Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report (2018) / Chapter Skim
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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
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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.
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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.
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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.
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B.4 Table B.1. Geotextile Strength Property Requirements1,2,3,4 for Permanent Erosion Control Geotextiles (after AASHTO 2006)
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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.
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B.6 Figure B.1. Flow chart summary of the FHWA filter design procedure.
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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)
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B.8 b. Determine armor stone placement technique (i.e., maximum height of drop)
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B.9 If geotextile and soil retained by it can move, use: B = 0.5 (B.6)
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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.
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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.
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B.12 DEFINE A Geotextile function(s)
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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.
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B.15 d. SURVIVABILITY A Class 1 geotextile will be specified because this is a critical application.
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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.
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B.17 2. Place geotextile loosely, laid with machine direction in the direction of anticipated water flow or movement.
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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)
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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.
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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)
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B.22 Geotextile filters have several general advantages over conventional gravel filters (Barrett 1966)
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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.
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B.24 (g) Construction factors.
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B.25 Table B.3. Minimum Geotextile Fabric Physical Property Requirements (from Moffatt & Nichol (1983)
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B.26 Table B.4. Determination of EOS and POA for Geotextiles (from Moffatt and Nichol (1983)
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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.
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B.28 Table B.5. Construction Limitations: Quarrystone Revetment1 (from Moffatt and Nichol (1983)
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B.29 Table B.6. Construction Limitations: Block Revetments and Subaqueous Applications (from Moffatt and Nichol (1983)
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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.
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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.
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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.
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B.33 Internal Stability Criterion. If the filter material has a wide gradation, there may be loss of finer particles causing internal instability.
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B.34 The filter design guidance of de Graauw et al.
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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.
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B.36 Koerner, R.M. and Welsh, J.P., 1980.
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