This paper considers the studies carried out to assess the geological, hydrogeological and rock mass conditions along the alignment of two tunnels to be excavated near the commercially exploited San Pellegrino thermal springs. The predictions made through an extensive investigation programme carried out in advance of tunnelling are described and compared with those obtained during the TBM excavation of a pilot tunnel. Considerations are also reported on the second, widening to full size, phase of tunnelling.
Ce texte donne une description des etudes effectues afin d'evaluer les conditions geologiques et hydrogeologiques du massif rocheux Ie long deux tunnels qui seront excaves dans une zone près de I'exploitee source thermale de San Pellegrino Terme (Italie). Les previsions envisagees à travers un programme d'investigations effectue avant Ie creusement de tunnels, sont decrites et comparees avec les donnees obtenues pendant l'excavation du tunnel pilote avec tunnelier. Les considerations sur la deuxième phase d'agrandissement jusqu' à la section finale des tunnels sont egalement exposees,
Dieser Artikel beschreibt die Studien, die zur Definition der geologischen und hydrogeologischen Situation sowie des Zustands der Felsmassen durchgefuehrt wurden, die langs der beiden kommerziell genutzten Tunnel in der Nahe der Thermalquellen San Pellegrino liegen. Die Vorab-Anlayse basiert auf großangelegten Untersuchungen, die vor der Grabung der Tunnel durchgefuehrt wurden und dann mit der Ergebnissen der Grabung eines Pilotstollens verglichen wurden, der mit einer TBM-Frase hergestellt worden war. Außerdem werden Betrachtungen zur zweiten Phase der Verbreiterung der Tunnel angestellt.
The prediction of the geological and hydrogeological conditions associated to underground civil works is of primary importance when facing the need of groundwater protection. In such a context, the present paper considers the influence of the excavation of two tunnels on the San Pellegrino thermal springs, which are located along the right flank of the valley of the river Brembo, adjacent to the town of San Pellegrino Terme (Italy). Because of some concern of the San Pellegrino company about the possible effects of tunneling on the commercially exploited thermal springs in the area (San Pellegrino and Pracastello springs), an extensive investigation programme has been carried out (deep boreholes drilling, geophysical investigations, geological mapping) to define the geological, hydrogeological, and geotechnical conditions. A numerical model based on the Finite Element Method (FEM) was also created to provide insights on the groundwater problem. Based on the site studies described above the geological section of Figure 2 (Prediction) was derived. Also reported in the same Figure 2 is the geological section as revised following the excavation of the TBM pilot tunnel (Pilot Tunnel Results). As shown, from south to north the Frasnadello tunnel has been excavated through the dolomite/limestone sequence (up to chainage 350 m), thrusted over the argillite/marl sequence. The Antea tunnel has been excavated entirely in a tectonic block of dolomite. The contact between the argillite/marl sequence and the Antea dolomite block is tectonic, with a fault along the Sambusso valley which has down thrown the argillitic sequence. In the northern edge of the Antea tunnel, down to a depth of about 300 m, the borehole S4 has encountered gypsum of the San Giovanni Bianco Formation. The comparison of prediction and actual conditions based on the pilot tunnel results shows that the main differences are mostly linked to the dip of the major faults, less inclined than predicted.
