Abstract: In view of the two typical modes of water inrush disasters, progressive fracturing of rock mass and seepage failure of filling structure, the mechanism of progressive fracturing of rock mass under the combined effects of dynamic disturbance, excavation unloading and high water pressure is described. The seepage failure mechanism of the variable strength-variable permeability-variable viscosity of the filling structure under osmotic pressure is also expounded. For the variable viscosity mechanism of water inrush caused by seepage failure of filling structure, a qualitative simulation study on the effect of fluid viscosity on seepage failure mechanism is carried out using the DEM-CFD coupled simulation method. The effects of fluid viscosity on the average contact force, flow rate (flow velocity), porosity, particle migration process, migration trajectory and critical hydraulic gradient of the simulation model are analyzed. The results show that the critical hydraulic gradient of fluid with low viscosity is smaller than that with high viscosity. In other words, seepage failure of filling structure is more likely to occur under the action of fluid flow with low viscosity; the average contact force is especially sensitive to the response of critical value of the hydraulic gradient, however it is difficult to be accurately reflected by the flow rate. Considering only the variable viscosity mechanism of water inrush due to seepage failure (regardless of the effect of increasing permeability), as the viscous medium flows into water, the fluid viscosity would increase, but the flow velocity would decrease, and the combined action of these two changes would actually hinder the development of seepage failure process. Finally, the phenomenon of water inrush process in engineering scale is simulated using DEM-CFD method, and the formation and expansion process of the dominant channel of water inrush is reproduced. The problems of parameter selection and quantitative analysis are identified to realize the simulation of water inrush mechanism.

Key words: water inrush disasters, mechanism, fluid viscosity, seepage failure, DEM-CFD, numerical simulation