For normal soils, water in bentonite slurry penetrates into the sandy walls and leaves behind a layer of bentonite particles on the surface of the soils. The bentonite particles form the filter cake of low permeability on the excavated faces. The filter would be formed only when slurry pressure is greater than the pore water pressure in excavated soils. The filter cake serves as impermeable layer and allows the application of full hydrostatic pressure of bentonite slurry on the excavated surface of soils.
For soils with low permeability such as clay, there is little water passage from slurry to excavated clay surface so that filter cake would hardly be formed. As a result, slurry pressure simply applies on clay surface.
This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.
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Is Bentonite successful in stabilizing trenches excavated for Slurry Walls in Sand?