Numerical simulation of groundwater in karst tunnel areas based on MODFLOW-CFPv2

Southwest China is a typical karst development area, and the groundwater system is extremely complex, with a high degree of heterogeneity and complexity. The karst geological features in this area usually show a fissure-pore dual-media structure, and this geohydrological feature leads to the obvious nonlinear behavior of groundwater flow patterns. These special geological conditions not only pose significant challenges to the development and utilization of groundwater resources, but also complicate the accurate prediction of groundwater dynamics. In particular, when the tunnel construction is conducted in karst areas, it is easy to affect the groundwater flow field and produce water inrush disasters, which will have a far-reaching impact on the local groundwater resources, ecological environment, and even social and economic activities. Therefore, it is of great practical significance to study and understand the law of groundwater flow in karst areas and to accurately predict the amount of groundwater inflow and flow modes during tunnel construction, so as to ensure the safety of engineering construction, the protection of water resources, and the prevention of groundwater pollution.

In this study, the MODFLOW-CFPv2 dual-media coupling model has been adopted. This model integrates two mechanisms-pipeline flow and matrix flow-enabling effective simulation of the complex water flow interaction between the fractured pipeline and the surrounding matrix in karst systems. The introduction of this model enables researchers to more accurately describe the laminar and turbulent flow characteristics in karst groundwater systems, and provides a powerful tool for in-depth understanding and simulation of the dynamics of karst groundwater. By integrating Gempy 3D geological modeling technology, this study further improves the fitting ability of the model to the complex geological structures of karst areas, thereby providing a more realistic depiction of groundwater flow characteristics within the tunnel area. During the simulation, key parameters, including the hydraulic conduction coefficient, recharge coefficient, pipeline size, and friction coefficient, have been calibrated through actual geological and hydrological data to ensure the accuracy and reliability of the model results.

The main objective of this study is to quantitatively evaluate the water inflow and its influence on the groundwater flow field during tunnel construction in karst areas through numerical simulation, so as to provide theoretical support for the hydrological management of tunnel construction. In order to ensure the reliability of the simulation results, a large number of field monitoring data were used for model calibration. Specifically, the long-term monitoring data of borehole water level and spring flow were used as calibration objective functions to reflect the hydrological trend before and after tunnel construction. After calibration, the results showed that the Pearson correlation coefficient between the simulated water level and the measured water level is more than 0.98. This indicates that the model can accurately reflect the actual groundwater dynamics and provides a reliable basis for subsequent predictions.

Simulation results revealed a significant variation in water inflow during tunnel construction. At the beginning of tunnel excavation, the predicted peak water inflow exceeded 20,000 m³·d^-1 due to the drastic changes in the groundwater flow field caused by excavation, indicating a substantially increased risk of water inrush disasters. The simulation also showed that the drainage channel formed after the tunnel excavation caused a rapid drop in groundwater levels, far exceeding natural fluctuations. Even after the completion of the construction, groundwater levels had not returned to pre-excavation levels and remained approximately two meters lower, indicating a lasting impact of tunnel construction on the hydrological system. These findings provide important evidence for the hydrological management during tunnel construction, highlighting the need to consider the long-term effects of such projects on groundwater flow and water levels, especially in karst areas.Furthermore, this study revealed the profound impact of tunnel construction on groundwater flow and spring flow variations. As the tunnel excavation progresses, groundwater is rapidly discharged through karst pipelines, leading to significant changes in regional groundwater flow and spring discharge. The research findings indicate that during tunnel construction, groundwater flow is influenced not only by fissure flow but also interacted with matrix flow, further complicating the groundwater flow patterns. By analyzing the model results, researchers can more accurately identify the extent of the impact of tunnel construction on the groundwater flow field and propose corresponding hydrological management measures.