Effect of anisotropy on the coefficient of sliding friction in Schistose rocks

Abstract

Foliated schist samples were sawcut at 45° to th cylinder axis and tested at 500 bars confining pressure, 25°C, and 10 −4sec −1 shortening rate, to examine the effect of anisotropy on the coefficient of sliding friction (μ). Cylinders were cored from sample blocks in a variety of orientations to produce sawcut-to-foliation (STF) angles ranging from 0° to 180° in 15° increments. The variation of μ with foliation orientation is sinusoidal. Maximum values of μ occur when the foliation is parallel to principal stress planes ( STF = 30°−45° and 105°−120°; τ = 0), and minimum values of μ correspond to planes of maximum resolved shear stress ( STF = 75° and 165°, τ = τ). Sliding-surface damage and gouge development vary inversely with μ. Surface damage results from the interaction of asperities with the sawcut surface, and failure and cataclasis of asperities produces gouge. Precursor events visible on force-time records, and partial loading tests terminated before megascopic sliding, indicate that surface damage and gouge are generated before megascopic sliding on the sawcut. Apparently, high resolved shear stress produces slip along the foliation prior to sliding on the sawcut. Foliation slip generates gouge at the sawcut interface; and the effect of the gouge is to lower μ.