Unloading deterioration and mechanism of rock under different loading and unloading stress paths

In order to study the failure mechanism of deep coal seam floor under unloading, pseudo-triaxial mechanical tests were carried out under different loading and unloading stress paths. The relationship between rock elastic modulus, generalized Poisson ratio and confining pressure under different stress paths was analyzed and fitted. The rock fracture mechanics model was constructed, and the stress concentration at the end of the branch crack under different loading and unloading paths was analyzed. The deterioration mechanism of rock mechanical parameters during unloading was analyzed from the aspects of deviatoric stress, energy and acoustic emission events. ① The research showed that the degree of deterioration of the elastic modulus of the sample and the change range of Poisson ratio under different axial compression loading methods were in the order of axial compression> axial compression remain unchanged> unloading axial pressure. Under the same axial pressure and different confining pressure unloading rates, the elastic modulus deterioration degree and Poisson ratio change of the sample were in a positive correlation with the unloading rate. ② Compared with the increase of the deviator stress at the unloading point, it changed in a positive correlation with the deterioration degree of the elastic modulus of the rock sample and the change of Poisson's ratio. During the unloading process of the confining pressure, the energy accumulated in the path that the axial load or remain unchanged was larger than the stress path of axial unloading. When the confining pressure was unloaded to a certain extent, the internal closed micro-cracks and primary fissures were reopened, and the energy used for crack propagation increased rapidly, and the ratio of change to elastic modulus and the Poisson’s ratio increased significantly, and the number of acoustic emission event ringing showed a characteristic of non-linear growth. ③ Under different loading and unloading paths, the deviatoric stress was the fundamental cause of rock instability and failure. The faster the deviatoric stress increased, the more obvious the degradation degree of rock elastic modulus and Poisson ratio changed. Under the action of loading and unloading, the deterioration degree of rock mass was the most serious under the influence of advanced abutment pressure, which was consistent with the fact that the water inrush accidents in the floor of working face mostly occur near the working face.