The Saint Martin La Porte access adit (Lyon–Turin Base Tunnel) revisited

Abstract

This paper presents the monitoring data collected over a period of five years from January 2006 to December 2010, during which the Saint Martin La Porte access adit of the Lyon–Turin Base Tunnel was excavated through a Carboniferous Formation, a highly heterogeneous, overstressed and in cases anisotropic rock mass exhibiting a squeezing behaviour. The interest of these data stems from the information which became available along a significant tunnel length in the same rock mass, where a novel excavation and support method was adopted with highly deformable elements inserted in the shotcrete lining which allowed for controlled deformation, rate of deformation and stress in the rock mass and in the supports. A description of the geological and geomechanical conditions met and of the tunnel response in different cross sections is given in terms of developed convergences, rates of convergence, strain distributions around the tunnel and ahead of the face, including the state of stress in the supports. In conclusion an attempt is made to characterise the Carboniferous Formation in terms of the extent of squeezing and in view of the future excavation of the Base Tunnel.