Brenner Base project fuels exploratory drive debate 12 Sep 2019

A special feature of the Brenner Base Tunnel (BBT) is the 5m diameter exploratory tunnel that runs between and 12m below the two main tubes. The excavations currently underway provide information on the rock mass, reducing construction costs and times to a minimum, and will be essential for drainage when the BBT becomes operational. During a visit to the Tulfes-Pfons Lot, TunnelTalk had the chance to talk to Konrad Bergmeister, Austrian CEO of BBT SE, about the exploratory tunnel in more detail.

Brenner Base railway system between Austria and Italy

“Having excavated more than 70% of the exploratory tunnel, we are now sure that it was the right decision to explore the unknown rock mass of this part of the Alps before we start construction of the main tunnels,” explained Bergmeister. He identified four main advantages to having the tunnel, the primary one being the geological, hydrological and geotechnical exploration of the rock mass encountered. The first step was to explore the ground with vertical drillings and to identify certain geological parameters without knowing exactly how the rock would behave. “We have to distinguish very clearly between a tunnel that runs very close to the surface, and the construction of a base tunnel,” said Bergmeister. “In our specific case – as we also learned from the construction of the other base tunnels like the Lötschberg and Gotthard in Switzerland – we knew that even through all the different investigations that are normally done in the preparation phase, only some rudimentary estimations of the possible rock mass behaviour can be achieved. With the exploratory tunnel we get a much better investigation of the rock mass and its behaviour.”

A second advantage is the water drainage system. The exploratory tunnel creates a completely independent water drainage system that will be used not only during the construction phase but also during the operational phase. This allows periodical cleaning of the drainage system, and there is no need to stop operation during maintenance.

Impressive structure of the rock directly behind the face

Thirdly, the exploratory tunnel allows the possibility to access geologically difficult zones of the main tunnels in advance of their regular TBM excavation. For example, in the case of difficult geo-hydrological zones, it is possible to insert specific injection mortar directly from the exploratory tunnel into the vicinity of the main tunnel in advance of TBM excavations, preventing dangerous incidents and expensive interruptions. It is also possible to construct vertical shafts or ramps directly to the position of difficult geotechnical stretches of the main tunnels and to excavate the main tunnel from the exploratory tunnel by drill and blast in advance of the TBMs for the same reasons. “We have an independent logistic system in a completely independent connection tube to the surface, which is quite useful for safety purposes but also in terms of costs,” explained Bergmeister.

A fourth advantage was presented during the preparation phase for the rail equipment. Long railway tunnels face the problem that all rail equipment has to be placed in the connection tunnels or directly in the main tunnels, meaning that one main tunnel has to be closed during inspection or maintenance. “In our case, we are now studying what equipment can be placed in the exploratory tunnel. Then operation in the main tunnels will be completely independent of maintenance issues,” added Bergmeister.

Konrad Bergmeister explaining the conception of the exploratory tunnel

The total costs of the exploratory tunnel are about 12% of the total costs of the Brenner Base Tunnel, an issue that has been repeatedly questioned. “Nevertheless, we have been able to show that by using these advantages, the additional construction costs result in savings in terms of reduced risks and costs for the main tunnel, and an optimised life cycle management, as the exploratory tunnel will serve as a service tunnel during operation.” concluded Bergmeister.

Construction update

Excavation of the Tulfes-Pfons Lot of the Brenner Base Tunnel completed in July 2019, which included the construction of a 15km stretch of exploratory tunnel between the Ahrental junction point and the town of Pfons. Several fault zones were encountered over the first 13km of the exploratory tunnel, which led to ten overbreaks with a wide variety of forms and geological/geotechnical conditions. The largest overbreak showed a cavity volume of about 5,500m³ and a cavity height of 18m, pressure on the gripper of more than 400bar, and water ingress of over 70l/s.

The use of an open TBM for the 15km exploration proved to be a wise choice, and the methods developed during construction to handle the unexpected overbreaks proved successful. In spite of the significantly increased costs due to geotechnical reasons, the construction schedule that had originally been contractually agreed upon was only exceeded by about 10%.

Work on the Pfons-Brenner Lot began in late autumn 2018 and work on the eastern main tunnel started at the beginning of July 2019. Excavation of the main tubes includes 32km by TBM and 5km by drill and blast. Four of seven drives are currently under construction and work is expected to end in early 2025.