The importance of overpressure timing and permeability evolution in fine-grained sediments undergoing shear

We summarise new experimental data which clarify the hydrological behaviour of clayey sediments undergoing shear, and which emphasise the importance of the relative timing of deformation and overpressure. Fluid flow in fine-grained materials subjected to shear is strongly dependent on the previous stress history of the material. Underconsolidated sediments, such as those that retain high porosities through retarded dewatering and consequent excess pore-fluid pressure generation, deform by bulk volume loss which reduces permeability. The deformation fabrics are pervasive in style, weakly developed, and without a significant influence on hydrological properties. In contrast, sediment that has been consolidated and then subjected to excess pore-fluid pressure deforms dilatively along discrete, brittle shear zones, which markedly affect the permeability. Moreover, below a certain threshold of effective stress, these localised zones prompt a markedly non-linear increase in hydraulic conductivity, a laboratory demonstration and quantification of the concept of fracture permeability.