Silt fence installation and correct function depend on common sense placement, adherence to plan details and use of specified materials. Sediment and erosion control plans often place silt fence and outlets incorrectly. Further, contractors frequently ignore plans details and instructive notes. In addition, installers sometimes substitute convenient materials for those that are specified.
Use these tips as supervisory or quality control inspection guidelines. Require such inspections during installation. Reject poorly installed silt fence. Poor installation equals poor performance.
Silt Fence Installation-Placement
Importantly, use a surveyor to layout silt fence and silt fence outlet locations.
Study the layout.
With design layout in mind, make sure the silt fence is positioned correctly on the terrain. However, proper site positioning depends on the correct design. If the original topo information is incorrect, then the plan will be incorrect. Consequently, the silt fence layout will conform to the design rather than to actual topography. For example, the surveyor uses data from the plan to layout the silt fence. However, small topo errors might place silt fence in wetlands or through streams.
Consider outlet placement. Surveyors layout silt fence according to plans provided to them. Small topo errors often cause design misplacement of silt fence outlets. As a result, surveyors transfer the error to the field layout. In my experience, misplaced outlets are fairly common. Consider alternate locations that allow outlet drainage. Properly placed outlets allow drainage, thus reducing the possibility of silt fence blowouts. Place an outlet anywhere along the silt fence that holds water. Areas that appear to hold a larger volume of runoff may require extra or longer outlets.
If an outlet placement is not necessary, that is it provides no drainage, consider eliminating or relocating it to a low area. Consider company change order procedures,
Remember, silt fence installation requires equipment such as tractors or trenchers. Equipment occupies space so clearance is necessary between any clearing limit line and the silt fence line.
In addition, consider future structure. Decide if silt fence design impacts other construction. If so, consider any negative impacts to moving the measure. Decide what clearances may be necessary to effect the subsequent construction.
Note any discrepancies and alert the designer for redesign or other advisements for correction.
Silt Fence Installation-Plan Details and Notes
Study plan details and notes. Different designers design differently. Details, notes, and materials change according to jurisdictional regulations, job conditions, and regulator preference as well.
Insist that features be built according to detail. The detail for silt fence outlets provides a picture of the measure in addition to specified dimensions. Compare the detail to on site construction. Detail non-compliance often results in failure of the measure. Additional risks include complete rebuild, increased maintenance time and labor, and citation during a regulatory inspection.
Plan notes related to details provide deeper or additional explanations not easily shown in a drawing. The detail and associated notes are usually boxed together on the plans.
Print a reduced size set of plans to bring to the field. Compare the installed measure to the detail. If the installation and detail don’t match, then alert installers, project managers or design engineer, whatever is appropriate for your job.
Silt Fence Installation-Compaction
Require compaction of soil disturbed by silt fence installation to ensure effectiveness.
Silt fence installation, either trenched or sliced, loosens the soil. Runoff along the fenceline and even head pressure from standing water potentially undermine silt fence, if the soil is not compacted. As stated in the EPA Stormwater Best Management Practices guidelines, “This infiltration leads to water seeking pathways under the fence, which causes subsequent soil erosion and retained sediment washout under the fence.”
Plan details and notes, regulatory rules, and/or manufacturers installation specifications recognize the need for compaction.
Again, the following is from the EPA Stormwater Best Management Practices in regards to the Static Slicing Method of silt fence installation. “The static slicing machine pulls a narrow blade through the ground to create a slit 12” deep and simultaneously inserts the silt fence fabric into this slit behind the blade.” Also,”Compaction is achieved by rolling a tractor wheel along both sides of the slit in the ground 2 to 4 times to achieve nearly the same or greater compaction as the original undisturbed soil.”
Require this compaction before post installation. As a practical matter, posts with fabric attached prevent effective compaction and invite fabric damage.
Finally, reject outlet stone already fouled by quarry fines or acquired dirt. Fouled stone turns and outlet into a dam. As a result, outlet failure is common.
Silt Fence Installation-Material Tips
First, plans, details, and notes specify material types, sizes, and placement.
Second, check posts for proper material and length. Often, different soil types require posts of different lengths.
Third, post spacing decreases without wire backed fabric.
Fourth, install fabric on the project side of the post.
Fifth, attach fabric to posts with three zip ties. Require all three ties within the top eight inches of the fabric. Attachment requires holes be made in the fabric. Ultraviolet degradation and pressure expand these holes. Holes below the level of standing or running water will leak both water and sediment.
Sixth, check for proper material heights above grade.
Seventh, check that outlet hardware cloth, stone, wattles, compost sock meet specs.
Silt Fence Installation-Regulatory Inspections
Inspectors expect proper performance. From my experience, regulatory inspectors do not count outlets or measure their placement from the plan. Inspectors look for failures. Inspectors cite offsite sedimentation.
Plan compliance, usually, equates to proper measure functionality. Proper measure function usually equates with successful regulatory inspections.
Silt Fence Installation-Image Review
Before I close consider the image above compared to its supporting text.
This looks like an installation failure to me. Water stands here so an outlet should have been placed in this location. Wooden stakes were used that were probably not specified. Moving or standing water caused undermining. Installers missed a whole section of fabric or ,maybe standing water blew out the section. Additionally, the fabric is overspanned, wire backing appears missing in this view. Bad installation and inattention to detail doomed this silt fence from the start. Someone reinforced the silt fence with wattles then filter sock. At least someone was inspecting.
Again, for the EPA’s take on this subject Click Here.
Finally, feel free to post comments or additional helpful hints.