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HomeMy WebLinkAboutElement 13 BMP-C233BMP C233: Silt Fence Purpose Use of a silt fence reduces the transport of coarse sediment from a construction site by providing a temporary physical barrier to sediment and reducing the runoff velocities of overland flow. See Figure 4.2.12 for details on silt fence construction. Conditions of Use Silt fence may be used downslope of all disturbed areas. Silt fence shall prevent soil carried by runoff water from going beneath, through, or over the top of the silt fence, but shall allow the water to pass through the fence. Silt fence is not intended to treat concentrated flows, nor is it intended to treat substantial amounts of overland flow. Convey any concentrated flows through the drainage system to a sediment pond. Do not construct silt fences in streams or use in V-shaped ditches. Silt fences do not provide an adequate method of silt control for anything deeper than sheet or overland flow. Figure 4.2.12 – Silt Fence Design and Installation Specifications Use in combination with sediment basins or other BMPs. Maximum slope steepness (normal (perpendicular) to fence line) 1H:1V. Maximum sheet or overland flow path length to the fence of 100 feet. Do not allow flows greater than 0.5 cfs. Steel fence posts or equivalent The geotextile used shall meet the following standards. All geotextile properties listed below are minimum average roll values (i.e., the test result for any sampled roll in a lot shall meet or exceed the values shown in Table 4.2.3): Table 4.2.3 Geotextile Standards Polymeric Mesh AOS (ASTM D4751) 0.60 mm maximum for slit film woven (#30 sieve). 0.30 mm maximum for all other geotextile types (#50 sieve). 0.15 mm minimum for all fabric types (#100 sieve). Water Permittivity (ASTM D4491) 0.02 sec-1 minimum Grab Tensile Strength (ASTM D4632) 180 lbs. Minimum for extra strength fabric. 100 lbs minimum for standard strength fabric. Grab Tensile Strength (ASTM D4632) 30% maximum Ultraviolet Resistance (ASTM D4355) 70% minimum Support standard strength fabrics with wire mesh, chicken wire, 2- inch x 2-inch wire, safety fence, or jute mesh to increase the strength of the fabric. Silt fence materials are available that have synthetic mesh backing attached. Filter fabric material shall contain ultraviolet ray inhibitors and stabilizers to provide a minimum of six months of expected usable construction life at a temperature range of 0°F. to 120°F. One-hundred percent biodegradable silt fence is available that is strong, long lasting, and can be left in place after the project is completed, if permitted by local regulations. Refer to Figure 4.2.12 for standard silt fence details. Include the following standard Notes for silt fence on construction plans and specifications: 1.The contractor shall install and maintain temporary silt fences at the locations shown in the Plans. 2.Construct silt fences in areas of clearing, grading, or drainage prior to starting those activities. 3.The silt fence shall have a 2-feet min. and a 2½-feet max. height above the original ground surface. 4.The filter fabric shall be sewn together at the point of manufacture to form filter fabric lengths as required. Locate all sewn seams at support posts. Alternatively, two sections of silt fence can be overlapped, provided the Contractor can demonstrate, to the satisfaction of the Engineer, that the overlap is long enough and that the adjacent fence sections are close enough together to prevent silt laden water from escaping through the fence at the overlap. 5.Attach the filter fabric on the up-slope side of the posts and secure with staples, wire, or in accordance with the manufacturer's recommendations. Attach the filter fabric to the posts in a manner that reduces the potential for tearing. 6.Support the filter fabric with wire or plastic mesh, dependent on the properties of the geotextile selected for use. If wire or plastic mesh is used, fasten the mesh securely to the up-slope side of the posts with the filter fabric up-slope of the mesh. 7.Mesh support, if used, shall consist of steel wire with a maximum mesh spacing of 2-inches, or a prefabricated polymeric mesh. The strength of the wire or polymeric mesh shall be equivalent to or greater than 180 lbs. grab tensile strength. The polymeric mesh must be as resistant to the same level of ultraviolet radiation as the filter fabric it supports. 8.Bury the bottom of the filter fabric 4-inches min. below the ground surface. Backfill and tamp soil in place over the buried portion of the filter fabric, so that no flow can pass beneath the fence and scouring cannot occur. When wire or polymeric back-up support mesh is used, the wire or polymeric mesh shall extend into the ground 3-inches min. 9.Drive or place the fence posts into the ground 18-inches min. A 12– inch min. depth is allowed if topsoil or other soft subgrade soil is not present and 18-inches cannot be reached. Increase fence post min. depths by 6 inches if the fence is located on slopes of 3H:1V or steeper and the slope is perpendicular to the fence. If required post depths cannot be obtained, the posts shall be adequately secured by bracing or guying to prevent overturning of the fence due to sediment loading. 10.Use steel or equivalent posts. The spacing of the support posts shall be a maximum of 6-feet. Posts shall consist of either: No. 6 steel rebar or larger. ASTM A 120 steel pipe with a minimum diameter of 1-inch. U, T, L, or C shape steel posts with a minimum weight of 1.35 lbs./ft. Other steel posts having equivalent strength and bending resistance to the post sizes listed above. 11.Locate silt fences on contour as much as possible, except at the ends of the fence, where the fence shall be turned uphill such that the silt fence captures the runoff water and prevents water from flowing around the end of the fence. 12.If the fence must cross contours, with the exception of the ends of the fence, place gravel check dams perpendicular to the back of the fence to minimize concentrated flow and erosion. The slope of the fence line where contours must be crossed shall not be steeper than 3H:1V. Gravel check dams shall be approximately 1-foot deep at the back of the fence. Gravel check dams shall be continued perpendicular to the fence at the same elevation until the top of the check dam intercepts the ground surface behind the fence. Gravel check dams shall consist of crushed surfacing base course, gravel backfill for walls, or shoulder ballast. Gravel check dams shall be located every 10 feet along the fence where the fence must cross contours. Refer to Figure 4.2.13 for slicing method details. Silt fence installation using the slicing method specifications: 1. The base of both end posts must be at least 2- to 4-inches above the top of the filter fabric on the middle posts for ditch checks to drain properly. Use a hand level or string level, if necessary, to mark base points before installation. 2. Install posts 3- to 4-feet apart in critical retention areas and 6- to 7- feet apart in standard applications. 3. Install posts 24-inches deep on the downstream side of the silt fence, and as close as possible to the filter fabric, enabling posts to support the filter fabric from upstream water pressure. 4. Install posts with the nipples facing away from the filter fabric. 5. Attach the filter fabric to each post with three ties, all spaced within the top 8-inches of the filter fabric. Attach each tie diagonally 45 degrees through the filter fabric, with each puncture at least 1-inch vertically apart. Each tie should be positioned to hang on a post nipple when tightening to prevent sagging. 6. Wrap approximately 6-inches of fabric around the end posts and secure with 3 ties. 7. No more than 24-inches of a 36-inch filter fabric is allowed above ground level. Compact the soil immediately next to the filter fabric with the front wheel of the tractor, skid steer, or roller exerting at least 60 pounds per square inch. Compact the upstream side first and then each side twice for a total of four trips. Check and correct the silt fence installation for any deviation before compaction. Use a flat-bladed shovel to tuck fabric deeper into the ground if necessary. Figure 4.2.13 – Silt Fence Installation by Slicing Method Maintenance Standards Repair any damage immediately. Intercept and convey all evident concentrated flows uphill of the silt fence to a sediment pond. Check the uphill side of the fence for signs of the fence clogging and acting as a barrier to flow and then causing channelization of flows parallel to the fence. If this occurs, replace the fence or remove the trapped sediment. Remove sediment deposits when the deposit reaches approximately one-third the height of the silt fence, or install a second silt fence. Replace filter fabric that has deteriorated due to ultraviolet breakdown.