Dilatancy and the Strength of Rocks containing Pore Water under Undrained Conditions

Summary Experiments have been carried out at atmospheric temperature and confining pressures up to 5 kb in which rocks, of different porosities, and fitted with flexible jackets, have been deformed up to 6 per cent axially while containing pore water under undrained conditions. Application of a confining pressure and axial compressive stress creates a pore pressure, which weakens and embrittles the rocks under these conditions. The initial pore pressure may take any value up to that of the confining pressure, depending on the amount of water available inside the jacket and sample. Dilatancy produced by crack propagation during shear deformation resulted in dilatancy hardening which prevented or delayed mechanical instability (as manifested by stress drops). The greatest amount of dilatancy occurred during the fracturing process, but dilatancy decreased as the effective confining pressure increased. Only a small amount of dilatancy occurred during subsequent movement on faults, and the diIatancy tended to reach a stable value at deformations of ∼5 per cent (∼1.5 mm of movement on a major fault surface). There is evidence of small quasicyclic variations of dilatancy at greater deformations, which produce small stress drops of ∼50 b. The implications for crustal faulting and earthquake premonitory effects are discussed.