VELOCITY- AND TIME-DEPENDENT STRESS TRANSIENTS IN SIMULATED FAULT GOUGE

Abstract

During the shearing of fault gouge, strength changes are often observed when the rate of strain is varied. A strain rate increase is characterized by a transient stress peak before reaching a final equilibrium value. Conversely, a stress drop and a gradual rise to the equilibrium strength accompany changes to slower rates. To study the phenomena, frictional sliding experiments were performed at normal stresses to 100 MPa on granite samples sawcut at 30° and filled with a layer of Ottawa sand. Pore water volume and shear stress were measured as the displacement rate along the inclined gouge layer was alternated between 2.2 × 10–3 and 2.2 × 10–5 mm/sec. Pore volume increased at the fast rate and decreased at the slower rate, verifying that gouge dilation is a function of strain rate. Pore volume changed until the critical void ratio of the granular material was reached for a particular rate of strain. Because of this, the degree of consolidation of the grains must also depend on rate. Soil mechanics studies have shown that densely packed sand typically exhibits a peak in shear stress before reaching a steady-state value, whereas the strength of loosely packed sand slowly rises to the final value, as we have also observed in these experiments. The dense sand is initially overconsolidated relative to the equilibrium level, whereas the loose sand is initially underconsolidated relative to this level. Therefore, the transient stress behavior must be due to the overconsolidated state of the gouge at the new rate when the velocity is increased and to the underconsolidated state when the velocity is lowered. Because time-dependent processes will also cause the gouge to compact, natural fault gouges may exhibit similar transient behavior as they become overconsolidated and stronger with time.