Characterization of rock mass structure for tunneling from outcrop survey and comparison with excavation evidence

This paper refers to the characterization of a geo-structural context traversed by a Aosta-Monte Bianco motorway tunnel. The tunnel was excavated in metamorphic Alpine rocks (mainly calcschist, micaschist and metabasite). Two different characterization methods have been used: a) outcrop geologic mapping and structural analysis with both weighted subjective sampling and systematic sampling: b) systematiC pilot hole structural survey. The outcrop survey allowed one to reliably extrapolate the surface lithologic contacts to the tunnel level. All the structural data have been treated with analytical and computational (simulation) procedures that are useful for geometrical rock mass modelling. A comparison between the surface and underground explorations show an acceptable agreement for joint distribution and at a lesser extent for the fracturing degree. The characterization obtained from the surface survey also gives a suitable appraisal of the tunnell stability condition.