The heart of a filtration geotextile’s function lies in its pore structure. Selecting a fabric with the correct pore size is a precise engineering task to ensure water flows through while soil is retained. Two key metrics govern this: Apparent Opening Size (AOS) and Filtration Opening Size (FOS).

Apparent Opening Size (AOS - ASTM D4751): This is the most common spec. It is defined as the approximate diameter of the largest spherical particle that will effectively pass through the geotextile. It is expressed as an equivalent sieve size (e.g., AOS = Sieve No. 70 or 0.212 mm). A smaller AOS number indicates smaller pores, suitable for retaining finer soils. A general rule is that the AOS should be smaller than the D85 size (the size at which 85% of the soil is finer) of the soil to be filtered.

Filtration Opening Size (FOS - ISO/EN 12956): More common in Europe, FOS is determined by a wet-sieving method with granular soil. It is considered by many engineers to better represent the geotextile’s behavior under hydraulic gradients with real soil. It is expressed as O90 or O98 (the pore size at which 90% or 98% of the soil is retained).

Filter Design Principles: A good filter must satisfy two opposing criteria:

Soil Retention: The pores must be small enough to prevent excessive migration of the base soil particles (“piping”).
Permeability and Anti-Clogging: The pores must be large enough, and the fabric’s permeability must be high enough, to allow water to pass freely without creating a buildup of water pressure or clogging over time.

This is a balancing act. A fabric that is too tight will blind and block; one that is too open will allow soil loss. The choice depends on the soil type (gradation, cohesion) and hydraulic conditions. For critical drainage applications behind retaining walls or in erosion control, proper filter design is essential for long-term performance.

At HZ Geotextile, our filtration geotextiles come with certified AOS and permeability data. Our technical team can assist in interpreting your soil reports to recommend the optimal filter fabric, ensuring your system functions as designed for decades. Trust the science behind the fabric at www.hzgeotextile.com.