Uniaxial and controlled-lateral strain tests on selected sedimentary rocks

Abstract

Cylindrical specimens of several sandstones, carbonates, and quartz-rich siltstones, exhibiting a wide range of porosities, are deformed in a computer-controlled hydraulic loading apparatus. They are loaded with axial stresses up to 400 MPa under conditions of uniaxial strain or limited lateral expansion. The resulting differential stresses lead to partially inelastic behavior arising from cataclastic processes and intragranular flow. Deviations from elasticity depend strongly on composition, cementation, and porosity. Rocks with a strong structural framework show the least deviation. No macrofracturing occurs in the uniaxial-strain tests. Curves of axial versus radial stress are always convex during loading, but during unloading the curves for brittle rocks remain nearly linear. The curvature increases with increasing porosity and decreasing cementation in sandstones as well as with increasing clay content in siltstones. Texture is important in limestones; plots for fine-grained limestones have less curvature than those for coarse-grained limestones of similar porosity where twinning is common. When a rock in uniaxial strain begins to compact significantly, its stress path seems to approach the envelope of ultimate strength defined by tests at constant confining pressure. Stress-strain curves for uniaxial-strain tests deviate most from linearity for the highly porous rocks. Hysteresis in the curves between loading and unloading increases with increasing porosity and decreasing cementation, and is largest when intragranular flow is active. Compaction is greatest in highly porous limestones where structural breakdown and twinning play a role. Such limestones approach the ductile state with apparent Poisson's ratios near 0.5. The Poisson's ratios remain low for sandstone and dolomite, with lowest ratios for sandstones of intermediate porosities. The ratio of radial to axial stress is less than 0.3 for sandstones and dolomite with their strong structural frameworks, but it nears 0.6 for porous limestones and clay-rich siltstones. The loading path for the controlled-radial strain tests requires the change in radial strain to be directly proportional to the change in confining pressure...