Ground settlements above tunnels in fractured crystalline rock: numerical analysis of coupled hydromechanical mechanisms

Vertical settlements with magnitudes reaching 12 cm were measured in fractured crystalline rock several hundred metres above the Gotthard highway tunnel in central Switzerland. Such magnitudes of surface subsidence were unexpected, especially in granitic gneisses and appear to be related to large-scale consolidation of fractures resulting from fluid drainage and pore pressure changes following tunnel construction. This paper focuses on the mechanisms involved in the development of such surface displacements and presents the preliminary results of 2-D discontinuum (i.e. distinct-element) and 2-D continuum modelling (i.e. finite-element). Results show that settlements are most sensitive to horizontal joints, as would be expected, but that vertical fractures also contribute to the settlement profile through a Toisson ratio' effect. However, these models also suggest that fracture deformation alone cannot explain the total subsidence measured. As such, 2-D poro-elastic finite-element models are presented to demonstrate the contributing effect of consolidation of the intact rock matrix.