THE EFFECT OF WATER ON STABILITY OF FRICTIONAL SLIDING OF FAULT

The friction behavior of faulted granite samples, with size of Φ 20 mm×40mm and saw cut at an angle of 35° to σ 1 axis, was experimentally studied under various water containing conditions and tri axial compression. The stability during stable sliding or stick slip was emphasized. Three types of polished fault surface are considered: (1) two surfaces contact directly without any gouge in between, used to simulate the friction behavior of brittle fault; (2) the preset fault is filled with 0 25g quartz powder with mean grain size of 10μm, used to simulate the friction behavior of a fault containing brittle gouge; (3) the preset fault is filled with 0.25g clay fault gouge with grain size of 10μm, used to simulate the friction behavior of a fault containing ductile gouge. The water content ranges from dry to water saturated, and the pore pressure ranges from 20 to 150MPa. The following conclusions can be drawn from the results of the experiment: (1) The mode of faulting is strongly influenced by the existence of water. For dry polished surfaces without any gouge, there is a certain range of confining pressure in which stick slip behavior exists. This range of confining pressure may shrink quickly when water enters and forms pore pressure on sliding surface, i.e. the friction behavior turn to stable sliding. For samples with quartz powder, frictional behavior changes from stable sliding to stick slip when dry gouge becomes wet, and return to stable sliding again when gouge is saturated and undrained. In this case, there exists also a certain range of confining pressure in which stick slip behavior exists. For samples with clay gouge, frictional sliding keeps stable under any humidity and pore water pressure. (2) Experimental study also shows that stress drop of stick slip is dependent on the water content. For samples without gouge, stress drop decreases with increasing water content in rock. For samples with quartz powder gouge, however, stress drop increases quickly at first and then slowly with increasing of water content in gouge under drained condition; under undrained condition, stress drop is small at first, then gets larger, and becomes small again when the content of water in gouge increases. When the samples are loaded with pore water pressure, frictional sliding turns to stable and the stress drop tends to zero. (3) Water influences the velocity dependence of frictional sliding of the tested rock samples. The velocity dependence of samples without gouge is velocity weakening at any water content. For the samples filled with quartz powder, it is velocity weakening when the water content ranges from dry to wet, i.e. at different humidity, and becomes velocity strengthening when pore pressure exists on the fault; for the samples filled with clay fault gauge, it is always velocity strengthening. (4) Based on velocity weakening model of friction instability and by considering the order of fault zone from no any gouge, to containing quartz powder gouge and clay gouge as a transition from brittle to ductile regimes, a conceptional model has been established for the relationship between the stability of faulting and the influential factors including fault property, water content and confining pressure (depth). Brittle fault displays velocity weakening in most cases of various water contents, and stick slip appears in the region of low water content, while stable sliding appears when the water content becomes high (high pore pressure). Semi brittle or semi ductile fault displays velocity weakening when water content is relatively low, and stick slip may exist; but it will displays velocity strengthening when high pore pressure appears, and the sliding will be stable. Ductile fault always displays velocity strengthening at any water content, and the sliding is always stable. (5) A conclusion can be drawn from the experimental results of this study in the light of pore pressure theory: Entering of water will decrease the stability of the fault system but the existence of water may