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  Impact of the Rock Mass Quality on the cost of the Conventional Excavation: The experience acquired during the execution of the Brenner Base Tunnel

  The production of a tunnel, defined as the ratio between excavation length and the relevant time, as is known, is closely related to the existing state of stress on site, to the type of rock mass and to its structure, i.e., more generally, to its quality. High rock mass quality gives higher productions, while poor rock mass quality leads to have low production, with a law that does not have direct proportionality.To upgrade the quality of the rock mass using consolidation elements and to install support or pre-support systems require much more time in a poor rock mass compared to a rock mass having good quality.Moreover, in addition to the installation of consolidation elements, poor quality rock masses often require also the installation of confining or pre-confining systems to control deformations and or support or pre-support elements to sustain loads derived both from de-confining and mass forces.Nowadays there are several reference guidelines for modeling the rock mass reactions to its excavation. From these models is possible to define the type and the quantity of elements to be applied to control the reaction of the rock mass; however, little is known about the costs associated to the excavation in the different quality conditions of the rock mass.Based on the ongoing experience in the excavation of the Brenner Base Tunnel, which at the end of the works will be the longest railway tunnel in the world, the present paper summarizes the costs sustained for typical excavation transversal section in different rock mass conditions.The costs are related to the conventional excavation made using the Italian Methodology for tunnel excavation, which is nowadays also a worldwide reference for conventional cyclical excavation. They are correlated to the encountered rock mass quality. From this correlation, the variation law, which links the rock mass quality with the related costs for an excavation in acceptable safety conditions, can be seen.

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