SUMMARY:

The excavation of a road tunnel in a region with complex hydrogeological conditions and the presence of valuable mineral water springs is the subject of this case history paper. The tunnel comprises two sections, Frasnadello and Antea, linked by a bridge, and has a length of 2400 m. Located in the Brembana valley, near the town of San Pellegrino Terme, to the North of the city of Bergamo (Italy), the tunnel was excavated in 1996–97 through Dolomite, Limestone and Argillite Formations, by a large diameter (11.8 m) Tunnel Boring Machine (TBM). The excavation took place as the final enlargement of an exploratory pilot tunnel (3.9Êm in diameter), which was excavated again by a TBM, more than two years in advance of full size excavation. The content of the paper is considered to be well in line with the main theme of the GeoEng 2000 Conference as geo-engineering concepts and methods are applied to engineering practice, through the use of a large spectrum of the geo-engineering disciplines. The first part of the paper describes the series of investigations (including geological, geomorphological and geostructural studies, borehole logging, and geophysical surveying) undertaken in order to tie-down the geology and to increase the understanding of the hydrogeology in the area of the mineral water springs, also to derive a predictive picture of the rock mass conditions along the tunnel alignment. In addition, a numerical modelling study was performed in order to evaluate the influence of tunnel excavation on the springs discharge. Then, the attention is moved to the full-size excavation of the main tunnel. A description is given of the stress analyses carried out in order to assess the stability conditions and to evaluate the stress distribution in the primary lining, in terms of rock-structure interaction considerations.

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