Geotextile Characteristic Opening Size (COS) Tester

Theory and Method

A geotextile filter works by letting water through while holding soil particles back, so the size of its openings decides its behavior. The wet sieving method treats the fabric itself as the sieve. A single layer of geotextile is clamped without tension across the ring, a measured quantity of graded granular material is spread over it, and the deck vibrates while water sprays continuously onto the bed. Particles small enough to negotiate the fabric structure wash through into a collection pan. Drying and grading that collected fraction yields a passing curve, and the particle diameter at which 90% of the passed material is finer becomes the characteristic opening size, written O90.

Water does the work that separates this method from dry sieving. Fine particles carry surface charge and cling to fibers and to each other, and a dry bed will hold them in the fabric structure long after they should have passed. Continuous spray breaks those adhesive and electrostatic bonds, floats fines toward the openings, and clears particles that have lodged partway through. The result reads finer and more realistically for needle-punched nonwovens, whose thick, tortuous structure traps material badly under dry conditions.

Clamping without tension matters just as much. Stretching a nonwoven pulls its fiber network apart and widens the pores, so a tensioned specimen reports an opening size larger than the fabric will show under a soil load. The COS holds the specimen flat across the Ø176 mm ring with no applied strain. Vibration at 1.5 mm amplitude over a 3 mm total travel, driven at 50 Hz, keeps the granular bed circulating so that individual particles meet many different openings during the run rather than settling over one spot.