THE TUNNEL SYSTEM
The Brenner Base Tunnel is a complex tunnel system. It includes two main tunnels running parallel between 40 and 70 metres apart.
• These tunnels are single-track, meaning that train traffic is one-way.
• The two tunnels are connected at 333-metre intervals by cross-passages. These short passages can be used as escape routes in case of emergencies.
• The tunnel system also includes access tunnels, emergency stops, a cross-over point, interconnecting tunnels and an emergency tunnel.
A unique characteristic of the Brenner Base Tunnel is the third tunnel that runs between and below the two main tunnels, the exploratory tunnel. Its original function was to provide information on the geological and the structure of the rock mass for the excavation of the two main tunnels, and during the construction phase it became a fundamental part of construction site logistics.
Once operations commence, it will become a service tunnel and carry out several important functions, such as water drainage and access route for maintenance work.
Lateral access tunnels in Ampass, Ahrental, Wolf bei Steinach am Brenner and Mules connect the BBT with the outside world.
Smaller cross passages connecting the two main tubes are located every 333 m.
In case of unforeseen events, trains can use the safe underground emergency stops (Innsbruck, St. Jodok, Trens).
Here, trains can switch to the other tunnel tube and thus change to the other track if necessary.
HOW IS THE BBT BEING BUILT?
The choice of tunnelling method depends on the geological conditions of the rock, geotechnical findings and forecasts, as well as construction logistics and economic considerations. The length of the route to be excavated and the available construction time both influence the choice of excavation method.
So some sections of the tunnel are excavated by drilling and blasting whilst others are built using mechanical construction methods, i.e. with a tunnel boring machine (TBM).
DRILL AND BLAST
Drill & blast is a flexible tunnelling method. It is suitable for difficult and changing rock conditions as well as for different and complex cross-sectional geometries. The stages for drilling and blasting, on the other hand, are predetermined. First, blast holes are drilled. These are filled with explosives, and then the blast is carried out. After the excavated material has been removed, the excavation is stabilised with shotcrete, anchors, lattice arches and structural steel mats. Once a blasting cycle is complete, the process starts again from the beginning.
MECHANICAL EXCAVATION
Tunnel boring machines (TBMs) are our ‘giants in the mountain’. They drill through the rock with a rotating drill head and secure the engineering works with anchors, reinforced rods and shotcrete (see diagramme) or by installing precast concrete elements, the so-called tubbing rings. The advantage of mechanical excavation lies in the high daily production rate. The most important part of the TBM, which is actually a high-technology underground logistics plant, is its cutter head. This has a diameter of several metres and consists of several cutter bits that press against the rock and break it into small pieces. The trailing structure behind the drilling head, or back-up, ensures the supply of the tunnelling equipment and the disposal of the excavated material. The machine also includes rock stabilisation, ventilation and dust collection equipment.
