Recent innovations in yield-control support systems allow to increase the rate of advance when tunneling in difficult conditions is associated with severely squeezing rock. Such systems which imply the insertion in the lining of highly deformable concrete elements are being adopted successfully in tunneling projects using conventional excavation methods. The Saint Martin access edit excavated in a Carboniferous Formation along the Base Tunnel of the Lyon-Turin rail line is presented as a case study. Numerical analyses are discussed to compare the results of computed and measured performance of a typical monitored section and to find out possible optimizations of the support system adopted.
Tunnel construction in squeezing rock is very demanding due to the difficulty in making reliable predictions at the design stage. Squeezing conditions may vary over short distances due to rock heterogeneity and fluctuations in rock mass properties. Indeed, the selection of the most appropriate excavation-construction method to be adopted is highly problematic and uncertain. In deep tunnels, whenever squeezing conditions are anticipated, conventional tunneling appears to be yet the method most often used. Conventional tunneling in squeezing rock generally takes place with a slow rate of advance. However, if the work at the face is well planned and appropriate stabilization measures are implemented, excavation can proceed at an acceptable rate of advance. A clear need to develop appropriate technological systems that help increase such a rate of advance is to be recognized (Cantieni & Anagnostou, 2009).
The Saint Martin La Porte access edit is a vital part of the early works for the Lyon-Turin Base Tunnel, which is at the centre of the axes linking the North and South, and East and West Europe and is to be excavated between the portals in Italy and France.