A displacement based approach for reinforced tunnel faces under undrained conditions

Tunnel faces excavated under particularly difficult ground conditions are commonly supported. Among the different techniques, the introduction of linear inclusions in the advance core is very popular. The design of this reinforcement system is nowadays based on either empirical or very simplified methods not allowing the estimation of face displacements (extrusion). For this reason, these methods cannot be employed for deep tunnel faces in cohesive soils under undrained or "partially drained" conditions where, owing to the absence of a failure mechanism, a displacement based design approach has to be employed. In this paper, the authors present a displacement based approach to design face reinforcements for deep tunnels under undrained conditions. This approach, based on the employment of a tunnel face non-dimensional characteristic curve, allows to estimate the face displacements once the tunnel geometry, material properties and reinforcement pattern (number, length, diameter) are assigned.