Composite hazards prevention with breaking coal seam and roof by super high pressure water jet

Coal seam with high ground stress and low permeability is high likely to outburst especially as it has high rockburst possibility.In order to prevent this problem,this paper studied the technique of ultra-high pressure water jet.The ultra-high pressure water jet could cut the coal seam transversely and longitudinally.The transverse cut refers to the stress relief of coal body,and the longitudinal cut refers to the stress relief of rock.The failure mechanism of coal and rock under ultra-high pressure water jet cutting is analyzed theoretically.With the ultra-high pressure water jet cutting,shear stress and tensile stress occur in coal and rock mass,which accelerates the generation and expansion of cracks in coal and rock mass.Then,coal and rock would break along the axial and radial direction of water jet cutting.Similar simulation experimental data show that the cutting depth is 152 cm with 100 MPa water pressure,2.5 mm nozzle diameter,60 r/min rotary speed of drill pipe,10 min water jet.Numerical experimental data show that the pressure relief is better with 2 m of two water jet point in a hole,and 8 m of two water jet holes.The ultra-high pressure water jet technique was applied in the 220108 working face in Xinji No.2 Coal Mine.The extraction of gas in single hole with ultra-high pressure water jet cutting increases 3.5 times of single hole without ultra-high pressure water jet cutting,what is more,the permeability coefficient of coal seam increases 30 times,the time of gas extraction is shortened by 38%,the pressure of coal seam is reduced by 35%,the pressure of shield is distributed from 20 to 25MPa.No high energy seism occurs during the breakage of rock strata.Those demonstrate that the ultra-high pressure water jet significantly reduces the rock and gas composite dynamic disaster.