Experimental and numerical study on mechanical behavior of rock beam fracture under unloading with different thicknesses and spans in deep mining working face

Abstract

The stability of the roof in coal mining is crucial for ensuring safe extraction. Studying the mechanical behavior of rock beams under various conditions is essential for improving coal mining safety. However, research on the dynamic response of rock beams under sudden unloading remains limited. This study utilized a self-developed bidirectional loading and unilateral unloading test system to simulate how sudden lower strata subsidence induces the fracture of upper hard rock beams. Bottom unloading experiments were performed on rock beams with varying thicknesses and spans. The experiments recorded surface crack development and internal damage evolution using high-speed photography and acoustic emission monitoring. The results show that rock beams experience multiple stress reductions after unloading, with the largest reduction occurring in the first stage. Flexural deformation was observed, becoming more pronounced as the thickness-span ratio decreased. Greater thickness increased shear cracks and crack expansion angles, while larger spans promoted tensile cracks, arched crack formation, and notable rock spalling. Acoustic emission analysis showed that signal count and energy increased with thickness and span. Finally, discrete element numerical simulations revealed the critical controlling role of harder rock strata in rock beam failure: when the harder strata are at the top, cracks are sharp, and shear failure is more likely; when they are at the bottom, the overall failure range expands, and cracks tend to form arches. These findings improve the understanding of dynamic rock beam fracture under sudden unloading and offer theoretical guidance for roof stability control in deep mining.

1. Introduction

Coal is the primary energy source in China and will continue to dominate the country’s primary energy structure for the foreseeable future. With large-scale coal mining, shallow resources are being gradually depleted, and coal mines are increasingly transitioning to deeper mining operations [1 −3]. After mining a working face, the inability of the overlying hard rock strata to collapse results in large exposed areas and significant delamination spaces, leading to stress concentration in the mining area. Sudden fractures and instability in the suspended rock strata can easily trigger significant deformation phenomena, potentially causing mine quakes and rock bursts [4, 5] Therefore, studying the evolution of fractures and the instability characteristics of overlying rock strata is crucial for controlling overburden strata and preventing mining hazards [6, 7].

The fracture and movement patterns of the roof in mining areas are major factors in controlling dynamic disasters such as rock bursts. Scholars at home and abroad have conducted extensive research in this field [8, 9]. Theoretically, various theories have been proposed, including cantilever beam theory, pressure arch theory, masonry beam theory, and key stratum theory [10 −13]. SUN et al [14], based on key stratum theory and overburden mechanical analysis, proposed a new analytical solution—an asymptotic hyperbolic subsidence model (AHSM), to describe the movement and failure within overburden strata. ZHANG et al [15], based on elastic thin-plate theory, developed a mechanical model for the movement of direct roof strata in the working face, revealing the influence of size variations in suspended structures on roof movement and its fracture mechanics mechanism. Many scholars have studied the fracture characteristics of mining area roofs through field monitoring [16 −18]. MONDAL et al [19] used various monitoring instruments to calculate the fractal dimension, Dc, for monitoring the stress level and crack conditions in overburden strata, enabling spatiotemporal predictions of roof collapse. DENG et al [20] revealed the large and small periodic patterns of hard roof fractures through the temporal distribution of microseismic events and hydraulic support pressure monitoring.