The Impact of Hsueh-Shan Tunnel Construction on the Hydrogeological Environment in Northern Taiwan

The Hsueh-Shan tunnel, the fourth longest tunnel in the world, was suffered many collapses due to huge groundwater ingression and was ultimately opened to the public in June, 2006, after 15-year construction. Since the commencement of construction of tunnel, a long-term monitoring project to measure the groundwater ingression into the tunnel was conducted to ensure the safety of tunnel structure. According to the measured data obtained from the monitoring project, the average total flux of ingressive groundwater is about 450 l/sec. In order to assess the influence of this huge amount of ingressive groundwater on the regional hydrogeology near the tunnel area, a hydrogeological conceptual model was developed. We use MODFLOW-2005 to simulate groundwater flow and use the automated parameter estimation method to calibrate the model. The data on geography, geological structure, and groundwater hydrology are compiled to develop the conceptual model and the measured flux of ingressive groundwater is used to calibrate the model. The regional hydrogeological characteristics, before and after the tunnel excavation, and the water resources are evaluated by this calibrated conceptual model. The result shows that the ingression of groundwater into the tunnel is almost reach the steady state and the total amount of water is mainly contributed by about 2% loss of the total inflow of the Feitsui Reservoir. Besides, the condition of linings in the tunnel plays an important role in the safety of tunnel structure. Therefore, the long-term monitoring project should be continuously conducted to ensure the distribution of water resources and the safety of tunnel structure.