Brenner Base Tunnel Lot "Mules 2-3"

Italy

Being built under the Brenner Pass is what will one day become the longest high-speed underground rail link in the world: the Brenner Base Tunnel. The project is part of the Trans-European transport network TEN-T, nicknamed “the European underground”, and more precisely of the Scandinavian-Mediterranean corridor. The “Mules 2–3” section was started in September 2016 and it is the tunnel’s longest one: it includes over 17 of the 64 kilometres of the railway line that will run beneath the Alps.

Once the new high-speed line has been completed, the time needed to cross the Brenner will be down to a third of the current travelling time. Once the “Mules 2–3” section has been finished, all the work on the tunnel in Italian territory will have been completed, joining Italy with Austria.

Most of the excavation has been carried out by three Tunnel Boring Machines (TBM). These machines are capable of excavating tunnels in
a mechanized way and in complete safety. The TBMs, commonly known as “moles”, excavate the rock with a rotating cutting wheel equipped with cutters. These remove the excavated material, and line the tunnel that was just made by installing prefabricated segments.

In line with tradition, the moles were given the names of women, and they will excavate a total of 42 kilometres of tunnels. The first and smallest of the three, Serena, with a diameter of 6.85 metres, began in late April 2018 to excavate an exploratory tunnel and analyse the ground conditions: the reconnaissance has already contributed to cutting construction costs and time. The excavation work of the exploratory passageways will continue until the end of 2021. The two main TBMs, both 10.71 metres in diameter, are excavating in parallel the two tunnels of the future railway lines: Flavia and Virginia began one month apart from each other, one in April and the other in May 2019.

Also, about 20 kilometres of tunnel are excavated using the traditional method, that is, with explosives. The phases of the traditional excavation are consequential and start with the boring of the holes on the excavation front. Once finished, the holes are filled with explosives, contained in gelatinous 600-mm cartridges. By carrying out the tamping hole by hole, NONEL (non-electric) detonators are applied to the explosive charges, connected to each other via detonating fuses. With the blasting, triggered by a small fire featuring a detonator with a non-electric principal wave, the combustion is set off from as far as the peripheral holes.

While building the “Mules 2–3” section, over 6 million cubic metres of excavation material will be extracted, 30% of which can be reused virtuously as aggregate for making building site concrete. The aggregate is transported to the Hinterrigger plant, where the castings factory is located, via a system of conveyor belts 80 kilometres long: this helps to reduce the impact on the roads and the surrounding environment.

There are 7 different buildings sites spread out across some 20 kilometres on the surface of the mountain pass. Some twenty service trains, over more than 100 kilometres of railway line, have been set up to allow the 1,000 workers to reach the excavation front in just under an hour. A gargantuan task and a great logistical challenge planned down to the smallest detail.

The excavation beneath the Alps with covers that reach up to 1,500 metres is particularly complex: the tunnel is excavated through the quartz phyllites, the shales, and the gneiss. The work also stretches across very high-pressure strata, subject to very high aquifer pressure, in extremely challenging geological and hydrogeological conditions.

The Brenner Base Tunnel is undoubtedly a work that will enter the literature of excavations at an international level thanks to the technical solutions and the innovations adopted. Considering the geography and their function, the Brenner Tunnels will play a key role in European mobility.

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