As part of the Lyon–Turin project, the excavation of the Saint-Martin-La-Porte access adit between 2003 and 2010 encountered severe operational difficulties in the coal schists section associated with an extreme squeezing condition. Careful instrumentation of displacement measurements of the tunnel walls along different directions clearly showed important anisotropic behaviour, in accordance with the preferential directions of the main mechanical weakness planes. In this paper, the analysis of the convergence measurements is performed for the sections between chainage 1250 and 1385 where the largest displacements have been recorded. By assuming an elliptic deformation of the tunnel walls, a preliminary geometrical treatment of the raw data is presented to determine the principal axes of deformation. A new anisotropic time-dependent constitutive model, which includes ubiquitous joints of specific orientation embedded in a viscoplastic medium, is then proposed. 3D numerical simulations with the new model are carried out to back analyze the convergence measurements.

1 Introduction

During the excavation of Saint-Martin-La-Porte access adit, extreme squeezing conditions occurred when the tunnel encountered the Carboniferous Formation, exhibiting strong time-dependent and highly anisotropic convergence of cross-section. The squeezing behavior observed in this access gallery has been analyzed in several studies, emphasizing either the time-dependent behavior (Barla et al. 2008, 2011; Pellet 2009) or the anisotropic behavior of rock mass (Russo et al. 2009, Vu et al. 2013a,b). The present paper aims to propose a new model that considers both anisotropic behavior and explicit time-dependent deformation of the rock mass to investigate the squeezing behavior of the Saint-Martin-La-Porte access adit.

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