How do you determine the appropriate non-woven geotextile for a specific soil?

Soil Type and Particle Size Distribution

First things first, you've got to get your hands dirty, literally. The type of soil you're working with is the single most important factor. Think of the geotextile as a filter; its job is to let water pass through while keeping soil particles in place. If the holes in the filter (the geotextile's pore size) are too big, fine soil particles will wash right through, leading to erosion and failure. If the holes are too small, they'll clog up with silt and clay, preventing water drainage and causing pressure buildup. We use a concept called apparent opening size (AOS) or equivalent opening size (EOS), measured in millimeters or US Sieve sizes, to match the geotextile to the soil.

For coarse-grained soils like gravels and sands, you need a geotextile with a larger opening size. A common rule of thumb is that the AOS should be less than or equal to the D85 size of the soil. The D85 is a particle size where 85% of the soil particles are finer. For instance, a sandy gravel with a D85 of 2.0 mm would typically require a geotextile with an AOS of no more than 1.0 to 2.0 mm (approximately US Sieve No. 10 to 18).

For fine-grained soils like silts and clays, the challenge is different. These particles are tiny and can easily clog a standard geotextile. Here, you need a geotextile with a very small opening size, but one that is also "open" enough to allow for adequate water flow over the long term without blinding (clogging at the surface) or clogging internally. For these soils, you often select a geotextile with an AOS in the range of 0.15 to 0.60 mm (approximately US Sieve No. 100 to 40). The key is to ensure the permeability of the geotextile is significantly greater than that of the soil, typically by a factor of 10 or more, to ensure flow capacity is maintained.

Project Function: What is the Geotextile Supposed to Do?

A NON-WOVEN GEOTEXTILE isn't a one-trick pony. The specific job it has to perform on your site will dictate its required physical and mechanical properties. You might be using it for one primary function or a combination of several.

Separation: This is one of the most common uses. You're placing the geotextile between two different layers of soil, like a soft subgrade and a clean gravel road base, to prevent them from mixing. Over time, vehicle loads can push the gravel down into the soft soil, ruining your road. The geotextile acts as a physical barrier. For separation, the key property is often puncture resistance and elongation. The geotextile needs to be tough enough to withstand the installation of the overlying material without tearing, and it needs to be able to stretch (elongate) to bridge soft spots in the subgrade. A medium-weight non-woven, say 200 to 300 grams per square meter (gsm), is often sufficient for many separation applications.

Filtration: As we discussed with soil types, this is all about allowing water to flow across the plane of the geotextile while retaining soil particles. This is critical in drainage applications like behind retaining walls, in French drains, or around subsurface pipes. The balance between retention (AOS) and permeability (flow rate) is paramount. Non-woven geotextiles are often preferred for filtration because of their tortuous flow paths, which are excellent at retaining particles while maintaining flow.

Drainage: This function is about water flowing within the plane of the geotextile itself. Non-woven geotextiles, due to their felt-like structure, have a significant thickness which creates voids for water to travel along. This is useful for relieving pore water pressure, for example, on a hillside or in a sports field. The key property here is transmissivity, which is a measure of in-plane flow capacity. It's a function of thickness and permeability. Heavier weight non-wovens (e.g., 300-500 gsm) have greater thickness and thus higher transmissivity.

Reinforcement: While woven geotextiles are often stronger, non-woven geotextiles do provide a degree of reinforcement, especially in applications where the soil-geotextile interaction is beneficial, like in unpaved roads over very soft ground. The primary property is tensile strength, measured in kilonewtons per meter (kN/m). For light reinforcement, a non-woven with a tensile strength of 8-12 kN/m might be used, but for significant reinforcement, woven or geogrid products are typically specified.

Protection: Here, the geotextile is used to protect a delicate geomembrane, like a pond liner, from puncture by sharp stones in the subsoil. This requires high puncture resistance and cushioning ability, which is a strength of thick, needle-punched non-woven geotextiles. A common weight for protection geotextiles is 400-600 gsm.

Physical and Mechanical Properties: The Specification Sheet

Once you know your soil and the required function, you translate that into specific numbers from a product data sheet. Don't just buy by weight (gsm); the weight is a general indicator, but the tested properties are what guarantee performance.

• Grab Tensile Strength (ASTM D4632): Measures the force required to break a specific width of the fabric. Important for installation survivability and reinforcement. Look for values like 900 N (Newtons) or higher.
• Puncture Resistance (ASTM D4833): Measures resistance to a plunger pushing through the fabric. Critical for separation and protection. Values are often in the range of 400-800 N.
• Trapezoid Tear Resistance (ASTM D4533): Measures resistance to tearing forces. Important for survivability. Values might be 300 N or more.
• Permittivity / Permeability (ASTM D4491): Permittivity is the flow capacity normalized for thickness (sec⁻¹). Permeability is the flow rate (cm/sec). For filtration, you need a permittivity that is an order of magnitude greater than the soil's.
• Apparent Opening Size (AOS) (ASTM D4751): The most critical property for filtration, as discussed. It's listed as O95, meaning 95% of the openings are smaller than this size (e.g., O95 = 0.21 mm).
• UV Resistance: If the geotextile will be exposed to sunlight for more than a few months, it needs to be stabilized against ultraviolet degradation. Look for a percentage of tensile strength retained after a certain number of hours of exposure (e.g., 70% strength retained after 500 hours per ASTM D4355).

Site-Specific and Environmental Conditions

Finally, you have to consider the real-world environment where the geotextile will live.

Chemical Compatibility: Is the soil or groundwater acidic, alkaline, or contaminated with hydrocarbons or other chemicals? Polypropylene, the most common polymer for non-woven geotextiles, has excellent chemical resistance to a wide range of pH levels (2 to 13) and is inert to most biological processes. However, if you're dealing with severe chemical exposure, you need to verify the polymer's resistance charts.

Installation Stresses: How will the geotextile be placed? If it's being unrolled by hand on a smooth surface, survivability requirements are low. If it's being placed on a rocky subgrade with large, angular stones and covered with a 500mm thick layer of dumped gravel from a haul truck, you need a very robust, high-strength geotextile to survive the installation without being punctured or torn. This often means specifying a higher weight or strength class than the design function alone would require.

Long-Term Performance: What is the design life of the project? A temporary access road might only need to last 6 months, while a landfill cap needs to perform for decades. Long-term creep resistance and clogging potential are critical considerations for permanent structures. This is where consulting with a geotechnical engineer or an experienced manufacturer is invaluable. They can provide data on long-term flow rates and strength reduction factors.