Numerical investigation of groundwater outbursts near faults in underground coal mines

Permeable geologic faults in the coal seam can cause intermittent production problems or unexpected amounts of groundwater outburst from the underlying aquifers. With the acknowledgment of the basic mechanism for groundwater outbursts, the groundwater outburst along the fault zones in coal mines are numerically investigated using RFPA, a numerical code based on FEM. The fracture initiation, propagation, and coalescence in the stressed strata and the seepage field evolution in the stress field are represented visually during the whole process of groundwater outburst. The numerically obtained damage evolution shows that the floor strata could be classified as three zones, i.e. mining induced fracture zone, intact zone and fault reactivation zone, in which the intact zone is the key part for resisting groundwater outburst and directly determines the effective thickness of water-resisting rock layer. With understanding of the evolution of stress field and seepage flow in floor strata, the groundwater outburst pathway is calibrated and the transformation of floor rock mass from water-resisting strata to outburst pathway is clearly illuminated. Moreover, it is shown that geometrical configuration, including inclination angle of faults and seam drop along faults, have an important influence on groundwater outburst. Finally, based on geological, hydrogeology survey and numerical results, the mechanism analysis of groundwater outburst in an engineering case is studied, which can provide significantly meaningful guides for the investigation on mechanism and prevention of groundwater outburst induced by faults in practice.

Research Highlights
► This study provides supplementary information on the stress distribution and failure-induced stress re-distribution that cannot be observed directly in situ or in experiments, within the areas of floor rock mass with the influence of fault.
► This study gives an interpretation of the fracture initiation, propagation, and coalescence in the stressed strata and the seepage field evolution in the stress field during the whole process of groundwater outburst. The transformation of floor rock mass from water-resisting strata to outburst pathway is clearly illustrated with the fracture evolution.
► This study makes an assessment of safety regarding water-resisting floor rock mass containing a fault with different configuration, including inclination angle and seam drop. And concretely illustrate the influence of fault configuration on groundwater outburst by a case study.