Consolidation settlements above deep tunnels in fractured crystalline rock: Part 2—Numerical analysis of the Gotthard highway tunnel case study

Abstract

A recent high precision levelling survey several hundred metres above the Gotthard highway tunnel in central Switzerland has revealed up to 12 cm of subsidence. Subsidence of this magnitude in relation to a deep tunnel excavated in fractured crystalline rock is unexpected and appears to be related to large-scale consolidation resulting from groundwater drainage and pore-pressure changes around the tunnel. This is a concern for the 57 km long Gotthard Base Tunnel currently under construction, as its alignment will pass through similar rock mass conditions and under several important concrete dams. Thus, the assessment and prediction of potential surface displacements are of paramount importance. This paper, the second of two parts, presents results from an extensive and thorough numerical modelling study focussing on the hydro-mechanical processes responsible for the measured subsidence above the Gotthard highway tunnel. Results derived from 2-D continuum and discontinuum numerical models (i.e. finite- and distinct-element, respectively) show that discrete fracture deformation and poroelastic consolidation of the intact rock matrix both contribute to the observed subsidence. Moreover, the explicit inclusion of geological structures in the distinct-element models enabled a better fit of the width and shape (asymmetry, small-scale inflections, etc.) of the measured subsidence profile to be achieved. Continuum models, although able to reproduce the maximum settlement when constrained by field observations, could not reproduce the asymmetric shape of the subsidence profile leading to under prediction of vertical displacements away from the centre of the subsidence trough.