An innovative technology of directional roof slitting by composite blasting and an engineering application

Cutting a hard roof is an effective means to change strata movements and control strong ground pressure in coal mining faces. In order to achieve directional，weak⁃disturbance and large⁃spacing seam making，an innova⁃ tive technology of directional roof slitting by composite blasting (DRSBCB) was developed. Firstly，the composition of a high energy perforating gun，a key equipment for this technology，was systematically introduced. Then the process of directionally roof cutting by the DRSBCB was analyzed. The directional crack formation mechanism of the DRSBCB was clarified by the AUTODYN，that was shaped charge jet and high⁃pressure fracturing. The LS⁃DYNA software was used to build numerical models of shaped charge jet and rock slitting by a high⁃pressure gas，and the size of perfora⁃ ting charge and the amount of booster were optimized by the models. A ground experiment of the DRSBCB was carried out. The experimental results revealed the characteristics of crack propagation between adjacent holes，and verified the rationality of perforation charge size and booster amount. An industrial experiment of the DRSBCB was carried out in the 8311 workface of the Tashan Coal Mine. Vertical stress peak values at different depths of coal pillars of the 2312 gob⁃side roadway were monitored during the advancing process of the 8311 workface. Then，the following conclu⁃ sions are drawn: 1 The mechanism of the DRSBCB is that the pressure on the rock penetration surface by the shaped charge jet is far greater than the rock strength，so the rock is broken to form directional channels. The high⁃ pressure gas，generated by the secondary deflagration，enters the channels and forms a continuous and flat fracture sur⁃ face along those channels under the action of ‘air wedge’. 2When the diameter of perforating cartridge is 40 mm and the booster is 4 pieces，the DRSBCB technology can achieve the splitting of medium sandstone strata at a 1.5 m borehole spacing. 3The field measurement shows that the vertical stress peak of the pillars near the goaf roadway de⁃ creases by 31% on average after the DRSBCB is adopted，which effectively controls the roadway surrounding rocks.