Pressure relief mechanism and experiment of directional hydraulic fracturing in retaining roadway

Stress of upper coal pillar is the force source of strong deformation of retaining roadway. For surrounding rock control of retaining roadway with serious deformation, traditional blasting pressure relief technology has some problems such as high safety risk, environment pollution and serious surrounding rock damage. Aiming at the above problems, pressure relief mechanism of directional hydraulic fracturing in retaining roadway was proposed, that is, through expansion of hydraulic fracture, weak structural surface is generated in roof rock, overall strength of the roof rock is reduced, and the weakened hard roof is timely broken and caved under action of mining stress, so as to reduce stress level of retaining roadway. Taking 4312 fully mechanized coal mining face of Changping Coal Mine of Shanxi Jincheng Anthracite Mining Group Co., Ltd. as test site, boreholes scheme and boreholes layout parameters of directional hydraulic fracturing were determined. The fracturing results show that transverse grooving depth is 5 mm and grooving effect is good. During fracturing step by step, pressure required for fracture expansion increases with increase of distance between fracturing site and borehole. Surface displacement and coal pillar stress of fractured and non-fractured sections of retaining roadway were monitored. The monitoring results show that deformation of retaining roadway is mainly caused by two side walls deformation, and the average displacements of two side walls and roof and floor of the fractured section are about 40.79% and 69.80% lower than those of the non-fractured section. Stress of coal pillar in the non-fractured section appears peak point when approaching working face, while stress of coal pillar in the fractured section appears peak point at about 200 m away from back of cut hole of working face, which indicates that directional hydraulic fracturing transfers stress peak position of upper coal pillar.