As ubiquitous as it is on construction sites, silt fence gets little respect. “They [construction crews] put up sediment fence, and it’s a waste of money and time,” remarks Bill Resch, vice president of the Friends of the Rocky Fork Watershed. “They usually give it to the last hired and least skilled worker.” Yet silt fence is often the primary sediment control BMP in the early phases of construction. Recent testing by the Environmental Evaluation Technology Center (EvTEC) examined to what extent the efficacy of silt fence depends on the method used to install it. An independent laboratory, TRI/Environmental Inc. of Austin, TX, supervised the research under contract to EvTEC. A seven-day field testing program, conducted at four sites with various soil types, compared trenching and slicing methods of installation in terms of performance (water retention) and installation time. Both methods were tested with variations in amount of backfill, degree of compaction, and volume of runoff. Three different trenching regimens were used, from minimum-specification installation to “Spec++” installation that included rare–on most sites–practices such as hand-cleaning the trench before installation of the fabric. Several problems are commonly associated with traditional trench-based installation methods: the trench is not properly backfilled, the soil is not compacted, debris in the trench interferes with installation, or the silt fence isn’t installed to a uniform depth. Static slicing, which originated in the mid-1990s, involves mechanically installing silt fence while simultaneously optimizing the soil for compaction. Compaction by wheel-rolling is then followed by posting and tying up the fabric. EvTEC’s results? Runoff retention tests showed that silt fence installed with the static slicing and the Spec++ trenching methods performed similarly, and both far outperformed the other trenching methods. The Spec++ required nearly triple the time effort to achieve those comparable results, however, and any less effort showed considerably poorer performance. The determining factor was compaction. Higher densities were directly correlated to higher performance–that is, water retention. Installing posts before attempting to compact the soil significantly reduced the ability to compact the soil and thus reduced performance. The slicing method was faster and required less manual labor than even the quickest and dirtiest trench installation. Static slicing also resulted in the most uniform installation. Excessive seepage and washout–which have led to the notoriety of silt fence on so many sites–occurred primarily with low compaction and inadequate backfill from the minimum specification effort.The testing dealt with methods of installation rather than particular types or brands of equipment, so the verification protocol can be used for future installation technologies as well. Tom Carpenter, president of Carpenter Erosion Control, which offers the tommy Silt Fence Machine, feels the testing confirmed what many users have already found for themselves. “The static slicing method is already part of the ASTM D6462 standard,” he notes, “so it’s an easy matter to incorporate this method into most project specifications.” D6462 is the American Society for Testing and Materials Standard Practice for Silt Fence Installation.
Hamilton Water
Profile Products
Andrea Weissenbuehler
