Volumetric strain of rock salt under triaxial low-frequency cyclic loading

The surrounding rock salt of underground gas storage cavern is subjected to long-term triaxial fatigue stress. The deformation property of rock salt under triaxial cyclic loading is of great importance to the stability of the rock salt cavity. A series of laboratory tests are performed to explore the deformation behavior of eight rock salt specimens under various confining pressures, stress levels and loading frequencies. The axial low-frequency cyclic stress is applied on each salt specimen while the confining pressure is kept steady. The test parameters are processed by the dimensionless method. The effects of the strength of stress ratio (ratio of generalized shear strength to spherical stress), amplitude of stress ratio, level of the maximum stress, loading frequency and loading cycles (N) on the volumetric strain (ε_v) of rock salt are analyzed. The nonlinear curve fitting is carried out using the function ε_v =λlgN+ε_v0 for a curve of each specimen on the volumetric strain with the cycles. Further, the expressions for the parameters λ and ε_v0 with the amplitude of stress ratio, the maximum stress and loading frequency are obtained, respectively. The analysis of standardization regression coefficient indicates that the maximum stress is the key factor that influences the volumetric deformation of rock salt, and the stress amplitude ratio comes the second. It is noted that the rock salt exhibits dilatancy under high maximum stress.