Dilatancy Characteristics and Crack Evolution of Deep-Buried Marble under Unloading Confining Pressure

The fracture process of unloading confining pressure in deep-buried rock is related to crack propagation. The triaxial unloading tests under different initial confining pressures of 25,50 and 80 MPa were carried out on the MTS815 Flex Test GT Rock Mechanics Test Platform for the 2400 m deep-buried marble of Jinping II. The deformation and dilatation characteristics and energy conversion law in the process of unloading confining pressure were analyzed. Based on this, the crack evolution law in the deformation and failure process of deep-buried marble under different initial confining pressure conditions was explored. The research results indicate as follows: The deformation and failure process of deep-buried marble under unloading confining pressure has phase characteristics. The crack initiation stress and dilatancy damage stress were determined by the transverse strain difference method and the maximum point of volumetric strain. Based on this, the volume deformation process is divided into stages. And the volumetric deformation process is divided into two stages: stable dilatancy and accelerated dilatancy. The initial confining pressure has an effect on dilatancy of marble, with increase of initial confining pressure, the dilatancy property decreases but the ability to bear dilatancy increases. The propagation of cracks is accompanied by dissipation of energy. In order to clarify the dilatancy failure mechanism of deep buried marble under unloading confining pressure, the crack evolution law in the process of deformation and failure was obtained by integrating the characteristics of plastic strain and energy dissipation. It is found thatinitial confining pressure controls the type and development of cracks. With increase of initial confining pressure, the cracks develop more fully and the proportion of sliding cracks increases, and the failure mode changes from tensile shear failure to shear failure. The ransverse crack strain rate POC₃ was proposed to characterize the crack evolution trend in the unloading confining pressure stage. It is pointed out that the turning point of POC₃ curve corresponds to the extension of the main crack in the rock, and most of them are about 60% of the unloading value of the limit confining pressure. According to this, prediction of unloading confining pressure failure of deep buried rock can be conducted.