Myths & Facts #3 Clayey soils need geotextile filter?

Presence of small amounts of clay in sandy silty soils can contribute to filter clogging process. It had been observed from my past laboratory experiments that the cohesiveness of clay mineral plays an important role in accelerating the filter internal clogging especially at low flow rate (below 1.0E-6 cm/sec).

A solution for filtering sandy silty soils with little to some clay content (10%wt.~ 20%wt.) is application of a thick nonwoven geotextile (GT) with a layer of fine sand placed on upstream side of the filter fabric. Affinity of clay mineral to sand grains and the increased tortuosity of the pore structure would help preventing clay accumulation on GT openings already partially clogged by fine sand and silt particles.

If clayey soils present sufficient plasticity and its saturated hydraulic conductivity less than 1.0E-6 cm/sec, GT filter will not be required in underdrain system. In lieu of GT, granular filter has been used as the best filter if required for preventing the clayey soil erosion which was vindicated throughout many previous dam case studies.