Tunnelling on a key section of the Brenner Base Tunnel has stayed on schedule despite encountering several challenges caused by the varied geology along the route.

At last week’s Tunnelling Festival conference, BTC JV senior project engineer Alberto Paddeu said that all three tunnel boring machines (TBMs) working on the Mules 2-3 lot of the Brenner Base Tunnel (BBT) have progressed on schedule

BTC JV completed this section’s exploratory tunnel on time a few weeks ago, after three and a half years of tunnelling.

The main west tunnel is also 67% complete, with 9.5km excavated, while the main east tunnel is 75% done, at 10.5km.

Paddeu said the tunnelling is expected to be finished by the end of next year.

BTC JV, comprising Ghella, Webuild and PAC, was awarded the Mules 2-3 lot in May 2016.

BBT is part of the Trans-European Transport Network (TEN-T), which is the biggest mobility plan under construction in Europe. The Scandinavian-Mediterranean corridor of TEN-T is the longest one at more than 9,000km. BBT is one of the crucial parts of this corridor, because it will connect Italy to northern Europe with a new base tunnel under the Alps.

Once completed, the BBT will be 64km long, stretching from its southern portal in Fortezza to the Tulfes portal outside Innsbruck, forming the longest railway tunnel in the world.

The Mules 2-3 construction lot is the biggest single contract awarded by BBT. It is characterised by long and deep drives without any intermediate stations.

Paddeu explained that the main purpose of the BBT is to reduce the slope of the historical Brenner line from 2.5% to 0.5%.

“And as a consequence of this we will improve the travel time for passengers but also increase the load capacity for commercial trains,” he said.

“Our client BBT has also estimated a 14 year period of tunnel operativity to offset the carbon dioxide emissions associated with the construction of the tunnel.”

The concept of the tunnel is to have single track, twin bore rail tunnels with a 70m distance between them, and cross passages every 333m. The exploratory and drainage tunnel is set in between the two main tunnels at a lower level.

The Mules 2-3 lot overall comprises two 14km long single track tunnels excavated with two 10.71m diameter TBMs, one 14km exploratory tunnel excavated with a 6.85m diameter TBM, 42 cross passages, two 5km single and double track tunnels excavated with the drill and blast method, a 3km access tunnel excavated with drill and blast, and a 600m long emergency stop with 17 connecting cross tunnels also excavated with drill and blast.

The BBT TBMs have faced a wide variety of rock formations with an associated variety of mechanical behaviours, including the Periadriatic fault.

Paddeu listed the main geological difficulties as:

“I’d like to point out that we have the two extreme opposites of rock mechanical behaviours; we go from 50MPa of uniaxial compressive strength in some sections to 220-2230MPa on the orthogneiss, which is one of the hardest parts of the project,” Paddeu noted.

After 3km of excavation, the TBM working on the exploratory tunnel was 900m below the surface and encountered a critical fault zone, composed mainly of quartz schist, which could cause caving during the TBM’s advance.

“It was clear for us that we had to consolidate the tunnel face and the tunnel profile,” Paddeu said. As a solution, the team used tube-à-manchette consolidation, inserted fibre glass pipes and injected a cementitious mix. The injections were successful, and the machine could thus proceed with the excavation.

“Then with the same TBM of the exploratory tunnel, we were just in the middle of a critical area […] where rocks were mainly composed of phyllite quartzite and mica schist. We reached one of the deepest points of the project, up to 1,600m below the surface, and the combination of all these factors induced a squeezing phenomenon, which is the fast closure of the tunnel profile. The rocks were squeezing and blocking the TBM shields,” Paddeu explained.

The team had to create a lateral rescue tunnel by cutting a entrance from the tunnel lining and then proceeding with a drill and blast method.

The situation was very delicate, Paddeau stressed, as “the rocks were not only loading the shields of the TBM but were also loading the centrings being installed”.

“We think this is one of the most difficult jobs we have done here in the Mules 2-3 lot,” he added.

When the TBM on the main west tunnel entered the critical fault zone with fine material composed mainly of lime schist poor in carbonate, there was a sudden release of material from the tunnel face and tunnel profile. This caused a cutterhead and shield blockage, which Paddeu called the “worst thing you can imagine”.

The first thing the team had to do was to consolidate the tunnel face and profile. It adapted a micropiling technique for this, installing steel casing pipes with a down-the-hole hammer and eccentric drill bits, together with grouting.

Secondly, it decreased the frictional forces around the shield using an “innovative” technique for TBMs, which is however commonly used for concrete demolition. It involved inserting a nozzle that can exert 1,600bar of water pressure.

“Not only bad news though,” Paddeau added, “to our knowledge we have done a TBM excavation record, with the TBM of the main east tunnel. We were able to excavate 860m in one month, which means 28m per day every single day.”

The project set the record in March of this year.

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