Experimental Study on Damage Characteristics of Coal Samples under True Triaxial Loading and Dynamic Unloading

Abstract

Abstract Coal bursts are one of the formidable hazards in underground coal mines, yet it is still not fully explored due to the complex stress environment that exists during mining. To better understand the bursting mechanism of coal under real-time mining conditions, it is necessary to develop an experimental method capable of reproducing in situ stress and loading-unloading paths of coal in in situ conditions. In this study, a self-developed true triaxial testing system was used to investigate the damage and failure characteristics of coal samples under true triaxial loading and dynamic unloading conditions. Acoustic Emission (AE) monitoring was used to capture the fracturing of the loaded coal. Passive Velocity Tomography (PVT) and Cumulative AE Energy Density (CAEED) were used to analyse damage evolution characteristics of the coal samples under true triaxial loading conditions. A high-speed camera was used to record the failure of the coal samples when the minimum principal stress σ3 was suddenly unloaded. It was found that continuous coal damage occurred primarily during the true triaxial loading period. The peak number of AE events in the coal samples increased and then dropped as σ1 levels increased. High and low wave velocity zones in the coal samples represent regenerations of the high-density zone and fracture emergence, respectively. Significant energy release zones transferred and expanded as the triaxial loading level increased. Under significant triaxial loads, the coal samples failed and were severely damaged, and the dynamic unloading of σ3 caused ejections of coal fragments at low velocities. The outcome of this study provides in-depth understanding of the failure mechanism of coal under in situ conditions.