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