Key Factors in the Face Stability Analysis of Shallow Tunnels

Stability of the excavation face in shallow tunnels and in poor ground is at present a relevant problem in tunnelling. A new calculation procedure is illustrated in this study for the analysis of the effect on the face stability in shallow tunnels of the following key factors: pre-support structures, presence of groundwater, free length of the tunnel (length of the unsupported span close to the excavation face). The procedure is based on the Limit Equilibrium Method (LEM) applied to the ground core ahead of the face and it is able to offer also a detailed evaluation of the interaction between each reinforcement element at the face and the surrounding ground. The originality of this procedure is the fact that it is an unified one and permits to take into account all of the following terms: fiberglass longitudinal dowels, non supported tunnel length, pre-support structures and water level ahead of the excavation face. The main result of the calculation concerns the safety factor of the excavation face. Based on a real case (Biella tunnel), a parametric study has been developed to show the influence of the several elements that affect the tunnel face stability in shallow tunnels.