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Abstract The hydraulic fracturing field tests to weaken the hard K3 upper roof above coal layer were conducted when severe problems occurred in the open-air and crossheading strata at Tashan Coal Mine. The water pressures in the boreholes and water oozing from bolt locations at two sides of roadways were monitored during the process of hydraulic fracturing to analyze the range of influence of hydraulic fracturing. The characteristics of weakening the hard roof by hydraulic fracturing were analyzed according to the step size of periodic pressure，the dynamic loading factor，the deflections of roof，the displacements of roadways，the statistical analysis of rib wall and gas discharge before and after hydraulic fracturing. It was found that the range of grooving fractures produced by hydraulic fracturing was no more than 55 m. Three types of the pressure-time curve of crack propagation in the coal and rock mass were obtained. The step sizes of periodic pressure are 18.69 and 16.76 m respectively and the average periodic dynamic pressure factors are 1.69 and 1.53 respectively before and after hydraulic fracturing. Both the deflection of roof and the displacement of roadways were greatly reduced and the roof of the roadways remained unbroken. The average depths of coal wall rib spalling were reduced from the original 31.18 m to 7.06 m. Besides，the gas release is relatively stable. The gas density at the upper corner was reduced from the original 0.20% to 0.15%. Meanwhile，the completeness and strength of the roof at the working front were weakened and roof was dropped as working front proceeds.
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Received: 22 August 2013
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2014, Vol. 33 
