Failure characteristics of two porous sandstones subjected to true triaxial testing

True triaxial tests have been carried out on two porous sandstones, Coconino (n=17.5%) and Bentheim (n=24%) to investigate the effect of the intermediate principal stress (sigma(2)) on compressive strength, failure-plane angle and failure mode. These rocks were selected because they are similar to sandstones forming hydrocarbon reservoirs, as well as to those being considered for CO2 sequestration. In both rocks the increase in strength as sigma(2) rose above a given sigma(3) was found to be similar to that established in crystalline rocks, but the maximum rise in strength was considerably smaller. Failure mode in Coconino sandstone was in the form of a steeply inclined shear band, or fault, at lower sigma(3), with the slope becoming gentler as sigma(3) increased, culminating in multiple parallel and conjugate bands at the highest sigma(3) attempted (=150MPa). Bentheim sandstone failure mode experienced a similar evolution in a narrower range (sigma(3) between 0 to 80MPa), but as loading continued, failure was in the form of shear-enhanced compaction bands with angle dropping to similar to 45 degrees. and lower, and reaching 0 degrees at sigma(3) = 150MPa (creating pure compaction bands). Failure-plane angle in both rocks increased by up to 15 degrees as sigma(2) was raised above a given sigma(3).