Effects of loading rate on strength and deformability of Maha Sarakham salt

Uniaxial and triaxial compression tests have been performed to assess the influence of loading rate on the compressive strength and deformability of the Maha Sarakham salt. The salt specimens with a nominal dimension of 5.4 x 5.4 x 5.4 cm(3) are compressed to failure using a polyaxial load frame. The lateral confining pressures are maintained constant at 0, 3, 7, 12,20 and 28 MPa while the axial stresses are increased at constant rates of 0.001, 0.01, 0.1, 1.0 and 10 MPa/s until failure occurs. The salt elasticity and strength increase with the loading rates. The elastic (tangent) modulus determined at about 40% of the failure stress varies from 15 to 25 GPa, and the Poisson's ratio from 0.23 to 0.43. The elastic parameters tend to be independent of the confining pressures. The strains induced at failure decrease as the loading rate increases. Various multiaxial formulations of loading rate dependent strength and deformability are derived. The variation of the octahedral shear stresses and strains induced at dilation and at failure with the applied shear stress rates can be best described by power relations. The distortional strain energy at dilation and at failure from various loading rates varies linearly with the mean normal stress. The proposed empirical criteria are applied to determine the safe maximum withdrawal rate of a compressed-air energy storage cavern in the Maha Sarakham salt formation. The strain energy criterion that considers both distortional and mean stress-strains at dilation tends to give the most conservative results. (C) 2012 Elsevier B.V. All rights reserved.