Evaluation of hydrogeological impact of tunnel engineering in a karst aquifer by coupled discrete-continuum numerical simulations

Anthropogenic disturbance of karst groundwater systems by large-scale underground engineering activities is an important topic but is difficult to address due to the compound influence of the heterogeneity and construction progression. In this study, we adopt the coupled discrete-continuum modeling approach where both the tunnels and karst conduits are treated as discrete channels. Based on extensive site characterization, groundwater observations, and tunnel discharge data, we establish a three-dimensional coupled model for a karst aquifer where deeply-buried, long tunnels are being constructed. We perform high-resolution simulations to quantitatively evaluate the spatial and temporal characteristics of the tunnel construction impact on a karst groundwater system. The simulation results satisfactorily reproduce the tunnel discharge time-series. We show that the excavation and lining schedule is an important factor to consider when evaluating the hydrogeological impact. During tunnel construction, the groundwater drainage through the tunnels (5.3 x 10(7) m(3)) accounts for a significant part of the total groundwater discharge (similar to 11%) in the studied area. The continuous water influx to the tunnels results in widespread drawdown with an area of impact (with drawdown >5 m) covering 64.0% of the entire study area after four years of construction. The total discharge through karst conduits decreases by about 30% in response to the tunnel construction. Our simulation results predict that after completion of lining the groundwater system will slowly approach a groundwater level 5-10 m below the undisturbed level. This work demonstrates the use of coupled discrete-continuum models to study regional groundwater flow in karst aquifers affected by tunnel engineering activities and also provides a guidance for the impact assessment of other underground engineering projects in mining and transportation industries. The simulation methodology may be exploited for optimization of the construction plans to minimize hydrogeological impact and to ensure construction safety.