Effects of loading rate on transverse isotropic responses of bedded gypsum

The objective of this study is to determine the time-dependent compressive strength and elastic properties of transverse isotropic gypsum. The results are needed for the stability analyses and design of mine slopes during excavation and after decommissioning. Over one hundred prismatic specimens of micro-crystalline gypsum with different bedding plane orientations have been subjected to uniaxial and triaxial compression tests with confining pressures up to 20 MPa. The major principal stresses are applied under constant rates (∂σ1/∂t) of 0.001, 0.01, 0.1, and 1.0 MPa/s. Results indicate that transverse isotropic effect occurs under all confinements and loading rates where the rock strengths are lowest when the normal to bedding planes make an angle (β) of 60° with the major principal axis. The lowest elastic and shear moduli are obtained at β = 0°, and the highest are at β = 90°. Increasing the confining pressures and decreasing the loading rates significantly reduce the degree of anisotropy. Slow loading under high confinement reduces the gypsum pore spaces, dislocates the crystals, and gradually tightens the inter-crystalline boundaries along the bedding planes. Coulomb criterion is capable of describing the rock compressive strengths where the cohesion is defined as a polynomial function of angle β and ∂σ1/∂t. The energy criterion suggests that under such low loading rate, the elastic and shear moduli are insensitive to the confining pressure.