Starting Underground Excavation Under Unfavourable Conditions

摘要

Modern road layouts are the cause today of increasing difficulties in underground constructions. Wernare not always free to apply traditional design schemes and methods because of the groundrnmorphology and of the existing and irremovable infrastructures. Moreover, the presence, along therntunnel alignment, of low vertical or lateral overburden, road and building underpasses or obliquernportals with low angle of incidence in respect of ground slopes, seriously interferes with the designer’srnwork. This report presents new design solutions for these problems with special care to static andrnenvironmental demands. We describe here some of our latest and completed works along with some ofrnour projects for roads and motorways in Italy. Our two examples describe the execution of the portalsrnof two tunnels with an axis alignment at a low angle of incidence on steep rocky slopes. We decided,rnin this case, to limit the surface ground excavation otherwise necessary when starting to workrnunderground in full section conditions, by a progressive opening of the cavity, in order to modify thernnatural morphology as less as possible. In these cases, the outer tunnel post is placed out of the groundrnfor a significant length and the excavation is only in part driven underground. This means that it isrnabsolutely necessary to assure the stability of the slope portion while removing its foot, during thernexcavation progression. In order to do this, specific structures, named pre-portals, are placed upon thernslope profile. In other cases, appropriate solutions are applied upon the same slope surface, above therntunnel crown, to provide a suitable confinement to the ground. For more details you can visit ourrnwebsite www.sinc.it.