Current Projects / Base tunnel Lyon - Turin / Yieldable shotcrete
lining at the St. Martin la Porte access gallery

The application of bearing yet yielding shotcrete shells is based on the engineering principle: In squeezing rock, the rock pressure decreases with increasing rock deformation.

An implementation of this principle are tunnel linings made of shotcrete shells with integrated deformable hiDCon® - elements. The result is a bearing yet yielding lining that reduces rock pressure.

The shape and number of hiDCon® elements per cross section depends on the amount of permitted radial deformation. The hiDCon® elements resist the shotcrete lining with a predetermined bearing capacity so the shotcrete does not become overstressed. The hiDCon® elements are designed so that after exhausting their working range only small additional deformations develop. The lining resists these (small) deformations with the full bearing capacity of the shotcrete shells.

The installation of the lining support within the access gallery, St. Martin la Porte, is being carried out in several steps because of the large amount of convergence (up to 2m). After full face excavation of the tunnel the primary excavation support is a thin sprayed shotcrete layer and steel arches with sliding connections. Behind the tunnel face, the partially converged tunnel sections are being reprofiled and a second step support system installed. This support system consists of new steel arches with sliding connections, beam shaped hiDCon - elements placed between the sliding connections and a 20 to 30 cm thick sprayed shotcrete layer.

The second step support system is able to absorb the additional tunnel convergence with increased bearing capacity so as to halt later deformations. In the final step (about 80 m behind the face) the final tunnel lining is being concreted in place.