Some measurements of permeability and effective stress on a heterogeneous soil mixture: implications for recovery of inelastic strains

The results of permeability variations in response to both changes in isotropic mean effective stress and internal fluid pressure gradients for an artificial silty clay are presented. While changing confining stress induces a typical elasto-plastic response, with a small increase in permeability during unloading, raising the fluid pressure results in a more pronounced increase in permeability, the magnitude of which is similar to that seen during virgin compression. The behaviour is interpreted as reflecting compressibility contrasts between rigid quartz grains and more compressible clays. Fluctuating the confining pressure has a more direct influence on the less compressible quartz particles, with a subsequent smaller change in permeability upon unloading. Conversely, raised pore fluid pressure affects the clay matrix more directly and can therefore prompt a different permeability response. The results have implications for predicting the permeability characteristics of overconsolidated soils that are heterogeneous in nature.