Rock strength and permeability under compression-shear coupling corresponding to different loading angles

The Ordovician limestone aquifer is widely developed in the basement of the Carboniferous Permian coal bearing strata in North China. Coal mining is threatened by both water inrush disasters and water resource loss from the floor confined aquifer. The hydraulic characteristics of the rock under compression and shear coupling action are the basis for evaluating the water resistance performance of the mining floor strata on high inclined confined aquifer. Using theoretical analysis and discrete element numerical calculation methods, the rock instability criterion for the Mohr circle rotation under different compression and shear ratios was established, the mechanism of rock strength attenuation under compression and shear coupling action was revealed, the stress threshold determination method for comprehensive indicators such as crack angle, crack propagation speed, and improved volume strain was proposed, and the characteristics of rock microcrack propagation and permeability evolution under different loading angles, osmotic pressure differences, and confining pressure were clarified. The main conclusions are as follows. The reduction of rock bearing capacity under compression and shear coupling action is manifested in strength reduction and elastic modulus increase. The elastic modulus of rocks shows a trend of first slowly increasing (0°−15°), then rapidly increasing (15°−30°), and then decreasing (30°−45°) with the increase of loading angle, but is still greater than its initial value. There is a linear negative correlation between rock strength and loading angle, and the reduction amplitude is proportional to confining pressure. The rate of strength reduction under 2 MPa confining pressure is 1.9 times that without confining pressure, controlled by the equivalent internal friction angle. As the loading angle increases, the degree of rock failure decreases and the post peak stress drop transitions from brittle to plasticity. Under compression and shear coupling action, the induced initiation of tensile cracks and the dominant unstable propagation of tensile and shear composite cracks explain the inherent mechanism of nonlinear evolution of initiation stress and damage stress thresholds under different compression and shear ratios. The position of permeability rebound is between the threshold of initiation and damage stress, and as the loading angle increases, the distribution of tensile and shear composite cracks (dominant seepage paths) is more concentrated. The rock strength decreases with the increase of osmotic pressure difference, and the rate of decrease is negatively correlated with the loading angle. The peak permeability of rocks decreases with the increase of loading angle, and the decreasing trend changes from linear to nonlinear with the increase of osmotic pressure difference. When the loading angle is large, the control effect of osmotic pressure difference and confining pressure on rock strength and permeability weakens.