Roof cutting mechanism of dense drilling in gob-side entry retaining and determination method of key parameters

To achieve the safe and stable structural condition of the roof along the goaf under the roof cutting by dense drilling method, the stress distribution and fracture process of the roof strata between normal and dense drilling conditions were studied. Based on rock fracture theory and elastic theory, the stress distribution of the surrounding rock between dense drilling and the fracture process were analyzed before and after mining, clarifying the cutting mechanism of dense drilling and the interaction mechanism of concentrated stress between adjacent drilling. The calculation formula for the tensile-shear stress of the surrounding rock between dense drilling in the arc-shaped triangular panel structure at the end of the working face was derived. Based on this, the influence of different parameters on the tensile-shear stress of the surrounding rock between drilling was analyzed, and a determination method for the key parameters of roof cutting by dense drilling holes along the goaf was proposed. Research has shown that the dense drilling mainly increases the concentration of the surrounding rock's tensile and shear stress by changing the stress conditions around the densely drilled holes before and after mining, causing the expansion and connection of fractures between drilling and forming key rock layers for cutting. Before mining, the drill holes are mainly affected by horizontal compression stress, and after mining, the stress gradually changes to the tensile and shear stress caused by the rotation and subsidence of the goaf roof, and the state of the surrounding rock between the holes transitions from the weakening stage to the formation of crack stage, and then to the fracture stage. The magnitude of the tensile and shear stress of the surrounding rock between the densely drilled holes mainly depends on the thickness of the key rock layer and the ratio of the hole diameter to the hole spacing, and is negatively correlated with the hole height and spacing, and positively correlated with the angle and diameter of the drill holes. Based on this, a method for determining the key parameters of intensive drilling and cutting is proposed, and a plan for intensive drilling and cutting in the goaf with hard roof conditions on the 3124 N working face of Longtan Mine is designed, with a hole length of 8.3 m, an angle of 15°, a hole diameter of 48 mm, and a spacing of 500 mm. The deformation of the goaf roof after cutting is controllable and the overall stability is good, which proves that the arrangement parameters of intensive drilling and cutting are reasonable, and the determination method for the key parameters of roof cutting by dense drilling holes is effective and feasible.