THE APPARENT FRICTION OF GRANULAR FAULT GOUGE IN SHEARED LAYERS

Data are presented from a series of experiments on layers of granular quartz gouge in the double-direct-shear geometry at a normal stress of 25 MPa. The apparent friction of a layer, defined as the ratio of the applied shear and normal stresses, shows considerable variability dep ending on the thickness of the layer and the particle size distributio n of the gouge. Measurements of layer thickness during the experiments also show that the layers thin as shearing proceeds. Layer thinning i s attributed to the processes of extrusion and densification, and by a nalyzing these processes the observed variations in apparent friction are reconciled with a constant coefficient of internal friction for qu artz gouge. For thinning due only to extrusion, the prinicipal assumpt ion made is that the deviatoric stress and plastic strain rate are coa xial. When densification is also admitted, a simple flow law with one adjustable parameter is required to relate the volumetric and shear st rain rates. The results and analysis show that (1) extrusion and densi fication must be considered when interpreting the measured frictional properties of gouge layers; (2) shear localization is inhibited in the se experiments; and (3) the apparent friction of a natural fault zone may be less than sin phi, where tan phi is the coefficient of internal friction of gouge in the fault zone.