[1] The analysis of dilatant and compactant failure in many sedimentary and geotechnical settings hinges upon a fundamental understanding of inelastic behavior and failure mode of porous carbonate rocks. In this study we acquire new mechanical data on the Indiana and Tavel limestones, which show that the phenomenology of dilatant and compactant failure in these carbonate rocks is similar to that of the more compact Solnhofen limestone as well as sandstones. Compressibility and porosity are positively correlated. Brittle strength decreases with increasing porosity and the critical stresses for the onset of pore collapse under hydrostatic and nonhydrostatic loadings also decrease with increasing porosity. Previously, two micromechanical models were used to interpret mechanical behavior of Solnhofen limestone: viewing cataclasis and crystal plasticity as two end-members of inelastic deformation mechanisms, the wing crack and plastic pore collapse models were applied to brittle and ductile failure, respectively. Synthesizing published data for carbonate rocks with porosities between 3% and 45%, we investigate to what extent the same micromechanisms may be active at higher porosity. Application of the plastic pore collapse model indicated that crystal plasticity cannot be the only deformation mechanism. To arrive at a more realistic interpretation of shear-enhanced compaction in porous carbonate rocks cataclastic processes must be taken into account. We infer that mechanical twinning dominates in the more porous limestones and chalk, while dislocation slip is activated in the more compact limestones.

[1] We studied the mechanics of compactant failure in four sandstones associated with a broad range of failure modes in the brittle-ductile transition. While Berea and Bentheim sandstones can fail by compaction localization, homogeneous cataclastic flow dominates failure modes in Adamswiller and Darley Dale sandstones at high effective pressures. We acquired new experimental data to complement previous studies, focusing on the strain hardening behavior in samples under drained conditions. The initial yield stresses were identified as the critical stresses at the onset of shear-enhanced compaction, subsequent yield stresses were considered to depend on hardening given by plastic volumetric strain. The yield stresses were described by elliptical yield caps in the stress space, and we compared the cap evolution with two constitutive models: the critical state model and the cap model. Bentheim sandstone showed the best agreement with both models to relatively large strains. Darley Dale sandstone showed the best agreement with the associated flow rule as prescribed by the normality condition, which is implicitly assumed in both constitutive models. Shear-enhanced compaction in Bentheim and Berea sandstones was appreciably more than that predicted for an associative flow rule, with the implication that a nonassociative model is necessary for capturing the inelastic and failure behavior of these sandstones over a broad range of effective pressures. With reference to the nonassociative model formulated by Rudnicki and Rice, bifurcation analysis would predict the transition of failure mode from shear band to compaction band and ultimately to cataclastic flow, in qualitative agreement with the experimental observations.Citation: Baud, P., V. Vajdova, and T. Wong (2006), Shear-enhanced compaction and strain localization: Inelastic deformation and constitutive modeling of four porous sandstones,