Time Dependent Deformations in Squeezing Tunnels

This paper deals with full face excavation of large size tunnels in rock masses of very poor quality which exhibit squeezing problems. Three different time dependent constitutive models of increasing complexity (a viscoelasticplastic model, an elastic-viscoplastic model, and an elastic-plastic-viscoplastic model) are used to analyze the tunnel response during excavation of the Saint Martin La Porte access adit, along the Lyon-Turin Base Tunnel. The interest of the paper stems from the innovative excavation-construction method adopted and the use of yielding support elements incorporated in the primary lining in order to cope with the squeezing problems encountered in crossing a Carboniferous Formation, a highly heterogeneous, disrupted and fractured rock mass, which is often affected by faulting. The tunnel response is back-analysed by using a semi-analytical solution and finite difference modelling in plane strain and axisymmetric conditions when accounting for the time dependent behaviour of the rock mass. The results of modelling are compared with convergence monitoring data and strain distributions measured around the tunnel. The aim is to highlight advantages and disadvantages of different time dependent constitutive models in reproducing the observed tunnel behaviour in both short term and long term conditions. The information provided is of relevance in relation to modelling the tunnel response in severe to extreme squeezing conditions, with acceptable level of confidence, and to the need to optimize in such conditions the type of support adopted and the excavation/support sequence implemented during face advance.