Effect of confining pressures on transverse isotropy of Maha Sarakham salt

Abstract

Uniaxial and triaxial compression tests have been performed on prismatic specimens of rock salt under confining pressures (σ3) up to 40 MPa. The specimens contain different bedding plane orientations with respect to the major axis. Results indicate that transverse isotropic effects occur under low confinement where the salt strength is lowest when the normal to bedding planes makes an angle (β) of 60° with the major principal axis. The lowest intrinsic elastic modulus is obtained at β = 0°, and the highest is at β = 90°. The apparent elastic parameters measured for 0° < β < 90° agree well with those predicted by Amadei's solutions. The confining pressures rapidly increase the elastic and shear moduli normal to bedding plane strike, and eventually they reach those parallel to the bedding planes under σ3 about 30 MPa. Loading the salt under high confinement induces some plastic deformation, as evidenced by the evolution of Ramberg and Osgood's parameters with the confining pressures. The confining pressures gradually tighten the inter-crystalline boundaries along bedding planes, and hence the applied differential stress can no longer recognize their transverse isotropic textures. The salt strengths for all bedding orientations become equal under σ3 above 30 MPa. The proposed exponential equation is capable of describing the salt dilations and strengths under unconfined to highly confined conditions. Distortional strain energy (Wd) induced at dilation linearly increases with mean strain energy (Wm). The Wd-Wm relation for β = 60° can represent the dilation of the salt from low to high confinements where the mechanical responses of the rock transitionally change from transverse isotropic to isotropic behavior. The Wd-Wm relations for β ≠ 60° are applicable only for confining pressures less than 30 MPa.