Similarity simulation study on the mining response law of artificial stress relief layer in coal seam roof

The "L"-shaped fracturing well on the ground breaks the thick and hard roof, forming an artificial stress relief layer above the coal seam, which can significantly reduce the risk of rockburst. Constraining at the engineering site, it is difficult to reveal the structural and load changes of the overlying rock during the mining after the formation of the artificial stress relief layer. Using OFDR distributed optical fiber, pressure box, and three-dimensional optical digital speckle monitoring, prefracturing of the thick and hard roof area is carried out to investigate the structure and load response law of the overlying rock after the formation of the artificial stress relief layer. The results show that fracturing effectively weakens the integrity and strength of thick and hard rock layers. During the process of advancing the working face, the overlying rock can collapse in a timely and orderly manner, and the cantilever area and length are reduced. The "F"-shaped structure of the long cantilever beam under non-fracturing conditions is transformed into the "F"-shaped structure of the short cantilever beam after fracturing. After fracturing, the fracture of rock layers no longer follows the formula of ultimate fracture step distance, but is controlled by the fracturing cracks. The fractured layer and nearby key layers should be fractured in advance. The fracturing cracks develop again under the action of mining stress, and a phenomenon of slow and advanced development of cracks in the local overlying roof rock is observed. The fractured blocks of overlying rock fall along fractures, resulting in a decrease by 17% in the step distance of mining-induced pressure. At the same time, no large-scale collapse of the overlying rock occur due to the simultaneous rupture of the key layer and the basic roof in the middle and high positions after fracturing. During the process of working face advance, the artificial stress relief layer formed by the top plate fracturing acts as a barrier, blocking the hard transmission of loads between the upper and lower layers. At the same time, cracks develop inside the fracturing layer. During the transition from triaxial to biaxial loading, the structure deteriorates continuously, thereby reducing the deformation of the overlying rock and the stress concentration of coal seam in advance.