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PERFORMANCE TESTING OF VARIOUS SILT FENCE INSTALLATION TECHNIQUESby 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. AbstractAn independent evaluation of silt fence performance was performed by the Civil Engineering Research Foundation’s Environmental Technology Evaluation Center. The primary objective of the evaluation was to perform well-defined field tests to provide data on the performance of trench-based silt fence installations and installations using static slicing. Several objectives were established to test the silt fence installation techniques, including:
The field evaluation included 51 test segments reflecting different soil types, different installation methods, and different hydraulic conditions. Testing was primarily performed at one site with a soil type that was predominately made up of silty clay. The large number of tests allowed alternative schemes to be evaluated in order to further define the benefits of each installation type (slicing vs. trenching) under a variety of conditions. Various amounts of backfill, degrees of compaction, spacing of posts, volumes of runoff, and types of soil were evaluated. Additionally, installation sequence, such as installing posts before versus after compaction, was evaluated. Performance, as measured by water retention, and efficiency, as measured by installation time, were evaluated. The field-testing lasted approximately 1 week and the data has been compiled into a comprehensive Verification Report. This paper provides details of the evaluation, including data summaries, discussion of results, and recommendations for future silt fence installations. Key Words: silt fence; static slicing; trenching; runoff; installation IntroductionSilt fence is commonly placed at the perimeter of a construction site to prevent off-site sediment discharges associated with storm water runoff. The bottom of the fence is embedded in a shallow trench to prevent the storm water from flowing underneath. The silt fence causes the sediment-laden runoff water to pond behind it, allowing sediments to settle out on-site. This paper describes an independent field evaluation of silt fence installation techniques conducted by the Civil Engineering Research Foundation’s Environmental Technology Evaluation Center. Compete details of the evaluation are available in CERF Report #40565. Trench Based InstallationsTypical trench-based installation specifications allow the sequence of installation steps as well as the details of each step to vary at the discretion of the installer. For instance, the installer can trench, install fabric, backfill, compact, and then install posts and tieup the fabric, or he can install the posts before installing the fabric. Also, the installer can use only the soil available on the upstream side of the fence, or he can make the special effort to obtain sufficient backfill to slightly over-backfill the trench so that, when compacted, it is completely full. The amount of compaction is not usually addressed. Since there is no "standard"; trench-based installation procedure, this study used the following three classifications for trench-based installations based on the likelihood of obtaining a fully backfilled and densely compacted trench: "Minimum"; Installation (Spec)—Silt fence installations with these steps and order:
"Better"; Installation (Spec+)—A better installation of silt fence would include; 1) fabric installation, use of available backfill, and compaction before setting and driving posts, or 2) over-backfilling the trench, or 3) posting and then mechanically compacting the filled trench. "Best"; Installation (Spec++)—The best silt fence installation would include multiple enhancements, such as, hand-cleaning the trench prior to installing the fabric and mechanical compaction of an over-filled trench and posting as the final action. Installation Using Static SlicingStatic slicing is defined as the insertion of a narrow custom-shaped blade at least 10 in into the ground, and simultaneously pulling silt fence fabric into the opening created as the blade is pulled through the ground. The blade imparts no vibration or oscillatory motion. The tip of the blade is designed to slightly disrupt soil upward, preventing horizontal compaction of the soil and simultaneously creating an optimum soil condition for future mechanical compaction. Compaction follows (typically two passes on each side of the fabric) using a tire on the tractor used to pull the slicing machine. Post setting and driving, followed with attaching the fabric to the post, finalizes the installation. Field Evaluation ProgramIn early August 2000 the Environmental Technology Evaluation Center (EvTEC) performed an evaluation of silt fence installation methods at test sites located within the greater Des Moines area. Representatives of TRI/Environmental, Inc., oversaw the field operations and acted as EvTEC’s independent oversight for the project. The field evaluation included 51 test segments reflecting different soil types, different installation methods, and different hydraulic conditions. Sites The two primary sites used for the evaluation were recently graded for development purposes. Site #1 was comprised largely of fill material excavated from a nearby property and transported to this site. The fill was placed and compacted to relatively gentle grades ranging from 2–10 percent. The fill was high in clay content and moist to the touch. Site #2 was generally cut with grades reflecting the natural terrain of the area. Slopes were in the range of 2–6 percent. The soil reflected a higher composition of silts and organics (topsoil). Two additional sites were selected to facilitate the limited evaluation of uniquely challenging soil conditions. Site #3 was an old fertilizer plant that had thick layers of coarse stone in old roadbeds. Site #4 was a very wet, vegetated area at the lower end of a multi-acre drainage area. As shown in Table 1, 36 tests were conducted on site #1 using 30 "smile,"; or arc segment, installations. Twenty of the smiles used some variation of trenching while ten smiles were installed using the static slicing method. Six tests involved re-application of water to a previously tested "smile."; An additional six tests were conducted on site #2 using the smile configuration in order to investigate the effects of soil type on fence performance. On site #2, two smiles were sliced and four smiles were trenched. Ten 100 ft straight sections were constructed on site #1 to evaluate installation efficiency. Additional straight sections were installed on sites #2, #3, and #4 to evaluate slicing on a steep slope, in rocky soils, and through wetlands, respectively. Test Configurations and Materials For the performance evaluation, a "smile" installation was used to demonstrate occurrences that stress silt fences at a specific point. For the productivity assessment, straight-line segments were used. Table 2 outlines the tests performed and the associated variables. Certain conditions were maintained throughout the testing, including:
Additionally, ten straight-line segments, representing nine different installation techniques, were installed to facilitate an economic "efficiency" evaluation. For the straight-line installation, no runoff was introduced; these 100 ft segments were constructed to provide productivity information for comparison purposes. Evaluation Criteria Three trench-based installation methods "classified" as "minimum-spec," "better-spec+," and "best-spec++" trenching, as well as, static slicing were evaluated and compared. Variation in amount of fill, compactive effort, and sequence of tasks was evaluated. These variations were evaluated for one primary soil type. Limited additional data was collected on installations in a second soil type.
Of primary importance in the evaluation was the performance, or water retention, of the installed silt fence when installed and subjected to runoff. Retention was expected to be adversely effected by excessive seepage under the fence (undermining). This undermining was expected to be related to compaction of soil within the trench and/or adjacent to the fabric. Therefore, along with retention, the degree of compaction achieved was evaluated for each soil type and each installation sequence. In addition to performance, economics were assessed, as was the importance of equipment maneuverability. The primary data collected included:
REVIEW OF RESULTSInstallation Efficiency Figure 1 provides a comparison of the installation efficiency associated with various installation techniques. As expected, installation efficiency improved with more mechanized installation practices. The trenching methods all required significant manual labor, including hand-shoveling to both clean out and to backfill the trench, holding the fabric in place while trying to attach it to the posts, holding the loose fabric in the trench while backfilling, and for the posting/tying effort. In Figure 1, it is clear that the installation time associated with any type of installation that includes trenching is substantially greater than the slicing technique. Among various trenching installations, those involving hand cleaning of the trench, hand transfer of the soil, and/or installation of posts prior to fabric installation and soil compaction took considerably longer to construct. If posts are installed before backfilling, fabric installation can be especially problematic—and time consuming. In this case, the fabric must be held and stretched between posts while being tied to the posts, a task made difficult by wind or too few laborers. Figure 1 also reveals that the time associated with digging a trench is comparable to the entire time required to slice the soil, simultaneously install the fabric and subsequently compact the soil with four passes. The time required to set and drive posts and tie-up fabric is basically the same for all installation techniques if fabric installation, backfilling (if required) and compaction are completed first. In general, installation efficiency using the static slicing method was superior to all trenching techniques. On average slicing is more than twice as fast an installation technique than is trenching. Slicing ranged from 1.75 to 4 times faster depending on the trenching-based installation technique. Slicing productivity is approximately 1200 manseconds per 100 ft. This translates into 0.33 manhours per 100 ft of installation using a two-man crew. Retention Performance In general, improved performance was related to more rigorous installation efforts. More rigorous installation efforts include setting and driving posts and tying fabric to posts after compaction, over-backfilling the trench, mechanically compacting the filled trench, or any combination of these enhancements. The minimum specification installations consistently performed poorly, while the installations that included better installation techniques performed much better. Figure 2 provides a comparison of the retention performance of the various classes of installation. The figure presents the relative performance of all tests, including a variety of flow conditions and installation configurations and soil types, while the evaluation also compared the relative performance of only those runs using standard conditions and only those tests performed at the "new site" (thus NS). Installations meeting minimum trenching-based specifications and better trenching-based specifications performed much more poorly than did the best trenching-based installations. This appears to be directly related to the degree of compaction achieved in the trench as shown in Figure 3. Significantly lower compaction was achieved when posts were installed before compaction, when inadequate compaction effort was provided, and when there was insufficient backfill in the trench. While the best trenching-based installations were able to retain 50 percent of the initial water level, less rigorous trenching-based installation techniques retained less than 20 percent as measured by the retained water level. In limited testing of the more erodible Site #2, the best trenching-based installation technique lost only 10 percent of its water level while the minimum specified installation experienced a 75 percent water level reduction. In all cases static slicing produced silt fence installations as good as or better than the very best trench-based installations.
Importance of Other Variables Post Spacing A few tests were conducted using various post spacing. The post spacing was found to effect the permeability of the silt fence. Closer post spacing results in higher permeability while, conversely, larger post spacing reduced flow through the fabric. Visual examination of silt fence with varying post spacing appeared to indicate that at close spacing the hydraulic pressure on the fence produced less stretching of the fence between posts. This tended to keep openings in the fabric from being closed off. The opposite appeared with larger post spacing. The fence heights at midpoint between the posts were clearly much lower than at the posts. This appeared to stretch the fence in a "Chinese finger holder" type way, reducing the size of the fence openings. It should be noted that there is also a trade-off between post spacing and the volume of runoff that can be retained. Closer spacing of posts prevents sagging of the fence at the mid-point between posts, enabling the silt fence to pond, or retain greater volumes of water. Closer spacing of posts may be more appropriate, especially when retaining highly sediment laden runoff. The sediments in the water will quickly block, or blind off, the fabric openings and the runoff will quickly impound behind the fence. Closer spacing of posts will both minimize fence sagging— increasing capacity—and maximize fence support— which will be needed to resist the increased pressure of the retained water. Amount of Runoff During the testing program, greater amounts of runoff were applied in some tests to examine the effects of the amount of runoff. Slicing and Spec++ held up well under a higher head pressure created by the greater volume, without the water undermining the installation. The greater hydraulic load stretched and tightened the fabric, without causing undermining. Spec and Spec+ failed to hold the higher pressure and experienced increased seepage under the fabric. As mentioned earlier, Spec and Spec+ cannot be compacted as well as slicing and Spec++ installations because the posts are typically installed first, interfering with mechanical compaction. Soil Type Not surprisingly, soil type was found to make a difference in the fence performance. Comparison was made for testing from both Site #1, a site composed of silty clay soils, and from Site #2 (NS), characterized by clay loam soils. The clay loam soil was more erodible, producing more sediment and thus more quickly blinding the silt fence fabric. While the silt loam soils were more easily compacted, when only modestly compacted the silt loam backfill was prone to piping. This highlights the importance of compaction in an erodible soil. When high levels of compaction are achieved, which is the case for slicing and the best trenching-based installation techniques, high levels of performance can be expected. Where only modest compaction is achieved, as is represented by the minimum specification (spec) for trenching-based installations, much poorer performance can be expected. The majority of tests were run in clayey soil that tended to break up into clods at the in situ moisture content. The voids between clods in the trench backfill proved to provide channels for water to seep along the trench and, at points, beneath the silt fence. When using static slicing very little soil disruption takes place. Thus, clayey soils are less likely to break into clods and fewer and smaller seepage channels are formed within the soil. Similarly, the slicing installation in the clay loam soil (Site #2) minimized disruption of the soil. It should be noted that, in this evaluation, trenches were hand-cleaned prior to fabric placement and backfilling. This procedure was used to optimize trenching-based installation performance but is commonly skipped in the "real world." Thus, it is likely that typical trenching-based installations would have many clods left in the trench, creating many more channels for seepage to undermine the fence. Compaction Two primary objectives of the evaluation were to determine if 1) greater compaction, and therefore higher density, of the soil adjacent to the embedded silt fence relates to performance, and 2) greater compaction is obtained adjacent to the embedded silt fence using the static slicing technique. An additional aim of the evaluation was to examine different methods for measuring compaction at the base of the silt fence and determine if it is possible to correlate density (or other measurements) with relative compaction. If possible, this could provide a practical QC tool for site inspectors to assure satisfactory silt fence installation. Though not easily quantified, the benefits of compaction appear obvious from Figure 3. The consistent trend shows that maximum compactive effort (Static Slicing and Spec++) significantly outperforms minimum compactive effort (Spec and Spec+). A significant correlation was found between the cone penetrometer readings and the nuclear density measurements. This may indicate that the much easier, and less expensive, hand penetrometer can be effectively used for field quality assurance. An additional observation can be made concerning the importance of compaction on the performance of static slicing. Test #22 was run on a slicing-based installation that was not compacted. It performed very poorly. This indicates that compaction is critical for slicing as well. ADDITIONAL OBSERVATIONSAs noted in Table 1, additional test segments were installed on three sites to evaluate the practical benefits of static slicing when installing silt fence using tight radii and in uniquely challenging conditions, such as on steep slopes, in rocky soils, and through wetlands. Maneuverability Often an effective silt fence installation requires that the end of the silt fence run be turned up-slope to assure containment of runoff. These up-turns or Jhooks, as they are often called, require the installation equipment to be able to make tight radius turns. Similar maneuverability may be beneficial when installing silt fence in subdivisions and around other obstacles. Tests #21, 27, 33, and 34 demonstrated that slicing is much more effective when used in tight radius installations than is a minimum trenching-based installation. Steep Slopes A 3:1 slope was available on Site #2 to evaluate the relative ease of installing silt fence by static slicing versus trenching. In both cases, the steepness of the slope tended to encourage the equipment to drift down-slope. But, the static slicing apparatus and tractor were much more resistant to this down-slope drift, apparently because the inserted blade helped anchor the equipment on the slope and because the ability to install at a faster rate maintains the equipment’s momentum across the slope. Conversely, trenching across the slope is very difficult to maintain in a straight line. The trenching bar must be forced down to cut the trench, which in turn tends to pick the back end of the trencher up off the slope. This decreases contact with the slope and allows the trencher to drift downslope. In comparison to trenching, static slicing provides much straighter, faster installation of silt fence across steep slopes. Rocky Soils Site #3 provided very rocky soil conditions in which to compare static slicing and trenching. While large buried rocks are able to disrupt both installation methods, static slicing appeared to be significantly more resistant to being "kicked" out of the ground, tending rather to bend around the obstruction or lifting the obstruction itself out of the ground. Additionally, it was apparent that the chain on the trenching machine would be damaged by digging in rocky soils and would require more maintenance. Also, a large number of rock fragments fell back into the trench, requiring the trench to be shoveled clean prior to fabric installation. In rocky conditions, the static slicing method provides a more dependable installation (presumably with fewer maintenance problems) than does trenching. Saturated Soil Site #4 was an area surrounding a small drainage way at the low point of a site that was under development. The area extended to the base of the rather steep graded area of the site. In order to provide some relatively flat area adjacent to the silt fence to provide retention area, the silt fence was placed within an area, which was saturated with water. The wet, organic soils and abundant vegetation made it practically impossible to remove the soils from a trench, install fabric, and then replace and compact clean soil in the trench without first stripping the area or importing clean fill. Conversely, the static slicing apparatus was able to "insert" the fabric deep into the wet soils and compaction was performed without substantially disrupting the soil. CONCLUSIONSThis evaluation was initially envisioned as a simple comparison between silt fence installed using the "traditional" trenching-based installation procedures and silt fence installed using the more fully mechanized static slicing method made possible with the Tommy silt fence machine. It quickly became apparent that there is no such thing as a single "standard" trenchingbased installation procedure. At best, there is minimum specification such as ASTM D 6462 which reflects common practices and "implies," but does not explicitly require, important installation details, such as complete backfilling of the trench and thorough compaction of backfill. This and other "minimum" specifications allow installation of posts prior to fabric installation and trench backfilling causing interference with thorough compaction efforts. Thus current specifications may inadvertently encourage trenching-based silt fence installations that provide unsatisfactory performance. Both the static slicing method and the "best" trenching installation technique (Spec++) performed quite well in runoff retention tests. Trenching techniques meeting only minimum specification requirements fared quite poorly. Runoff retention tests measured the ability of an arc segment of installed silt fence to retain runoff. Poorly performing test segments generally experienced excessive seepage and, in the worst case, subsequent "blow-out" of soil in the trench. No blow-outs were experienced by segments installed using slicing or the "best" trenching techniques. Those segments installed using the minimum specification requirements generally experienced both excessive seepage and blow-out. Corresponding density and retention data 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 effected 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. Still, it should be noted that the installations using static slicing also required reasonable compaction efforts to perform properly. During the field testing compaction densities were measured with a nuclear density gauge, and a handheld cone penetrometer. There was a significant correlation between the cone penetrometer readings and the nuclear density measurements. This may indicate the much easier, and less expensive, hand penetrometer can be effectively used as a field quality assurance tool. As far as installation efficiencies go, the static slicing method provided much quicker, easier, and higher quality installations than any trench method installation attempted. The static slicing method also offers practical advantages over traditional trenching-based methods, including maneuverability, ease of installation on steep side slopes, through rocky soils, and in saturated soils. From the field testing performed for this evaluation, there appear to be two possible ways to achieve maximum silt fence performance—static slicing or the "best" trenching-based installation. Yet, there is no clear, generally accepted specification to obtain this "best" trenching-based installation and all trenchingbased installations require considerably more time than do slicing-based installations. The static slicing method is included in ASTM D 6462 and, should be strongly considered for incorporation into future project specifications where the existing trench specification is vague or loosely defined. ACKNOWLEDGMENTThis research was supported by Carpenter Erosion Control, Ankeny, Iowa.
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