SHEARING OF SATURATED CLAYS IN ROCK JOINTS AT HIGH CONFINING PRESSURES

Saturated clays are sheared between rock joints at various pore water pressures and at confining pressures up to 3 kb (300 Mpa). Sliding on these joints is stable. For a given claythe shear stress required to initiate sliding increases linearly with the effective normal stress across the sliding surfacewith a slope of 0.08 ± 0.01 for joints filled with saturated montmorillonite0.12 ± 0.01 with saturated chlorite0.15 ± 0.01 with saturated kaoliniteand 0.22 ± 0.02 with saturated silty illite. Thus at high confining pressures the shear stress required to initiate sliding on joints filled with saturated clays are very much smaller than that required to initiate sliding on clean rock joints or on joints filled with dry gouge materials. In the crustsaturation of gouge materials along active faults would greatly lower the frictional resistance to faulting and would stabilize fault movement. Different fault behaviors such as stable creep along some faults and intermittent but sudden slip along others may reflect in part different degrees of saturation of fault zones at depth. Copyright 1979 by the American Geophysical Union.