Starting underground excavations under unfavorable conditions

Abstract

Modern road layouts are the cause today of increasing difficulties in underground constructions. We are not always free to apply traditional design schemes and methods because of the ground morphology and of the existing and irremovable infrastructures. Moreover, the presence, along the tunnel alignment, of low vertical or lateral overburden, road and building underpasses or oblique portals with low angle of incidence in respect of ground slopes, seriously interferes with the designer's work. This report presents new design solutions for these problems with special care to static and environmental demands. We describe here some of our latest and completed works along with some of our projects for roads and motorways in Italy. Our two examples describe the execution of the portals of two tunnels with an axis alignment at a low angle of incidence on steep rocky slopes. We decided, in this case, to limit the surface ground excavation otherwise necessary when starting to work underground in full section conditions, by a progressive opening of the cavity, in order to modify the natural morphology as less as possible. In these cases, the outer tunnel post is placed out of the ground for a significant length and the excavation is only in part driven underground. This means that it is absolutely necessary to assure the stability of the slope portion while removing its foot, during the excavation progression. In order to do this, specific structures, named pre-portals, are placed upon the slope profile. In other cases, appropriate solutions are applied upon the same slope surface, above the tunnel crown, to provide a suitable confinement to the ground. For more details you can visit our website www.sinc.it. (A) This paper was presented at Safety in the underground space - Proceedings of the ITA-AITES 2006 World Tunnel Congress and the 32nd ITA General Assembly, Seoul, Korea, 22-27 April 2006. For the covering abstract see ITRD E129148. Reprinted with permission from Elsevier.