Record of 860m in one month set by the 10.65m diameter double shield TBM

Gripper TBM from Herrenknecht completes 16.63km section of exploratory tunnel on north Austrian side in July 2020

On the Mauls/Mules 2-3 contract of the Brenner Base railway tunnel between Austria and Italy, three Herrenknecht TBMs are excavating two main tunnel drives from south to north, and an exploratory and later service tunnel 12m below. In March 2021, a new record of 860m in one month was set by one of the 10.65m diameter double shield TBMs excavating the main tunnels, achieving a peak advance rate for JV Astaldi-Ghella and Brenner Base Tunnel project owner, Austrian Italian BBT SE. The machine achieved an average daily advance rate of 27.7m with a daily best of 36.75m.

By early April 2021, the two main running tunnel TBMs had completed more than 50% of the total distance toward their contract and lot boundary. The double shield TBMs were chosen for their good performance in stable geology and to errect the one-pass precast concrete segmental lining as they progress. Dr Matthias Flora, Sales and Project Manager at Herrenknecht, congratulated the jobsite team, recognising their achievements and their professionalism.

At a total length of 64km, the Brenner Base Tunnel will be the longest underground rail link in the world. It crosses beneath the Alps between Innsbruck in Austria and Franzensfeste/Fortezza in South Tyrol, Italy, with low gradients along the route and beneath a maximum mountain overburden of up to 1,750m.

Two large TBMs have launched to excavate main running tunnel drives of the Mules Lot of the 55km long Brenner Base Tunnel project between Austria and Italy. The twin 10.65m diameter double shield Herrenknecht TBMs take the total to three shields engaged to bore the majority of the contract’s excavation after establishment of words by extensive drill+blast work. The third machine working on contract is currently excavating the service tunnel.

Mules lot TBMs underway on Brenner Baseline

Mules Lot contractor Astaldi/Ghella JV has been advancing the 20km of the contract’s twin tunnels, a total of 40.3km of main running tunnel excavation,and the parallel exploratory/service tunnel, since award of contract in late 2016. In early 2017 the JV began pushing out from the existing tunnels using drill+blast in both directions.

Working from the previously excavated Mules adit the JV is excavating the twin southward main tunnels and the exploratory/service tunnel by drill+blast to reach the boundary with the adjacent Isarco Lot, and in the opposite northward direction it has advanced the main tunnels by drill+blast to the Trens multifunctional emergency stop complex. It is from here that the two large TBMs have been launched to continue north to the border with Austria and interface with the Pfons-Brenner Lot. Drill+blast excavation of the south drives is expected to finish by about mid-2020, according to client delivery partner BBT-SE.

Second main drive TBM launches at Mules

The 6.82m diameter service tunnel TBM was launched in May 2018, some 600m north of the Mules adit, and the machine is due to reach the adjacent contract interface in 2021. Its progress, combined with that of the earlier drill+blast works, has achieved a total of nearly 5km completed of the total 14.8km additional service tunnel to be built in the Lot.

The main tunnel TBMs, launched in April and May, are expected to reach the end of their drive by 2022 carrying on from almost 7km of main tunnel drill+blast excavation on the lot. The shields are erecting a 450mm thick segmental lining in 1.75m long concrete rings as they progress through mainly calcareous schists and gneiss geology beneath the mountainous overburden of between 590m and 1,750m.

Early works on Mules Adit under the Mules 2-3 Lot were complete between 2011 and 2015, and were advanced to explore and open up the section of the mega rail route that crosses the Periadriatic Seam, a major fault line at the boundary of two tectonic plates. These early excavations across the fault completed a total 3.7km of the twin running tunnels and about 1.5km of the exploratory/service tunnel.

There is one large lot yet to be awarded on the mega baseline project. This is the Pfons-Sillschlucht or Sillschlucht-Ahrental Lot at the northern end of the route in Austria. The Lot involves a total 7km of parallel main running tunnel excavation for a total of 14km, and the parallel service tunnel from the existing Ahrental Adit, and complete the section to reach Innsbruck (Fig 1). Timing of the procurement process has yet to be decided, said a spokeswoman for BBT-SE who added that it will not be announced in 2019. Earlier expectations had anticipated a start of procurement in 2017, which was later expected to get underway sometime this year (2019). This schedule has since been delayed further.

Drill+blast established start of the Mules Lot and will complete the southern drives to the Isarco Lot interface

Other works underway on the major rail baseline project are on the Pfons-Brenner Lot, the Tulfes-Pfons Lot, and the Isarco Underpass Lot.

The Pfons-Brenner Lot was awarded in late 2017 to the Porr-Hinteregger-Condotte-Itinera JV almost a year after the consortia was selected due to appeals. The scope of works call for construction of a total of 46km of tunnels - 37km for the 18.6km of parallel main running tunnels and 8.8km of the exploratory/service tunnel. Excavations will involve both TBM and drill+blast excavation with the TBMs focused on the main tunnels.

The package also includes excavation of the St Jodok emergency stop complex, comprising enlarged main running tunnels plus cross passages at 90m intervals, instead of the 333m distance between cross passages on parallel running tunnel route that correspond to the length of the passenger trains that are to run on the new baseline rail route. The lot also includes a total of 6.2km of safety and logistics tunnels.

Mules Lot service tunnel TBM factory ready

Construction on Pfons-Brenner Lot began in 2018 and BBT-SE expects the works to last 74 months. While work on the main tunnels has yet to start, early progress in tunnelling has seen about 500m of the exploratory service tunnel excavated, again as a head start being obtain from prior adit construction. Almost 460m of the safety and logistics tunnels have been excavated.

The Tulfes-Pfons Lot and the Isarco Underpass Lot are both being advanced by the Salini-Impregilo/Strabag JV.

The Tulfes-Pfons Lot at the north, Austrian, end of the route includes a number of tunnels and excavation sites in a complex layout (Fig 1). The works advanced by drill+blast are now complete. The lot includes a total of 38km of tunnels, including a a total of almost 6km of the main running tunnels and a 15km long reach of the exploratory/service tunnel using a 7.93m diameter Herrenknecht open gripper TBM. Launched in September 2015, the TBM is also nearing its end point with only 500m remaining to complete. Other excavations in the Lot from the Ahrental Adit include a 9.7km long emergency tunnel, 6.9km of connecting tunnels, and a 1.8km long emergency stop tunnel.

Construction begins at Pfons-Brenner Lot

On the Isarco Lot, the construction challenge is varied with both soft ground conditions calling for ground freezing and jet grouting to take the main running tunnels under the Isarco River, and drill+blast once in the hillside. Of the 2.25km of twin running tunnels, for a total of 4.5km, more than half, or about 2.8km, have been completed to date. The lot also includes a total of 1.2km of connecting tunnels. Construction began in 2014 and is scheduled for completion by late 2022.

Early works also included excavation of the Wolf Adit on the Pfuns-Brenner Lot of the main line route. As well as the adit, the Wolf 2 Lot excavated about 1.2km of exploratory/service tunnel that lies between and 12m below the two main tunnels. A further 9km of exploratory tunnel will be excavated by drill+blast within the Pfons-Brenner Lot once awarded.

With drill+blast providing the excavation method to establish the adits and early works excavations, and advancing sections of the main running tunnels and exploratory tunnel, the four TBMs currently making progress on Tulfes-Pfons and Mules 2-3 Lots and those to contribute to the yet-to-be-awarded Pfons-Brenner Lot, are not the first to bite into rock on the project. Early works in 2008-2009 involved use of a 6.3m diameter Seli TBM to construct the southern-most 10.5km of the exploratory/service tunnel. The 5.6m i.d. tunnel was bored northward off the main tunnel axis and stopped short of the Periadriatic Seam. In the challenges faced, the drive gave geologists and project planners substantial data and experience to help plan and design the many works to come.