Time Dependent Modelling of Tunnels in Squeezing Conditions

The numerical analysis of tunnels in rock masses, which exhibit a squeezing behavior, is considered, with reference to the use of two elastoviscoplastic constitutive laws implemented in finite-difference and finite-element codes. Because of the fact that in the most severe squeezing conditions time dependence is observed during excavation and may last for a long time, both the reinforcement of the tunnel face and the support of the tunnel section are to be considered at the design and construction stages. As a consequence, the need arises to analyze the tunnel excavation-construction sequence in detail and to account for time-dependent behavior. Numerical analyses have been carried with the Saint Martin La Porte access adit, along the Lyon-Turin Base Tunnel, taken as a case study. The interest stems from the performance monitoring data available for a significant tunnel length in the carboniferous formation, which exhibits a severely squeezing behavior. Also of interest is the adoption of a novel yielding support system, which revealed to be successful in coping with the difficult conditions encountered. Following a series of preliminary back analyses, carried out in axisymmetric conditions and meant to calibrate the rock mass parameters, attention has been focused on the support system. The actual excavation-construction sequence has been simulated in detail to investigate the tunnel response and the stress changes in the support. The numerical results are compared with the monitoring data, and the potential of the two different elastoviscoplastic constitutive models in describing the tunnel behavior is assessed in detail.