Preliminary investigation of volume changes in crushed granite preceding stick‐slip failure

Abstract

A large granite sample containing a saw‐cut filled with crushed granite, modeling a natural fault with gouge, was loaded triaxially at confining pressure of 400 MPa. Changes in pore volume of the simulated gouge layer were measured during application of confining pressure and during axial loading to failure by sudden stick‐slip. Pore volume strain in this configuration, with a thin layer of gouge between rock surfaces, was significantly less with increasing confining pressure than in similar experiments in which the entire sample was of crushed granite. Our sample continued to decrease in volume upon application of axial load up to the point at which sliding on the saw‐cut started. At this point, the decrease in volume slowed down, and the volume increased just before failure. Failure occurred at a stress below that required to cause dilatancy in intact rock, suggesting that gouge might show premonitory changes before earthquakes that would not appear in intact rock surrounding a fault zone.