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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Suggested Citation:"Appendix B. Alternative Filter Design Procedures." National Academies of Sciences, Engineering, and Medicine. 2018. Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report. Washington, DC: The National Academies Press. doi: 10.17226/25302.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

APPENDIX B ALTERNATIVE FILTER DESIGN PROCEDURES B.1 FHWA National Highway Institute Geotextile Design for Riprap Revetments and Other Permanent Erosion Control Systems B.2 U.S. Army Corps of Engineers Filter Design Guidance

B.1 B.1 FHWA National Highway Institute Geotextile Design for Riprap Revetments and Other Permanent Erosion Control Systems B.1.1 Background As in drainage systems, geotextiles can effectively replace graded granular filters typically used beneath riprap or other hard armor materials in revetments and other erosion control systems. This was one of the first applications of geotextiles in the United States; woven monofilament geotextiles were initially used for this application with rather extensive installation starting in the early 1960s. Numerous case histories have shown geotextiles to be very effective compared to riprap-only systems and as effective as conventional graded granular filters in preventing fines from migrating through the armor system. Furthermore, geotextiles have proven to be very cost effective in this application. Since the early developments in coastal and lake shoreline erosion control, the same design concepts and construction procedures using geotextile filters have subsequently been applied to stream bank protection (see HEC 11 and HEC-23 (FHWA 1989 and 2009)), cut and fill slope protection, protection of various small drainage structures (see HEC 14 (FHWA 2006)) and ditches (see HEC 15 (FHWA 2005)), wave protection for causeway and shoreline roadway embankments, and scour protection for structures such as bridge piers and abutments (see HEC 18 (FHWA 2012, and HEC 23 (FHWA 2009)). Design guidelines and construction procedures with geotextile filters for these and other similar permanent erosion control applications are presented in the following sections. Hydraulic design considerations can be found in the AASHTO Model Drainage Manual (2005) and the above FHWA Hydraulic Engineering Circulars. B.1.2 Applications Riprap-geotextile systems have been used successfully for precipitation runoff collection and high- velocity diversion ditches. Geotextiles may be used in slope protection to prevent or reduce erosion from precipitation, surface runoff, and internal seepage or piping. In this instance, the geotextile may replace one or more layers of granular filter materials that would be placed on the slope in conventional applications. Erosion control systems with geotextiles may be required along stream banks to prevent encroachment of roadways or appurtenant facilities. Similarly, they may be used for scour protection around structures. A riprap-geotextile system can also be effective in reducing erosion caused by wave attack or tidal variations when facilities are constructed across or adjacent to large bodies of water. Finally, hydraulic structures such as culverts, drop inlets, and artificial stream channels may require protection from erosion. In such applications, if vegetation cannot be established or the natural soil is highly erodible, a geotextile can be used beneath armor materials to increase erosion resistance. B.1.3 Geotextile Filters Beneath Hard Armor: Design Concepts Geotextile filter design for hard armor erosion control systems is essentially the same, with a few exceptions, as the design for geotextile filters in subsurface drainage systems (see FHWA/NHI 2008). This section highlights those exceptions and discusses the special considerations for geotextile filters beneath hard armor erosion control systems.

B.2 Retention Criteria for Cyclic or Dynamic Flow. Many erosion control situations have cyclic or dynamic flow conditions, so soil particles may be able to move behind the geotextile if it is not properly weighted down and in intimate contact with the soil. Thus, unlike conventional filters, using a retention coefficient B = 1 may not be conservative, as the bridging network may not develop and the geotextile may be required to retain even the finer particles of soil. If there is a risk that uplift of the armor system can occur, it is recommended that the B value be reduced to 0.5 or less; that is, the largest hole in the geotextile should be small enough to retain the smaller particles of soil. Here, B is defined as the retention coefficient where B ranges from 0.5 to 2 and is a function of the type of soil to be filtered, its density, the uniformity coefficient Cu if the soil is granular, the type of geotextile (woven or nonwoven), and the flow conditions. For sands, gravelly sands, silty sands, and clayey sands (soils with less than 50% passing the No. 200 (0.075 mm) sieve per the Unified Soil Classification System), B is a function of the uniformity coefficient, Cu. For silts and clays (soils with more than 50% passing the No. 200 (0.075 mm) sieve), B is a function of the type of geotextile (see Section B.1.4, Step 5). Due to their random pore characteristics and, in some types their felt-like nature, nonwoven geotextiles will generally retain finer particles than a woven geotextile of the same AOS. Therefore, the use of B = 1 will be even more conservative for nonwoven geotextiles. In many erosion control applications it is common to have high hydraulic stresses induced by wave or tidal action. The geotextile may be loose when it spans between large armor stone or large joints in block-type armor systems. For these conditions, it is recommended that an intermediate layer of finer stone or gravel be placed over the geotextile and that riprap of sufficient weight be placed to prevent wave action from moving either stone or geotextile and to maintain the intimate contact between the soil and geotextile filter. Geosynthetic composites (e.g., geonet/geotextile composite) could also be considered beneath block-type armor systems to prevent movement of the geotextile filter as well as uplift on the block. For all applications where the geotextile can move, and when it is used as sandbags, it is recommended that samples of the site soils be washed through the geotextile to determine its particle-retention capabilities. Permeability and Effective Flow Capacity Requirements for Erosion Control. In certain erosion control systems, portions of the geotextile may be covered by the armor stone or concrete block revetment systems, or the geotextile may be used to span joints in sheet pile bulkheads. For such systems, it is especially important to evaluate the flow rate required through the open portion of the system and select a geotextile that meets those flow requirements. Again, since flow is restricted through the geotextile, the required flow capacity is based on the flow capacity of the area available for flow; or grequired = ggeotextile (Ag / At) (B.1) where: Ag = Geotextile area available for flow, and At = Total geotextile area The AASHTO M 288 Standard Specification for Geotextiles (2006) presents recommended minimum permittivity values in relation to percent of in-situ soil passing the No.200 (0.075 mm) sieve. The values are presented in Section B.1.4. These permittivity values are based upon the predominant particle sizes of the in-situ soil and are additional qualifiers to the permeability criteria.

B.3 Clogging Resistance for Cyclic or Dynamic Flow and for Problematic Soils. Since erosion control systems are often used on highly erodible soils with reversing and cyclic flow conditions, severe hydraulic and soil conditions often exist. Accordingly, designs should reflect these conditions, and soil-geotextile filtration tests should be conducted. Since these tests are performance-type tests and require soil samples from the project site, they must be conducted by the owner or the owner’s representative and not by geotextile manufacturers or suppliers. Project specific testing should be performed especially if one or more of the following problematic soil environments are encountered: unstable or highly erodible soils such as non- cohesive silts; gap graded soils; alternating sand/silt laminated soils; dispersive clays; and/or rock flour. For sandy soils with k > 10-6 m/s the long-term, gradient ratio test (ASTM D 5101) is recommended (note that the U.S. Army Corps of Engineers recommends a maximum allowable gradient ratio (GR) of three). For soils with permeabilities less than about 10-6 m/s, filtration tests should be conducted in a flexible wall or triaxial type apparatus to ensure that the specimen is 100% saturated and that flow is through the soil rather than along the sides of the specimen. The soil flexible wall test is ASTM D 5084, while the Hydraulic Conductivity Ratio (HCR) test (ASTM D 5567) currently is the standard test for geotextiles and soils with appreciable fines. The HCR test should be considered only with the modifications and caveats recommended in Chapter 1 of FHWA/NHI (2008). Other filtration tests discussed in Chapters 1 and 2 of FHWA/NHI (2008) should also be considered. Survivability Criteria for Erosion Control. The geotextile property requirements for survivability are summarized in Table B.1. As placement of armor stone is generally more severe than placement of drainage aggregate, required property values are higher for each category of geotextile. Furthermore, the specifications should require the contractor to demonstrate in the field that their proposed armoring placement technique will not damage the geotextile. Riprap or armor stone should be large enough to withstand wave action and thus not abrade the geotextile. The specific site conditions should be reviewed, and if such movement cannot be avoided, then an abrasion requirement based on ASTM D 4886, Standard Test Method for Abrasion Resistance of Geotextiles should be included in the specifications. Abrasion of course only affects the physical and mechanical properties of the geotextile. No reduction in piping resistance, permeability, or clogging resistance should be allowed after exposure to abrasion. It is important to realize that the survivability requirements in Table B.1 are minimum survivability values and are not based on any systematic research. They are based on the properties of geotextiles that are known to have performed satisfactorily in various hard armor erosion control applications. The values in Table B.1 are meant to serve as guidelines for inexperienced users in selecting geotextiles for routine projects. They are not intended to replace site-specific evaluation, testing, and design. Additional Filter Selection Considerations and Summary. To enhance system performance, special consideration should be given to the type of geotextile chosen for certain soil and hydraulic conditions. As mentioned above, special attention should be given to problematic, unstable, or highly erodible soils. Examples include non-cohesive silts, gap graded soils, alternating sands and silts, dispersive clays, and rock flour. Project specific laboratory testing should be performed especially for critical projects and severe conditions.

B.4 Table B.1. Geotextile Strength Property Requirements1,2,3,4 for Permanent Erosion Control Geotextiles (after AASHTO 2006).

B.5 In certain situations, multiple filter layers may be necessary. For example, a sand layer could be placed on the soil subgrade, with the geotextile designed to filter the sand only but with sufficient size and number of openings to allow any fines that do reach the geotextile to pass through it. Another special consideration for erosion control applications relates to a preference towards felted and rough versus slick surface geotextiles, especially on steeper slopes where there is a potential for the riprap to slide on the geotextile. Such installations must be assessed either through field trials or large-scale laboratory tests. Figure B.1 is a flow chart summarizing the FHWA filter design process. B.1.4 Geotextile Design Guidelines STEP 1. Evaluate critical nature and site conditions. a. Critical/less critical 1. If the erosion control system fails, will there be a risk of loss of life? 2. Does the erosion control system protect a significant structure, or will failure lead to significant structural damage? 3. If the geotextile clogs, will failure occur with no warning? Will failure be catastrophic? 4. If the erosion control system fails, will the repair costs greatly exceed installation costs? b. Severe/less severe 1. Are soils to be protected gap-graded, pipable, or dispersive? 2. Do the soils consist primarily of silts and uniform sands with 85% passing the No.100 sieve? 3. Will the erosion control system be subjected to reversing or cyclic flow conditions such as wave action or tidal variations? 4. Will high hydraulic gradients exist in the soils to be protected? Will rapid drawdown conditions or seeps or weeps in the soil exist? Will blockage of seeps and weeps produce high hydraulic pressures? 5. Will high-velocity conditions exist, such as in stream channels? NOTE: If the answer is yes to any of the above questions, the design should proceed under the critical/severe requirements; otherwise use the less critical/less severe design approach. STEP 2. Obtain soil samples from the site. a. Perform grain size analyses 1. Determine percent passing the No.200 (0.075 mm)sieve. 2. Determine the plastic index (PI). 3. Calculate Cu = D60/D10.

B.6 Figure B.1. Flow chart summary of the FHWA filter design procedure.

B.7 NOTE: When the protected soil contains particles passing the No.200 (0.075 mm) sieve, use only the gradation passing the No.4 (4.75 mm) sieve in selecting the geotextile (i.e., scalp off the + No. 4 (+4.75 mm) material). 4. Obtain D85 for each soil and select the worst case soil (i.e., soil with smallest B x D85) for retention b. Perform field or laboratory permeability tests 1. Select worst case soil (i.e., soil with highest coefficient of permeability k). NOTE: The permeability of clean sands (< 5% passing No.200 (0.075 mm) sieve) with 0.1 mm < D10 < 3 mm and Cu < 5 can be estimated by Hazen's formula, k = (D10)2 (k in cm/s; D10 in mm). This formula should not be used if the soil contains more than 5% fines. NOTE: Laboratory tests for permeability (hydraulic conductivity) are detailed in ASTM D 2434 for granular soils, D 5856 using a compaction-mold permeameter, and in D 5084 using a flexible-wall permeameter for soils with appreciable fines. Field tests include pumping tests in boreholes and infiltrometer tests. Standard procedures for several field tests are also in ASTM. NOTE: A good visual classification of the soils at the site will enable an experienced geotechnical engineer to estimate the permeability to the nearest order of magnitude, which is often sufficient for geotextile filter design. The following table, adapted from Casagrande (1938) and Holtz and Kovacs (1981), gives a range of hydraulic conductivities for different natural soils. Visual Classification Permeability or Hydraulic Conductivity, k (m/s) Clean gravel > 0.01 Clean sands and clean sand-gravel mixtures 0.01 < k < 10-5 Very fine sands; silts; mixtures of sand, silt, and clay; glacial tills; stratified clays 10-5 < k < 10-9 “Impervious” soils; homogeneous reasonably intact clays from below zone of weathering k > 10-9 “Impervious” soils, modified by vegetation, weathering, fissured, highly OC clays ≈ 5 x 10-5 < k < ≈ 5 x 10-8 STEP 3. Evaluate armor material and placement. Design reference: FHWA HEC-11, HEC-15, or HEC-23 (FHWA 1989, 2005, or 2009). a. Size armor stone or riprap Where minimum size of stone exceeds 4 in. (100 mm), or greater than a 4-in. (100 mm) gap exists between blocks, an intermediate gravel layer at least 6 in. (150 mm) thick should be used between the armor stone and geotextile. Gravel should be sized such that it will not wash through the armor stone (i.e., D85(gravel) ≥ D15(riprap)/5).

B.8 b. Determine armor stone placement technique (i.e., maximum height of drop). c. Consider alternate surface treatments such as with geocells. STEP 4. Determine anticipated reversing flow through the erosion control system. Here we need to estimate the maximum flow from seeps and weeps, maximum flow from wave action, or maximum flow from rapid drawdown. a. General case -- use Darcy's law q = kiA (B.2) where: q = Outflow rate (m3/sec) k = Effective permeability of soil (from Step 2b above) (m/sec) i = Average hydraulic gradient in soil (e.g., tangent of slope angle for wave runup) (dimensionless) A = Area of soil and drain material normal to the direction of flow (m2). Can be evaluated using a unit area. Use a conventional flow net analysis (e.g., Cedergren 1989) for seepage through dikes and dams or from a rapid drawdown analysis. b. Specific erosion control systems -- Hydraulic characteristics depend on expected precipitation, runoff volumes and flow rates, stream flow volumes and water level fluctuations, normal and maximum wave heights anticipated, direction of waves and tidal variations. Detailed information on determination of these parameters is available in the FHWA (1989) Hydraulic Engineering Circular No. 11. STEP 5. Determine geotextile requirements. a. Retention Criteria From Step 2a, obtain D85 and Cu; then determine largest opening size allowed. AOS or O95(geotextile) < B D85(soil) (B.3) where: B = 1 for a conservative design. For a less-conservative design and for soils with < 50% passing the No.200 sieve: B = 1 for Cu < 2 or > 8 (B.4a) B = 0.5 Cu for 2 < Cu < 4 (B.4b) B = 8/Cu for 4 < Cu < 8 (B.4c) For soils with > 50% passing the No.200 sieve: B = 1 for woven geotextiles B = 1.8 for nonwoven geotextiles and AOS or O95(geotextile) < 0.3 mm (B.5)

B.9 If geotextile and soil retained by it can move, use: B = 0.5 (B.6) b. Permeability/Permittivity Criteria 1. Less Critical/Less Severe kgeotextile > ksoil (B.7a) 2. Critical/Severe kgeotextile > 1 0 ksoil (B.7b) 3. Permittivity ψ Requirement for < 15% passing No.200 (0.075 mm) ψ ≥ 0.7 sec-1 (B.8a) for 15 to 50% passing No.200 (0.075 mm) ψ ≥ 0.2 sec-1 (B.8b) for > 50 % passing No.200 (0.075 mm) ψ ≥ 0.1 sec-1 (B.8c) 4. Flow Capacity Requirement qgeotextile > (At/Ag) qrequired (B.9) or (kgeotextile/t) h Ag ≥ qrequired qrequired is obtained from Step 4 (EQ. B.2) above (m3/sec) kgeotextile/t = ψ = permittivity (sec-1) h = Average head in field (m) Ag = Area of geotextile available for flow (e.g., if 50% of geotextile covered by flat rocks or riprap, Ag = 0.5 total area) (m2) At = Total area of geotextile (m2) c. Clogging Criteria 1. Less critical/less severe • From Step 2a obtain D15; then 1. For soils with Cu > 3, determine minimum pore size requirement, from O95 > 3 D15 (B.10) 2. For Cu < 3, specify geotextile with maximum opening size possible from retention criteria • Other qualifiers

B.10 For soils with % passing No. 200 > 5% < 5% Woven monofilament geotextiles: Percent Open Area > 4% 10% Nonwoven geotextiles: Porosity > 50% 70% • Alternative: Run filtration tests 2. Critical/severe Select geotextiles that meet the above retention, permeability, and survivability criteria; as well as the criteria in Step 5c.1 above; perform a long-term filtration test. Suggested filtration test for sandy soils is the gradient ratio (GR) test. The hydraulic conductivity ratio (HCR) test is recommended by ASTM for fine-grained soils, but as noted above, the HCR test has serious disadvantages. Alternative: Consider long-term filtration tests, F3 tests and the Flexible Wall GR test (see FHWA/NHI 2008, Section 1.5). d. Survivability Select geotextile properties required for survivability from Table B.1. Add durability requirements if applicable. Don't forget to check for abrasion and check drop height. Evaluate worst case scenario for drop height. STEP 6. Estimate costs. Calculate the volume of armor stone, the volume of aggregate and the area of the geotextile. Apply appropriate unit cost values. Grading and site preparation (LS) Geotextile (/yd2 {/m2}) Geotextile placement (/yd2 {/m2}) In-place aggregate bedding layer (/yd2 {/m2}) Armor stone (/ton {/kg}) Armor stone placement (/ton {/kg}) Total cost STEP 7. Prepare specifications. Include for the geotextile: A. General requirements B. Specific geotextile properties C. Seams and overlaps D. Placement procedures E. Repairs F. Testing and placement observation requirements STEP 8. Obtain samples of the geotextile before acceptance. STEP 9. Monitor installation during construction, and control drop height. Observe erosion control systems during and after significant storm events.

B.11 B.1.5 Geotextile Design Example DEFINITION OF DESIGN EXAMPLE • Project Description: Riprap on slope is required to permit groundwater seepage out of slope face, without erosion of slope. See figure for project cross section. • Type of Structure: small stone riprap slope protection • Type of Application: geotextile filter beneath riprap • Alternatives: (i) graded soil filter; or (ii) geotextile filter between embankment and riprap GIVEN DATA • See cross section • Riprap is to allow unimpeded seepage out of slope • Riprap will consist of small stone (2 to 12 in. {50 mm to 0.3 m}) • Stone will be placed by dropping from a backhoe • Seeps have been observed in the existing slope • Soil beneath the proposed riprap is a fine silty sand • Gradations of two representative soil samples Project Cross Section SIEVE SIZE (mm) PERCENT PASSING, BY WEIGHT Sample A Sample B 4.75 1.68 0.84 0.42 0.15 0.075 100 96 92 85 43 25 100 100 98 76 32 15

B.12 DEFINE A. Geotextile function(s) B. Geotextile properties required C. Geotextile specification SOLUTION A. Geotextile function(s): Primary - filtration Secondary - separation B. Geotextile properties required: apparent opening size (AOS), permittivity, and survivability DESIGN STEP 1. EVALUATE CRITICAL NATURE AND SITE CONDITIONS. From given data, this is a critical application due to potential for loss of life and potential for significant structural damage. Soils are reasonably well-graded, hydraulic gradient is low for this type of application, and flow conditions are steady state.

B.13 STEP 2. OBTAIN SOIL SAMPLES. a. VISUAL CLASSIFICATION AND GRAIN SIZE ANALYSES Visual classification (ASTM D 2488) indicates the fines are silty; therefore the soils are classified as silty sands, SM. Perform grain size analyses of the two soils. Plot gradations of representative soils. The D60, D10, and D85 sizes from the gradation plot are noted in the table below for Samples A and B. b. PERMEABILITY TESTS This is a critical application and soil permeability tests should of course be conducted. In this example, however, we will use only an estimated permeability, just to show how the design is done. STEP 3. EVALUATE ARMOR MATERIAL AND PLACEMENT. a. Small stone (2 to 12 in. {50 mm to 0.3 m}) riprap will be used (no wave action or significant surface flow). b. A placement drop of less than 3 ft (1 m) will be specified. STEP 4. CALCULATE ANTICIPATED FLOW THROUGH SYSTEM. Flow computations are not included within this example. The entire height of the slope face will be protected. This is conservative and for better appearance. STEP 5. DETERMINE GEOTEXTILE REQUIREMENTS. a. RETENTION AOS < B D85 Determine uniformity coefficient, Cu, retention coefficient B, and the maximum AOS. Soil Sample D60 ÷ D10 = Cu B = B × D85 > AOS (mm) A B 0.20 ÷ 0.045 = 4.4 0.30 ÷ 0.06 = 5 8 ÷ Cu = 8 ÷ 4.4 = 1.82 8 ÷ Cu = 8 ÷ 5 = 1.6 1.82 × 0.44 = 0.8 1.6 × 0.54 = 0.86 Worst case for retention is Soil A, so Sample A controls (see table in Step 2.a, above); therefore, AOS < 0.8 mm

B.14 b. PERMEABILITY/PERMITTIVITY This is a critical application, therefore, kgeotextile > 10 x ksoil For this example, let’s estimate the soil permeability (using Hazen's formula, but recognizing that it is applicable only for clean uniform sands and is much less accurate for soils with 25 to 15% fines. k ≈ (D10)2 where: k = approximate soil permeability (cm/sec); and D10 is in mm. ksoil = 2.0 (10)-3 cm/sec for Sample A = 3.6 (10)-3 cm/sec for Sample B Therefore (with rounding the number), kgeotextile > 4 (10)-2 cm/sec Since 15% to 25% of the soil to be protected is finer than No. 200 (0.075 mm), the permittivity is: ψgeotextile > 0.2 sec-1 c. CLOGGING As the project is critical, a filtration test is recommended to evaluate clogging potential. Select geotextile(s) meeting the retention and permeability criteria, along with the following qualifiers: Minimum Opening Size Qualifier (for Cu > 3): O95 > 3 D15 O95 > 3 × 0.057 = 0.17 mm for Sample A 3 × 0.079 = 0.24 mm for Sample B Sample A controls, therefore, O95 > 0.17 mm Other Qualifiers, since greater than 5% of the soil to be protected is finer than No. 200, from Table B.1: for Nonwoven geotextiles - Porosity > 50 % for Woven geotextiles - POA (Percent Open Area) > 4 % Then run a filtration test to evaluate long-term clogging potential. As the material is quite silty, the gradient ratio test (ASTM D 5101) may take up to several weeks to stabilize. After testing, geotextiles that perform satisfactorily can be prequalified. Alternatively, geotextiles proposed by the contractor must be evaluated prior to installation to confirm compatibility.

B.15 d. SURVIVABILITY A Class 1 geotextile will be specified because this is a critical application. Effect on project cost is minor. Therefore, from Table B.1, the following minimum values will be specified except for the UV resistance. Because this is a critical project and there is a potential for exposure between riprap, we will increase the UV resistance for this example. <50% Elongation >50% Elongation Grab Strength 315 lb (1400 N) 200 lb (900 N) Sewn Seam Strength 280 lb (1260 N) 180 lb (810 N) Tear Strength 110 lb (500 N) 80 lb (350 N) Puncture Strength 620 lb (2750 N) 433 lb (1925 N) Ultraviolet Degradation 70% strength retained at 500 hours Complete Steps 6 through 9 to finish design. STEP 6. ESTIMATE COSTS. STEP 7. PREPARE SPECIFICATIONS. STEP 8. COLLECT SAMPLES. STEP 9. MONITOR INSTALLATION, AND DURING & AFTER STORM EVENTS. B.1.6 Geotextile Cost Considerations The total cost of a riprap-geotextile revetment system will depend on the actual application and type of revetment selected. The following items should be considered: 1. Grading and site preparation 2. Cost of geotextile, including cost of overlapping and pins versus cost of sewn seams 3. Cost of placing geotextile, including special considerations for below-water placement 4. Bedding materials, if required, including placement 5. Armor stone, concrete blocks, sand bags, etc. 6. Placement of armor stone (dropped versus hand- or machine-placed) For Item No. 2, the cost of overlapping includes the extra material required for the overlap, cost of pins, and labor considerations versus the cost of field and/or factory seaming, plus the additional cost of laboratory seam testing. These costs can be obtained from manufacturers, but typical costs of a sewn seam are equivalent to 1.2 to 1.8 yd2 (1 to 1.5 m2) of geotextile. Alternatively, the contractor can be required to supply the cost on an area covered or in-place basis. For example, U.S. Army Corps of Engineers Specifications (CW-02215 1977) require measurement for payment for geotextiles in streambank and slope protection to be on an in- place basis without allowance for any material in laps and seams. Further, the unit price includes furnishing all plant, labor, material, equipment, securing pins, etc., and performing all operations in connection with placement of the geotextile, including prior preparation of banks and slopes. Of course, field performance should also be considered, and sewn seams are generally preferred to overlaps. For many erosion protection projects, the decision whether to sew or overlap is left to the contractor, with a bid item for the geotextile based on the area to be covered.

B.16 Another important consideration for Items 2, 4, and 6 is the difference between Moderate versus High Survivability geotextiles (Table B.1) and its effect on the cost of bedding materials and placement of armor stone. Class 1 geotextile materials typically cost 20% more than Class 2 materials. To determine cost effectiveness, benefit-cost ratios should be compared for the riprap- geotextile system versus conventional riprap-granular filter systems or other available alternatives of equal technical feasibility and operational practicality. Average cost of geotextile protection systems placed above the water level, including slope preparation, geotextile cost of seaming or securing pins, and placement is approximately $2.50-$5.00 per square yard, excluding the armor stone. Cost of placement below water level can vary considerably depending on the site conditions and the contractor's experience. For below- water placement, it is recommended that prebid meetings be held with potential contractors to explore ideas for placement and discuss anticipated costs. B.1.7 Geotextile Specifications In addition to the general recommendations concerning specifications in Chapter 1 of FHWA/NHI 2008, erosion control specifications must include construction details (see Section B.1.8 below), as the appropriate geotextile will depend on the placement technique. In addition, the specifications should require the contractor to demonstrate through trial sections that the proposed riprap placement technique will not damage the geotextile. Many erosion control projects may be better served by performance-type filtration tests that provide an indication of long-term performance. Thus, in many cases, approved list-type specifications may be appropriate. To develop the list of approved geotextiles, filtration studies should be performed using problem soils and conditions that exist in the localities where geotextiles will be used. An approved list for each condition should be established. In addition, geotextiles should be classified as High or Moderate Survivability geotextiles, in accordance with the index properties listed in Table B.1 and construction conditions. An example specification (a combination of the AASHTO M288 (2006) geotextile material specification and its accompanying construction/installation guidelines) is provided in FHWA/NHI 2008. B.1.8 Geotextile Installation Procedures Construction requirements will depend on specific application and site conditions. Photographs of several installations are shown in Chapter 2, Figure 2.30, Final Report. The following general construction considerations apply for most riprap-geotextile erosion protection systems. Special considerations related to specific applications and alternate riprap designs will follow. General Construction Considerations 1. Grade area and remove debris to provide smooth, fairly even surface. a. Depressions or holes in the slope should be filled to avoid geotextile bridging and possible tearing when cover materials are placed. b. Large stones, limbs, and other debris should be removed prior to placement to prevent fabric damage from tearing or puncturing during stone placement.

B.17 2. Place geotextile loosely, laid with machine direction in the direction of anticipated water flow or movement. 3. Seam or overlap the geotextile as required. a. For overlaps, adjacent rolls of geotextile should be overlapped a minimum of 1 ft (0.3 m). Overlaps should be in the direction of water flow and stapled or pinned to hold the overlap in place during placement of stone. Steel pins are normally 3/16-in. (5 mm) diameter, 18 in. (0.5 m) long, pointed at one end, and fitted with 1½-in. (38 mm) diameter washers at the other end. Pins should be spaced along all overlap alignments at a distance of approximately 3 ft (1 m) center to center. b. The geotextile should be pinned loosely so it can easily conform to the ground surface and give when stone is placed. c. If seamed, seam strength should equal or exceed the minimum seam requirements (see Section 1.6 FHWA/NHI 2008). 4. The maximum allowable slope on which a hard armor-geotextile system can be placed is equal to the lowest soil-geotextile friction angle for the natural ground or stone- geotextile friction angle for cover (armor) materials. Additional reductions in slope may be necessary due to hydraulic considerations and possible long-term stability conditions. For slopes greater than 2.5 to 1, special construction procedures will be required, including toe berms to provide a buttress against slippage, loose placement of geotextile sufficient to allow for downslope movement, elimination of pins at overlaps, increase in overlap requirements, and possible benching of the slope. Care should be taken not to put irregular wrinkles in the geotextile because erosion channels can form beneath the geotextile. 5. 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 Chapter 2, Figure 2.31, Final Report. 6. Place revetment (cushion layer and/or riprap) over the geotextile width, while avoiding puncturing or tearing it. a. Revetment should be placed on the geotextile within 14 days. b. Placement of armor cover will depend on the type of riprap, whether quarry stone, sandbags (which may be constructed of geotextiles), interlocked or articulating concrete blocks, soil-cement filled bags, filled geocells, or other suitable slope protection is used. c. For sloped surfaces, placement should always start from the base of the slope, moving up slope and, preferably, from the center outward. d. In no case should stone weighing more than 90 lbs (40 kg) be allowed to roll downslope on the geotextile. e. Field trials should be performed to determine if placement techniques will damage the geotextile and to determine the maximum height of safe drop. As a general guideline, for High Survivability (Class 1) geotextiles (Table B.1) with no cushion layer, height of drop for stones less than 220 lbs (100 kg) should be less than 3 ft

B.18 (1 m). For High Survivability (Class 1) geotextiles (Table B.1) or Moderate Survivability (Class 2) geotextiles with a 6-in. (150 mm) thick small aggregate cushion layer, height of drop for stones less than 220 lbs (100 kg) should be less than 3 ft (1 m). Stones greater than 220 lbs (100 kg) should be placed with no free fall unless field trials demonstrate they can be dropped without damaging the geotextile. f. Grading of slopes should be performed during placement of riprap. Grading should not be allowed after placement if it results in stone movement directly on the geotextile. As previously indicated, 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 Chapter 2, Figure 2.32, Final Report and are reviewed in the following subsections (from Christopher and Holtz 1985). Cut and Fill Slope Protection. Cut and fill slopes are generally protected using an armor stone over a geotextile-type system. Special consideration must be given to the steepness of the slope. After grading, clearing, and leveling a slope, the geotextile should be placed directly on the slope. When possible, geotextile placement should be placed parallel to the slope direction. A minimum overlap of 1 ft (0.3 m) between adjacent roll ends and a minimum 1 ft (0.3 m) overlap of adjacent strips is recommended. It is also important to place the up- slope geotextile over the down-slope geotextile to prevent overlap separation during aggregate placement. When placing the aggregate, do not push the aggregate up the slope against the overlap. Streambank Protection. 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). Construction procedures essentially follow the procedures listed above. 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. Roll ends should be overlapped 3 ft (1 m) and offset as shown in Chapter 2, Figure 2.32, Final Report. The upslope roll should overlap the downslope roll. The geotextile should be placed along the bank to an elevation well below mean low water level based on the anticipated flows in the stream. Existing agency design criteria for conventional nongeotextile streambank protection could be utilized to locate the toe of the erosion protection system. In the absence of other specifications, placement to a vertical

B.19 distance of 3 ft (1 m) below mean water level, or to the bottom of the streambed for streams shallower than 3 ft (1 m), is recommended. Geotextiles should either be placed to the top of the bank or at a given distance up the slope above expected high water level from the appropriate design storm event, including whatever requirements are normally used for conventional (nongeotextile) streambank protection systems. In the absence of other specifications, the geotextile should extend vertically a minimum of 18 in. (0.5 m) above the expected maximum water stage, or at least 3 ft (1 m) beyond the top of the embankment if less than 18 in. (0.5 m) above expected water level. 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 pit or trench at the toe. Alternative toe treatments are shown in Chapter 2, Figure 2.31, Final Report. The trench methods in Figures 2.31a and 2.31b require excavating a trench at the toe of the slope. This may be a good alternative for new construction; however, it should be evaluated with respect to slope stability when a trench will be excavated at the toe of a potentially saturated slope below the water level. Keying in at the top can consist of burying the top bank edge of the geotextile in a shallow trench after placement of the armor material. This will provide resistance to undermining from infiltration of over-the-bank precipitation runoff, and also provide stability should a storm greater than anticipated occur. However, unless excessive quantities of runoff are expected and stream flows are relatively small, this step is usually omitted. The armoring material (e.g., riprap, sandbags, blocks, filled geocells) must be placed to avoid tearing or puncturing the geotextile, as indicated above. Wave Protection Revetments. Because of cyclic flow conditions, geotextiles used for wave protection systems in most cases should be selected on the basis of severe criteria. The geotextile should be placed in accordance with the procedures listed above. If a geotextile will be placed where existing riprap, rubble, or other armor materials placed on natural soil have been unsuccessful in retarding wave erosion, site preparation could consist of covering the existing riprap with a filter sand. The geotextile could then be designed with less rigorous requirements as a filter for the sand than if the geotextile is required to filter finer soils. 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 the steps described above, except that a 3 ft (1 m) overlap distance is recommended by the Corps of Engineers for underwater placement (Chapter 2, Figure 2.32, Final Report). 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

B.20 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 Chapter 2, Figure 2.31, Final Report. Also, a key trench should be placed at the top of the bank, as shown in Figure 3.41a, 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 the recommendations stated in the general construction criteria above. Riprap or cover stone can also be placed underwater by cranes or bottom dump barges. Scour Protection. Scour, because of 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 for geotextile selection. An extremely wide variety of transportation-associated structures are possible and, thus, numerous ways exist to protect such structures with riprap geotextile systems. Typical applications are shown in FHWA HEC-23 (2009). In all instances, the geotextile is placed on a smoothly graded surface as stated in the general construction requirements above. 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 cohesionless 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. The geotextile should normally be placed with the machine direction parallel to the anticipated water flow direction. Seaming and/or overlapping of adjacent rolls should be performed as recommended in general construction requirements above. When roll ends are overlapped, the upstream ends should be placed over the downstream end. As necessary and appropriate, the geotextile may be secured in place with steel pins, as previously described. 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 (i.e., provided the air contained in the geotextile can be readily removed by submersion) while those less than 1 g/cm3 will float. Riprap, precast concrete blocks, bedding materials if used, or other elements placed on the geotextile should be placed without puncturing or tearing the geotextile. Drop heights should be selected on the basis of geotextile strength criteria, as discussed above.

B.21 B.2 USACOE Engineering and Design for Coastal Projects B.2.1 Background This section provides guidance from Chapter 4 (Materials and Construction Aspects) and Chapter 5 (Fundamentals of Design) from the September 2011 update (Change 3) to the Corps' Engineering Design Manual EM 1110-2-1100 on Coastal Engineering Projects (USACOE 2011). Following this overview background section the Corps geotextile filter design guidance and granular filter design guidance for coastal engineering are extracted and summarized to supplement the FHWA guidance for filter design presented in Chapter 2, Section 2.2.3 of the Final Report. Coastal application of woven geotextile fabric began in the mid-1950s. In the United States geotextiles were first used as a filter for an ocean-front concrete block revetment in 1958. Dutch coastal engineers first used geotextiles in 1956, and they continued development of geotextiles as work began on the massive Delta Works Scheme (John 1987). During the 1960s, geotextiles became well-established as replacements for granular filters due in part to extensive use in the Delta Works Scheme. It is estimated that over 10 million m3 of geotextile were used in the Dutch flood protection project. Initially, use of geotextiles was not cost- effective, and applications were limited to sites that lacked local sources of good granular fill material. Presently, the use of geosynthetics has become more widespread. Although the most common geotextile application is to serve as a filter, these fabrics can also serve the following functions: (a) Separate different soil layers (b) Reinforce soil banks against lateral movement (c) Control erosion of banks (d) Provide drainage (e) Cap and/or contain contaminated dredged material (see for example Chapter 2, Figures 2.34 and 2.35, Final Report) Generally, geotextile filter fabrics should allow water to flow through while retaining the soil. Other coastal applications, such as bank reinforcement, rely on high fabric tensile strength. Examples of geotextile use in coastal construction can be found in Barrett (1966), Dunham and Barrett (1974), Koerner and Welsh (1980), and John (1987). B.2.2 Geotextiles Geotextile Fabrics. Plastic filaments or fibers can be woven or needle punched into strong fabrics called "geotextiles" that are often used as filter cloth beneath hard armor systems. Other names for these types of fabrics include filter fabrics, construction fabrics, plastic filter cloth, engineering fabrics, and geotechnical fabrics. The most frequent use of geotextiles in coastal construction is as a filter between fine granular sands or soils and overlying gravel or small stone that forms the first under layer of a coastal structure such as a revetment. The purpose of the geotextile is to retain the soil while permitting flow of water through the fabric. Chapter 2, Figures 2.30 and 2.31 of the Final Report illustrate typical usage of a geotextile fabric as a filter.

B.22 Geotextile filters have several general advantages over conventional gravel filters (Barrett 1966): (a) Filtering characteristics are uniform and factory controlled (b) Geotextile filter fabrics can withstand tensile stresses (c) Geotextile placement is more easily controlled, and underwater placement is likely to be more successful than comparable gravel filters (d) Inspection and quality control are quick and accurate (e) Local availability of filter materials is not a cost consideration, and often substantial savings can be realized Some potential disadvantages of geotextile filter fabrics are the following: (1) it is difficult to repair damaged fabric that underlays several layers of stone, (2) if improperly designed, some fabrics can be relatively impervious to rapid hydraulic transients, which could lead to uplift pressures over the fabric surface, and (3) the fabric is susceptible to undermining at the structure toe if not properly anchored. Geotextile Properties. Most geotextiles are made from one of the four main polymer families: polyester, polyamide, polypropylene, or polyethylene. Polyethylene has one of the simplest molecular structures, and its main attractions include low cost and chemical resistance. Polyamides (e.g., nylon) are roughly three times more expensive than polyethylene, and they exhibit moderate strength and chemical resistance characteristics. Polypropylenes are low cost and currently comprise the most widely used group of geotextiles (John 1987). Polyesters have the best tensile strength characteristics, the least long-term creep and high inherent ultraviolet light resistance, but these attributes come at a high cost. The relative differences between the four polymer families in terms of important physical properties are shown Table B.2. Within each main group there are many subgroups that can have significantly different characteristics than those attributed to the group as a whole. In particular, strength properties vary with manufacturing method. Table B.2. Comparative Properties of Geotextile Materials (from John (1987))

B.23 Engineering properties and overall suitability of geotextiles for specific applications depend as much on the fabric manufacture as the properties of the polymer. Fabrics are either woven, nonwoven, or a combination of the two. Several weaving methods are used in the manufacture of geotextiles, and each method achieves different results in the fabric. Fabrics made of monofilament yarns have relatively regular and uniform pore sizes and are more reliable for critical filtration applications where the higher cost is justified. Nonwoven geotextiles are made of discrete fibers bonded together by some method that often allows for a somewhat thicker porous fabric. Porosity may be achieved by punching holes in the fabric with needles to attain a more uniform filtering capability. Both woven and nonwoven fabrics have been used in coastal applications, but woven monofilament geotextiles are overwhelmingly preferred for coastal structures. B.2.3 Design Requirements For Geotextile Fabrics General Design Requirements. Use of a geotextile as a filter cloth requires that the fabric be permeable to water without allowing passage of retained soil particles or clogging. Flow of water through the geotextile must be at a rate that prevents excessive head loss or buildup of hydrostatic pressure. An effective filter requires a geotextile suited to the retained soil grain size and slope, groundwater, wave and water level loading, and particulars of the overlying stone layers. Selection of a geotextile may be difficult because of the wide range of fabrics available from a number of manufacturers; however, the specification should be based on properties such as transmissivity, porosity, etc. It may help to examine past performance of particular geotextiles in similar projects. Some combination of the factors listed below may influence the selection of a suitable geotextile fabric (Moffatt and Nichol 1983). (a) Tensile strength. Fabric tensile strength is needed to resist tearing when subjected to dynamic loads from waves, currents, and constant movement of structure under layers. For rubble structures, strong fabrics allow placement of larger stones directly on the geotextile, thus reducing the overall structure thickness. However, if large voids occur in the overlying structure layers, soil pressure and/or hydrostatic pressure may rupture the fabric. Fabrics that have sufficient burst strength will continue to retain the soil, thus reducing rehabilitation costs. (b) Elongation at failure. Excessive elongation will distort and enlarge the pores, changing the filtering characteristics and perhaps resulting in soil loss. (c) Puncture resistance. Geotextiles need good puncture resistance to survive placement of materials over the fabric during construction. The fabric also needs to resist puncturing due to movement of armor stone and under layer stone as the structure settles or as it responds dynamically to wave action. (d) Abrasion resistance. Constant wave action on a coastal structure causes movement of materials adjacent to the geotextile, and the fabric must withstand this abrasion over the life of the structure. Special care must be taken during construction to avoid fabric abrasion as materials are placed on the geotextile. (e) Durability. Geotextiles must perform consistently over the life of the structure. Durability depends on the chemical composition and construction of the fabric, physical properties of the finished fabric, exposure to deteriorating environmental conditions, and physical abuse experienced during service. (f) Site-specific factors. Some coastal applications may subject geotextile fabrics to freeze/thaw conditions or to high or low temperatures. It may be necessary to test the geotextile for survivability under these conditions. Also, fabric selection should account for any anticipated exposure to chemicals, acids, alkalis, or fuels.

B.24 (g) Construction factors. Placement of geotextiles in severe wave environments may be difficult, and fabric may be damaged or severely abraded during placement attempts. Excessive movement of under layer materials by waves may severely damage the fabric before more stable armor layers can be placed. Construction methods may need to be modified to minimize adverse wave exposure. Recommended Minimum Geotextile Physical Properties. Moffatt and Nichol (1983) presented recommended minimum values for various geotextile engineering parameters under three different loading conditions for coastal projects. These recommendations are reproduced in Table B.3. "Severe Dynamic Loading" refers to continued abrasive movement of materials adjacent to the fabric due to wave action. "Dynamic and Static Loading" results from more restrictive placement procedures that limit abrasion. "Stringent Placement and Drainage" refers to applications where placement and service life are nearly free of any abrasive movement of adjacent materials. Design values for specific candidate geotextiles should be determined according to test procedures given in the referenced ASTM standards in Table B.3 (also summarized in Moffatt and Nichol (1983)) Geotextile Filtering and Clogging Criteria. Geotextile filters in coastal structures may be exposed to rapid flow fluctuations including turbulent flows, high hydrodynamic pressure differentials, and sudden or periodic runup and rundown. The selected geotextile must be able to retain the soil, yet have openings large enough to permit rapid drainage without clogging. Calhoun (1972) conducted extensive tests to develop engineering criteria for geotextile fabrics, and these criteria have been verified through numerous field applications. The capability of a geotextile to retain soil while allowing water to pass is termed the ''piping resistance." Calhoun developed a procedure for determining the piping resistance based on the size of the retained soil and the equivalent opening size (EOS) of the geotextile. He also developed clogging criteria based on the fabric ''percent of open area" (POA). Values of EOS and POA are determined using the procedures described in Table B.4. For retention of coarse-grained soils containing 50 percent or less by weight of particles passing U.S. No. 200 sieve (0.074 mm diameter), the piping resistance (PR) for woven geotextile fabric is given by: EOS soilprotectedofDPR 85= (B.11) where D85 is the effective grain size (in mm) for which 85 percent of the soil (by weight) has smaller grain size. (Note EOS is expressed in millimeters.) Ideally, the value of the piping resistance should be unity or slightly greater to promote drainage and prevent clogging. As values of PR increase, flow resistance through the fabric also increases. Adequate clogging resistance is provided by geotextiles having an effective POA equal to or greater than 4 percent. If a percentage of the geotextile's surface is covered by flat smooth materials (e.g., articulating concrete blocks) without an intervening gravel layer, the necessary fabric POA must be increased proportionately. For example if one third of the fabric is to be covered by flat blocks, then the necessary geotextile POA must be increased by a factor of 3 to 12 percent to give an effective POA of 4 percent. Geotextiles adjacent to finer soils in which more than 50 percent of the grains (by weight) pass through the U.S. No. 200 sieve should have an EOS no larger than a U.S. No. 70 sieve (0.21 0 mm). Geotextiles with an EOS smaller than a U.S. No. 100 sieve (0.149 mm) should not be used as filter in coastal projects.

B.25 Table B.3. Minimum Geotextile Fabric Physical Property Requirements (from Moffatt & Nichol (1983)).

B.26 Table B.4. Determination of EOS and POA for Geotextiles (from Moffatt and Nichol (1983)). Equivalent Opening Size (EOS) Based on the Calhoun (1972) method, five unaged samples shall be tested. Obtain about 150 gm of each of the following fractions of a sand composed of sound, rounded-to- subrounded particles: U.S. Standard Sieve Numbers Sample I. Passing #10 and Retained on #20 Sample 2. Passing #20 and Retained on #30 Sample 3. Passing #30 and Retained on #40 Sample 4. Passing #40 and Retained on #50 Sample 5. Passing #50 and Retained on #70 Sample 6. Passing #70 and Retained on #100 Sample 7. Passing # 100 and Retained on # 120 The cloth shall be affixed to a standard sieve having openings larger than the coarsest sand used, in such a manner that no sand can pass between the cloth and the sieve wall. The sand shall be oven-dried. Shaking of the sample will continue for 20 min. Determine by sieving (using successively coarser fractions) that fraction of sand of which 5 percent or less by weight passes the cloth. The equivalent opening size of the cloth sample is the retained on U.S. Standard Sieve number of this fraction. Percent of Open Area (POA) Each of five unaged samples should be placed separately in a 50- x 50-mm (2- x 2-in.) glass slide holder and the image projected with a slide projector on a screen. Select a block of 25 openings near the center of the image and measure to the nearest 25.4 microns (0.001 in.) the length and width of each of the 25 openings and the widths of two fibers adjacent to each opening. The percent open area is determined by dividing the sum of the open areas of the 25 openings by the sum of the total area of the 25 openings and their adjacent fibers. B.2.4 Geotextile Installation Considerations Practical experience with geotextile filters in coastal projects has provided general guidelines for geotextile installation and maintenance. However, unique site conditions may dictate alternate techniques. Geotextile Placement. Successful use of geotextiles in coastal projects depends critically on initial placement of the fabric. The sequence of geotextile placement is determined somewhat by the specific project and application, but in general the following guidelines should be followed. (a) Geotextiles should be laid loosely, free of wrinkles, creases, and folds. This allows the fabric to conform to irregularities in the soil when heavier materials are placed on the fabric. Placing the geotextile in a stretched condition under tensile stress should be avoided.

B.27 (b) Fabric placement on slopes subjected to wave action should begin at the slope toe and proceed upslope with the upslope panel overlapping the downslope panel. For slopes subjected to along-structure currents, upstream panels should overlap downstream panels (see Chapter 2, Figure 2.32, Final Report). (c) When the slope continues beyond the protective armor layers, the filter should be keyed into a trench at the upper portion of the structure. Similar termination of the filter can be used at the structure toe as illustrated in Chapter 2, Figure 2.31 of the Final Report. (d) Horizontal underwater fabric placement should start at the shoreward end and proceed seaward. For scour protection the placement should start adjacent to the protected structure, and proceed to the outer limit of the protection. (e) Any overlying gravel layers must have sufficient permeability so as not to reduce flow through the geotextile. (f) Steel securing pins (when needed) should be 5 mm (3/16 in.) in diameter and have a head capable of retaining a steel washer having a 3.8-cm outside diameter. Pin length should be a minimum of 45 cm (1.5 ft) for medium to high density soil, and longer for looser soils. Pins should be placed at the overlap mid-point. Pin spacing along the overlap should be a maximum of 0.6 m (2 ft) for slopes steeper than 1-on-3, 1.0 m (3 ft) for slopes between 1-on-3 and 1-on-4, and 1.5 m (5 ft) for slopes flatter than 1-on-4. Additional pins should be used as necessary to prevent geotextile slippage. (g) Placement of overlying stones should begin at the toe and proceed upslope. Some projects may require stone placement in conjunction with geotextile placement to hold the fabric against wave or current action. (h) Care must be exercised in placing the overlying stone layers to avoid puncturing the geotextile. Tables B.5 and B.6 provide construction drop height limitations for quarry stone revetments, block revetments, and subaqueous applications. Loading conditions are the same as described for Table B.3. No stones over 440 N (100 lb) should be rolled downslope over the fabric. Geotextile Seams and Joins. Geotextiles can he obtained in fairly long lengths, but width is limited by practical considerations related to manufacture and transportation. Wider panels reduce the number of fabric overlaps (which is the most probable cause of error during placement). Overlaps that are not subjected to tensile loading should be at least 45 cm (1.5 ft) and staggered in above water applications where placement can be well-controlled. Underwater geotextile overlaps should be at least 1 m (3 ft). Geotextile panels can be joined before or during placement by either sewing or cementing the panels together at the seams. Generally, sewing is preferred for onsite joins. The most appropriate guidance on field joining of specific geotextile fabrics should be available from the manufacturer. More general guidance is given in various geotextile textbooks (e.g., Ingold and Miller (1988), John (1987)). Geotextile Repairs. Construction damage to geotextile filters is easily repaired by trimming out the damaged section and replacing it with a section of fabric that provides a minimum of 0.6 m (2 ft) overlap in all directions. The edges of the replacement fabric should be placed under the undamaged geotextile. If damage occurs to geotextile panels in which the fabric tensile strength is needed to reinforce soil slopes, the entire fabric panel should be replaced. Repairing damaged fabric underlying a rubble-mound structure is more difficult because the overlying stone layers must first be removed to expose the damaged filter cloth.

B.28 Table B.5. Construction Limitations: Quarrystone Revetment1 (from Moffatt and Nichol (1983)).

B.29 Table B.6. Construction Limitations: Block Revetments and Subaqueous Applications (from Moffatt and Nichol (1983)). B.2.5 Environmental Effects on Geotextiles Chemical and Biological Effects. Geotextiles are generally considered not biodegradable, and they are relatively unaffected by chemicals found in normal concentrations in the coastal zone. However, some chemicals, such as alkalis and fuel products, can rapidly destroy some plastic compounds. Although geotextiles are impervious to biological attack, marine growth on plastic structure components may induce additional drag forces or hinder smooth operation of moving parts. Bacterial activity in the interstices of geotextiles can clog the fabric and increase its piping resistance. Ultraviolet Radiation. Unless stabilizers have been added during manufacture, geotextiles will deteriorate when exposed to ultraviolet (UV) radiation. For most coastal structure filtering applications, geotextiles are exposed to sunlight for only a short period during construction before placement of overlayers, and the effects of UV radiation are minor. In some cases, it may be prudent to sequence construction to minimize exposure of geotextile fabric to sunlight.

B.30 Geotextiles can be exposed to UV radiation if the armor layer is relatively thin, allowing sunlight to penetrate through voids in the armor layer. Similarly, precast armoring blocks may have holes that allow light penetration. Storm damage to structure armor layers can expose geotextile filters to sunlight for extended periods before repairs can be initiated. In the above situations, UV radiation will ultimately destroy the geotextile unless the fabric has been stabilized. The relative ultraviolet radiation resistance of untreated polymer types is shown in Table B.1. Fire. Geotextiles will bum or disintegrate if exposed to fire or high temperatures, often releasing very poisonous gases. Some geotextiles will burn easily, some slowly, and others with great difficulty. Flame-retardant chemicals can be combined into the molecular structure of the geotextile materials. Temperatures above the polymer's melting point will alter the filtering characteristics of geotextile fabrics. Other Factors. Abrasion by overlying material (or debris in the case of exposed fabric) can tear fibers in geotextiles, weakening the fabric. Impact loading by waves, vessels, or other objects may puncture geotextile fabric, and ice formation may induce tensile stresses exceeding the material yield strength. Excessive ground motion accelerations due to seismic events may cause differential shifting of the armor layer or soil slope, resulting in tension failure of the geotextile filter. Finally, exposed geotextile or high- strength fabrics may be damaged by vandalism. B.2.6 Granular Filters In coastal engineering, filter layers are defined as layers that protect the underlying base material or soil from erosion by waves and currents without excessive buildup of pore pressure in the underlying material. Filter functions can be achieved using either one or more layers of granular material such as gravel or small stone of various grain sizes, geotextile fabric, or a combination of geotextile overlaid with granular material. This section covers the function and design of granular filters. Design criteria for geotextile filter cloth used in filter application are given above (see Sections B.2.1 through B.2.5) Filter Layer Functions. Filter layers are designed to achieve one or more of the following objectives in coastal structures. They can prevent the migration of underlying sand or soil particles through the filter layer voids into the overlying rubble-mound structure layers. Leaching of base material could be caused by turbulent flow within the structure or by excessive pore pressures that can wash out fine particles. Without a filter layer, foundation or under layer material would be lost and the stones in the structure layer over the filter would sink into the void resulting in differential settlement and decreased structure crest elevation. Filter layers can aid in the distribution of structure weight. A bedding filter layer helps to distribute the structure's weight over the underlying base material to provide more uniform settlement. A leveled bedding layer also ensures a more uniform base plate load on caisson structures. Filter layers can also reduce the hydrodynamic loads on a structure's outer stone layers. A granular filter layer can help dissipate flow energy whereas a geotextile filter will not be as effective in this regard.

B.31 (a) Granular filters are commonly used as a bedding layer on which a coastal structure rests, or in construction of revetments where the filter layer protects the underlying embankment. Filter layers are also needed in rubble-mound structures having cores composed of fine materials like sand or gravel. Stone blankets (used to prevent erosion due to waves and currents) also reduce leaching of the underlying sand or soil, but in this situation stability of the stone blanket material in waves and currents is an important design concern. (b) It is advisable to place coastal structures on a bedding layer (along with adequate toe protection) to prevent or reduce undermining and settlement. When rubble structures are founded on cohesionless soil, especially sand, a filter blanket should be provided to prevent differential wave pressures, currents, and groundwater flow from creating an unstable foundation condition through removal of particles. Even when a filter blanket is not needed, bedding layers may be used to prevent erosion during construction, to distribute structure weight, or to retain and protect a geotextile filter cloth. Bedding layers are not necessary where depths are greater than about three times the maximum wave height, where the anticipated bottom current velocities are below the incipient motion level for the average-size bed material, or where the foundation is a hard, durable material such as bedrock. (c) In some situations granular filters have several advantages over geotextile filters in coastal construction (Permanent International Association of Navigation Congresses (PIANC) 1992). • The filter elements (stone, gravel, sand, etc.) are usually very durable. • Granular filters provide a good contact interface between the filter and base material below and between the filter and overlying layers. This is important for sloping structures. • Granular bedding layers can help smooth bottom irregularities and thus provide a more uniform construction base. • The porosity of granular filters help damp wave energy. • Self-weight of the filter layer contributes to its stability when exposed to waves and currents during construction whereas geotextiles may have to be weighted under similar conditions. • The loose nature of the filter elements allows the filter to better withstand impacts when larger stones are placed on the filter layer during construction or the stones shift during settlement. • Granular filter layers are relatively easy to repair, and in some instances may be self-healing. • Filter materials are widely available and inexpensive. (d) The major disadvantage of granular filters is the difficulty of assuring uniform construction underwater to obtain the required thickness of the filter layer. (e) Placing larger armor stone or riprap directly on geotextile filter cloth is likely to puncture the fabric either during placement or later during armor settlement. Placing a granular filter layer over the geotextile fabric protects it from damage. In this application there is more flexibility in specifying the filter stone gradation because the geotextile is retaining the underlying soil.

B.32 B.2.7 Granular Filter Failure Modes Granular filter layers fail their intended function when: (a) The base layer is eroded through the filter layer. Erosion can occur either by outgoing flow washing out particles perpendicular to the base/filter interface or by wave- and current-induced external flows parallel to the interface. (b) The filter layer becomes internally unstable. Instability occurs in filters having a very wide gradation when the finer fraction of the filter grain-size distribution is flushed out of the layer between the coarser material. This could result in compaction of the filter layer, differential settlement of the overlayers, and gradual increase in layer permeability. (c) The interface between adjacent granular layers becomes unstable, and lateral shearing motion occurs between layers constructed on a slope. (d) The filter layer fails to protect the underlying geotextile fabric from punctures and loss of soil through the filter cloth. B.2.8 Granular Filter Design Criteria Design criteria for granular filters were originally based on the geometry of voids between packed, uniform spheres. Allowances for grain-size distributions (and many successful field applications) led to the following established geometric filter design criteria. Retention Criterion. To prevent loss of the foundation or core material by leeching through the filter layer, the grain-size diameter exceeded by 85 percent of the filter material should be less than approximately four or five times the grain-size diameter exceeded by the coarsest 15 percent of the foundation or underlying material: )5to4( d d )foundation(85 )filter(15 < (B.12) The coarser particles of the foundation or base material are trapped in the voids of the filter layer, thus forming a barrier for the smaller sized fraction of the foundation material. The same criterion can be used to size successive layers in multilayer filters that might be needed when there is a large disparity between void sizes in the overlayer and particle sizes in the material under the filter. Filter layers overlying coarse material like quarry spall and subject to intense dynamic forces should be designed similar to a rubble-mound structure underlayer with: )20to15( W W )foundation(50 )filter(50 < (B.13) Permeability Criterion. Adequate permeability of the filter layer is needed to reduce the hydraulic gradient across the layer. The accepted permeability criterion is: )5to4( d d )foundation(85 )filter(15 < (B.14)

B.33 Internal Stability Criterion. If the filter material has a wide gradation, there may be loss of finer particles causing internal instability. Internal stability requires: 10 d d )filter(10 )filter(60 < (B.15) Layer Thickness. Filter layers constructed of coarse gravel or larger material should have a minimum thickness at least two to three times the diameter of the larger stones in the filter distribution to be effective. Smaller gravel filter layer thickness should be at least 20 cm, and sand filter layers should be at least 10 cm thick (Pilarczyk 1990). These thickness guidelines assume controlled above-water construction. In underwater placement, bedding layer thickness should be at least two to three times the size of the larger quarrystones used in the layer, but never less than 30 cm thick to ensure that bottom irregularities are completely covered. Considerations such as shallow depths, exposure during construction, construction method, and strong hydrodynamic forces may dictate thicker filters, but no general rules can be stated. For deeper water the uncertainty related to construction often demands a minimum thickness of 50 cm. Bedding Layer Over Geotextile Fabric. In designs where a geotextile fabric is used to meet the retention criterion, a covering layer of quarry spalls or crushed rock (10-cm minimum and 20-cm maximum) should be placed to protect against puncturing by the overlying stones. Recommended minimum bedding layer thickness in this case is 60 cm, and filtering criteria should be met between the bedding layer and overlying stone layer. B.2.9 Examples Examples of typical granular filters and bedding layers are illustrated in Lee (1972), who discussed and illustrated applications of granular and geotextile filters in coastal structures. Design of filters for block-type revetments with large holes in the cover layer can be found in the PIANC (1992) reference. B.2.10 Observations on Granular Design The previous geometric granular filter criteria are widely accepted in practice, and they are recommended in cases when an appreciable pressure gradient is expected perpendicular to the soil/filter interface. However, these rules may be somewhat conservative in situations without significant pressure gradients and when flow is parallel to the filter layer. The need for reliable granular filter design guidance under steady flow and cyclic design conditions fostered research by Delft Hydraulics Laboratory in support of the Oosterschelde Storm Surge Barrier in The Netherlands. Stationary and cyclic flow both parallel to and perpendicular to the filter layer were investigated by de Graauw, van der Meulen, and van der Does de Bye (1984). They developed hydraulic filter criteria based on an expression for critical hydraulic gradient parallel to the filter/soil interface. This method assumes that erosion of base material is caused by shear stresses rather than groundwater pressure gradients; and where this is the case, the geometric filter requirements can be relaxed.

B.34 The filter design guidance of de Graauw et al. (1984) was expressed in terms of the filter d15, base material d50, filter porosity, and critical shear velocity of the base material; and acceptable values for the critical gradient were given by graphs for each of the flow cases. Design of a hydraulic granular filter requires good understanding of the character of flow within the filter layer, e.g., steady flow in channels. In these cases the method of de Graauw et al. (1984) can be used. More recent research aimed at improving granular filter design criteria was reported by Bakker, Verheij, and deGroot (1994). For additional discussion of current European practice for granular filter design see Chapter 2, Section 2.2.4 of the Final Report. B.2.11 Granular Filter Construction Aspects Granular filter construction above water creates no special problems, and accurate placement is straightforward. However, constructing a filter beneath the water surface is somewhat more problematic. If small-size filter material with a wide gradation is dropped into place, there is a risk of particle segregation by size. This risk can be decreased by using more uniform material and minimizing the drop distance. Another problem is maintaining adequate layer thickness during underwater placement. This has led to the recommended layer thickness being greater than required by the geometric filter criteria. Finally, filter or bedding layers placed underwater are exposed to eroding waves and currents until the overlayers are placed. Depending on site- specific conditions, this factor may influence the construction sequence or the time of year chosen for construction. It is common practice to extend the bedding layer beneath rubble-mound structures at least 1.5 m beyond the toe of the cover stone to help reduce toe scour. Some low rubble-mound structures have no core, and instead are composed entirely of armor layer and under layers. These structures should have a bedding layer that extends across the full width of the structure. B.2.12 References AASHTO, 2006. "Standard Specifications for Geotextiles - M 288," Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 25th Edition, American Association of State Transportation and Highway Officials, Washington, D.C. AASHTO, 2005. "Model Drainage Manual," 3rd Edition, American Association of State Highway and Transportation Officials, Washington, D.C. ASTM, 2006. "Annual Books of ASTM Standards," American Society for Testing and Materials, Volume 4.08 (I), Soil and Rock, Volume 4.09 (II), Soil and Rock; Geosynthetics, West Conshohocken, PA. Bakker, K.J., Verheij, H.J., and de Groot, M.B., 1994. "Design Relationship for Filters in Bed Protection," Journal of Hydraulic Engineering, American Society of Civil Engineers, Vol. 120, No. 9, pp. 1082-1088. Barrett, R.J., 1966. "Use of Plastic Filters in Coastal Structures," Proceedings of the 10th International Conference on Coastal Engineering, American Society of Civil Engineers, Vol. 2, pp. 1048-1067.

B.35 Calhoun, C.C., Jr., 1972. "Development of Design Criteria and Acceptance Specifications for Plastic Filter Cloth," Technical Report S-72-7, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. Casagrande, A., 1938. "Notes on Soil Mechanics--First Semester," Harvard University (unpublished), 129 pp. Cedergren, H.R., 1989. "Seepage, Drainage, and Flow Nets," Third Edition, John Wiley and Sons, New York, NY, 465 p. Christopher, B.R. and Holtz, R.D., 1985. "Geotextile Engineering Manual," FHWA-TS-86/203, March, 1044 p. deGraauw, A.J., van der Meulen, T., and van der Does de Bye, M.R., 1984. "Granular Filters: Design Criteria," Journal of Waterway, Port. Coastal, and Ocean Engineering, American Society of Civil Engineers, Vol. 110, No. 1, pp. 80-96. Dunham, J.W. and Barrett, R.J., 1974. "Woven Plastic Cloth Filters for Stone Seawalls," Journal of the Waterways, Harbors, and Coastal Engineering Division, American Society of Civil Engineers, Vol. 100, No. WW1, pp. 13-22. FHWA, 2012. "Evaluating Scour at Bridges," Hydraulic Engineering Circular No. 18, Fifth Edition, Federal Highway Administration, FHWA HIF-12-003. FHWA, 2009. "Bridge Scour and Stream Instability Countermeasures, Experience, Selection, and Design Guidance," Hydraulic Engineering Circular No. 23, Third Edition, Federal Highway Administration, FHWA NHI-09-112. FHWA/NHI, 2008. "Geosynthetic Design and Construction Guidelines," Reference Manual, National Highway Institute Course No. 132013, Publication No. FHWA NHI-07-092, Washington, D.C. FHWA, 2006. "Hydraulic Design of Energy Dissipators for Culvert and Channels," Hydraulic Engineering Circular No. 14, Federal Highway Administration, FHWA-NHI-06-086. FHWA, 2005. "Design of Roadside Channels with Flexible Linings," Hydraulic Engineering Circular No. 15, Third Edition, Federal Highway Administration, FHWA-IF-05-114. FHWA, 1989. "Design of Riprap Revetment," Hydraulic Engineering Circular No. 11, Federal Highway Administration, FHWA-IP-89-016. Holtz, R.D. and Kovacs, W.D., 1981. "An Introduction to Geotechnical Engineering," Prentice- Hall, p. 210. Ingold, T.S. and Miller, K.S., 1988. "Geotextiles Handbook," Thomas Telford, Limited, London. John, N.W., 1987. "Geotextiles," Blackie and Sons, Ltd., London. Keown, M.P. and Dardeau, E.A., Jr., 1980. "Utilization of Filter Fabric for Streambank Protection Applications," TR HL-80-12, Hydraulics Laboratory, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS.

B.36 Koerner, R.M. and Welsh, J.P., 1980. "Construction and Geotechnical Engineering Using Synthetic Fabrics," John Wiley and Sons, NY. Lee, T.T., 1972. "Design of Filter System for Rubble-Mound Structures," Proceedings of the 13th International Conference on Coastal Engineering, American Society of Civil Engineers, Vol. 3, pp. 1917-1933. Moffatt and Nichol, Engineers, 1983. "Construction Materials for Coastal Structures," Special Report SR-10, U.S. Army Engineer Waterways Experiment Station, Coastal Engineering Research Center, Vicksburg, MS. Permanent International Association of Navigation Congresses, 1992. "Guidelines for the Design and Construction of Flexible Revetments Incorporating Geotextiles in Marine Environment," Report of Working Group No. 21 of the Permanent Technical Committee II, Supplement to Bulletins No. 78/79. Pilarczyk, K.W., 1990. "Design of Seawalls and Dikes - Including Overview of Revetments," Coastal Protection, K. Pilarczyk, ed., A.A. Balkema Publishers, Rotterdam, The Netherlands. U.S. Army Corps of Engineers, 2011. "Coastal Engineering Manual: Engineering and Design," EM 1110-2-1100, Change 3, Department of the Army, Washington, D.C. U.S. Army Corps of Engineers, 1977. "Civil Works Construction Guide Specification for Plastic Filter Fabric," Corps of Engineer Specifications No. CW-02215, Office, Chief of Engineers, U.S. Army Corps of Engineers, Washington, D.C.

Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 254: Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report documents the research effort of NCHRP Research Report 887: Guidance for Underwater Installation of Filter Systems. The project provides guidance on design procedures, material testing requirements, installation alternatives, and quality checklist items for both granular and geotextile filters. Filters are an important countermeasure for stream instability or bridge scour and are essential to the successful long-term performance of hydraulic countermeasures and other erosion countermeasures.

In addition to this guidance, a training manual for an underwater filter installation workshop is available as NCHRP Web-Only Document 254, Volume 2.

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