Analytical modeling and field verification of roof failure mechanisms in steep extra-thick coal seams under permafrost conditions

The redistribution of in-situ stress within the coal and surrounding rock mass leads to progressive deformation and eventual structural failure during mining process. Understanding the failure mechanisms of overlying strata in steeply inclined extra-thick coal seams is critical for improving safety and optimizing mining strategies under complex geological conditions. In this study, taking 20# steep extra-thick coal seam in Jiangcang No. 1 coal seam located in Qinghai Province, China, as an engineering background, each mining level is subdivided into three segments, and a mechanical model based on elastic thin plate theory is developed to derive the expressions for both initial and periodic weighting step distances of the overlying strata. The results indicate that the immediate roof experiences its first failure during the second segment of the first mining level, characterized by a vertical O-X fracture pattern when the working face advances to 38 m. The corresponding periodic weighting step distance is calculated to be 23.48 m. Following the collapse of the immediate roof, the accumulated caved materials fill the voids above the coal seam, effectively reducing the suspension span of the main roof. As a result, the main roof remains intact after the completion of the first mining level. In the second mining level, the main roof undergoes its initial failure in the second segment at an advance distance of 97 m, exhibiting a horizontal O-X failure mode along the strike direction. The periodic weighting step distance for the main roof is subsequently determined to be 59.96 m. These findings clarify the distinct failure modes and weighting characteristics of steep extra-thick seams, providing a theoretical basis for safer coal recovery and for protecting the integrity of overlying frozen soil layers in cold regions.