Characteristics and evolution of karst collapse columns in the Huainan coalfield

Karst collapse columns (KCCs) seriously affect the mining safety of deep coal seams. This study systematically summarizes and analyzes the development of KCCs, and classifies the different development stage to identify their development stages and evolutionary process in the Huainan coalfield. The evolution models for KCC development are given, combining with the exploration strata data from boreholes, the groundwater flow data of regional field, the hydrogeochemical data from the relative aquifers of KCC, and the hydrodynamic parameters. The results show that first types of KCCs are discovered in Liuzhuang and the Pansan mine, which is broken and disorganized, with a high degree of filling and cementation, and with the low permeability and velocity, and lower storage capacity. The KCCs in the Xieqiao and Zhangji mine have various morphologies and size differences. Their internal rocks are broken and semi-cemented, with a coefficient of permeability between 0.2 and 0.5 m/d and a specific discharge between 0.1 and 0.2 L/s•m. The KCCs are located in the cone of depression and are part of a sink area with moderate water-richness, where upper and lower aquifers have close hydraulic connections with mixed water quality. The internal core of the KCCs in the Gubei Mine is close to the center of the cone of depression which is highly broken and disorganized, with a high permeability, and a specific discharge more than 1.5 L/s•m. The intensity of runoff increases as its vertical depth also increases, especially the local areas are a higher hydraulic conductivity. Based on comprehensive hydrogeological characteristics, the KCCs development is divided into three stages: growing stage, declining stage, and dead stage. According to their characteristics of different stages, a series of evolutionary processes are established. Combining the sedimentological, karstological, and hydrogeological theories, some measures have been taken for prevention and control of mine water hazard in the various developmental stages. Hence, this research not only provides a new classified approach for KCC stages, but also an essential reference for a better understanding the mechanism of water inrush of KCCs in Northern China.