Presentation on theme: "Properly installed silt fence is the most effective temporary sediment control device available 100 ft. of effective silt fence can retain 180 tons of."— Presentation transcript:
1 Properly installed silt fence is the most effective temporary sediment control device available 100 ft. of effective silt fence can retain 180 tons of sediment.
2 Properly installed silt fence detains water for sedimentation to occur
3 Sediment control devices primarily collect larger particles of soil because clays and silts are very small. Large particles cause further erosion – thus a multiplying effect - once introduced into a stream.
4 Goal of silt fenceSilt fence, for maximum efficacy, should detain water for sedimentation to occur.Occasionally it may be used to divert stormwater to a storage area.Silt fence, at a minimum, should retain large soil particles and prevent loss from the site.
5 Two most common problems Placement issues – won’t pond water or insufficient quantity for the areaInstallation issues - inadequate backfill and/or compaction – one weak area allows the fence to fail
6 Effective silt fence works PlacementInstallationSupportEffective silt fence workslike a chain…Each link must work;if one link fails,the system failsQuantityCompactionAttachment
7 A failure on any segment of the system makes the entire silt fence a complete waste of money!
8 PLACEMENT refers to specific locations on each construction site, and to specific designs (layouts) at each location.
9 The ends of a “smile” must always be long enough to pond water and sediment.
17 Stop sediment before it reaches the pavement. stormwater inlet
18 Ditch checksDo not install silt fence in a continuous flow situation, or on the discharge of an outlet.Install on the inlet side of a culvert.A properly installed, compacted, and supported silt fence will retain significant sediment in a ditch situation.Shallow posting (wood posts?) and poor compaction will not be effective.
19 25 tons of sediment captured per fence in a ditch situation
25 On-site adjustment is mandatory Grading plans and PPP’s are often drawn in an office many months prior to disturbance.The ESC contractor must adjust the plan to properly control the site, or at a minimum notify the site managers of potential problems.
26 Do not use silt fence:Where you can not create a storage area for runoff, such as on steep slopes, on small areas, or in short segments.Where you can not prevent water running around the endIn a V-shaped or shallow channel
27 Effective silt fence works PlacementInstallationSupportEffective silt fence workslike a chain…Each link must work;if one link fails,the system failsQuantityCompactionAttachment
28 QUANTITY relates to area of control QUANTITY relates to area of control. The volume of water from a large site can not be controlled in one run of silt fence. Multiple locations or storage areas are often required.
29 Large areas oftenneed additionalruns installed inthe interior toreduce the volumeof water reachingthe perimeter fences.
30 Long runs should be avoided. They tend to accumulate sediment in one area,causing premature overflow.
31 Long runs often concentrate water and then overflow
32 Breaking up a run more than doubles storage area
33 Multiple runs slow runoff velocity and add storage area on site.
34 Proper Quantity100 ft. of silt fence per 10,000 sq. ft. of disturbed area seems to be an adequate rule of thumb for sediment control and economics.
35 Effective silt fence works PlacementInstallationSupportEffective silt fence workslike a chain…Each link must work;if one link fails,the system failsQuantityCompactionAttachment
36 INSTALLATION relates to how the fabric is placed in the soil, appropriate depth of placement, and appropriate backfill for an effective silt fence.
37 Improper installation often leaves silt fence blowing in the wind.
48 EPA co-sponsored research has shown compaction is the critical factor in silt fence effectiveness. Trenches must be over-backfilled and mechanically compacted –BEFORE installing postsSlicing installation must be mechanically compacted before installing posts
51 EPA co-sponsored research ASTM and ASHTO specifications are vague on backfill, compaction, and chronological order of posting - resulting in poor performance.
52 TRI Environmental, Inc. national field study of random installations 70% of trench installations had no backfill or no compactionThe remaining 30% of trench installations had an average compaction of only 45% of the undisturbed soil strength
53 TRI Environmental, Inc. national field study of random installations Less than 8% of sliced installations were not compacted.The remaining 92% of sliced installations had an average compaction of 86% of the undisturbed soil strength.
54 Effective silt fence works PlacementInstallationSupportEffective silt fence workslike a chain…Each link must work;if one link fails,the system failsQuantityCompactionAttachment
55 relates to a support system that will not fall over under the load PROPER POST SPACINGAND POST DEPTHrelates to a support system thatwill not fall over under the loadof a full silt fence.
56 24 inches into compacted ground. 60" steelT-post (1.33# per foot)24”abovegroundbelowSupport Post SpacingPost should be spaceda maximum of 6 ft.apart and driven24 inches into compacted ground.
57 EPA co-sponsored research Trench installations were adversely affected by the inability to compact effectively when the posts were installed first and when insufficient backfill was placed in the trench.
58 Properly supported silt fence will carry 24 inches of sediment.
59 Wood post spacingWood posts may be more difficult to drive an adequate depth into compacted soil, requiring closer post spacing to achieve adequate silt fence support.
60 wood posts are difficult to install to a proper depth
61 ATTACHMENTProper attachment is the glue of the system, combing the strength of the fabric and the support posts into a unified structure. It must be adequate to support 18 inches of sediment.
62 Attachment must be adequate to support a fully loaded silt fence
63 AttachmentFor steel posts, recommend 3 plastic ties per post, located in the top 8 inches of the fabric, with each tie hung on a post nipple, placed diagonally to attach as many threads as possible.For wood posts, recommend several staples per post using a wood lath to overlay the fabric for extra strength.
64 A properly installed silt fence can retain several tons of sediment, thus saving millions of tons across the country.
65 Tell-tale sign of a problem If there is minimal sediment or water behind a silt fence, either water is running around the end, or there is a washout needing repair.
66 Wire-backed fence problems Doubles the cost of silt fenceCreates voluminous, expensive disposalDisposal creates environmental harmModern fabrics do not need wire supportImproper post spacing for fence support is the real problem
67 Disposal pile of wire-backed silt fence from just one job
69 Alternatives to wire-backed fence Possibly more runs of silt fence to reduce the potential volume/loadUse of steel posts spaced 4 feet apart and 24 inches deep
70 Maintenance of silt fence Once the fabric is clogged with sediment, it no longer functions as designed – so removing sediment does not result in a performing fabric.What is to be done with the removed sediment and how do you physically handle it?Solution – leave the sediment in the silt fence and build a new silt fence above or below to collect additional sediment.
71 Cleaning simply creates a low volume sediment basin
73 Inspector Responsibilities Inspectors must know proper placement and installation conceptsMust not approve improper installations for payment.Improper installation includes silt fence that does not pond water.
74 Effective silt fence works PlacementInstallationSupportEffective silt fence workslike a chain…Each link must work;if one link fails,the system failsQuantityCompactionAttachment
75 A Study of Silt Fence Installation Techniques and Associated Efficiency and Performance Issues
76 Environmental Technology Silt Fence StudyEvTECEnvironmental TechnologyEvaluation Center
77 Verification of Performance Silt Fence StudyVerification of PerformanceThe field evaluation included 51 test segments reflecting different soil types, installation methods, and hydraulic conditions.Various amounts of backfill, degrees of compaction, spacing of posts, volumes of runoff, and types of soil were evaluated.
78 Silt Fence StudyAdditionally, 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.
79 Nuclear Density Measurements Silt Fence StudyNuclear Density Measurements
85 Silt Fence StudyThe static slicing method was found to provide storm water retention as good as or better than the ‘Best’ trenched installation and far superior to common installations.
86 Silt Fence StudyThe ‘best’ installation far exceeded the standard ASTM and ASHTO specifications which are vague as to backfill, compaction, and chronological order of posting.
87 Silt Fence StudyThe conclusion was clear that when the enhancements of the ‘Best’ trenched installations were not performed - the trenched installation performed poorly.
88 Silt Fence StudyTrench installations were adversely affected by the inability to compact effectively when the posts were installed first and when insufficient backfill material was placed in the trench.
89 Silt Fence StudyThe ‘Best’ trenched installation requires nearly triple the installation effort to achieve similar effectiveness as static slicing.Trenching techniques meeting only minimum or marginally enhanced specification requirements fared quite poorly.
90 Silt Fence StudyPerformance trends provide a clear indication that a greater level of compaction (i.e. higher density obtained) corresponds to better performance (i.e. greater water retention).There was a significant correlation between the cone penetrometer readings and the nuclear density measurements.
91 Silt Fence StudyPoorly performing test segments experienced excessive seepage and, in the worst case, subsequent “blow-out” of soil in the trench.Segments installed using slicing or the “best” trenching techniques experienced no blowouts.Those segments installed using the minimum specification requirements experienced both excessive seepage and blowout.
92 Silt Fence StudyAdditionally, the static slicing method of installation was found to be much more efficient, and therefore, cost effective technique for silt fence installation when compared to a range of traditional trench-based procedures.
93 Silt Fence StudyStatic slicing ranged from 1.75 to 4 times faster than all trench-based installation techniques.As far as installation efficiencies go, the static slicing method provided much quicker installations than any trench method installation attempted.
94 Silt Fence StudyThe static slicing method offers practical advantages over traditional trenching-based methods, such as maneuverability, minimal soil-handling and hand labor, consistent depth and compaction, and ease of installation in windy conditions, on steep side slopes, through rocky soils, and in saturated soils.
95 Silt Fence StudyMechanical installation by static slicing minimizes the hand labor requirements, as well as the potential backfill and compaction problems associated with trenching. As a result, static slicing can be expected to provide uniform, dependable installations.
96 Silt Fence Study Conclusions There appear to be two possible ways to achieve maximum silt fence performance – static slicing or the “best” trench-based installation.The combination of maximum performance and maximum productivity can be achieved in one method – static slicing. The static slicing method is included in ASTM D 6462.
97 Silt Fence Study Conclusions In all cases, static slicing produced silt fence installations as good as or better than the best trench-based installations.Both static slicing and the best trench-based installations substantially outperformed “typical” trench-based installations.