The Brenner Base Tunnel is a 64 km long underground railway connection under construction between Italy and Austria. The project mainly consists of two main one-track tunnels and a central exploratory tunnel 12 m below them. Considering that the three tunnels intersected the same rock masses and fault zones, the paper analyses the scale factor displayed by selected machine parameters in relation to the tunnel diameter, TBM characteristics and rock mass conditions. Final considerations about the performance of the three TBMs, also in the context of comparable case-histories, are addressed.
The BBT is a system of underground structures currently being excavated to build a high-speed railway connection between the town of Fortezza (Italy) and the city of Innsbruck (Austria). The project mainly consists of two main tubes (MT) and an exploratory tunnel (ET) excavated in advance (Figure 1). The 14.2 km long Italian stretch of the ET, already completed in January 2022, was driven by a DS-TBM with a cutterhead diameter of 6.85 m. The main tunnels are currently being excavated by two twin DS-TBMs with a larger diameter of 10.71 m; in particular, on 9th January 2023 the excavations of the east and west MTs, respectively, were 95 % and 75 % complete.
The tunnels were driven under extremely complex geological-geomechanical conditions, with overburdens varying between 700 and 1,700 m. The forecast and as built geological models are shown in Figure 2: the tectonic units are labelled above the profile and the single litho-stratigraphic units are represented by the different colours (Skuk et al. 2023).
To investigate the influence of TBM diameter and characteristics, seven 500 m long homogeneous zones from a geological and geomechanical point of view were selected (Figure 2, Table 1). Each of them was characterised by regular excavation conditions in terms of rock-cutterhead interaction and observed ground behaviour, which strongly depend on intact rock strength (UCS, BTS) and abrasiveness (CAI), properties and orientation of main discontinuities and tunnel overburden.
