Megaproject spotlight: constructing the world’s longest railway tunnel
Extensive, efficient and well-running transport infrastructure is essential to maintaining a country’s prosperity and competitiveness. They’re also critical to connecting supply chains and allowing economic development through the mobility of people and freight. The more dense and well-connected transport systems are, the greater the economic and social opportunities, and this is precisely what the Trans-European Transport Network (TEN-T) hopes to achieve; narrow gaps, remove bottlenecks and reinforce social and economic integration within the continent. The TEN-T can be characterised by the increasing development of transport infrastructure megaprojects of impressive scale and magnitude, and a standout is the Brenner Base Tunnel (BBT).
The Brenner Base Tunnel is a horizontal trajectory railway tunnel between Innsbruck, Austria and Fortezza, Italy, spanning just under 64 km. It will be an impressive fixture of the Scandinavian-Mediterranean Corridor and a priority axis of the TEN-T connecting Helsinki to Valletta, that’s nearly 5000 km! Once completed, it will be the longest underground rail tunnel in the world and an engineering marvel.
The BBT comprises a flat railway tunnel with two parallel bores and an exploratory tunnel beneath them. The 1371-meter-high Brenner Pass is a mountain pass through the Alpine region that forms the border between Austria and Italy and is currently one of the most important transit routes between Northern and Southern Europe. Despite its winding roads, narrow valleys, and extreme gradients, over 2 million trucks cross the Brenner Pass annually, with upwards of 50 million metric tonnes of freight travelling through the existing rail line and roads.
Bordering two EU countries means that border enforcements rarely happen. Even with this freedom of movement, freight and leisure traffic still cause long traffic jams resulting in significant air and traffic pollution.
The solution? One of the most complex, ambitious, and groundbreaking megaprojects connecting traffic and freight between two sides of the content. The BBT will eliminate the challenging route of the existing railway alignment and significantly improve sustainable mobility in the region and across the continent. An exploratory tunnel was first dug 12m deeper than the proposed tubes of the rail tunnel to explore the rock mass, once completed, it will also be used as a drainage and service tunnel.
Major specs of the Brenner Base Tunnel are summarised below:
Tunnels, particularly in urban areas, are critical to the supply, provision and maintenance of essential infrastructure like road and rail supporting our transport, water, sewer and utility systems. Keeping up with the increasing demand for underground infrastructure means that modern tunnels are being built in increasingly constrained and demanding environments. The Brenner Base Tunnel is no exception.
What makes this record-breaking project even more momentous is that it is being built in Alpine regions. Alpine areas have tectonically deformed rocks and geologically challenging sections known as fault zones. The principal challenge is when excavation activities have to cross fault rocks. Fault rocks are characterised as weak rocks, which means they have a low compressive strength from about 2-20 MPa. The presence of fault zones intermittent throughout the composition of an underground structure can lead to significant challenges in delivery.
When rock masses are weak or fractured, they can behave like soils and can easily be disturbed by underground works, heavy structural elements, and hydrogeological challenges. As a result, construction teams can often face all sorts of ground instabilities. Therefore, to produce a safe and rigorous design of deep alpine tunnels, a comprehensive understanding of the site geology is essential.
This is commonly done through specimen preparation, field investigations and laboratory testing. In the case of the BBT, each advance of the tunnel boring machines (TBM) was documented particularly in geologically challenging sections like fault zones and in sections with homogenous geology, where every second advance of the TBM was documented. This is known as geological mapping. By mapping the rock face, critical physical parameters such as hardness, weathering, and groundwater conditions were measured, recorded, and managed through tunnel database software. Since different types of rock require different cement specifications to stabilise the tunnel, geology must be documented and analysed at regular intervals. Information obtained here directly influences the safest excavation methods for the operational railway tunnels, the safety of the site, tunnel support mechanisms, long-term behaviour and can allow for more accurate project costing.
So, in all matters crucial to megaproject delivery, designers are interested in considering the characteristics and properties of rock masses and faults at the earliest possible stage of underground construction. In one of the two exploratory tunnels, the Aica-Mules tunnel, along its 10.4 km span, 18 faults were found. At around 2.5 km into the tunnel, an unexpected fault was encountered, which caused some deformation and cracking in concrete segments. At about 6km into the tunnel, a 50 m deep fault was found along with significant water inflows. Deformations in certain areas along the tunnel axis almost caused a four-month shutdown of the TBM. Rock samples were taken continuously and sent for laboratory testing. From nearly 200 samples taken from this tunnel, an average rock density of 2.67 kN was determined with compressive strengths around 130 MPa. Samples were also taken of weaker rocks with an average compressive strength of 35 MPa. Whilst the boring of the exploratory tunnel set the project at an initial cost disadvantage, the comprehensive data and information on prevailing rock types, fault zones, and geologically hazardous sections are compensatory advantages .
I’ve scratched the surface of the day-to-day challenges facing this project but it is worth mentioning just how experts and multi-disciplinary teams are rising to it, I get into that next.
Technical excellence & milestones reached
Once completed, the BBT will boast a complex system of rail tunnels, composed of two main single-track tunnel tubes, each 8.1m wide and over 40m apart, connected with cross passages. The implementation of these tunnels is characterised by planning and construction lots, where exploration and construction take place. A third of the pilot tunnel is driven through by conventional blasting, with the remainder using either tunnel boring machines (TBM) or shield machines during exploration.
By 2011, most of the preliminary work was undertaken with approximately 15km of the exploratory tunnel driven through. By then, financing and the start of the building phase were approved, along with financing of preparatory construction work and further prospection. The main construction work for the main tunnels began in 2016.
A highlight of this megaproject was the use of a massive belt conveyor system to remove and relocate the excavated material. The conveyor system can remove 5,000 tonnes of rock per hour. Without this innovative feature, almost 200 trucks with a load capacity of 26 tonnes each would have had to drive in and out of the tunnel every single hour to handle the amount of excavated material. Another win was that this conveyor system could be monitored and operated from a control centre.
The first major achievement for the project happened on July 6, 2020, with the breakthrough of the exploratory tunnel, a definitive milestone in part due to the speed of the TBM, which tunnelled over 61 metres per day. This 26 m2 cross-section of the exploratory tunnel is unique in that it is crucial to both the construction and operational phases of the BBT. During construction, it provides essential geological and geotechnical information about the rock mass, weaknesses of fracture zones and the location of any fault zones. This ensures that the main tunnels' construction risks are significantly reduced and provides information that can help optimise against areas of specific concern and sections where methods and materials can be altered, leading to potential cost savings.
Since stoppages on main tunnel construction can cost up to and over €40,000 EUR a day, excavation work was also done to eliminate surprises when the main tunnels are bored. A team of geologists, geotechnical engineers, and hydrogeologists continually assess data from the exploratory tunnel and provide an updated design basis for the main tunnels. This forecast was initially done through surface mapping and point exploration through deep boreholes and then augmented with the information obtained from the exploratory tunnel. Particular attention is paid to the consideration of fault zones that the exploratory tunnel encounters, with data, parameters and essential geological, hydrological, and geotechnical information forming a fault catalogue that is then used to form a model of the fault zones for the main tunnel forecast.
This learning process is key to gaining significant insights into geotechnical conditions. Being able to correlate this data with the parameters of the TBMs means that mechanised processes can be optimised more precisely during main tunnel excavation. This could be adapting the lining of the boring machines or even the partial omission of time-consuming probe drilling in areas of homogenous ground conditions. It was found that after 50% of the exploratory tunnel had been dug, important optimisations of upcoming forecasts were made . A major advantage of the exploratory tunnel as it stands today is that is it able to be the sole location for maintenance, cleaning, and inspection works completely independent from the main tunnels, without requiring their closure at any time during operation. This would not have been possible had it not been for the findings gained during the construction of the exploratory tunnel.
Another innovative technique used in this project for the portion of the tunnel under the Isarco River, was a ‘special freezing technique’, adopted to address the unique complexities faced. One of the greatest risks during ground stabilisation is that it can get washed away when it is near a moving water source. These risks were mitigated by drilling the ground under the river and injecting it with chemicals and cement mixtures which slowed the speed of the water. Drilled holes were then filled with liquid nitrogen, freezing the soil surrounding the injection hole for about 1 m on all sides. This makes the space between the river and the tunnel below impermeable to water providing greater resistance, strength and stability to the ground surrounding the tunnel.
The era of megaprojects is here, and many countries are accelerating major projects hoping for an infrastructure-led recovery out of economic downturns.
With more and more megaprojects in the pipeline, the aims are often the same: spend big and spend quickly, create jobs and stimulate the economy. Yet time and time again, history has shown that megaprojects take too long before they prove to be an effective stimulus and are often prone to ‘mega’ cost overruns. I’ve actually written an article about that too, check it out here!
Yet, the Brenner Base Tunnel has been in the pipeline for a very long time, with preliminary projects and prospection starting in 1999. Whilst all risk characteristics of megaprojects apply to this project, if forecasted and executed accurately and accordingly, the BBT would represent an important and strategic step forward for sustainable mobility in Italy and Austria and Europe. The transfer of freight and leisure traffic from road to rail will significantly reduce CO emissions, increase transport safety, reduce noise exposure and air pollution and be an iconic piece of infrastructure in Europe. Additionally, it creates a connecting axis for travel from Finland to Malta and is one of the numerous sustainable mobility projects across the continent aimed at delivering systemic modal shifts in transport.
I’ll end on the myriad opportunities and innovations megaprojects like BBT can create. Researchers are looking into the prospect of tunnel construction being used as an environmentally friendly source of energy! When tunnelling into a mountain, the structure often has to pass through or be designed to contend with groundwater and mountain water. The latter refers to water coming from rain or flowing from the mountain surface. This water seeps into a tunnel structure and has to be drained. Mountain water is warmed by the earth’s heat, creating substantial geothermal energy. Unlike solar panels, which achieve maximum efficiency when the sun is shining, or wind turbines which need wind to provide energy, geothermal energy is always there. Right now, discharged water from underground tunnels is usually cooled before being reintroduced into the natural environment.
But what if we don’t cool it down? What if this heat is used as a source of energy? This is the challenge that researchers are currently working on. The prospect of using naturally heated tunnel water to provide a source of energy for households and entire neighbourhoods in cities like Innsbruck is groundbreaking. This is made more viable by how the project has undergone construction; an exploratory tunnel independent of the operational tunnel means that measures for heat recovery of drained water are realisable. But it is still early days, the tunnel structures in the BBT are still under construction, and its geothermal potential can only be estimated at this point, but as we often like to say: watch this space!
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 Gattinoni, Paola, Pizzarotti, Enrico Maria & Scesi, Laura, 2016. Geomechanical characterisation of fault rocks in tunnelling: The Brenner Base Tunnel (Northern Italy). Tunnelling and underground space technology, 51, pp.250–257.
 Bergmeister, Konrad & Reinhold, Chris, 2017. Learning and optimization from the exploratory tunnel – Brenner Base Tunnel. Geomechanik und Tunnelbau, 10(5), pp.467–476.