Triaxial compression tests were run on seven orientations of three rocks with pervasive planar anisotropy—slate, phyllite, and schist—to evaluate the effects of cohesion and granularity on anisotropic strength variation and deformational mode. The rocks show a gradation in the percentage of platy minerals, and in the size, shape, and percentage of quartz grains present. Experiments were carried out at confining pressures of 500, 1,000, and 2,000 bars, and all tests were duplicated. The tests on slate were duplicated in two different apparatuses, with and without specimen end lubrication, to evaluate the effects of testing conditions on the experimental results. The strength data obtained were analyzed in terms of cohesive strength and internal friction. Cohesive strength increases with increasing granularity, but the coefficient of internal friction becomes the principal contributor to the strength variation. The coefficient of internal friction does not appear to change significantly with increased granularity; moreover, it appears to be less sensitive to material variability than does the cohesive strength. Cohesive strength becomes the more significant contributor to strength variation with increased percentage of platy minerals. Cohesion can have a pronounced effect on the mode of deformation. Curves of differential stress at failure versus anisotropy orientation are concave upward with pronounced strength minima for maximum compression at 30° to 45° to the anisotropy, depending upon the granularity and percentage of platy minerals. Maximum strength occurs in the 90° or 0° orientation, also as a function of material characteristics. The observed strength variation among different orientations is in excellent accord with the extended variable cohesive strength theory of Jaeger (1960). Strength for a given orientation increases linearly for phyllite at pressures up to 2,000 bars; the increase is nonlinear for slate above 1,000 bars. Significant shear stress developed on the ends of specimens under certain testing conditions that, for one orientation, resulted in the rotation of the maximum principal stress from the vertical loading direction by as much as 30°. As a consequence, for these test conditions measured values of strength for this orientation can be about 25 percent lower than the true strength. Lubrication of specimen ends and a rigid loading piston are believed to give the most accurate results.

Abstract Disposal of waste fluids into the subsurface may affect the mechanical properties and strength of the rock mass in two important ways. When fluid is chemically inert, strength and ductility are reduced by increasing pore pressure. When fluid is chemically active, the strength of the rock mass is further reduced through modification of the cohesive strength of its constituent grains in contact with the fluid. Several experiments reveal that the strength of rocks and propagation of minute surface cracks are highly dependent on the moisture content of the rock. Dilute solutions of aluminum and ferric iron salts, in addition to water, react with the surface structure of quartz and silicates and weaken the surface silicon-oxygen bonds by hydrolysis. The result is a reduction in surface energy, surface cohesion, and breaking strength. The coefficient of internal friction, however, remains unaltered. The frictional characteristics of already-broken rocks may be significantly altered by the introduction of chemically active fluids. Because of such manmade earthquakes as those near Denver and Rangely, Colorado, it is obvious that more sophisticated tests of rock properties should be made, particularly regarding the influence of pore-fluid pressure and chemistry, if problems caused by unexpected rock failure are to be eliminated.