Study on fracture and stress variation characteristics of overlying rock with weakly consolidated gravel layer

The western Inner Mongolia region is characterized by widespread weakly cemented soft rock strata, where the presence of thick overlying gravel layers exerts a significant influence on roof failure and stress distribution. Understanding the fracture behavior and stress distribution characteristics of weakly cemented gravel-bearing layers is critical for assessing roof stability and determining the extent of pressure release zones in this region. Using Hongqingliang Coal Mine as a case study, this research integrates theoretical analysis, laboratory experiments, numerical simulations, and field monitoring, focuses on the lithological and spatial distribution features of 3-1 coal overburden, and the study investigates the fracture mechanisms and modes of the overlying roof in weakly cemented gravel-bearing strata. The evolution of roof fractures and associated stress variations are analyzed with engineering validation through micro-seismic monitoring. The findings reveal that the roof fracture process in weakly cemented gravel-bearing strata occurs in four distinct stages: caving zone subsidence, stratified fracture of the gravel, overall roof failure, and roof bending-sagging. The strain response in the gravel layer follows a distinct “slow increase - rapid increase - sharp decrease-stabilization” pattern, with failure progressing of “stratified fracture, rotation, and compression into hinged rock blocks”. The gravel layer plays a pivotal role in controlling roof failure, with the overlying strata bending and sinking in a coordinated manner, exhibiting relatively low damage and maintaining overall continuity. Tensile failure is predominant in the gravel layer, and after fracture, roof fissures and the pressure release zone stabilize vertically, with the fissure development height reaching approximately 52.8 meters. The pressure release zone extends to about 79 meters, as indicated by the distribution of roof stress concentration factors. During the micro-seismic monitoring period, the fracture degree of the gravel layer was large, and the single energy release was high, with a maximum of 8 723.36 J, which occurred 249 times; the upper rock layer was relatively low but dense, and the microseismic events occurred more frequently, with a total of 658 times, and the maximum energy release was 7 432.52 J.