Groundwater Drainage-Induced Rock Mass Deformation During the Gotthard Base Tunnel Excavation in the Swiss Alps: 3D Modelling and Comparison to Field Measurements

Abstract

ABSTRACT: The Gotthard Base Tunnel (GBT), constructed between 2000 and 2011, is a 57 km long and up to 2.5 km deep high-speed railway tunnel located in the Swiss Alps. Significant ground surface displacements up to 10 cm were observed during and after the tunnel construction. To gain a better understanding of the physical mechanisms underpinning such conspicuous ground deformations, we develop a 3D fully-coupled hydro-mechanical model to simulate the excavation-induced groundwater drainage, stress perturbation, and poroelastic response in faulted rock masses over both space and time. First, we construct a geological model with topographical features, lithological units, and natural faults realistically represented. We constrain the material properties of faults and rock masses based on extensive laboratory testing results and site characterisation datasets. We then simulate the tunnelling process with the resulting coupled hydro-mechanical responses of the faulted rock captured and ground surface displacements analysed. The simulation results show a good agreement with the field monitoring data of both ground surface displacement and tunnel settlement. The research findings from our work have important implications for many groundwater drainage-related geoengineering activities such as underground excavation in alpine mountains and fluid withdrawal in subsurface reservoirs. 1 INTRODUCTION The Gotthard Base Tunnel (GBT) is a 57 km long and up to 2.5 km deep high-speed railway tunnel located in the Central Alps of Switzerland (Fig. 1). The excavation work of the two tunnel tubes started in 2000 and successfully completed in 2011. The Sedrun and Faido sections of the GBT are located in crystalline basement rocks of the Gotthard Massif, where three arch dams are located on the ground surface at the Nalps, Curnera and St. Maria hydropower lakes. Due to the significant concern regarding the safety of these nearby hydropower infrastructures during and after the GBT excavation, a comprehensive in-situ monitoring system was installed to closely monitor the surface displacement and tunnel inflow/settlement. According to the monitoring data, up to 10 cm surface displacements and 50L/s tunnel inflow took place during the construction period (Loew et al., 2015).