Analysis of water inrush at Dongyu coal mine in China from an old water-logged goaf associated with a syncline fractured zone

Water inrush from old water-logged goafs is a major cause of disaster in coal mines. The conditions at Dongyu Mine (Shanxi Province, China) were analyzed from three aspects: water source, pressure and flow channels. The water-inrush process has three stages: slow seepage, sudden increase and decrease, and slow decrease. The mechanisms and causes were analyzed, and the relationships among pressure, velocity, porosity and suspended-particle concentration were revealed. High pressure forms easily at the syncline axis, due to water-level difference caused by the syncline structure of two high wings of strata and a low middle section. A tensile fractured zone can form at the axis under a bending moment, which provides a major water-conducting channel. In the slow seepage stage, the relationship between water inflow and time is exponentially nonlinear, due to the positive-feedback process, in which the hydraulic conductivity and effective water-conducting area become larger after the water flow removes the gravels, and this leads to further flow and removal of more gravels. In the sudden increase and decrease stage, the shear force cannot restrain the upwarping of the stratum according to the thick plate model. Maximum displacement of upwarping is 0.02 m. The opening increment is 0.01 m. A small opening can cause the fractured zone to loosen and lose stability, causing large-scale water inrush. In the slow decrease stage, decreasing water level and pressure lead to decrease of water inflow. Numerical simulation showed that when pressure continues to decrease by the same amount, the velocity will decrease more.