Soil retention is a predominant function of geotextiles in drainage and filtering applications. Pore size is the key parameter that controls the ability of the geotextile to retain the soil. The Apparent Opening Size (AOS) also called the Equivalent Opening Size (EOS) was developed by US Army Corps of Engineers for evaluating this property. AOS is the size of the holes in the geotextiles and it provides an index to the size of the largest opening through the geotextile. In conducting this test, the geotextile fabric is placed (instead of a wire mesh) in a sieving frame by using a special clamping arrangement. The test involves sieving rounded particle sizes for which 5% or less by weight, pass through the geotextile. The AOS is defined as 'Retained On' size of that fraction expressed as a standard sieve number(size). The test is widely used for relative comparison amongst the geotextiles. Thus AOS is a means of correlating geotextile pore structure to an equivalent screen mesh size. This test is standardised by ASTM, ISO, IGS (International Geotextile Society).The apparatus consists of a 20 cm dia brass frame with clamp, a receiver and a lid.

Soil retention is a predominant function of geotextiles in drainage and filtering applications. Pore size is the key parameter that controls the ability of the geotextile to retain the soil. The Apparent Opening Size (AOS) also called the Equivalent Opening Size (EOS) was developed by US Army Corps of Engineers for evaluating this property. AOS is the size of the holes in the geotextiles and it provides an index to the size of the largest opening through the geotextile. In conducting this test, the geotextile fabric is placed (instead of a wire mesh) in a sieving frame by using a special clamping arrangement. The test involves sieving rounded particle sizes for which 5% or less by weight, pass through the geotextile. The AOS is defined as 'Retained On' size of that fraction expressed as a standard sieve number(size). The test is widely used for relative comparison amongst the geotextiles. Thus AOS is a means of correlating geotextile pore structure to an equivalent screen mesh size. This test is standardised by ASTM, ISO, IGS (International Geotextile Society).The apparatus consists of a 20 cm dia brass frame with clamp, a receiver and a lid.