Failure characteristics of two porous sandstones subjected to true triaxial stresses: Applied through a novel loading path

We performed an extensive suite of true triaxial experiments in two porous sandstones, Bentheim (porosity approximate to 24%) and Coconino (17%). The experiments were conducted using a novel loading path, which maintains constant Lode angle () throughout the test. This path enabled the examination of the effects of Lode angle and mean stress on failure (sigma(oct,f)). Our tests covered sigma(3) magnitudes between 0 and 150MPa and of at -30 degrees (axisymmetric extension), -16 degrees, 0 degrees, +11 degrees, +21 degrees, and +30 degrees (axisymmetric compression). Test results revealed the respective contribution of each of the two stress invariants to failure stress, failure plane angle, and failure mode. In both sandstones, the shear stress required for failure increases with mean stress but decreases with when shear failure mode dominates. However, the dependence of failure stress on mean stress and is reversed when the compactive failure mode is in control. The compactive failure mode was evident in Bentheim sandstone when compaction bands were observed under high mean stress. The Coconino sandstone did not reach the compactive failure regime within the maximum confinement applied. The failure plane angle monotonically decreases with increasing mean stress and . For Coconino sandstone, failure plane angle varies between 80 degrees and 50 degrees for sigma(oct,f) between 50 and 450MPa whereas it drops to 0 degrees as sigma(oct,f), approaches 250MPa in Bentheim sandstone. We employed the bifurcation theory to relate the stress conditions at failure to the development of failure planes. The theory is in qualitative agreement with the experimental data.