Permeability of Compacted Clay

The effects of molding water content, density, degree of saturation, method of compaction, and thixotropic hardening on the permeability of compacted silty clay have been determined. The formation of a dispersed structure in samples compacted wet of optimum may result in a coefficient of permeability two or three orders of magnitude less than for the same soil compacted dry of optimum. The actual decrease in permeability wet of optimum appears to correlate well with the degree of shear strain applied to the soil during compaction. In line with this, it was found that for samples compacted wet of optimum kneading compaction gave significantly lower values of permeability than did static compaction. Thixotropic hardening was accompanied by an increase in permeability, a result compatible with the concept that thixotropic hardening involves a change to a more flocculent structure. As much as a five-fold increase in permeability may accompany an increase in saturation from the as-compacted state to the fully saturated condition. Because of the great variability in permeability with compaction conditions, selection of an appropriate value for use in problems involving seepage or pore pressure dissipation will be difficult.