Effects of Complex Triaxial Unloading Confining Stress Conditions on the Mechanical Behaviour and Fracture Mechanism of Sandstone

The effect of complex triaxial unloading confining pressure conditions on the mechanical behaviour and fracture mechanism of sandstone is investigated. Constant deviatoric stress direct and cyclic unloading confining pressure tests are carried out under two initial damage states. Based on the evolution law of deformation parameters, the influence of the initial damage and unloading rate on the mechanical properties of sandstone samples is expounded. The mechanism of tensile fracture development caused by the unloading of the confining pressure is proposed, and the engineering disasters that may be induced by this mechanism are discussed. The results show that the cyclic unloading confining pressure tests can replace direct unloading pressure tests to study the evolution process of the mechanical parameters of samples. The unloading confining pressure test under low damage conditions is more conducive to the study of unloading confining pressure failure. The unloading confining pressure produces obvious radial expansion and tensile cracks in the sample. With decreasing confining pressure, the bulk modulus of the samples under the two damage conditions gradually decreases. Under low damage conditions, the dilatancy angle decreases first and then increases, while under high damage conditions, it gradually increases. The unloading rate does not affect tensile crack development but does affect compression-shear crack development, resulting in different strain responses of the two damaged specimens. The generation mechanism of macroscopic tensile cracks is as follows: the residual stress generated by the unloading of the confining pressure causes random tensile microcracks to nucleate, and the increase in microcrack density provides power for its development. The airfoil cracks gradually bend in the direction of the first principal stress, and the microcracks develop directionally and interconnect with each other to form macroscopic axial tensile cracks. The unloading of confining pressure produces tensile cracks perpendicular to the unloading direction, which may induce rock bursts and landslide disasters. The research results provide an important scientific basis for underground rock engineering construction and slope engineering prevention. The unloading rate does not affect tensile crack development but does affect compression-shear crack development.The residual stress generated by the unloading of the confining pressure causes random tensile microcracks to nucleate, and the increase in microcrack density provides power for its development.The unloading of confining pressure produces tensile cracks perpendicular to the unloading direction, which may induce rock bursts and landslide disasters.