Influence of the excavation rate on the mechanical response of deep tunnel fronts in cohesive soils

In conventional tunnelling, the mechanical response of the tunnel front is a main concern and depends on both the geometry (tunnel diameter and cover) and soil mechanical properties. Moreover, in case of excavations in soils characterized by a low value of permeability, even the time factor plays an important role: in case of particularly problematic soils, the displacements of the front are expected to progressively increase with time and tunnel fronts stable under short term conditions potentially either develop unacceptable displacements or become unstable under long term conditions. In this paper, the mechanical response of deep tunnel fronts excavated in a homogeneous cohesive soil stratum are analysed by both performing experimental 1g small scale model tests and non-linear 3D FEM analyses. The numerical results are obtained by assuming the material to be isotropic, homogeneous and characterized by an elastic-perfectly plastic constitutive relationship. The results are presented in terms of the tunnel front characteristic curve, defined in analogy with the well-known characteristic curve for the tunnel cavity and by employing a suitable non-dimensional variable depending on the excavation time, the soil hydraulic/mechanical properties and the tunnel geometry. Finally, the authors introduce a rapid procedure allowing the front displacements estimation without performing ad hoc numerical analyses.