Numerical Simulation of Water–Silt Inrush Hazard of Fault Rock: A Three‑Phase Flow Model

Excavation in underground mines could activate faults that may cause a large amount of water ingress when connected with the aquifer. Water inrush liquefies the materials within the fault to form fault rock. Fault rocks have numerous effects because their mechanical and physical properties differ from those of the surrounding rocks. They decrease the waterproof ability of the fractured zones, damages the integrity of the strata formation, facilitate the formation of water-conducting pathways and increase the permeability of the granular rocks. Therefore, fault rock is a typical hazardous material that can cause severe causalities, economic losses and environmental degradation.

Water-silt inrush in the tunnel is a common problem when the aquifer is connected with the fault. It causes the movement of the water-silt fluid and the fine rock particles to form a water-rock-silt three-phase flow in the fault fracture zone. This portrays a close relationship between the three-phase flow and the water-silt inrush hazards in the fault rock. Fluid-solid flow in soils and rocks has become a significant issue in geofluid-induced hazards. Understanding the hydraulic properties in the three-phase seepage field is imperative for effective control and prevention of the water-silt inrush.

Lately, numerical simulation methods have been adopted to overcome the limitations of existing water-solid flow tests in studying the hydraulic mechanisms of fluid-solid flows. Regardless, the influence of silt migration on the hydraulic properties of fault rocks is unclear, partly due to the complex nature of the three-phase flow. To this noted, Professor Dan Ma, Dr. Hongyu Duan, Professor Jixiong Zhang and Dr. Xianwei Liu from China University of Mining & Technology, together with Dr. Zhenhua Li from Henan Polytechnic University, numerically investigated the hydraulic characteristics of the fault rock formed due to the water-split inrush hazards.

Briefly, the authors started by proposing a one-dimensional radial three-phase flow model of water-rock-silt and defining the continuity, mass conservation, erosion constitutive and momentum conservation equations. The momentum conservation and the migration of the rock particle were described by non-Darcy flow and erosion constitutive equations, respectively. The accuracy of this model was verified by comparing the numerical results with the laboratory test results of the porosity and volume discharge evolution rate. The work is published in the journal, Rock Mechanics and Rock Engineering.

The authors comprehensively compared the numerical and laboratory test results for different cases to establish the variation in the model accuracy. The model accuracy first decreased before increasing and again decreasing after 1400s for the fitting of the sample porosity temporal evolution during seepage. For the fitting of porosity distribution after seepage, the model accuracy increased when the fluid outlet distance became much closer. Lastly, the model accuracy was much higher at around 200s and 1600s for the volume discharge rate evolution fitting.

Although the proposed model with silt flow and that without silt flow exhibited similar variation trends, the model with no silt in the mixture fluid exhibited the highest accuracy. Furthermore, testing values exhibited low dispersion while higher silt concentration caused higher standard deviations in the repeated test results. Further comparison of the cases under different silt concentrations confirmed that higher silt concentration played a key role in inhibiting the erosion effects. This was attributed to the corresponding increase in the density of the mixed fluid.

In summary, the water-silt inrush hazard of the fault rock was successfully simulated to obtain the associated temporal-spatial distribution. An increase in the permeability and porosity of the fault rock caused by the fluidization and constant outward migration of the rock particles near the fluid outlet during the three-phase flow was reported. Its effects were enhanced by the gradual formation of water-conducting pathways in the fault rock. Due to the water resistance to silt and fluidized rock particles, the pore pressure exhibited a nonlinear spatial distribution. In a statement to Advances in Engineering, Professor Dan Ma said the new model will be a powerful tool in preventing and controlling water-silt inrush in underground mines and tunnels.