Failure mode and weakening effect of water on sandstone

Previous studies have shown that brittle strength of a rock is generally re duced in the presence of water. However, for siliciclastic rocks, there is a paucity of data on the water-weakening behavior in the cataclastic now re gime. To compare the weakening effect of water in the brittle faulting and cataclastic flow regime, triaxial compression experiments were conducted on the Berea, Boise, Darley Dale, and Gosford sandstones (with nominal porosi ties ranging from 11% to 35%) under nominally dry and saturated conditions at room temperature. Inelastic behavior and failure mode of the nominal dry samples were qualitatively similar to those of water-saturated samples. At elevated pressures, shear localization was inhibited, and all the samples failed by strain hardening. The compactive yield strengths (associated with the onset of shear-enhanced compaction) in the saturated samples were lowe r than those in the dry samples deformed under comparable pressure conditio ns by 20% to 70%. The reductions of brittle strength in the presence of wat er ranged from 5% to 17%. The water-weakening effects were most and least s ignificant in the Gosford and Berea sandstones, respectively. The relation between water weakening and failure mode is consistently explained by micro mechanical models formulated on the basis that the specific surface energy in the presence of water is lowered than that in vacuum by the ratio lambda . In accordance with the Hertzian fracture model the initial yield stress i n the compactive cataclastic flow regime scales with the grain-clushing pre ssure, which is proportional to lambda(3/2). In the brittle faulting regime damage mechanics models predict that the uniaxial compressive strength sca les with lambda(1/2). In the presence of water the confined brittle strengt h is lower due to reductions of both the specific surface energy and fricti on coefficient.