Mechanical response and damage evolution mechanism of multi-layer combined coal mass under static and dynamic loading

Abstract

During underground excavation, structural instability and failure of coal-rock masses are major causes of engineering accidents. This study focuses on the coal mass ahead of the excavation face and divides into a multi-layer composite structure to systematically analyze the mechanical response characteristics of the combined coal mass under static and dynamic loading. Drop hammer impact tests were conducted to investigate the crack propagation and failure morphology of the combined coal mass. The results show that, under static loading, single-layer coal mainly exhibited X-shaped shear failure, whereas the combined coal mass showed V-shaped layered spalling failure. Under dynamic loading, compressive failure dominated in the central region, while shear failure mainly occurred at the four corners, and the damage was more pronounced along the minor axis direction. Two crack patterns were observed on the coal surface, simple radial cracks and complex intersecting radial and circumferential cracks. Under the combined effects of rectangular geometry and impact loading, the radial cracks of the specimen show a structural tendency to preferentially propagate along the minor axis direction. Under impact loading, the combined coal mass exhibited an overall spindle-shaped failure morphology in the axial direction. In the damage zone, the superposition of tensile stress and energy accumulation made the coal mass more susceptible to failure, resulting in a greater number of radial and circumferential cracks. These findings provide a scientific theoretical basis for revealing the incubation mechanisms of coal-rock dynamic disasters and optimizing disaster prevention and control measures under mining-induced disturbance conditions.