Failure characteristics of two porous sandstones subjected to true triaxial stresses

We conducted an extensive suite of true triaxial experiments in two porous sandstones, Bentheim (porosity approximate to 24%) and Coconino (17.5%). Our experiments demonstrate that failure of both sandstones is not only a function of sigma(3) but also of sigma(2). For a given sigma(3), sigma(1) at failure (sigma(1,peak)) increases as sigma(2) is raised above sigma(3) between tests. The sigma(1,peak) reaches a peak as sigma(2) is about halfway between sigma(3) and sigma(1) and then gradually decreases such that when sigma(2)approximate to sigma(1,peak), it approaches its initial magnitude when sigma(2)=sigma(3). For a constant sigma(3), failure-plane angle increases with sigma(2) by a maximum of less than 10 degrees as sigma(2) rises from sigma(2)=sigma(3) to sigma(2)=sigma(1,peak). The effect of sigma(2) on both failure level and failure-plane angle is stronger in the lower-porosity Coconino sandstone than in the Bentheim sandstone. The sigma(2) dependence of failure mode in the Bentheim is different than Coconino over the same sigma(3) range. Both sandstones failed dilatantly at low sigma(3) magnitudes. However, at high sigma(3) (100-120MPa), Bentheim sandstone developed shear-enhanced compaction bands, followed by pure compaction bands at sigma(3)=150MPa. Compaction bands were not observed in the Coconino. Microscopic observations via SEM reveal that tensile microcracking is dominant when shear banding occurs (under low sigma(3)), while pervasive grain crushing and pore collapse inside compaction bands are observed at high sigma(3).