Critical state plasticity theory applied to deformation of porous sandstone

A need exists for a sound constitutive model that would fully describe deformation and stability of porous rock. A plasticity-based constitutive model, CSP, has proven successful in describing deformation of unconsolidated sediments and therefore represents a logical starting point towards developing a model for lithified sediments. In order to test this model, a series of deformation experiments was performed on Massillon sandstone, a quartzofeldspathic Permian sandstone with a measured porosity of 22 percent. This material was chosen because its high porosity would allow it to exhibit a wide range of deformational behavior under the stress states allowable by the apparatus used. Data taken during deformation include axial and radial stresses, axial and volume strains, and rate of acoustic emission (AE).; The CSP model provides a theoretically-based explanation for the well-documented 'brittle-ductile' transition of rock in crustal environments. Thin-section observations are used with experimental data to examine deformation mechanisms, such as grain crushing, and how they relate to stability of failure. Specimens exhibiting dilatant volume strain are shown to fail in an unstable manner, with a single discrete fracture being the resulting deformation-induced structure.; The AE data were used to determine initial yielding, and the initial yield surface, while convex outward as the model predicts, does not have the elliptical shape that the simplest form of CSP would suggest. Constitutive anisotropy is identified and accounted for in determining plastic strain increment data. In a general sense, the model accurately predicts the nature of volume strain increments at yield. Experimental results also show that the yield surface does expand outward with compactive volume strain, and that the critical state line exists and appears to have a linear form. The ratio of incremental shear to incremental volume strain is used with initial yield data to suggest that the normality condition is appropriate to at least a small degree.