The method, mechanism and application of preventing rock burst by artificial liberation layer of roof

The development of free layer(protective layer)mining in rockburst mines can fundamentally improve the stress concentration of mining face, and thus reduce the impact risk of mining activities in a large space. In order to achieve the regional pressure relief goal of a single coal seam with rockburst tendency that can be mined without a liberated layer, a method of “artificial liberated layer” for the pressure relief and prevention of rockburst in the thick and hard roof area of the main disaster zone above the coal seam created by hydraulic fracturing is put forward. The engineering mechanical model is established, and the regional pressure relief mechanism and engineering test research are comprehensively carried out by using theoretical analysis, engineering acceptance and other methods. The results show that in the load supply process of rockburst, the foundation static load concentration necessary for rockburst can be changed by the regional transformation of the structure and load of overlying strata of coal seam. For the roof controlled rockburst mines, in the different stages of development, preparation and mining, a regional hydraulic fracturing is carried out for the thick and hard roof, that affects the occurrence of rockburst, to transform the overburden structure and load distribution. Therefore, the roadway excavation and mining face are in the low stress area under the fracturing coverage, and the effect of “artificial liberation layer” is realized for mining a single coal seam under rockburst threat. The mechanism of hydraulic fracturing in the roof area to relieve pressure and prevent rock burst is that compared with that before fracturing, the thick hard roof plate overlying the coal seam is artificially fractured regionally, making its long beams become short beams and large blocks become small blocks in the strike, so it does not have the function of large-area cantilever and reduces the dynamic load brought by the roof fracture caused by the increase of suspended overburden volume. In the inclined direction, the overburden load changes from hard transmission to soft transmission, reducing the overall static load level of the coal seam under the fracturing area. The first surface regional fracturing test for creating an “artificial liberated layer” was carried out in No.401102 fully mechanized caving face at the Mengcun Coal Mine in China. After the fracturing operation, the micro-seismic events during mining presented a high-frequency and low-energy distribution, the fracture fragmentation of roof rock was reduced, the roof water fell evenly in the goaf, the water inflow of the working face was reduced, the pressure and deformation of the working face and two roadways were significantly reduced, and the pressure relief effect of the liberated layer was achieved.