Water injection softening modeling of hard roof and application in Buertai coal mine

A hard roof implies a large hanging-roof and high-frequency dynamic strata behavior during mining, which poses a great risk to the safety of personnel and equipment. To solve this problem, a water injection softening method was used in the Buertai coal mine in Inner Mongolia, China, which has a hard sandstone roof. Comprehensive experimental tests and numerical simulations were conducted to investigate the water injection softening effect on the hard roof. Uniaxial tests were conducted on water-softened rock specimens to establish the relationship between the water content and mechanical properties, and the permeability was correlated with the pore water pressure and axial stress. A hydromechanical coupling model was developed and implemented in the numerical model by introducing the water-softening elastic modulus and the dynamic porosity model. Numerical modeling of water injection into the hard roof was conducted to characterize the roof behavior and the dynamic flow field under water injection softening. The results showed that (1) the water weakened the strength and elastic modulus of the rock. As the pore water pressure increased, the permeability increased exponentially. However, with increasing axial stress, the permeability decreased exponentially. (2) The Terzaghi effective stress principle and static equilibrium equation were combined to derive the Biot three-dimensional consolidation control equation with the strain tensor and pore water pressure, which can effectively describe the coupling relationship between the pore water flow and the elastic deformation of the elastic body of a porous medium. Expressions for the porosity of the elastic body of the porous medium were determined. Porosity was expressed as a function of the Biot coefficient, volumetric strain, pore water pressure, and bulk modulus, which made the model more comprehensive and reasonable. (3) The pore water pressure, strain, and permeability were higher near the water injection hole. The respective increases in pore water pressure, strain, and permeability were rapid at the beginning and diminished with subsequent water injection.