Mechanism of bedding cracking and development laws of lateral fracture in overlying strata induced by longwall mining

The subsidence and deformation of strata induced by longwall mining will bring the bending stress inside the strata, which may result in bedding cracking for its relatively low shear and tension strength and forming lateral water-courses. Water in goaf areas may inrush through these watercourses and become a safety risk to the normal operation of the nearby mining face, so the development laws of the lateral fractures induced by longwall mining are analysed. First, the deflection equation of mining overlying strata based on the Winkler elastic foundation beam theory is established. Then the corresponding overlying pressure and foundation coefficients of strata above the coal pillar and goaf are determined according to the different abutment and damage, and the calculation formulas of the subsidence, rotation, moment and shear of every stratum are provided. Based on this, the mining-induced bedding fractures are divided into two classes, i. e. tension separation fractures and shear dislocation fractures, and the failure criteria of the two different fractures are proposed, and the calculation methods of the fractures position, length and width are provided. Finally, taking No.15102 working face at the Lingzhida coal mine as the research background, the deflection curve and bending stress of the overlying strata induced by mining are calculated and the development laws of the lateral bedding fractures are investigated. The results show that there are only shear dislocation fractures developing with a length of more than 50 m above the pillar, which is longer than the width of the pillar between No.15102 and No.15101 working face. The fractures above the goaf boundary include tension separation fractures and shear dislocation fractures. The lateral bedding fractures above the center of the goaf are mainly the tension separation fractures. Drilling imaging method has been adopted to observe the lateral fractures induced by No. 15101 mining, which is used to verify the theoretical analysis. According to the research, the lateral fractures developing along the bedding are the main watercourses that may cause water inrush from No.15101 goaf. Therefore, boring and grouting in front of and behind the heading faces of No. 15102 mining roadways were operated as well as water draining in No. 15101 goaf to effectively reduce water sprinkling in the roofs of No.15102 mining roadways, which can ensure the normal excavation of the roadways and safety mining of No.15102 work face.