The tunneling industry is abuzz about the upcoming Brenner Base Tunnel Project. The tunnel route will run below the Alps with a maximum cover of 1,600 m. In such conditions, choosing the right tunneling method requires much forethought. Geology in high cover tunnels is often complex and TBM excavation often proves to be the optimal solution versus traditional methods like drill & blast. Both excavation methods are considered for this project. TBMs have proven themselves in deep tunnels worldwide, and are often faster, safer and more cost-effective than their conventional counterparts, as well as more customizable. This paper will explore the advantages of mechanical excavation and the best types of TBMs for the Brenner Base Tunnel Project, a comparison of mechanical excavation versus drill & blast, and important considerations for ground support in high cover conditions. A case study of the high-cover Olmos Trans-Andean Tunnel will also be presented.

1 Project Background
1.1 About the Brenner Base Tunnel

The oft-mentioned Brenner Base Tunnel will become the longest underground structure in the world. The 64 km long tunnel will run from Tulfes/Innsbruck, Austria to Fortezza, Italy, making it arguably one of the longest – if not the longest – underground railway tunnels in the world. The tunnel route is a challenging one, traveling below the Brenner Pass in the Alps mountain range with a maximum cover of around 1,600 m. When complete in 2026, twin 8.1 m i.d. tubes will run single-track trains just 70 m apart from one another, connected every 333 m by cross passages.

Excavation on the massive scale required to build the Brenner Base Tunnel necessitates customized machinery, skilled crews, and precision planning. Besides the challenges of variable rock types, groundwater and high overburden towards the center of the alignment, it will be prudent to plan for rock bursting and squeezing conditions.

The route also crosses a major fault zone where the European and Adriatic tectonic plates press together. These anticipated conditions gave rise to the practical need for an exploratory tunnel, which will provide additional design and programming data for the excavation of the main tubes. Once completed, the exploratory tunnel will be used for drainage during construction and eventually as the service tunnel during operation. The exploratory tunnel could also carry power and data cables.

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