Specifying a filtration geotextile by its Apparent Opening Size (AOS) alone is like prescribing medicine based only on the patient’s weight—it’s a starting point, but woefully incomplete. True filtration design is a systematic engineering process that balances soil retention, adequate permeability, and long-term clogging resistance to ensure the system functions for its design life. For critical applications like dam filters, landfill leachate collection, or coastal revetments, this process is non-negotiable.
Here is a step-by-step framework for designing a geotextile filter:
Step 1: Characterize the Base Soil
This is the foundation. Obtain a representative grain-size distribution (GSD) curve of the soil to be protected. Identify key diameters: D85 (the size at which 85% of the soil is finer), D50, and D15. Also, note the soil’s plasticity (PI) and cohesion, as fine, cohesive soils behave differently from granular soils.
Step 2: Apply Soil Retention Criteria
The primary rule: the geotextile must prevent excessive migration of the base soil particles. The most common retention criterion is:
O95 (Geotextile) ≤ B * D85 (Soil)
Where O95 is the geotextile’s pore size (similar to AOS), and B is a factor based on soil type and hydraulic conditions. For steady-state, normal flow conditions:
B ≈ 1 for uniform granular soils (Cu < 3)
B ≈ 0.5 to 1 for well-graded soils
Stricter criteria (smaller B) are used for severe cyclic or reversal flow.
Step 3: Verify Permeability Criteria
The filter must be more permeable than the soil to prevent a buildup of water pressure. The rule of thumb is:
kg (Geotextile) ≥ 10 * ks (Soil)
Where kg is the geotextile’s permeability and ks is the soil’s permeability. For fine soils with very low ks, this can be challenging. A thick non-woven geotextile with high porosity is often required to meet this criterion.
Step 4: Assess Clogging Potential
This is the most complex step. Retention and permeability are necessary but not sufficient. A fabric that retains all particles might blind quickly. Several methods are used:
Gradient Ratio Test (ASTM D5101): A lab test that simulates flow under a hydraulic gradient. A Gradient Ratio (GR) ≤ 3 is generally considered indicative of non-clogging behavior.
Porosity and Thickness: Empirical guidelines suggest that for fine, non-cohesive soils, a geotextile with a porosity > 30% and sufficient thickness performs well.
Experience and Testing: For critical applications (e.g., dam cores), long-term performance tests or reliance on documented successful case histories with similar soils is essential.
Step 5: Consider Survivability and Durability
Finally, the fabric selected must survive installation and the chemical environment. This may require selecting a heavier weight or a different polymer.
Example Application: A Retaining Wall Drain
Soil: Silty sand (D85 = 0.2 mm, ks = 1x10⁻⁴ cm/s).
Retention: Choose a geotextile with O95 ≤ 1 * 0.2mm = 0.2mm (AOS ~#70 sieve).
Permeability: Need kg ≥ 1x10⁻³ cm/s. A standard non-woven easily exceeds this.
Clogging: A needle-punched non-woven with moderate thickness would be selected, and its GR test data would be reviewed.
Filtration is a classic geotechnical engineering problem. Partnering with a manufacturer that provides comprehensive pore size distribution and hydraulic test data is crucial. At HZ Geotextile, our technical data sheets provide the O95, permeability, and thickness parameters you need to begin this design process. For your most critical drainage and erosion control systems, design with confidence using our engineered filter fabrics. Access our technical resources at www.hzgeotextile.com.
