Mechanical and acoustic emission characteristics of rock: Effect of loading and unloading confining pressure at the postpeak stage

The disturbance caused by excavation to surrounding rock structures during underground mining under complex geological conditions at great depth induces rock failure due to the effect of stress concentration and unloading. Therefore, the design, construction and maintenance of structures (i.e., pillars, drifts, chambers) are based not only on peak strength, but also on postpeak behavior and, subsequently, residual strength of rocks. Therefore, better understanding the characteristic features of rock at the postpeak stage is of paramount importance for underground construction and ground support design activities. Argillaceous limestone specimens loaded to the residual phase in the conventional triaxial compression test under different initial confining pressures were continuously loaded by loading and unloading cycles. Analysis of the variation characteristics of stress and strain were complemented with acoustic emission (AE) monitoring for investigating the effects of the confining pressure and the loading and unloading cycles on the mechanical properties of the rock in the residual phase. Besides, the yielding premonition information about the rock in the loading confining pressure stage was analyzed and calculated. The results show that because the deformation during the axial loading mainly comes from the sliding between the fracture planes, the bearing capacity is related to not only the confining pressure, but also the unloading path; the elastic modulus in the axial elastic deformation stage is larger than the corresponding elastic modulus of the prepeak stage, and the axial deformation presents good feature of elastic-plastic deformation; the bearing capacity of rock in the residual phase was provided by the frictional sliding between the elastic rock block under confining pressure. The precursor information about the rock yielding to plastic deformation in the loading confining pressure test of the failure rock could be confirmed based on the evolution characteristics of the AE amplitude.