The mechanical mechanism of key strata fracturing under different water pressure of the fourth aquifer in loose layers

For the prevention and control of water inrush disasters in coal mining under loose and pressurized aquifers, our study delves into the fracturing dynamics between water pressure in the fourth aquifer of the loose layer and the movement of the main key strata. Based on on the key strata theory and the medium thick plate theory, we analyze the statistical relationship among water pressure, loose base ratio and loose layer thickness in the fourth aquifer of 18 mines in 6 typical thick loose layer mining areas. Our findings reveal a significant positive correlation between loose base ratio, loose layer thickness and water pressure in the fourth aquifer of the loose layer. Using the medium-thick plate theory, our analysis demonstrates a notable transition of fracture mechanics mechanism of the key strata from tensile to shear fracturing with the increase of water pressure in the fourth aquifer of different loose layers. Numerical simulation analysis validates this fracturing shift of the key strata from tensile to shear fracture. As water pressure increases, the proportion of shear stress increases gradually, causing the structure transition between key blocks from a continuous and stable masonry beam structure to a discontinuous and easily unstable stepped rock beam structure. Furthermore, the increased water pressure in the fourth aquifer of the loose layer may accelerate the transformation of the originally stable masonry beam structure in the goaf into a stepped rock beam structure, thereby promoting the development of water conducting fracture zones. Consequently, the key strata rock block structure exhibits the coexistence of masonry beams and stepped rock beams. The boundary lines of rock movement with different pine base ratios show obvious exponential curves characteristics. As the pine base ratio increases, the boundary lines of rock movement shift from straight lines to exponential curves, and the displacement during the advancing process of the working face displays clear zoning characteristics. © 2024 Editorial Office of Journal of Mining and Strata Control Engineering. All rights reserved.