Fractal characteristics and fracture mechanisms of flying gangue in longwall workings of the steeply dipping seam

The flying-gangue hazard has long threatened the safety of personnel and mining equipment in steeply dipping/inclined coal seams. A flying gangue maybe produces a large number of “subgangues” after impacting on the longwall floor, it’s damage is sharply increased. Revealing the rupture response and fragmentation mechanism of the flying gangue under dynamic loading is one of the important contents to grasp the characteristics of dynamical damage for flying gangue hazard. This study was carried out through the combination of Split Hopkinson Pressure Bar test, model test and numerical simulation test. The results show: The failure mode of coal-rock under high-speed impact can be divided into three types: intact, single crack, and severe fragmentation. Fractal dimension D the coal specimen has a logarithmic relationship with the strain rate. Coal with lower uniaxial compression strength is more severely broken and has a larger fractal dimension D under the same strain rate conditions. The larger the coal size, the smaller the fractal dimension D is. The fractal dimension D of coal has a logarithmic relationship with strain rate regardless of whether the shape of the specimen is a disk or a cube. The “shape-rounded” phenomenon of the cube block is prone to appear during the movement. This phenomenon can be attributed to three parts: First, because the impact orientation between the block and slope or equipment is not strictly perpendicular compared with a drop test, the stress wave propagation within the specimen is dispersed to some extent during an impact. Second, because the block shape is not strictly regular, the stress is easily concentrated on a non-smooth surface. Finally, due to local shear failure in the contact area, a damaged area is formed. The corners of the flying-gangue are more susceptible to collision and direct loading, resulting in localized disintegration. This study provides a method for risk assessment and for determining the principles of protective systems in underground steep coal seams. In contrast, the flying-gangue involves material, geometry and other sources of uncertainty, stratification, fracture distribution and the degree of development by mining disturbance has a greater impact on the fractal characteristics of its crushing tends to be more complex. In the next step, the effect of the above factors on the crushing characteristics of the flying-gangue will be considered.