Coal mining collapse analysis of total caving method based on FLAC3D and UAV aerial surveying

Objective Total caving coal mining in the upper reaches of the Yellow River in Inner Mongolia has resulted in large areas of subsidence, causing frequent ecological and environmental problems on the surface. Methods For the study of the surface crack development rule of the goaf and collapse stress and the displacement field evolution characteristics, for a string of the Chuancaogedan Coal Mine 6102 working face as the research object. The distribution range and rule of surface cracks were found by UAV aerial survey, a FLAC3D numerical model was constructed to analyze the variation of stress and displacement of surrounding rock in goaf. Results The results of the analysis were combined with the results of UAV mutual authentication. The results show that (1) The collapse is mainly in the form of ground fissures, which are mainly distributed in the northwest and midwest of the mine. There are two types of ground fractures in the working face. One type of arc-shaped stepped fracture group is distributed in parallel and appears at intervals of 5-20 m. Most of them form staggered platforms with a step height of 15-130 cm, and the fractures develop in the direction of the vertical working face advance at a deviation angle of 3°-5°. For the other kind of linear edge crack belts, exhibiting severe tensile damage, were observed along the working face peripheral, belt development can occur, a few can be spread in the inside of the working face, and the distance from the outermost crack to the working face is 38.53 m. (2) During the excavation of the underground coal seam, an obvious saddle-shaped tensile stress concentration area appears in the roof of the goaf, and with the advance of excavation, the tensile stress in the surface concentration area first increases and then decreases, with a maximum value of 0.181 MPa. (3) Complete collapse occurred at the top of the goaf, with the maximum surface vertical displacement of approximately 5.5 m located at the middle of the goaf. The maximum surface horizontal displacement was located above the coal pillar of the goaf, with a maximum value of 1.93 m. (4) Numerical simulation results regarding settlement and crack angles were found to align closely with the UAV survey data. Conclusion The research results can provide a reference for the solution of ecological environmental problems caused by coal seam mining.