Study on Overburden Migration Law During Working Face Mining After Grouting Reconstruction of Unconsolidated Aquifer

To clarify the migration and structural evolution of mining-induced overburden following grouting reconstruction of the Fourth Aquifer, the inner section of Panel 1022-2 in Wugou Coal Mine was taken as the engineering background. The evolution law of overburden movement and the development characteristics of the caving zone were systematically investigated via theoretical analysis, similar-material simulation, and numerical simulation. In addition, the maximum caving-zone height of Panel 1022-2 was calculated based on the measured caving-to-mining ratio of the adjacent Panel 1010-1. The results show that following grouting reconstruction of the Fourth Aquifer, the water inflow and permeability coefficient decreased significantly, the mining-induced water-body grade was classified as Grade III, and the required coal pillar type was converted from a waterproof safety coal (rock) pillar to an anti-collapse safety coal (rock) pillar. The bedrock failure morphology evolved sequentially from a symmetrical trapezoid to a stepped shape and finally to an asymmetrical saddle shape, with a maximum caving-zone height of 19.0 m, whereas the Fourth Aquifer evolved from fracture initiation and bed separation to asymmetrical overall subsidence. Overburden migration is jointly controlled by bedrock thickness and the mechanical properties of the unconsolidated layer, presenting a distinct three-stage evolution pattern. As the size of the reserved safety coal (rock) pillar decreases, the overburden failure mode changes from overall plastic failure under relatively thick bedrock, to semi-block failure with longitudinal fractures penetrating to the base of the Fourth Aquifer and transverse fractures and interlayer separation initiating inside the aquifer, and finally to intensified failure under thin-bedrock conditions. Based on field analogy with Panel 1010-1, the maximum caving-zone height of Panel 1022-2 was calculated to be 19.73 m, which is in good agreement with the numerical and similar-material simulation results, verifying the reliability of the three-stage overburden evolution law and the caving-zone height evaluation.