Comparative study on development characteristics of high-strength mining fissures in different overburden types

The development height of overburden fractures is a key factor for the prediction and prevention of water inrush from the roof of a mine. The overburden strength type and mining intensity are the main influencing factors for the development characteristics of overburden fractures.When predicting the damage height of overlying strata, it is important to choose the correct classification standard of rock mass and empirical formula for different mining method. The comprehensive methods of water consumption quantity and borehole TV observation were used to test the fracture development characteristics of overlying rock under high-strength mining conditions in hard and weak formations. It is concluded that the hard rock formations have developed mining fractures and the flushing fluid consumption changes intermittentlyor completely leaks, and the inclination of the rock formation fractures is large, which causes a large impact angle of the rock formation mining propagation. However, for the weak strata, the rock is softened and collapsed easily. The TV imaging of the boreholes is blurred, and the collapse zone is difficult to observe. It is necessary to adopt a comprehensive method to distinguish the vertices of the “two zones”. For the short distance and extra-thick coal seam mining, the comprehensive mining thickness is needed to predict the fissure zone height of lower seam. But the formula of comprehensive mining thickness is no longer applicablebased on the “Guidelines for coal pillar retention and coal mining in buildings, water bodies, railways and main shafts”.For this reason, by constructing a mathematical model of the relationship between the height of the caving zone and the interlayer spacing in the short-distance ultra-thick coal seam mining, a comprehensive mining thickness calculation formula suitable for the geological conditions is proposed. Due to the influence of the coal seam spacing, the height of the comprehensive caving zone increases, and the upper group of coal has little effect on the comprehensive mining thickness, resulting in a significant increase in the caving ratio.It is concluded that the comprehensive caving ratio of hard roof high-strength mining is 9.39-9.62 and the proportion of fissure zone height and mining height is more than 17.80 for the hard strata, which is two times of weak strata. It shows that the strength type of the overburden has a significant impact on the development characteristics of the overburden failure cracks in high-strength mining, and the formula has strong applicability. The research results can provide references for the observation of the overburden failure height and the field practice of water damage prevention.