Compression tests have been performed on cylindrical specimens of bedded gypsum under confining pressures up to 15 MPa. The specimens contain different bedding plane orientations. Results indicate that transverse isotropic effect occurs under all confinements where the strength is lowest when the normal to bedding planes makes an angle (β) of 60° with the core axis. The lowest modulus is obtained at β = 0°, and the highest is at β = 90°. Confining pressures rapidly increase the elastic and shear moduli normal to bedding plane strike, toward those parallel to the beds. Loading gypsum under high confinement may induce plastic deformation by dislocation climb mechanism, which gradually tightens the microcracks and pore spaces along bedding planes. Coulomb criterion is capable of describing the rock compressive strengths where the cohesion is defined as a polynomial function of β. Distortional strain energy induced at failure coincides with the results obtained from Coulomb criterion.
