The failure of natural rock mainly forms along the joint planes; therefore, the existence of joints heavily affects the rock strength. The influences of joints with a different dip angle and cross double joints on rock mass strength were analyzed using theoretical calculation and particle flow PFC2D software to explore the mechanical properties of joints on rock failure. Results show that the strength of a jointed rock has obvious angle-range effects in the uniaxial compression text, and the peak strength of rock specimens is typical of the "U" type with the increase of the joint dip angle. For the rock mass of cross double joints, the rock mass strength is determined by the strength of joints when the joint dip angle of either joint is between 45° and 75°. The rock mass strength is determined by the superposition of the effects of these two joints when the joint dip angles of the two joints are both between 45° and 75°; the influences of joints on the rock mass strength are not obvious when neither of the joint dip angles of the two joints is between 45° and 75°. The conclusions obtained in the study have significant implications for understanding the laws of joint rock strength.

Gas extraction is an effective measure for reducing gas accidents in the colliery; however, extracting gas in coal with low permeability is difficult. To obtain the influential law of mining-induced stress on the permeability of coal and improve the effect of gas extraction, this study took the 17218 face of Zhangji Coal Mine under Huainan Mine Industry Group as the engineering background to set the measurement station at both the return airway and haulage roadway of the face, establish the FLAC3D numerical calculation model, and comprehensively analyze the evolution laws of mining-induced stress, gas flow, gas extraction amount, and coal mass failure characteristic during the actual mining process at the face. Research results showed that during the actual mining process at the face, gas flow and mining-induced stress showed the same changing law, and the permeability of coal mass was controlled by mining-induced stress. The peak of coal stress at the side of the return airway, the degree of stress concentration, and its scope of influence were all large. The coal failure scope and degree at the side of the return airway were all large and the permeability of coal mass was high. The permeability of coal mass in the zone in which the stress was reduced at the face was the highest. The gas concentration could be significantly reduced if on-site technicians extract the gas in the stress reduction zone. The research findings enrich the theoretical research on coal permeability and provide the basis for effective gas extraction.