The soon-to-be-completed Gotthard base tunnel at 57km is the world’s longest rail tunnel. But without its little brother, the 15.4km Ceneri base tunnel, the Gotthard Alp Transit Project would not be able to achieve its ambition of creating a continuous flat rail link between Zurich and Milan and reducing the journey time between the two cities from 4 hours 10 minutes to 3 hours.
The Ceneri base tunnel is Switzerland’s second largest tunnel currently under construction and, along with Gotthard and the already completed Lötschberg base tunnel, forms part of the New Rail Link through the Alps (NRLA). Ceneri tunnel is being built by AlpTransit Gotthard AG, under contract from the Swiss federal government, as is the Gotthard base tunnel. The estimated investment for the Ceneri base tunnel is CHF 2.58bn (USD 2.69bn).
It is anticipated that 270-340 trains per day will be able to travel the new route – up substantially on the 220-260 that were able to run on the previous mountain route. Trains will be able to reach up to 250km per hour. Also, once the NRLA is complete, 2,000 tonne loads on goods trains will be able to travel non-stop through Switzerland, without the addition of a pushing locomotive and freight transport capacity will increase from around 20 million tonnes per year to 50 million tonnes per year.
Ceneri base tunnel is in the Swiss Canton of Ticino and passes under Monte Ceneri between Camorino in the Magadino Plain and Vezia, near Lugano.
At the request of Ticino, a Locarno- Lugano link is also being implemented to serve regional traf_ c – travel between these two destinations will reduce from 55 minutes to 22 minutes.
A 5.5m diameter, 3.1km-long exploration tunnel near Sigirino was driven via drill and blast between 1999 and 2003 to gain geological data, with orthogneisses and other gneisses being the predominant finding. This exploration tunnel as well as an extensive geotechnical investigation was used to determine the definitive horizontal route of the twin, single-track tubes.
The routes of both the Gotthard and Ceneri base tunnels are curved – a fact determined not just by geology but also by geography, such as the position of dams, access routes to construction sites, depth of the overlying rock and land use.
Drill and Blast
The decision was made that the Ceneri base tunnel would be excavated entirely by drilling and blasting – with one exception. In 2008 an access adit tunnel, also at Sigirino and running parallel to the exploration bore, was dug using a Robbins TBM.
This 9.7m diameter, 2.3km-long tunnel joins up with approximately the halfway point of the main rail tunnel bores and is dimensioned to accommodate the conveyor belt system, site vehicles and construction ventilation. A system of chambers was constructed at the junction with the main tunnels, the largest of which houses the concrete production plant. Space at the access portal is very constricted and locating part of the construction site inside the mountain mitigates noise pollution for local residents.
The TBM method was chosen largely for noise abatement reasons with respect to the nearby municipality of Sigirino and excavation of the adit tunnel was achieved in 10 months and with the use of 30 cutter rings (daily advance rates averaged 18.5m). Robbins found the geology of the tunnel alignment to be good for TBM boring, with no squeezing ground or large water inflows encountered.
However according to AlpTransit project manager Paolo Vicentini, the foreseen geology of the main drives did not lend itself to the use of a TBM through the whole length.
"It would only have been possible to excavate about 4.5km of the southern drive by TBM," he said. "The main contractor, Consorzio Condotte Cossi, offered both excavation methods but the drill and blast was more economical and the bid included a guarantee that the completion time goals we had set would be reached.
"And, indeed, the breakthrough at the south was achieved in March this year, almost a year ahead of the original schedule."
Work on the two main tunnel tubes began from the intermediate chamber system in 2010, with the majority of the excavation being carried out simultaneously in both directions.
However, to minimise time and costs, inward drives from portals at Vigana/ Camorino and Vezia are also being excavated.
Restrictions have been observed in order to reduce transmitted vibrations and noise, especially in the south drives where there is thin cover and near the portal where it is densely populated.
Blasting has been controlled by reducing explosive loading and fragmenting the detonations and, at the two portals, it was forbidden to work at night and during the weekend.
In addition, during excavation of the trench for the cut-and-cover track of the south portal, the main blasting and excavation activities were only allowed during the summer months in order not to disturb a nearby school.
The north portal is located in the Vigana/Camorino area and here, in loose rock, the tunnel had to pass just nine metres underneath the A2 motorway.
A monitoring and alarm system was installed to warn of any unexpected subsidence in the road surface. Shortly after this portal, both tubes feature branch-off caverns, which will allow the crossing of the Magadino Plain at some point in the future.
Provision is also made for future extensions near the south portal, which is at Vezia. Around 2.5km north of this portal – and still inside the mountain – there is a branch off at Sarè, which will allow the future extension of the tunnel south to Chiasso and Como.
Driving in the solid rock here was performed with minimal amounts of explosive in order to protect populated areas and buildings nearby – such as the 18th century Villa Negroni. Protective construction methods were also called for further on where the tunnel passes just four metres above the new Vedeggio-Cassarate road tunnel of the Lugano bypass.
Spoil situation
As the majority of the Ceneri base tunnel is being excavated from the mid-way point, the material preparation and storage facilities are located at Sigirino. The transfer tower in front of the portal of the access tunnel, silos connected to the works track, conveyor belts and the installation for producing the rock aggregates for concrete production were set up between 2007 and 2009.
The dump for the excavated material can hold 7.5 million tonnes and accommodates spoil from both Vezia and Vigana portals, as well as Sigirino. An estimated 7.9 million tonnes are expected to be excavated from the whole Ceneri tunnel system.
Almost all the spoil – 99.9 per cent – is removed via a network of conveyors supplied and installed by the Comestei joint venture of Agir Aggregat and Ferrari.
"The total length of the conveyors is 30km," said Carl Ulrich Wassermann, of Agir, adding that the system runs from the crusher at the drill and blast area to the final landfill, about 2km from the portal.
Approximately 300m of tunnel length at the Vezia and Vigana portals has been mucked separately, leaving the vast majority to be extracted from the Sigirino access tunnel.
Agir supplied a hanging conveyor in this access tunnel, to allow more room for traffic, while in the main tunnel tubes, ground-standing conveyors were chosen for reasons of speed and ease of installation.
The design capacity of the band systems is 1,000 tonnes per hour but when all four drill and blast areas were mucking at the same time, Wassermann said a maximum of 1,100 tonnes per hour was recorded on one of the conveyors.
He also reported that there have been no conveyor breakdowns as such, although there has been some downtime as a result of power failures. And some major repairs have been necessary due to excavators and trucks running into the conveyors.
Agir performed the functions of crushing, sorting, washing, interim storage and final depositing of the spoil, including the management of the material that could not be reused – building roads, installing water pipes and fences, and planting, transforming it into embankments and landscaped areas.
It erected 10 silos, each with a capacity of 800 tonnes, plus another 10, each with a capacity of 1,400 tonnes. The silos have a diameter of 9.3m.
The aggregate production site is outside the access tunnel next to this silo array and Agir also handled the transport of these aggregate components back into the tunnel to the chamber housing the concrete production plant.
This plant is required to produce 800m3 of ready-mix concrete a day and crews work around the clock in teams of two for three eight-hour shifts in order to accomplish this.
Almost 100 per cent of the cement, along with primary aggregates and concrete needed for the tunnel lining and bed are being supplied by Holcim.
Around 0.4 million tonnes of aggregate for the Ceneri tunnel are from recycled spoil, with another 1.6 million tonnes of primary aggregates delivered by train from Holcim’s Hüntwangen quarry. Around 390,000 tonnes of cement is being supplied from Holcim’s plants at Siggenthal and Eclépens.
The twin bores of the Ceneri base tunnel are 8.5m diameter, are set 40m apart and are joined by cross passages every 325m. These cross passages – 46 in total – house infrastructure systems and also serve as self-rescue evacuation routes into the opposite tube.
Unlike the much longer Gotthard base tunnel, it is not deemed necessary to construct multi-function stations at intermediary points along its 15.4km length. Also in contrast to Gotthard, this tunnel does not have a ventilation centre. Instead, more than 50 jet fans mounted near the portals and in the middle of the tunnel will provide the necessary ventilation.
The main tunnelling works were split into three contracts – two smaller ones for the north and south portals and one larger contract for the main drives north and south from Sigirino.
The joint venture of Matro sud (Pizzarotti SA, Bellinzona CH; Pizzarotti SpA, Parma I; Implenia Schweiz, Wallisen CH; Cossi SpA, Sondrio I; Ennio Ferrari, Lodrino CH, LGV, Bellinzona CH; Rodio AG, Urdorf CH) is fulfilling the contract for the north portal, while the south portal is being handled by the CPV Consorzio portale Vezia joint venture (CSC SA, Lugano CH; Pizzarotti SA, Bellinzona CH). The Conzorzio Condotte Cossi joint venture (Condotte D’Acqua and Cossi) won the main CHF 987m (USD 1bn) contract in 2009 and has been digging the four headings, north and south, with Sandvik jumbos and Rowa suspended backup trains.
Maximum overburden is 800m and the southern drive proceeded well – despite hitting a problematic area of disturbed rock 1km into the drive, which necessitated major ground support.
"We expected we would have complications in this section of the tunnel," said Vicentini. "The face of the drive in the south-east pipe came down, breaking into the tunnel and bringing work to a stop for a couple of days while we explored the situation and developed an intervention measure.
"Luckily there were no injuries as the contractor anticipated the problem, pulled back its equipment and waited for the breakdown to happen."
The disturbed rock zone extended in to the south-west pipe and while there was no breakdown, there was some major movement of the walls. "We had to reprofile both the tunnels because we had too much convergence," said Vicentini. "And we had to wait a longer time before we were able to realise the concrete invert of the final lining."
A smaller disturbed zone also caused a slight delay in the excavation of the caverns. "Again, we had to reprofile the tunnel and reinforce a few metres with longer anchors," said Vicentini. "We also had to break the excavation stage down into more phases. We had hoped to be able to excavate the big phase of the cavern in two steps -an upper and a lower. In the end, especially in the west cavern, we had to break it down into four steps – three in the upper part and one in the lower part. This took a little time but the works continued in security and we recovered the delay after this."
Schedule expectations
In fact, despite these hold-ups, breakthrough at the southern end of the western bore was achieved on 17 March this year, 13 months ahead of schedule and with a deviation of just 20mm horizontally and 10mm vertically from the planned alignment. A short while later, on 30 March, tunnellers broke through on the southbound eastern bore.
Breakthrough of the northern bores is planned for the beginning of 2016 although an area of faulted ground has caused delays.
"The geology that was encountered was expected but the disturbed section was bigger than foreseen," said Vicentini. "In addition to that, in many cases the orientation of schistosity and fracturation was particularly unlucky compared to the direction of excavation (parallelismus). This resulted in the application of shorter blast steps and heavy safety measures (meshes, anchors, shotcrete and steel ribs) and so progress in the drives was slowed down."
The original plan in the north section was to line the tunnel after excavation had finished but, to regain time, the contractor has installed the final concrete and foil lining in tandem with excavation.
Another measure, in a shorter section, was to open a third excavation face towards the north while, in the east pipe, the drive was about 400m behind.
A further delay has been caused not by the construction works but by a dispute over the railway system tenders. AlpTransit said that while it believed that the original opening date of December 2019 was still feasible, the risk of failing to meet this target was sufficient that it felt it had to push the opening of the Ceneri base tunnel back to December 2020.