Research on the unloading effect of hydraulic jet cutting in hard coal seam

The key to preventing and controlling rock burst disasters in coal mines lies in achieving rapid pressure relief within coal seams. High-pressure hydraulic jet cutting is an effective technique for coal seam destressing and burst prevention. By investigating the pressure relief effect of hydraulic jet cutting boreholes, reasonable and efficient parameters for hazard mitigation can be determined. This study analyzes the coal breaking process and failure types induced by hydraulic jets, dividing the breaking process into two stages: the hydraulic hammer pressure stage and the stagnation pressure stage. Based on the hydraulic jet momentum theorem and the Boussinesq-Flamant model, a theoretical formula for the maximum cutting depth is established. Taking the 3_lower coal seam of Jining No.3 Coal Mine as the research object, surface cutting depth tests are conducted to verify the relationship between jet pressure and cutting depth. Test results show that the cutting depths at pressures of 80 MPa, 90 MPa, and 100 MPa are 1.24 m, 1.46 m, and 1.52 m, respectively. The mechanism of static load relief is analyzed theoretically, and the relationship between roof subsidence and the abutment pressure of the coal in the slotted zone is established. The dynamic load blocking effect of the slot is analyzed, and the pressure relief and burst prevention mechanism of cutting is elaborated. Ultra-high pressure water jet cutting and pressure relief tests are carried out in the auxiliary transport roadway of the 123 lower 06 longwall face in Jining No.3 Coal Mine, and the cutting radius as well as radial and axial stress relief effects are investigated. Field tests demonstrate that, under the conditions of the 3 lower coal seam, the cutting radius reaches 1.55 m at a jet pressure of 100 MPa. The radial pressure relief influence range of the borehole is 2-3 m, and a significant pressure relief effect is observed at a borehole depth of 10 m in the axial direction. At a distance of 5 m from the borehole collar, the internal stress of the coal increases slightly due to stress transfer from the slot, and the stress influence range of the slot along the borehole axis is 3 m. These findings provide reasonable parameter guidance for large-scale pressure relief in rock burst-prone coal seams.