Shear-induced permeability reduction and shear-zone development of sand under high vertical stress

To evaluate the permeability perpendicular to the shear zone with shear-zone development in silica sand under high vertical stress, we investigated the effects of shear displacement on permeability variation by performing a laboratory study involving ring-shear testing. The permeability changes under low and high vertical stresses were drastic and gradual, respectively, with increasing shear displacement. The shear-zone thickness under each set of stress and shear displacement conditions was determined by observing the particle crushing and compaction in the microstructure. The pore-diameter reductions were dependent on both the shear displacement and stress. The clear relations between the cumulative pore-diameter frequencies and median grain sizes in samples with shear zones were determined through the measured vertical stresses. In the grain-size analyses of the shear zone samples, the reductions of the sand size fraction and median grain size with shear could reflect the shapes of the permeability variations in two distinct groups of shear and stress conditions in the tests. The correlations identified between the cumulative pore-diameter frequencies and median grain sizes in the shear-zone samples were obtained under vertical stresses. The results of this study are expected to enhance understanding of the permeability mechanisms in fault and slip zones in sandy sediments, enabling the estimation of permeability and groundwater movement in landslides as well as the development of preliminary simulations for production operations in oil and natural gas reservoirs.