Deformation evolution and dissipated energy characteristicsof marble under pre-peak unloading conditions

During the rock unloading failure process,the deformation evolution is an important feature and the dissipation of accumulated energy in the rock is the essence of unloading failure. The deformation and energy evolution characteristics of marble are studied based on MTS815.3 testing system in terms of three unloading paths,three unloading rates and three unloading points. The results show that the unloaded damaged rock samples can be obtained when the volumetric strain is positive when the confining pressure is 0. The volumetric strain is divided into three stages during the unloading process:the stabilization phase,the slow reduction phase and significant expansion phase. The dilatancy angle is positively correlated with the unloading point during unloading process. The deformation modulus (generalized Poisson's ratio) decreases (increases) slowly first,then decreases (increases) rapidly in the test. The larger the unloading point,the larger the deformation modulus (generalized Poisson's ratio) is and the unloading paths have no significant effect on the deformation modulus and generalized Poisson's ratio. Dissipative energy is positively correlated with unloading point. The absorption energy and dissipation energy of different unloading paths decrease with the increase of the unloading rate,and eventually stabilize. The final absorption and dissipation energy are about 0.27 MJ/m³ and 0.16 MJ/m³. The failure mode is mainly controlled by the unloading rate. When the unloading rate is from low to high,the failure mode of the specimen changes from tensile shear failure to shear failure accompanied with spalling.Unloading points and unloading paths play a role in the degree of rock fracture. The research conclusions have a certain reference value for the excavation of deep caverns and the optimization of support schemes.