Experimental study on controlling strong strata behavior in mining face by cracking roof with directional hole hydraulic fracturing

In an attempt to address the strong strata pressure problems frequently occurring during the mining process in No. 42107 working face of Buertai Mine of Shendong Mining Area, the studies was carried out to investigate the feasibility of directional hydraulic fracturing technique using theoretical analysis, numerical simulation and site measurement. The results show that strong strata pressure at working face is mainly cause by the 2.2-meters-thick siltstone, the No.2 key stratum located in No.4-2 coal seam roof. Theoretically, strong strata pressure can be reduced to some extent by lowering the strength of No.2 key stratum. By reducing the friction of vertical joints in No.2 key stratum , the authors were able to simulate the cracking effect, and the simulating results indicated that with the reduction of friction, the bearing pressure in the coal body drops considerably by 18.3% approximately. In real operation, three directional boreholes were drilled from coal seam into No. 2 key stratum at No.42107 working face and based on the working resistance curves generated by the 11 hydraulic supports both before and after the occurring of cracking, the average support resistance reduced from 22 535 kN prior to cracking to 19 599 kN after cracking, and the periodic weighting interval decreased from 21.0 m to 16.9 m. Those results imply that strong strata pressure could be controlled by cracking the key strata through directional hydraulic fracturing technique.