Weakening mechanism and instability characteristics of coal pillar under mining disturbance and water immersion in water storage goaf

Using the goaf of coal mine as a temporary water storage can alleviate the mining drainage difficulty of water rich roof working face and speed up the process of mining and excavation. However, the instable characteristics of residual coal pillar in goaf under the influence of mining water immersion will affect the safe mining. Based on this, the weakening mechanism and instable characteristics of mining water immersion of coal pillar in this section were analyzed by field measurement and fluid solid coupling numerical simulation. The results of transient electromagnetic method combined with borehole stress meter and roadway surface displacement monitoring have shown that during the mining process of the first longwall face (P122106), the coal pillar stress was distributed in a saddle shape, which can form an effective support for the roadway roof. However, under the coupling of the adjacent longwall face (P122108) mining and water immersion of the goaf, the deformation of P122108 roadway increased greatly and the coal pillar was unstable. The progressive damage characteristics of coal pillar under mining disturbance and water immersion were obtained by using the self-developed fluid-solid coupling numerical simulation and quantitative index of damage degree. The influence of different goaf water level on coal pillar stability was studied. Water immersion weakening makes the coal pillar with a width of 35 m insufficient to isolate the reservoir in the goaf, causing water inrush in longwall face. In the mining process of the first longwall face, the damage degree of coal pillar was 53.4%; Under the influence of water storage in goaf and adjacent longwall face mining, the damage degree of coal pillar in coal pillar observation area reached 84.1%, which exceeded the critical damage degree of coal pillar instability by 65.7%. When the water level in the goaf dropped to 2 m and 4 m, although the damage degree of coal pillar decreased, which were 70.5% and 76.8%, respectively, it still exceeded the limit damage degree. Considering the actual conditions on site and the results of numerical simulation, the solution measures of draining the water in the reservoir of P122106 goaf and strengthening the support to reduce the damage and deformation of the surrounding rock of the alley were proposed. After adopting the above measures in the study mine, the deformation of coal pillar and roof was reduced by 39.6% and 30.8%, ensuring the safe production of the working face. © 2022, Editorial Board of Journal of Mining & Safety Engineering. All right reserved.