Durability design of anchors in tunnel engineering A case study for the Brenner Base Tunnel

In tunnel engineering specific anchors are used for local stabilization of the rock immediately after excavation. Since the structural inner shell usually is realized within a short period after excavation, the anchors are designed for a very limited service life. The durability of the anchors depends strongly on the local conditions of the surrounding rock i.e. presence of water, chemical composition of the water and geological/geotechnical uncertainties related with the rock mass. The 1.8 km long access tunnel "Mules" on the Italian side of the Brenner Base Tunnel has been excavated by drill and blast and is supported by shotcrete, reinforcement mesh and friction expansion anchors (Swellex). Since anchor corrosion can't be excluded probabilistic calculations are performed taking into account various corrosion scenarios. A monitoring program is developed in order to ensure the safety and reliability of the temporary anchors in the time interval between excavation of the tunnel and realization of the inner shell. The monitoring activities consist in visual inspection, convergence measurements and pullout tests on anchors. In the case that critical parameters are exceeded, rehabilitation and/or strengthening measures are recommended.