Numerical Modelling of Mining-induced Inrushes from Subjacent Water Conducting Karst Collapse Columns in Northern China

Collapse columns are geological features formed by chimney caving of karst voids. Collapse columns can cut through the rock mass and introduce vast geological complexity in coalfields. When a collapse column becomes accessible to a confined aquifer, water inrush disasters can be triggered. In northern China, inrushes related to collapse columns occur in regions where extensive Ordovician limestone underlies the coal measures. This limestone is strongly karstified and holds large quantities of water. The piezometric surface can be considerably higher than the coal extraction level. Numerical simulation was used to explore how the water inrush pathway forms, based on a demonstration model. First, the evolution of the stress field, damage field, and flow volume within the coal floor was analysed. Then factors such as the column height, mechanical strength of the coal floor, and hydraulic pressure in the aquifer were considered. In the demonstration models, if the height of the collapse column exceeded 35 m and the initial thickness of the aquiclude was less than 15 m, an inrush definitely occurred, even when the strength of the floor strata was relatively high. Finally, a practical case of water inrush in the Fangezhuang coal mine related to a collapse column was numerically investigated. Modelling of the fracturing process provided insight into evolution of the fracture zone and inrush processes that cannot be observed in the field and are difficult to evaluate using static stress analysis.