Causal mechanism of Gotthard Base Tunnel-induced ground deformation : Insights from 3D fully-coupled hydro-mechanical simulation and comparison to field measurements

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 reaching about 10 cm were observed during and after the tunnel construction. To gain a better understanding of the causal mechanism of such conspicuous ground deformations, we develop a three-dimensional (3D) fully-coupled hydro-mechanical model to simulate GBT-induced groundwater drainage, stress redistribution, rock mass consolidation, and fault zone deformation at the regional scale. First, we construct a geological model with topographical features, lithological units, and natural faults realistically represented. We constrain the material properties of fault zones and rock masses based on available extensive laboratory testing results and site characterisation datasets. We then simulate the tunnelling process over time, with the resulting coupled hydro-mechanical responses of faulted rock masses well captured and ground surface/subsurface displacements quantitatively analysed. The simulation results in general show a good agreement with the field monitoring data of ground surface displacement, subsurface tunnel settlement, and groundwater inflow into the tunnel. Our model indicates that ground surface displacements originate from GBT-induced water drainage and rock mass consolidation in the deep subsurface. Our results also show that the GBT construction could trigger faults to shear via drainage-induced pressure diffusion and poroelastic stressing. 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