In the context of the Saint-Martin-la-Porte survey project of the Lyon-Turin railway link, an access gallery (SMP2) was first excavated across a Carboniferous formation, where tectonized productive Houiller was met at a depth of 300 m. It exhibited a highly squeezing behavior, characterized by large, time-dependent and anisotropic deformation. Recently, a new survey gallery (SMP4) began to be excavated along the axis of the future base tunnel at a depth of about 600 m. Squeezing conditions have been met again when SMP4 crossed the same Carboniferous formation. Practical problems have been encountered related to the large and anisotropic closure of the cross-section and the instability on the tunnel face. On the basis of previous studies and numerical analyses of carried out for SMP2, the proposed methods are extended to SMP4 conditions. Numerical modeling is performed using FLAC3D code and the various phases of excavation and specific support installation are considered. The efficiency of the compressible elements in reducing the stress in the support system is demonstrated.
As part of the Trans-European Transport Network (TEN-T) Project, the Lyon-Turin railway is a key element of the Mediterranean Corridor. It will connect France and Italy through a 57.5 km base tunnel under the Alps. In Saint-Martin-la-Porte in France, a survey project is underway to study the geological environment of one of the most complex areas of the Lyon-Turin base tunnel (figure 1a). An access gallery (SMP2) was first excavated to reach the future base tunnel to provide access for the excavation of the base tunnel. A tectonized Carboniferous formation was encountered at a depth of 300-330 m, where productive Houiller was encountered. It presents a very heterogeneous stratified and fractured structure, composed of schist and/or Carboniferous schist, sandstone and a large proportion of clastic rock. Due to the poor mechanical properties of the rock mass, squeezing behavior was observed around the tunnel. It is characterized by large, time-dependent and often anisotropic convergence around the tunnel wall. Observation and convergence measurement show an elliptical deformation of the tunnel section with a high convergence up to 2 m, which produces severe problems for the excavation and support process. On the basis of the intensive field monitoring, a number of studies have been carried out on SMP2 to analyze the response of the rock masses and of the specific support systems during and after excavation (e.g. Barla et al., 2007; Russo et al., 2009; Vu et al., 2013; Descoeudres et al., 2015; Tran-Manh et al., 2015).