Onset of Pore Collapse and Dilatancy in Porous Sandstone Under True Triaxial Compression: Experimental Observation and Micromechanical Modeling 

In many geotechnical and tectonic settings, a fundamental understanding of the inelastic behavior of porous rocks under polyaxial compression is necessary. In this study, we present new true triaxial compression data obtained in the ductile regime on Bleurswiller sandstone with the size of 100mm X 50mm X 50mm. The deformed samples show a range of failure modes qualitatively similar to what was reported by earlier experimental studies performed in conventional conditions (axisymmetric compression). In particular, visual inspection and X-ray Computed Tomography imaging reveal compaction localization in all our deformed samples. The pore collapse model of Zhu et al. (2010) is extended to include the role of the intermediate principal stress and our new data for the onset of shear-enhanced compaction are in basic agreement with this extended model that includes three stress invariants. At constant minimum principal stress, the onset of shear-enhanced compaction tends to decrease slightly with increasing intermediate principal stress.Published true triaxial data obtained in the brittle regime highlights the impact of the intermediate principal stress on the onset of dilatancy. The predictions of the conventional sliding wing crack model extended to true triaxial conditions are in poor agreement with these data. Our analysis suggests that the observed discrepancies are related to the influence of the intermediate principal stress on the effective shear stress on the wing cracks. Another energetic approach pioneered by Wiebols & Cook (1968) shows a better agreement with the experimental results, and predicts that at constant minimum principal stress, the onset of dilatancy would not be a monotonic function of the intermediate principal stress. Our new data and analysis will help the interpretation of inelastic deformation under polyaxial compression in various geotechnical and tectonic settings.