Influence of the fibreglass reinforcement stiffness on the mechanical response of deep tunnel fronts in cohesive soils under undrained conditions

The fronts of tunnels excavated under particularly difficult ground conditions are commonly reinforced by inserting fibreglass pipes. This technique is particularly popular since it is very simple to adapt the reinforcement number/length according to the nature of soils encountered. In this paper, the authors illustrate the results of a 3D FEM numerical campaign aimed at analysing the influence of the reinforcements on the system response, conveniently summarized by employing a suitably normalized front characteristic curve. The numerical analyses were performed by assuming the material to be isotropic, homogeneous and characterized by an elastic-perfectly plastic constitutive relationship. Only undrained conditions are taken into account: the failure locus is defined according to the Tresca criterion and the flow rule is assumed to be associated. The numerical results show that the effectiveness of the inclusions is not related to the absolute value of the inclusion stiffness but to the value of a suitably defined non-dimensional variable. By employing this non-dimensional variable the authors show that it is possible to tailor the reinforcement stiffness according to the nature of the soil encountered.