Mechanism and Control Technology of Strata Behavior for Ultra-Thick Coal Seam Multi-Slice Mining

Multi-slice mining of the 70 m ultra-thick coal seam in East Junggar coalfield, China is marked by large-scale mining space expansion and frequent stress disturbances. To address those, this study uses theoretical analysis, physical simulation, and numerical simulation to explore the evolution of an overburden bearing structure and the control of strata behavior in multi-slice mining. The results (1) clarify the overburden fracture-hinging characteristics: fractured blocks in lower hard strata form beam-type hinges (early stage), the lower hinged structure weakens and the beam-type hinge structure moves upward in steps (middle stage), the continuous increase in the mined-out space leads to the transverse O-X fracture of far-stope rock strata, and broken rock blocks are extruded into shells (late stage); this study also proposes an identification method for the morphology of roof bearing structures (including beam structure, higher beam structure, and arch structure); (2) define the support-controlled strata range and load calculation method at different stages, and show that the support load “increases slowly under the near-stope roof bearing structure and tends to stabilize under the far-stope roof bearing structure” as the roof bearing structure moves upward; and (3) guided by the aims of avoiding cantilever beams and ensuring near-stope roof stability, lead us to propose the following measures: pre-splitting main roof (early stage); short working faces with reduced layered thickness and rapid advance (late stage); and goaf/separation grouting (whole process). The maximum support load drops from 20,017.5 kN to 16,192.5 kN, enabling lightweight support selection. This study provides theoretical guidance for support selection and roof control in the multi-slice mining of ultra-thick coal seams.