Energy-accumulated directional blasting technology and its application in gob-side entry retaining by roof cutting

Based on the principle of energy-accumulated directional blasting technology, the penetration condition of damage between blasting holes is established. The laws of blasting stress evolution, the development of directional cracks, and the directional blasting parameters are examined by the methods of theoretical analysis, numerical simulation, and field testing. Based on the results of the Finite Element (FE) numerical simulation, a tensile stress of 91.2 MPa is generated on the blasting hole wall in the designated direction, which can fracture the rock mass to form directional cracks. With the action of the detonation stress, the length and width of directional damage in the rock mass increase continuously. The combined roof structure of “lower short-arm beam and upper masonry beam” is created after mining by the use of directional blasting to presplit the roof. The combined roof structure is more conducive for the stability of the retained entry. Taking the 21304 working face of Chengjiao coal mine as the engineering background, the directional blasting parameters are determined by doing theoretical analysis and conducting field tests, such as the blasting hole depth of 8 m, the blasting hole angle of 15°, the spacing between blasting holes of 500 mm, the single-hole charge structure of “4 + 3 + 2 + 0,” with the sealing length set at 2 m. When the determined blasting parameters were used to retain the entry, the roof in the goaf collapsed along the structural surface formed by directional blasting and the gangues filled the entire roadway quickly. Further, the surrounding rock of the retained entry was stable at about 290 m behind the working face, and a good retaining effect was obtained at the entry. The research results are significant for improving the technical system of gob-side entry retaining by roof cutting (GERRC), and provide guidance for the application of GERRC.