Microcracking Process Characterization and Failure Time Prediction of Three Typical Rocks upon Uniaxial Compression Based on Acoustic Emission Activity

Mechanical stability of pillars is crucial for the safety of underground engineering. Understanding the failure process of rock pillars upon uniaxial compression may help predict its failure. This study investigates the failure process of sandstone, coal and granite under uniaxial compression using acoustic emission (AE) monitoring. A developed AE method, i.e., microcrack cumulative summation curve method, was applied to identify whether tensile or shear microcracks dominated during rock deformation to failure. This method was validated through Brazilian and direct shear tests. Our results showed that a) sandstone samples experienced shear faulting failure with AE analysis indicating an initial dominance of tensile microcracks, followed by shear microcracks, likely due to grains movements and intra- and inter-granular microcrack development; b) granite samples exhibited tensile failure, with transgranular microcracks developing along the axial compression direction, intense wing microcracks, and a potential transition from tensile to shear microcracking dominating process as indicated by AE analysis; c) coal samples displayed mixed shear-tensile failure, possibly dominated by the orientation of calcite veins where microcracks initiated, developed and coalesced, as suggested by AE and microstructure analysis. Using the power-law singularity theory for failure warning, our analysis demonstrated that lateral strain and AE count can accurately predict failure times for sandstone (15 similar to 22% early) and granite (8 similar to 12% early) samples. However, this method is less effective for coal samples due to their discontinuous failure processes. Our research highlights that combining microcrack development with failure prediction analyses effectively reveals physical failure processes in typical rocks, offering practical application for mining engineering.