Failure behaviors of fractured granite during loading and unloading under high confining pressure based on acoustic emission multi-parameter analysis

Abstract

To investigate failure and mechanical behaviors of fractured rocks during loading and unloading under conditions of high ground stress, a series of loading and unloading tests were conducted on granite containing prefabricated fractures under high confining pressure. Results indicate that the evolution of acoustic emission (AE) events of fractured granite during loading and unloading under high confining pressure is strongly correlated with the deformation and failure and the energy evolution process in the rock, the peak values of AE cumulative energy and AE cumulative count rate in the pre-peak deformation stage decrease first and then increase with the dip angle (the minimum value is reached at 60°), when the dip angle is 0° to 60°, the number of AE events increases abruptly due to closure of the prefabricated fracture in the early stage of loading(only about 30 % of the main fracture time). After the fractured granite enters the unstable crack propagation stage, the crack tips are subject to a dynamic cycle of stress concentration, crack propagation, and stress release, accompanied by significant fluctuations in the energy rate before and after the point of peak AE energy rate. Therefore, the cracks propagate in an intermittent or a stepped manner. Under the unloading path, the process of crack development, gathering, and coalescence in the granite happens earlier, and there are more high-energy AE events in the failure process. The value of the brittleness index Bd is larger under the unloading path under high confining pressure, which reflects more significant brittle failure characteristics. The research provides a basis for judging failure precursors of rocks using AE characteristics in the failure process.