Transient surface deformations caused by the Gotthard Base Tunnel

Abstract

The Gotthard Base Tunnel (GBT) is a 57km long and up to 2500m deep railway tunnel constructed between 2000 and 2011 in the Central Alps of Switzerland. As drainage of fractured rocks by deep tunnels accompanied by significant decrease in groundwater pressure causes large-scale deformations even in hard crystalline rocks, a comprehensive surface deformation and tunnel inflow monitoring system has been established and operated for more than ten years. This paper presents the results from this monitoring system and explains the observed hydro-mechanically coupled and transient rock mass behavior based on detailed assessments of geological, geomechanical and hydrogeological conditions and conceptual continuum models. The collected data show that significant tunnel-drainage induced surface deformations also develop in rock masses with moderate hydraulic conductivity (2E−9m/s) and small cumulative tunnel inflows (a few liters per second per kilometer). In this case deformations are caused by pore pressure reductions and rock mass deformations around the draining tunnel at depth, and not by groundwater table elevation changes. The pattern of surface settlements observed along the tunnel axis is very irregular (up to 11cm in 2013) and strongly influenced by hectometer scale hydro-mechanical heterogeneities of steeply dipping geological units striking at large angle to the tunnel axes. At the depth of the studied tunnel section (1500–2500m) about 50% of the surface settlements can be recorded. The surface settlements are connected to horizontal displacements and strains directed towards the tunnel axes or advancing tunnel face. The resulting horizontal displacement at the Nalps dam has reached about 65mm in 2013. Compressive strains in the order of 20–50 microstrain are typically observed within a corridor of about 1 to 1.5km width. Outside the reversal point of the settlement trough, extensile strains of similar magnitude develop.