Experimental study on damage law of coal seam under hydraulic fracturing and blast load

Abstract Compared with blast mining only, blast mining after on-site hydraulic fracturing can make the mining easier and obtain better mining outcomes. To explore the effects of hydraulic fracturing on the blasting damages in coal seam, blasting experiments were carried out under biaxial confining pressure using the synthetic coal briquettes. The coal briquettes with the same mechanical properties as coal seam were prepared and the mica sheets with different radii and thicknesses were added to simulate the internal hydraulic fractures of different radii and openings. The internal damage distributions and stress attenuations of the coal briquette specimens with different hydraulic fracture radii and openings after the blasting were then measured using a rock ultrasonic tester and a static-dynamic strainmeter. Based on the rock blasting theory, the effects of hydraulic fractures with different radii and openings on the blast fracture propagation and coal seam damage were analyzed. The following conclusions are drawn: (1) The increases in hydraulic fracture radius mainly enhance the damages in the vertical direction to the hydraulic fracture, and can increase the vertical range of the severely damaged area by 20–25 cm. The increases in the hydraulic fracture opening mainly cause more severe damages along the direction of the hydraulic fracture and increase the horizontal range of the severely damaged area by 30 cm. (2) The area of the severely damaged area caused by blasting increases by 550 cm 2 as the hydraulic fracture radius increased from 5 to 15 cm. As the hydraulic fracture opening increased from 2 to 10 mm, and the area of the severely damaged area caused by blasting increases by 650 cm 2 . Therefore, the hydraulic fracture opening has greater impacts on the severely damaged area. (3) The increase in the hydraulic fracture length reduces the compression phase attenuation of the blast stress in the radial direction. Both the increases of the hydraulic fracture length and opening increase the absolute value of the tensile phase in the radial direction. (4) Increasing the hydraulic fracture radius and opening can greatly promote the development of blast fractures and enhance the damages to coal seam. Therefore, the coal seam mining effect can be improved by increasing the radii or openings of hydraulic fractures to adjust the main action direction of blast fracture.