To evaluate the properties of rock masses, particularly the influence of friction across surfaces of mechanical discontinuity, sliding friction studies of triaxial compression have been made of all combinations of Tennessee Sandstone, Solenhofen Limestone, and Blair Dolomite. Right-circular cylinders, each with a polished saw-cut at 45° to the load axis, were deformed at confining pressures to 1.4 kb at room temperature, and at a constant rate of shortening of 10–4/sec.

For intact samples, dolomite has the highest ultimate strength and limestone the lowest, while for precut monolithologic samples, limestone has the highest and dolomite the lowest. Where sliding takes place by brittle behavior along the surface, the coefficient of sliding friction, μ, is 0.5 to 0.6 and is independent of confining pressure. Where ductile behavior accompanies sliding, μ decreases with increasing confining pressure and has a mean value of about 0.7. In dilithologic specimens, μ lies between those of the corresponding monolithologic samples. The rock with the more ductile behavior along the sliding surface controls the behavior of the composite specimen. For brittle sliding, μ is maximum after 0.1 to 0.2 cm of sliding at 350 bars confining pressure; for ductile sliding, μ reaches a maximum at 0.05 cm of displacement. This suggests that ductile behavior along the sliding surface increases the area of contact. Optical and scanning electron microscopy support this conclusion. At all pressures investigated, monolithologic dolomite shortened by stable sliding, sandstone by stable sliding followed by stick-slip, and limestone by stable sliding and faulting. Dilithologic specimens showed stable sliding in all cases except for dolomite-sandstone at 350 bars where stick-slip followed stable sliding.