Study on the Influence of Stress Relief in Close-Distance Coal Seam Goaf and Roadway Excavation Disturbance on Surrounding Rock

To investigate the influence of pressure relief in close-distance coal seam goaf and roadway excavation disturbance on surrounding rock stability, this study takes the large-section open-off cut of Ⅱ8224N as the research object and employs methods including RQD rock mass quality evaluation, FLAC3D numerical simulation, similar material simulation experiment, and field borehole camera observation. The results show that: The RQD rock mass quality evaluation indicates that the immediate roof siltstone (thickness: 3.8m) and the fine sandstone in the lower part of the main roof are classified as medium-quality Grade Ⅲ rock mass, while the fine sandstone in the upper part of the main roof and the mudstone at the floor of No. 73 coal seam are extremely poor Grade Ⅴ rock mass; The goaf of No. 73 coal seam exerts a significant pressure relief effect on the surrounding rock of the Ⅱ8224 open-off cut, which is located in a pressure relief zone with a pressure range of 2.5ཞ5.0MPa. A 3m-wide non-plastic failure zone exists between the goaf floor and the open-off cut roof, and no plastic failure occurs within the 7.4m anchorage range of the cable bolts. The rock bolt zone on the roof is a tensile stress zone, while the cable bolt zone is a compressive stress zone; After the excavation of the open-off cut, a new surrounding rock bearing layer is formed between the open-off cut and the goaf. Stepwise excavation induces secondary stress disturbance in the roof surrounding rock, and after roadway formation, the stress continues to increase with the most concentrated tensile stress in the middle part; The similar material simulation experiment shows that the roof of No. 73 coal seam fully collapses when the load exceeds 6.60kN, with caving angles of 65°and 60° respectively. The stress of the goaf floor converts to tensile stress after 300 minutes and tends to stabilize after 1200 minutes; Field borehole camera observation reveals that the overall integrity of the roof surrounding rock is good, with only a small number of circumferential fractures. The research results provide a certain theoretical basis and technical support for the surrounding rock control of large-section open-off cuts under close-distance goaf conditions.