Research on the influence of large diameter borehole pressure relief on surrounding rock strength and anchoring force

In order to study the effect of pressure relief of large diameter borehole on surrounding rock strength and anchoring force, according to the thick wall theoretical model, the corresponding relationship between the laboratory experiment and the strength change of surrounding rock in coal mine is established. The influence of large diameter borehole on surrounding rock strength was analyzed by FLAC3D, and it was found that the surrounding rock strength of 3320 working face in Liangbaosi Coal Mine decreased by 20%-25% after pressure relief. The mechanical properties of anchor bolts and anchoring agents used in coal mines are obtained through experimental tests. PVC, 6061 aluminum and 20# steel sleeves were used to study the relationship between different radial stiffness and anchoring force, the anchoring force is between 55.4 kN and 110 kN. According to the equivalent radial stiffness formula, it is determined that the bolt anchorage force weakened is about 10% by the pressure relief drilling with diameter of 150 mm. In order to maintain the equivalent control degree of surrounding rock, the support strength should be designed as 1.10 times of that without pressure relief drilling.