Abstract
Recent measurements of surface displacements 800 m above the Gotthard (N2) highway tunnel in central Switzerland have shown up to 12 cm of subsidence since the tunnel was constructed. Subsidence of this magnitude in a fractured crystalline rock mass is unexpected and appears to be related to large-scale consolidation resulting from groundwater drainage and pore pressure changes around the tunnel. With the recently initiated construction of the 57 km long Gotthard Base Tunnel, which passes through similar geological conditions as the Gotthard highway tunnel and underneath several important dams (the integrity of which could be adversely affected by differential settlements), understanding these processes and being able to predict the magnitude and extent of surface displacement becomes highly relevant. This paper focuses on the use of 2-D finite-element models to analyse the measured subsidence magnitudes recorded over the Gotthard highway tunnel, treating the fractured rock mass as an equivalent poro-elastic medium. The provisional results reported in this paper address the hydrogeological controls and their affect on the extent of the diffusion front and the resulting shape and magnitude of the subsidence profile.
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