Strength and damage failure characteristics of white sandstone under unloading confining pressure

In order to study the strength and deformation characteristics of white sandstone under unloading confining pressure,conventional triaxial tests and constant axial pressure and unloading confining pressure tests were carried out using RMT rock mechanics test system. The strength,deformation,stability time and damage characteristics of white sandstone were analyzed under unloading confining pressure. The main conclusions are as follows:under the same initial confining pressure,the yield stage decreases with the increase of unloading confining pressure rate,and the larger the initial confining pressure is,the more obvious the yield characteristic is. Under the same initial confining pressure,the axial deformation of white sandstone increases with the decrease of unloading confining pressure rate,which indicates that the brittleness decreases and the ductility increases and the internal damage becomes more serious during failure. With the decrease of unloading confining pressure rate,the confining pressure at failure increases,the confining pressure reduction decreases and the confining pressure ratio increases under the same initial confining pressure. The larger the initial confining pressure is,the longer the stability time of white sandstone is. However,with the increase of unloading rate,the stability time decreases rapidly. When the unloading confining pressure rate reaches 1 MPa / s,the influence of initial confining pressure on the stability time is very small. The reduction law of the unit confining pressure stability time is basically the same under different initial confining pressures,and it is little affected by the initial confining pressure. The stress path of unloading confining pressure weakens the cohesion of white sandstone, but strengthens the internal friction angle,the larger the unloading confining pressure rate is,the larger the internal friction angle is,the smaller the cohesion is. Based on Mogi-Coulomb strength criterion and octahedral shear stress,a general strength formula of white sandstone was obtained. In the process of unloading confining pressure,the number of acoustic emission ring is always at a low level,and there is no gradual increase process before failure,and the maximum ring counts occurs at the peak stress. The internal damage of rock sample develops slowly in the process of unloading confining pressure,and the rapid development of damage forms the main control fracture surface at the peak,and there is no gradual increase omen for acoustic emission,the failure suddenly occurs. The typical shear failure occurs in white sandstone with single fracture surface,and the fracture angle increases with the increase of unloading confining pressure rate.