filter Systems solution to stop scouring and provide protection for the long term [see Figure 2.15 and associated discussion]
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Findings 65 can be adapted to the individual application in form, strength, and permeability. Often geosynthetic containers result in lower costs compared to traditional construction methods.
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