A multi-axial strength criterion is developed to describe the distortional strain energy density of rock salt at failure as a function of the mean strain energy. The temperature effect on salt strength is implicitly considered by incorporating empirical relations between the elastic parameters and temperatures of the tested specimens. The proposed criterion agrees well with the test results obtained under temperatures ranging from 273-467 K. The proposed criterion is useful and practical for a conservative determination of the stability of compressed-air or gas storage caverns where the surrounding salt is subject to fluctuations of temperature during product injection and withdrawal periods.
Uniaxial and triaxial compressive strength tests have been performed using a polyaxial load frame to assess the influence of loading rate on the strength and deformability of three Thai sandstones. The applied axial stresses are controlled at constant rates of 0.001, 0.01, 0.1, 1.0 and 10 MPa/s. The confining pressures are maintained constant at 0, 3, 7 and 12 MPa. The sandstone strengths and elastic moduli tend to increase exponentially with the loading rates. The effects seem to be independent of the confining pressures. An empirical loading rate dependent formulation of both deformability and shear strength is developed for the elastic and isotropic rocks. It is based on the assumption of constant distortional strain energy of the rock at failure under a given mean normal stress. The proposed multiaxial criterion well describes the sandstone strengths within the range of the loading rates used here. It seems reasonable that the derived loading rate dependent equations for deformability and shear strength are transferable to similar brittle isotropic intact rocks.
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