A Study on the Characteristics of Complex Overhanging Roof Structures on the Go-Ahead Side in High-Intensity Mining and Roof Control: A Case Study of the 12-Shang-302 Working Face

Abstract In the Shendong Mining Area, under high-intensity mining conditions in shallow coal seams, the problem of intense mining pressure in open-face roadways caused by large-scale overhangs in thick, hard roof strata has become increasingly prominent. Taking the 12-Upper 302 working face at the Guoneng Shendong Burtay Coal Mine as the engineering context, this paper systematically investigates the structural characteristics of thick, hard roof overhangs on the open-face side and pressure relief control technologies by comprehensively applying on-site core sampling and testing, theoretical analysis of key strata, multi-point displacement monitoring, and roof pre-fracturing blasting tests. The results show that within a 40-meter range above Coal Seam 12 in the study area, there is one main critical layer and seven subcritical layers. Among these, the lower subcritical layer consists of coarse-grained sandstone with a thickness of 12.04 meters; it is located within a collapse zone with a height of approximately 16.8 meters. The overlying strata exhibit a “dual-zone” distribution comprising a collapse zone and a fracture zone, with the fracture zone reaching a height of approximately 54.6 meters. Multi-point displacement monitoring revealed that displacement changes were greatest at the lower measurement points and gradually decreased as the vertical height increased, indicating that the suspended roof structure is dominated by the critical lower layer. Fractures propagate in the direction of movement through a bottom-up layered transmission mechanism, while the direction of inclination forms a stepped lateral cantilever beam structure. The cantilevers exhibit periodic growth-fracture cycles of 8–12 m, and after fracturing, they transform into a masonry beam structure. Based on this masonry beam structure, this study elucidates the disaster-causing mechanism resulting from the coupling of static and dynamic loads in multi-key-layer composite suspended roofs. A long cantilever extending from a low-lying critical layer in a subsidence zone causes long-term concentration of high static loads on the coal pillar, while sequential fracturing of multiple subcritical layers in a fracture zone generates impact dynamic loads; the superposition of these two forces, transmitted through the narrow coal pillar, induces instability in the surrounding rock of the tunnel. Based on the characteristics of surrounding rock instability and failure, a zoned, graded, and targeted pre-fracturing stress relief plan is proposed. By employing a three-tiered drilling and fracturing method—low, medium, and high—the load is applied to the key controlling layer in the collapse zone and to sub-key layers at different levels within the fracture zone. This approach eliminates static loads at the roof and distributes dynamic loads, thereby achieving a layered, staged collapse of the roof. The study revealed the structural characteristics of the roof—characterized by “multiple key layers overlapping, low-level dominance, and a stepped cantilever”—in the goaf side under high-intensity mining, as well as the mechanisms underlying the resulting disasters. The proposed zoned and graded stress-relief plan can serve as a reference for the prevention and control of high mining pressure in goaf tunnels in mines with similar conditions.