Compaction, dilatancy, and failure in porous carbonate rocks

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.