Mechanical analysis and blasting mechanism of main roof initial fracturing in shallow depth mining face with large cutting height

While the “equal deep holes” blasting scheme was carried out at the 621-02 mining face of Nalinmiao No. 2 coalmine,China,the blasting effect was not ideal for the occurrence of a huge shockwave,a heavy rib fall and the safety valve opening of No. 30-90 supports during the main roof initial fracturing. Aimed at solving the problems,the authors analyzed the blasting mechanism of shallow depth mining face with a large cutting height,put forward a new blasting scheme that the blasting holes are ladder-like distributed,calculated the main roof initial fracturing distance after blasting and the rational blasting hole spacing. Furthermore,the authors comparatively analyzed the Mises stress evolution of two blasting schemes using the numerical simulation of nonlinear dynamic finite element software LS-DY- NA,and found that the new blasting scheme’ s effect is better. Field test results show that while the new blasting scheme was carried out at the 621 -03 mining face,its roof initial pressurized distance and weighting intensity were 60 m and 38. 5 MPa,while the 621-02 mining face’s were 70 m and 42. 0 MPa. In short,the new blasting scheme significantly reduced the roof initial pressurized distance and weighting intensity. At the same time,no roof cutting and shockwave accidents occurred and a safe-efficient mining was achieved at the 621-03 mining face.