Yanyan Duan

Residual coal pillars retained in the overlying seam represent an important source of stress disturbance that can intensify mine pressure responses in underlying thick coal seam mining. Taking the downward mining of the Carboniferous–Permian dual-system extra-thick coal seam in the Xiao Coal Mine as the engineering context, this study integrates theoretical modeling and numerical simulation to characterize the disturbance extent of the underlying seam, the stress field redistribution in interlayer rock strata induced by the residual coal pillar, the evolution of advanced abutment pressure, and the variation in surrounding rock stress fields as the working face advances underneath the remaining coal pillar. A mechanical model is developed to describe the stress propagation and superposition induced by the coal pillar. Numerical simulation reveals that the vertical stress peak in the interlayer strata reaches 21.79 MPa near the overlying seam floor and 21.71 MPa at the underlying seam roof, and the peak stress in the coal seam roof beneath the pillar reaches 17.0 MPa, approximately 2.2 times the in situ stress, decreasing to 7.8 MPa at 42.5 m from the pillar. As the working face advances from 70 m to 10 m away from the monitoring section, the vertical stress peak on the goaf side increases from 21.19 MPa to 23.69 MPa, and on the solid coal side from 20.73 MPa to 23.46 MPa; the peak position shifts downward by approximately 0.7 m. The high floor stress from the overlying pillar superimposes strongly with the advanced abutment pressure, and the maximum disturbance width occurs at the peak abutment pressure point. Based on these findings, an optimized working face layout is proposed: an internal offset distance of 42.5 m between the mining roadway and the overlying remaining coal pillar. Following implementation of the optimized support scheme and floor grooving measures, field monitoring indicated reductions of 49.5% in roof subsidence and 60% in floor heave, with the maximum floor deformation limited to 256 mm. These results provide a theoretical basis for roadway layout and ground pressure control in downward mining of extra-thick coal seams under similar conditions.

Ground subsidence and shaft lining deformation caused by compressed dewatered bottom aquifers in deep unconsolidated strata mining areas are critical engineering challenges, making the study of the seepage–soil deformation coupling mechanism during groundwater injection remediation vital. This study built a visual cylindrical model (1025 mm × 150 mm); formulated well-graded analogous materials based on the D₂₀ principle to simulate sandy gravel layers; embedded FBG sensors at 200/400/600 mm depths, combined with a dial indicator on the model top; and conducted two water injection–dewatering cycles. Results indicate: water injection generates excess pore water pressure, placing the entire model in a tensile stress state with top rebound; post-injection vertical stress redistributes (tension above the injection point, compression below, and an interlaced transitional band), validating the necessity of full-section injection; during the second injection–dewatering cycle, tensile strain at the upper monitoring point reaches 597.77 με, while compressive strain at lower depths reaches −253.90 με, internal deformation stabilizes within 6.5–10.0 days, injection improves the in situ stress state by reducing effective stress, and the deformation of the field strata remains in a stabilization period, with the stabilization time decreasing as the depth of the strata increases. This study clarifies the temporal evolution and representative spatial variation in internal strain at monitored depths during injection, providing theoretical and design references for optimizing water injection schemes to mitigate coal mine shaft damage.

Strong mine pressure often emerges when the working face of the lower coal seam in a closely spaced coal seam system passes through the remaining coal pillar in the overlying goaf. This study investigates the law of overburden movement and the manifestation of mine pressure during mining under the remaining coal pillar. A physical model measuring 2.5 × 0.2 × 1.503 m is constructed. Fiber Bragg grating sensing technology (FBG) and Brillouin optical time domain analysis technology (BOTDA) are employed in the physical model experiment to monitor the internal strain of the overlying rock as the working face advances. This study determines the laws of overlying rock fracture and working face pressure while mining coal seams beneath the remaining coal pillar. It analyzes the relationship between the pressure at the working face and the strain characteristics of the horizontally distributed optical fiber. A fiber grating characterization method is established for the stress evolution law of overlying rock while passing the remaining coal pillar. The experimental results indicated that the fracture angle of overlying rock gradually decreases during the mining stage through and after the coal pillar. In the mining stage through the coal pillar, the cycle pressure step distance of the working face is reduced by 33.3% compared to the stage after mining through the coal pillar. Initially, the strain pattern of the horizontal optical fiber is unimodal when pressure is first applied to the working face, and it transitions from unimodal to bimodal during periodic pressure. The peak value of fiber Bragg grating compressive strain and the range of influence of advanced support pressure are 3.6 times and 4.8 times, respectively, before passing through the remaining coal pillar. Finally, the accuracy of the FBG characterization method is verified by comparing it to the monitoring curve of the coal seam floor pressure sensor. The research results contribute to applying fiber optic sensing technology in mining physical model experiments.