Yes, non-woven geotextiles are effective for silt fence applications, but with a critical caveat: their effectiveness is highly dependent on the specific project conditions and the properties of the non-woven fabric chosen. They are not a one-size-fits-all replacement for the more traditional woven monofilament silt fences. The key lies in understanding the fundamental difference in how they function. Traditional woven silt fences are primarily filtering devices, allowing water to pass through while physically trapping sediment particles. Non-woven geotextiles, particularly those used in silt fences, act more as dewatering fabrics. They work by temporarily ponding water, which allows sediment to settle out, and then slowly releasing the clarified water through the fabric’s porous structure.
This dewatering mechanism makes non-woven geotextiles exceptionally effective for certain types of sediment. They excel in trapping very fine, clay-sized particles that can sometimes pass through the larger openings of a standard woven filter fabric. When a site has soils with a high clay content, a non-woven NON-WOVEN GEOTEXTILE can achieve higher turbidity reduction rates. However, this advantage comes with a significant engineering trade-off: the need for careful hydraulic head management.
The Science of Filtration vs. Dewatering
To understand why non-woven fabrics are situationally superior, we need to dive into the science. A standard woven monofilament geotextile might have an Apparent Opening Size (AOS) of 30 to 40 (approximately 0.425 mm to 0.250 mm sieve size). It works by creating a filter cake on its surface. As sediment-laden water hits the fence, larger particles are immediately trapped. These trapped particles then form a layer that itself acts as a filter for finer particles, a process known as “filter cake formation.”
Non-woven geotextiles, typically needle-punched, have a more complex, three-dimensional fibrous structure. Their AOS is generally much smaller, often in the range of 70 to 100 (approximately 0.210 mm to 0.149 mm). Because the openings are smaller and the flow path through the tangled fibers is more tortuous, water does not pass through as freely. This causes water to pond behind the fence. The ponding action is crucial. It drastically reduces the water’s velocity, allowing suspended solids to settle to the ground by gravity. The water that eventually exfiltrates through the fabric is therefore much clearer.
The performance of a non-woven silt fence is directly tied to its physical and hydraulic properties. These are not all created equal.
| Property | Typical Range for Non-Woven Silt Fence Fabric | Why It Matters for Performance |
|---|---|---|
| Grab Tensile Strength (ASTM D4632) | 120 – 250 lbs | Determines resistance to tearing and stress during installation and from water pressure. |
| Mullen Burst Strength (ASTM D3786) | 300 – 500 psi | Indicates resistance to punctures and the pressure of ponded water. |
| Apparent Opening Size (AOS) (ASTM D4751) | 70 – 100 (U.S. Sieve) | Smaller AOS improves fine sediment retention but reduces flow rate, increasing ponding risk. |
| Flow Rate (Permittivity) (ASTM D4491) | 0.5 – 1.5 sec⁻¹ | Lower permittivity means slower water release, requiring more careful management of ponded water depth. |
| Weight | 6 – 10 ounces per square yard | Generally correlates with durability and strength; heavier fabrics are more robust. |
When to Use a Non-Woven Silt Fence: The Ideal Scenarios
Given their dewatering nature, non-woven geotextiles are most effective in specific scenarios:
1. Fine-Grained Soils: On sites with silty or clayey soils, non-woven fabrics are often the best choice. Their ability to trap sub-micron particles is superior to standard woven fences. This is critical for complying with strict turbidity standards in environmental regulations.
2. Low-Flow, Long-Duration Events: For sites where runoff is generated from a large, relatively flat area, the flow rate is typically low but can last for a long time. A non-woven fence can effectively pond this water, allowing for continuous sedimentation without the risk of over-topping that might occur with a sudden, high-intensity storm.
3. Sediment Basin Outlets: A common and highly effective use is as a stand-alone barrier or as a filter sock at the outlet of a temporary sediment basin. The ponding action of the basin is complemented by the fine filtration of the non-woven fabric, providing a final polishing stage for the water before it leaves the site.
Critical Limitations and Installation Considerations
Ignoring the limitations of non-woven silt fences is a recipe for failure. The primary risk is over-topping and catastrophic breach due to inadequate flow capacity.
Hydraulic Head is Everything: The maximum ponding depth behind a non-woven fence is a critical design parameter. If the ponded water gets too deep, the hydraulic pressure can push the sediment cake into the fabric, blinding it and stopping flow entirely, or it can cause the fence to collapse. As a rule of thumb, the ponded water depth should not exceed half the height of the exposed fabric. For a standard installation with 12-14 inches of exposed fabric, this means water should not pond deeper than 6-7 inches. This requires proper site grading to spread flow evenly along the fence length and prevent localized deep ponding.
Unsuitable for High-Energy Flow: Non-woven fences should never be used in concentrated flow areas, such as swales or ditches. The low flow rate cannot handle the volume of water, leading to immediate over-topping and erosion around the ends of the fence. In these cases, a woven fabric or a specialized reinforced silt fence designed for concentrated flow is mandatory.
Installation is More Demanding: A proper installation is non-negotiable. The support posts must be sturdy (typically steel T-posts) and spaced closely (often 4-5 feet apart). The fabric must be trenched in deeply (at least 6-8 inches) and backfilled with compacted soil to prevent under-scouring. The fabric should be attached to the posts with staples or ties that provide a continuous connection, and the “upstream” face of the fabric must be oriented correctly to function as intended. A sloppy installation will fail, regardless of the fabric quality.
Cost-Benefit Analysis: Is it Worth It?
From a cost perspective, non-woven geotextile silt fence fabric is generally more expensive per linear foot than its standard woven counterpart. However, a simple price comparison is misleading. The true cost must include:
- Material Cost: Higher for non-woven.
- Installation Cost: Similar for both, assuming proper techniques are used.
- Performance Cost (Risk): This is the key differentiator. A failed silt fence can lead to off-site sedimentation, regulatory fines, site shutdowns, and costly remediation. The superior fine-sediment retention of a correctly installed and maintained non-woven fence significantly reduces this risk on sensitive sites. Therefore, the higher initial material cost can be easily justified by the avoidance of much larger potential liabilities.
The decision matrix is clear: if your project involves fine-grained soils and you are subject to stringent turbidity limits, the investment in a high-quality non-woven system is not just effective—it is a prudent and often necessary risk management strategy. For sites with sandy soils or where the primary concern is bulk sediment control from high-energy flow, a standard woven fence remains the more appropriate and cost-effective choice.