Directional high-level borehole in roof is an important means to control mining pressure-release gas in coal mine underground. Due to limitation of drilling device, the borehole diameter is small in once-through drilling, and borehole group arrangement is needed to achieve good drainage effect. In order to solve the above problem, a forward multi-stage large-diameter reaming technology for directional high-level borehole in roof was developed, which constructed directional pilot hole firstly and then enlarged the pilot hole level by level though rotating drill pipe in drilling machine driving reaming bit in borehole. Reaming diameter gradation parameters were analyzed and determined on the basis of equal torque principle, and drilling devices were developed and selected including spiral blade and combination reaming bit, reaming drill pipe with high-strength and high-toughness, high-power directional drilling machine, mud pump truck with high-pressure and large-flow, etc. The field test was carried out at W1304 working face of Sihe Mine of Shanxi Jincheng Anthracite Mining Group Co., Ltd. The test results show that the diameter of directional high-level borehole in roof can be increased from 96 mm to 153 mm by use of the forward multi-stage large-diameter reaming technology, the maximum return-air gas volume fraction is 0.47% during the working face mining, and gas drainage flow of 153 mm borehole is nearly three times than that of 96 mm borehole, which verifies 153 mm borehole has salient gas drainage effect.
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