Silt Fence Installation: Spacing, Trenching and When It Fails
A silt fence works by slowing sediment-laden overland (sheet) flow so the sediment settles out before the water passes through the fabric. The installation that makes it work: dig a trench along the line and bury the bottom edge of the fabric in it, backfilled and compacted, so water can't run under the fence this is the step most often skipped. Support the fabric on posts at close enough spacing to take the water load without sagging or collapsing. Site it along the contour to intercept sheet flow, not across concentrated channels it can't handle. It fails when the base isn't trenched in, the posts are too far apart, it's placed in flowing channels, or it's not maintained which is why installation and upkeep matter more than the fence itself.
A silt fence is one of the simplest sediment controls on a construction site, and one of the most commonly installed wrong. Stapled to posts and left sitting on the surface, it lets muddy water run straight under it. Installed properly, it holds sediment back and passes inspection. The difference is entirely in the installation. Here's how to get it right.
How a Silt Fence Works
A silt fence is a permeable geotextile barrier that intercepts sediment-laden sheet flow running off disturbed ground. It works by ponding the water briefly against the fabric so the sediment settles out, while the filtered water passes through slowly. It's a perimeter control it protects what's downhill (drains, watercourses, neighbouring land) from the sediment washing off the site. Its job is settling sediment from diffuse overland flow, not carrying a stream.
Trenching the Base In
This is the step that decides everything. The bottom edge of the fabric must be buried in a trench dug along the fence line, then backfilled and compacted, so water is forced to pond against and filter through the fabric rather than run underneath it. A silt fence stapled to posts and left sitting on the surface simply lets the water and the sediment flow straight under the bottom. If you take one thing from this, it's trench the base in.
Spacing and Support
- 1 Support the fabric on posts driven firmly into the ground.
- 2 Space the posts close enough that the fabric doesn't sag or the fence collapse under the water load.
- 3 Support the fabric adequately (backing or correct fabric) so ponded water doesn't blow it out.
- 4 Return the ends up-slope so water can't simply flow around the end of the fence.
When It Fails
Silt fences fail for predictable reasons: the base wasn't trenched in (water runs under), the posts are too far apart or too weak (it sags or collapses), it's been placed across a concentrated flow or channel it can't handle (it's for sheet flow), the ends weren't returned up-slope (water goes around), or it wasn't maintained sediment builds up and it's not cleared, or damage isn't repaired. A silt fence needs inspecting and maintaining through the works. See silt fence, silt sock or silt curtain for choosing the right barrier, and dewatering for handling pumped water. SSI Environmental supplies silt fences and sediment control across Ireland.
Frequently Asked Questions
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How do you install a silt fence correctly?
Dig a trench along the line and bury the bottom edge of the fabric in it, backfilled and compacted, so water can't run under the fence the step most often skipped. Support the fabric on posts at close spacing so it doesn't sag or collapse, site it along the contour for sheet flow, and return the ends up-slope. -
Why do silt fences fail?
Usually because the base wasn't trenched in (water runs under), the posts are too far apart (it sags or collapses), it's placed across a concentrated flow it can't handle, the ends weren't returned up-slope (water goes around), or it wasn't maintained. Installation and upkeep, not the fence itself, decide whether it works. -
What is a silt fence for?
It's a perimeter sediment control that intercepts sediment-laden overland (sheet) flow from disturbed ground, ponding the water so sediment settles out before the filtered water passes through. It protects downhill drains, watercourses and land from sediment washing off a construction site. It's for diffuse sheet flow, not concentrated channels.
Control It at Source, with SSI Environmental
SSI Environmental has supplied spill, sediment and site environmental protection products across Ireland since 1996, from silt fences and dewatering bags to drain guards and plant nappies. Tell us about your site and its planning conditions, and we'll help you specify the controls to keep sediment and pollution where they belong. Call 01 8665676.