Development Height of Roof Fractures in Shallow Buried Short-Distance Coal Seams Under Composite Water Bodies

To solve the problem of difficulty in accurately calculating the height of roof crack development in shallow mining under the threat of composite water body, a study was conducted using Gaotouyao coal mine in Dongsheng coalfield as an example. Firstly, taking the mining and geological conditions of coal seams 2-3 and 3-1 in Gaotouyao coal mine as the research background, physical similarity simulation and three-dimensional discrete element code (3DEC) numerical calculation were used to study the development height of shallow buried closed coal seam roof cracks under composite water bodies. Then, in response to the complexity of the evolution law of shallow buried closed strata roof cracks in composite water bodies, the estimation formula for the height of roof crack development was revised. Finally, by combining similar material physical simulation experiments and numerical simulation calculations, the accuracy and applicability of the revised formula were verified through on-site monitoring of the development height of roof cracks in the 2-3 and 3-1 coal seams of Gaotouyao coal mine. The research results indicate that: Under the influence of mining the No. 3-1 coal seam, roof fractures expanded and connected, and previously compacted fractures in the roof of the No. 2-3 coal seam re-developed due to repeated mining, resulting in an increase in fracture width. The evolution law of shallow buried closed strata roof cracks in composite water bodies was revealed. And field measurements showed that the roof fracture development height after mining the No. 2-3 coal seam at Gaotouyao coal mine was 47.0 m, and after repeated mining of the No. 3-1 coal seam, the roof fracture development height was 88.5 m. These results are close to the calculations obtained using the revised formula, validating the accuracy of the revised formula for estimating roof fracture development height in shallow, closely spaced coal seams under composite water bodies. The research results have certain theoretical guidance significance for the safe and efficient mining of shallow and close range coal seams.