Test of a liquid directional roof-cutting technology for pressure-relief entry retaining mining in Ningtiaota coal mine

The key technology of pressure-relief entry retaining mining (PERM) is roof directional cutting, traditional roof directional cutting adopts blasting directional cutting technology, which has many problems.The research is based on the PERM project in Ningtiaota coal mine, the liquid directional roof-cutting technology test is carried out to improve the roof slicing technology.The test target requires that the damage rate of rock mass between adjacent boreholes after liquid directional roof-cutting should reach above 90%, and the roof-cutting line should be a straight line, so as to ensure the roof-cutting effect in PERM project.The test system is mainly composed of pump station pressure system, abrasive supply device, cutting head conveying device, cutting head, directional device and abrasive used in field test.In the test process, four drilling holes are drilled in the roof rock layer in advance, and then cutting head is installed in the slot hole directionally through the conveying device and high-pressure sealing drill pipe.High-pressure water jet is used to make holes in the adjacent holes radially along the four slot holes, and all the slit holes are connected directionally, so as to form directional crack in the hole.The test results show that the cracks are basically straight, the maximum deflection angle of the cracks is 0°-10°, but in the process of cutting seams in the four holes, the relatively intuitive cracks in the holes have not penetrated through the hole wall of the whole borehole.The results show that the liquid directional roof-cutting technology cannot achieve the goal of forming 90% of the cutting line through the design requirements of PERM, and the parameters of the liquid directional roof-cutting technology still need to be further optimized.