Factors affecting the separation and filtration mechanism of geotextiles

14 Factors affecting the separation and filtration mechanism of geotextiles 

A notable problem, which readily accelerates the mixing of materials of stone and subgrade, is the movement of water when large loads periodically pressurise the surface of the stone layer. A pumping action occurs and the water movement erodes the subgrade and accelerates the migration of the 

The above factors are also applicable when geotextiles are used for filtration. However, with filtration the intention is to form a natural filter cake at the fabric/soil interface that will retain the fines consequently the pore-size requirement is different than that for the separation function. For filtration, the AOS of the textile must be smaller than the larger sizes of soil particles, but of sufficient dimension for the small soil particles to pass through, otherwise the geotextile will become clogged or blind and its permeability adversely affected. The fabric must have a pore-size distribution with a large number of larger size pores, so that a relatively unhindered water flow is maintained where some of the smaller size pores become blocked. 

Flow conditions affect the way in which soil particles interact with one another and with the filter. As a result retention requirements, given in terms of the AOS value and in relation to particle size distribution parameters, should be determined for unidirectional flow or multidirectional flow conditions by using a wet-sieving performance test. In considering the soil parameters, the particle size distributions of typical soil types can be placed into the three zones shown in Fig. 8.32. 

Zone 1 soils consist of more than 85% clay and silt (particles smaller than 0.075 mm) and often have sufficient cohesion and low permeability to allow 

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