For cost effective, durable silt control on a construction site, silt fences are an optimal solution. They are an essential tool, ensuring environmental protection and reducing risk of pollution to nearby watercourses or surrounding land.
To properly install a silt fence, you’ll need to understand the following:
What is silt?
Silt forms when materials are disturbed or soil or stone is eroded during construction works. If exposed ground is subject to heavy rainfall or adverse weather conditions silt run-off can reach nearby watercourses, impacting on biodiversity.
Silt can fall under two classifications: hazardous or non-hazardous.
Non-hazardous silt forms when inert material, such as soil or stone, is disturbed or eroded by construction works or weather and travels to a different area of a site. While the material is not immediately hazardous, its potential to reach watercourses makes it an environmental risk to biodiversity.
Hazardous silt forms from chemicals or materials with specific, specialist treatment requirements, such as adhesives or fuels, which can be disturbed or misplaced by construction works or weather.
Please note: This advice focusses on solutions for managing non-hazardous silt, to avoid it reaching or contaminating watercourses.
What is a silt fence?
A silt fence is used for surface water management. They are trenched into the ground to provide lateral resistance for water run-off, collecting silt for it to be correctly disposed of while water evaporates or sinks into the ground.
How much will I need?
You’ll need specialist silt fence fabric and support posts, or stakes, along with approved fixing methods, ranging from cable ties or zip ties to stainless steel nails or pins.
How do I install one?
To properly install a silt fence the fabric must be trenched into the ground at least six inches deep and backfilled, with soil compacted around it. The silt fence cloth must also be trenched, giving it a solid anchor and making it nearly impossible for water to get past.
Placing stakes every 1-1.5m will ensure resistance against heavy water flow. Knowing the expected volumes of waterflow and fabric strength is essential in deciding the optimal distance between fence posts – reducing risk of collapse.
Fabric must be well secured and placed the right side of the stakes, depending on the direction and velocity of water, to ensure they do not separate when capturing silt. An effective fence will be able to withstand 18 inches of standing water.
Where should it go?
Silt fences are typically placed on sloping ground or embankments, at the perimeter of a site and around streams or detention basins. Working to the natural contours of the ground on site is advised.
When should it be reviewed or removed?
Like all environmental protection measures, silt fences should be checked and maintained regularly, particularly after heavy rain. If stakes are broken, fabric is torn or gaps are developing between the fence and ground, it should be repaired or replaced immediately.
If you are using green silt fencing – popular for blending with the natural environment – spotting damage can be harder, so inspections should be more thorough. For easier inspections, black silt fencing is recommended.
As soon as build-up reaches around one-third to half the height of the fence, the silt or sediment should be removed.
Silt fences are a temporary measure so where ongoing silt control is required, permanent solutions should be designed into a site. Fences should be removed once construction works are completed for the fabric and stakes to be reused elsewhere.
Figure 1 – A Silt Fence that has been overloaded and is ready for replacement
The image above shows that the silt fence is working, however, the fence has collected so much silt that it is now clogged up and causing the build up of water. This highlights the importance in carrying out regular inspections of the silt fence to check for problems.
Figure 2 – Secondary Silt Fence Installation
Often a second Silt Fence is installed in areas with a ‘high flow potential.’ This second fence acts as a double barrier to ensure no silt can get through.
What if I get it wrong?
Good prior planning and ongoing maintenance will reduce the risk of your silt fence failing. However, if erosion and silt are not properly controlled contaminated water can affect surrounding land and watercourses, negatively impacting biodiversity and risking costly fines.
To ensure you can answer each of the questions above, speak to one of our consultants for information and advice on how to optimize your projects.