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A FIELD STUDY OF SILT FENCE INSTALLATIONS AROUND THE UNITED STATESby C. Joel Sprague
The following document is used by permission, courtesy of the International Erosion Control Association (IECA) Mr. Sprague is a senior engineer for TRI/Environmental, Inc., Austin, Texas. Mr. Sprague is based in Greenville, South Carolina, where he also consults for Sprague & Sprague Consulting Engineers. He is a registered professional engineer in North and South Carolina, Georgia, and Texas. He has authored numerous articles and technical papers on the development, testing, and application of erosion and sediment control materials and geosynthetics. AbstractIn 1999, a silt fence performance evaluation was performed by the Civil Engineering Research Foundation’s Environmental Technology Evaluation Center (EVTEC). The evaluation included 51 test segments reflecting different soil types, different installation methods, and different hydraulic conditions. In general, the static slicing method was found to provide storm water runoff retention consistently as good as or better than the besttrenched installations, and far superior retention to common trenched installations. Additionally, the static slicing method of installation was found to be a much more efficient, and therefore, cost-effective technique for silt fence installation when compared to a range of traditional trenching-based procedures. The EVTEC field evaluation was a controlled experiment in which the trench depth, soil types, backfill conditions, and most other installation parameters were pre-selected based on industry standards to facilitate valid comparisons and the resulting conclusions. In an effort to determine if the conclusions drawn from the EVTEC study were consistent with common field experience, a field study was performed involving the inspection of randomly selected silt fence installations around the United States to provide data on the “state-of-practice” of silt fence installation, including both static slicing and traditional silt fence installation methods. The field study was performed by TRI/Environmental, Inc., and included 56 sites in 12 states reflecting different soil types, different installation methods, and different hydraulic conditions. In general, the static slicing method of silt fence installation was found to provide more consistent results than common trenched installations. This consistency was manifested in more uniform silt fence fabric burial depth, substantially higher level of compaction of the soil adjacent to the buried fabric, and satisfactory performance. Conversely, the trenched installations exhibited frequent inadequacy of both burial depth and soil compaction and, often, indications of being undermined by runoff. This paper provides details of the evaluation, including data summaries, and discussion of results. Key Words: silt fence; static slicing; trenching; runoff; installation OverviewThis paper describes a field study involving the inspection of randomly selected silt fence installations in an effort to provide data on the “state-of-practice” of silt fence installation. The data should provide potential users and specifiers of silt fence with the ability to make informed decisions about the typical quality of different silt fence installation methods. The field study was performed by TRI/Environmental, Inc, and included 56 sites in 12 states reflecting different soil types, different installation methods, and different hydraulic conditions. In general, the static slicing method of silt fence installation was found to provide more consistent results than common trenched installations. This consistency was manifested in more uniform silt fence fabric burial depth, substantially higher level of compaction of the soil adjacent to the buried fabric, and satisfactory performance. Conversely, trenched installations exhibited frequent inadequacy of both burial depth and soil compaction and, often, indications of being undermined by runoff. BackgroundIn 1999, a field evaluation of the installation was performed. The performance of silt fence installed using the Tommy® Silt Fence Static Slicing Method was compared to the performance of silt fence installed using traditional trenching methods. This large-scale evaluation/verification was overseen and coordinated by the Environmental Technology Evaluation Center (EVTEC), a program of the Civil Engineering Research Foundation (CERF), the research and technology transfer arm of the American Society of Civil Engineers (ASCE). The field verification testing was supervised by TRI/Environmental, Inc. The field testing included 51 test segments reflecting different soil types, different installation methods, and different hydraulic conditions. In general, the static slicing method was found to provide storm water runoff retention consistently as good as or better than the best-trenched installations, and far superior retention to common installations. Additionally, the static slicing method of installation was found to be a much more efficient, and therefore, cost-effective technique for silt fence installation when compared to a range of traditional trenching-based procedures. The static slicing method also was found to offer practical advantages over traditional trenching-based methods, such as maneuverability, minimal soilhandling and hand labor, consistent depth and compaction, and ease of installation in windy conditions, on steep side slopes, through rocky soils, and in wet soil conditions. COMPACTION BENEFITSPerformance trends from the original field evaluation provided a clear indication that a greater level of compaction (i.e., higher density obtained) corresponds to better performance (i.e., greater water retention). System comparisons showed that slicing provided installations that had both higher densities and greater water retention than all trenching-based installations. Trenching-based installations were affected by the inability to compact effectively when posts are installed first, when insufficient backfill material is placed in the trench, or when inadequate compaction effort is provided. It was noted that the installations using static slicing also required reasonable compaction efforts to perform properly. During the original field evaluation, compaction densities were measured with a nuclear density gauge, and associated shear strengths were measured with a hand-held cone penetrometer. There was a significant correlation between the cone penetrometer readings and the nuclear density measurements. This suggested that the much easier, and less expensive, hand penetrometer could be effectively used as a field quality assurance tool. REMAINING QUESTIONS FROM THE EVTEC EVALUATIONThe original EVTEC field evaluation was a controlled experiment designed to limit the number of variables being tested. To this end, the trench depth, soil types, backfill conditions, and most other installation parameters were pre-selected based on industry standards to facilitate valid comparisons and the resulting conclusions. It still remained to be demonstrated whether the conclusions drawn from such a controlled experiment reflected common, widespread field experience. FIELD STUDY OBJECTIVESThe primary objective of the field study was to perform random inspections of actual silt fence installations to provide data on the “state-of-practice” of silt fence installation. With this data it would be possible to determine the relevance of the earlier evaluation described above and to provide potential users and specifiers with the ability to make informed decisions about using static slicing for the installation of silt fences at their local construction sites. FIELD STUDY DESCRIPTIONThe field study was performed by a TRI technician and included 56 sites in 12 states. The sites were randomly selected either from a selection of sites obtained from local silt fence installers or by chance observation when driving in a pre-selected city. The sites reflected many different soil types, installation methods, and hydraulic conditions. Details of the site inspections, including data summaries and discussion of results, follow. DATA COLLECTED FROM INSPECTIONSAll the inspections were made by an experienced technician. The inspector used a tape measure and hand-held cone penetrometer, and recorded measurements and observations on a data sheet. Additionally, pictures were taken of most sites. Each inspection included numerous measurements/observations, including:
Detailed data on all 56 sites is presented in Table 1. A summary of results is presented in Tables 2–4. The results are displayed graphically in Figures 1–3. DISCUSSION OF RESULTS— HIGHLIGHTS AND OBSERVATIONS1. 70% of silt fence installations using trenching had no backfill or no compaction. 2. The 30% of trenched installations having at least minimally compacted backfill had an average of 45% of the in situ soil strength in the backfill soil. (Range: 23% to 91%.) 3. Less than 8% of silt fence installations using static slicing had no compaction. 4. The 92+% of static sliced installations received compaction effort sufficient to achieve an average of 86% of the in situ soil strength in the backfill soil. (Range: 60% to 100%.) 5. At least 40% of trenched installations experienced (or will experience) undermining. 6. Static sliced installations experienced undermining only at points where the fabric had been damaged after installation. 7. 42% of static sliced installations had indications of effectively ponding runoff. 8. 27% of trenched installations had indications of effectively ponding runoff. 9. Generally, static slicing resulted in silt fence fabric and posts embedded more securely in the ground than when installing silt fence with trenching. 10. Five non-buried installations were counted as trench-based installations because they were prefab or hand-placed systems. CONCLUSIONSThis field study involved the random selection and inspection of silt fences around the United States and was performed by TRI/Environmental, Inc. The study included 56 sites in 12 states reflecting different soil types, different installation methods, and different hydraulic conditions. In general, the static slicing method of silt fence installation was found to provide more consistent results than common trenched installations. This consistency was manifested in more uniform silt fence fabric burial depth, substantially higher level of compaction of the soil adjacent to the buried fabric, and widespread indications of satisfactory performance. Conversely, the trenched installations exhibited less burial depth, substantially lower soil compaction and, thus, not surprisingly, many more indications of inadequate performance. The results appear to validate the findings of the earlier referenced EVTEC study as being applicable to the general use of silt fence around the United States, including the clear finding that substantial compaction of the soil adjacent to the buried silt fence is critical to satisfactory silt fence performance and is generally assured by static slicing. Additionally, it was found that most trench-based silt fence installations have insufficient backfill and/or backfill compaction.
ACKNOWLEDGMENTSThe author would like to thank TRI/Environmental Field Technician, Ken Bryce, for performing the field inspections. Funding for this study was provided by Carpenter Erosion Control, Inc.
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