Analysis of fracture mode transformation mechanism and mechanical model of rock strata with different thicknesses

The deformation, fracture, and movement mode of rock strata are greatly related to underground pressure, surface subsidence, etc. The fracture modes of the thin rock stratum and thick rock stratum are significantly different. How to uniformly describe the fracture modes of rock strata with different thicknesses and establish a mechanical model is a difficult theoretical problem in rock movement. In this paper, the fracture structures of mining strata in 23 typical working faces are investigated. In addition, the fracture mechanical factors and movement modes of the rock stratum are classified based on fracture mechanics, and the relationship between fracture structure and rock stratum thickness is analyzed. Based on the medium-thick plate theory, the fracture mechanical model of rock strata with different thicknesses is established. The fracture mechanical mechanism and fracture mode of rock strata with different thicknesses are revealed, and the corresponding criteria are given. The breakage and movement laws of rock strata with different thicknesses and fracture modes in the mining process are analyzed by numerical simulation. The research shows that when different mining conditions and different stress boundary conditions act on rock strata with different thicknesses, different fracture modes will occur. With the increase of the rock strata thickness, the shear stress of rock strata increases continuously, and the fracture mechanical mechanism of rock stratum presents the evolution law of tensile fracture, tensile shear fracture, and shear fracture. Also, the corresponding rock mass structures are masonry beam structure, layered fracture, and step voussoir beam respectively. Moreover, the statistical data show that the rock layers with a thickness span ratio greater than 0.5 are more prone to occur shear fractures and form step voussoir beams. Based on the above analysis, a mixed fracture model is proposed aiming at the possible tension shear failure of thick rock strata, and the rock fracture mechanical factors are further subdivided into three types: tensile fracture, tensile shear fracture, and shear fracture. There are seven types of fracture modes in rock strata in total. Tensile fracture often occurs in a thin layer, which will change to a shear fracture when the fault appears. Tensile fracture, tensile shear fracture, and shear fracture are typically fracture modes in a thick layer, in which tensile shear mixed fracture is divided into end tensile shear mixed fracture, layered linkage movement, and layered uncoordinated movement. Further research on the fracture modes of rock strata can provide valuable theoretical support for the control of ground pressure and the prevention of surface subsidence hazards in mining.