Experimental Study of True Triaxial Loading Effect on Ultrasonic Properties of Rock Salt

Because of low permeability and porosity as well as rock salt healing, salt domes are considered suitable sites for the storage of hydrocarbon, natural gas, compressed air, and nuclear waste. Applying high deviatoric stresses on rock salt leads to initiation and propagation of microcracks, and results in an increase of the permeability and reduction of the velocity of the ultrasonic wave in it. Therefore, one of the methods to determine the damage degree in rock salt is to measure ultrasonic wave velocities. In this study, initially, the degree of anisotropy and inhomogeneity of rock salt samples are investigated using the measurement of ultrasonic wave propagation velocities in three directions along the sides of the cubic samples. Then, the anisotropy degree of rock salt samples is reevaluated after true triaxial compression tests under different loading conditions. The results show that the most decrease in ultrasonic wave velocities is along the minimum principal stress, indicating that the dominant extension of micro cracks is perpendicular to the minimum principal stress direction. Also, enforcing deviatoric stresses on isotropic rock salt samples has led to anisotropy in rock samples. Therefore, in the design of storage caverns in salt domes, it should be noted that despite the basic assumption of the isotropic host rock, the deviatoric loading caused by the construction and operation of the cavern may lead to anisotropy in host rock.