Strength and deformability of a low-porosity sandstone under true triaxial compression conditions

Abstract

True triaxial tests (σ1≥σ2≥σ3) were conducted on the Zigong sandstone with a relatively low porosity of ~6.5% with the aim of assessing the influence of the intermediate principal stress σ2 on the strength and deformability. Three series of true triaxial tests (σ3= 0, 20, and 60 MPa) were performed. Within each series, σ2 was planned to be varied from σ2=σ3 to σ2=σ1 from test to test. For each test, σ1 was raised monotonically to failure while σ2 and σ3 were kept constant and the post-peak behavior has been captured. It is found that the strain in the σ3-direction is always larger than that in the σ2-direction under true triaxial stresses, which is attributed to the stress-induced tensile microcracks aligned with the σ1-σ2 plane. For a constant σ3, the dilatant strain in the σ2-direction is progressively suppressed and becomes compactive as σ2 raises from σ2=σ3 to σ2=σ1. Based on a modified Hoek-Brown failure criterion with three stress invariants, the best fitting brittle strength envelope reveals that the σ2 effect on rock strength, i.e., the ascending-then-descending trend for a given σ3, is the combined effect of mean stress and Lode angle. In the series of biaxial tests (σ3=0 MPa), macro tensile fractures that parallel to the σ1-σ2 plane develop regardless of σ2, displaying a failure plane angle of approximately 90°. As σ3 is raised, failure plane angle generally decreases. When σ3 = 20 MPa, the failure plane angle generally increases with σ2 while there is no evident dependence on σ2 when σ3 = 60 MPa. Comparisons with the other two sandstones with higher porosity reveal that porosity exerts critical control on the brittle strength and that the σ2 effect on deformability is also the result of two competing effects induced by mean stress and Lode angle. It is also found that the residual strength data facilitate the prediction of brittle-ductile transition pressure when σ2=σ3 in spite of relatively low σ3 level, which matches well with the empirical relation proposed by Mogi.1