Dependence of Strain-Rate Effects on Deformation Mechanism and Rock Type

Abstract

Results are reported for sixty-nine triaxial compression tests run on a marble, a lithographic limestone, a very fine-grained sandstone, and a siltstone at room temperature, dry, confining pressures of 1,000 and 2,000 bars, and at strain rates of $$10^{-3}$$, $$10^{-4}$$, $$10^{-5}$$, $$10^{-6}$$, and $$10^{-7}$$/second. A decrease in strain rate from $$10^{-3}$$ to $$10^{-7}$$/second at 2,000 bars confining pressure caused a decrease in strength at 2 percent strain of about 33 percent for the marble and 8.4 percent for the limestone, but had no significant effect on either the sandstone or siltstone. Similar results were obtained for the tests run at 1,000 bars. Over the same range of strain rates, the strength at 10 percent strain decreased by about 10 percent for the marble at 2,000 bars, but increased slightly more than 11 percent at 1,000 bars. The limestone showed a small increase at 2,000 bars. Again, there was no significant change for the sandstone and siltstone at either confining pressure. The observed increases in strength at 10 percent strain reflect the increase in strain hardening in both marble and limestone that results with decreasing strain rate. The results for sandstone and siltstone, and for limestone or marble at lower pressures, indicate that cataclastic flow is independent of time. The strength after several percent strain of the cataclastically deformed rocks was not affected at rates slower than $$10^{-3}$$/second. Rocks deforming by cataclastic flow in natural environments therefore may be able to sustain, over indefinite periods, high differential stress determined solely by the existing temperatures and effective pressures. Moreover, strength decreases with decreasing strain rate in rock deforming by intracrystalline gliding may be sufficiently small at low temperatures that these rocks could also sustain significant differential stress at extremely slow strain rates. As a consequence, it is possible that appreciable shear stress exists for long periods of time in rocks in the upper parts of the earth's crust, and that the mean pressure in these rocks is significantly higher than that calculated from the weight of the overlying rocks.