Water Conservation when Mining Multiple, Thick, Closely-Spaced Coal Seams: A Case Study of Mining Under Weishan Lake

To resolve issues of water resource conservation and avoid water inrush accidents, a case study was performed in the Gaozhuang coal mine, which lies under Weishan Lake. The two coal seams are 4 and 5 m thick, and are separated by 2–5 m. As the goaf gradually expanded with mining, some fractures that had formed at the centre of the goaf were gradually compressed and closed, while fractures at the periphery of the goaf did not. Therefore, the periphery of the goaf was recognized as the key region for water conservation mining. After the upper seam was mined, the hydraulically connected fractured (HCF) zone in the overburden had a maximum height of 6.9 times of the integrated mining height. After the lower seam was mined, the HCF zone increased to a maximum height of 9.8 times the integrated mining height. The maximum depth of ground surface fractures above the goaf was 1.1 times the integrated mining height. The thickness of the safety pillar to be left in place as protective strata was determined based on these values, and the hydraulic connection between the aquifers above the mine and the overlying lake was assessed. We then analysed the feasibility of in situ protection of the surface water and proposed techniques to prevent the water in the overlying sandstone from rushing into the work area.