Frictional behavior and constitutive modeling of simulated fault gouge

Presents an investigation of the frictional properties and stability of frictional sliding for simulated fault gouge. In these experiments gouge layers (quartz sand) were sheared under saturated drained conditions and at constant normal stress (50-190 MPa) between either rough steel surfaces or Westerly granite surfaces in a triaxial apparatus. Porosity φ was monitored continuously during shear. Measurements indicate that granular gouge exhibits strain hardening and net compaction for shear strains γ >0.5-1.0. For γ sliding occurs at approximately constant shear stress and net compaction from one load/unload cycle to the next ceases. Dilatancy occurs at 1/3 to 1/2 the shear stress required for sliding and d²φ/dγ² becomes negative at about the peak stress in a given loading cycle, indicating the onset of shear localization. Experiments with an initial gouge layer exhibit velocity strengthening, and initially bare granite surfaces exhibit velocity weakening. Data suggest that slip within unconsolidated granular material, such as some natural fault gouges, is inherently stable. -from Authors