NORWAY’S OSLO to Bergen railway, climbing 1,237m over the country’s north-south mountain spine, is usually listed as the ‘second most scenic’ in the world. Presumably the Canadian Rocky Mountains take the top spot.

Either way much of the passenger load along its 492km single track is from tourists both summer and in the winter skiing season, though it is also important for freight. But near to Bergen the main demand is from commuters into the coastal city, second largest to the capital and a major oil industry centre. Some 120 trains use the line daily.

It means the line is at full capacity and particularly through a 7.7km-long single bore tunnel just outside the city, the main bottleneck on the route. It passes underneath the Ulriken Mountain and was opened in 1965.

DOUBLE UP

Work is in hand now by the Norwegian National Rail Administration, Jernbaneverket, to double the track from the town of Arna into the centre, with the key part of the project being a second parallel bore to the first tunnel. Just 30m away from the first bore it will be slightly longer at 7.8km and linked to the first by both rail track cross over connections at one end, and 16 cross passages for safety and maintenance.

"Nine of those will also house mechanical and electrical equipment," says the Administration’s head project manager for the scheme, Hans-Egil Larsen.

It would be typical in Norway for a tunnel like this to be built using drill and blast. The technique suits the very hard rock typical of the Baltic Shield geology and the country has significant contracting experience in the method. The Ulriken Mountain is not out of the ordinary or in a soft ground area; in fact its rock is some of the hardest in the country says Larsen, a granite that is competent and unfractured. "It is at the high end of the hardness scale," he says.

But in fact the work will primarily be done by TBM, in this case a hard rock open type gripper machine from Herrenknecht, driving a 9.33m outer diameter tunnel. It will be the first rail tunnel using a TBM in Norway, and one of the first TBM drives made in the last 30 years.

CHOICE OF EXCAVATION METHOD

A primary reason for choosing the machine, apart from general advances in TBM technology, is that the new Norconsult designed tunnel lies close to the old one, just 30m away. Drill and blast methods would need to pay attention to the proximity of the original tunnel and the trains, which will continue to run throughout the project.

In fact nearly a dozen companies bidding on the project were given the option to use either TBM or drill and blast methods. But the best bid price, from a joint venture of Skanska’s Norwegian arm and the Austrian firm Strabag, proposed the TBM method.

"The difficulty otherwise is that there are only a few windows in each day when blasting is allowed," explains project manager Torbjørn Tveit Bakketun from the Skanska side of the JV. Vibration risks in the live tunnel rule out blasting as trains pass and they only stop for four 35-minute periods. Some blasting is needed however. The first 764m of the tunnel, as it leaves a new station being built at Arna involves a track layout that funnels several station platform lines into the two for the tunnel.

This section of the tunnel also includes track switches and obliquely running crossovers. "The cross section varies from 60m2 for single track to as much as 300m2 where there are twin tracks with a third tracks diverging," says Larsen.

"And of course there are connections for safety tunnels between the new bore and the old one to be made," he says. At some points the caverns needed are 28m across.

The D&B work was the first task for the joint venture when it mobilised and came on site last summer at the Arna end of the project.

"Actually it had been intended to start on the Bergen side but we talked it over with the client in an initial stage and it made more sense to site the operations at Arna," says Bakketun. The Bergen side has a greater density of houses and other buildings almost up to the portal and Arna less.

There are environmental issues to pay attention to. A nearby stream is a particularly favoured wild salmon fishing spot and the contractor has had to channel and culvert this underneath the station early on, as well as ensuring operations properly clean any water discharge from the site or the tunnel itself. "We have quite a high-tech processing plant for the water," says Bakketun.

Work forming the portal area began in August last year with tunnelling underway in November using two Atlas Copco three boom jumbo rigs to drill first of all a short side access adit and then the main bore in two directions, with the changing profiles needed for the blasting.

"We had one XF3 and one WE3 and the work has gone quite well," says Bakketun.

"We used Volvo articulated trucks to move the spoil, loaded with 350 wheel loaders with a side-tipping bucket." The spoil, some 135,000m3 from this section was placed nearby in a temporary store and then loaded onto road trucks for a 7km haul to a local quarry where it was crushed for sale. The hard material is useful for multiple proposes.

At the end of the drill and blast section the tunnel splits, one half for the second of the two diagonal train crossing points and the other a 30m stub tunnel in which the TBM drive will begin. Assembly is currently underway outside the tunnel.

Critical path

One advantage of beginning with this work was that it allowed time for the TBM to be ordered and delivered, says Bakketun. The machine, which is partly new and partly reconditioned, was factory ready in May and then delivered by ship over the summer, with a final move by truck from Bergen. "Some 90 loads were needed" he says, "including some large special loads at night." One of the biggest was the 151t central section of the cutter wheel. But an unusual assembly is being used for the machine. "Because we changed the start end of the tunnel from Bergen the original plan to put it together in a starting chamber was not possible," explains Bakketun.

Continuing drill and blast operations mean the machine must be assembled outside the tunnel. "We have prepared a small 50m long concrete pad just outside the main portal in a space close the Arna station area," says Bakketun. This is out of the way of the current operations which access the tunnel through the small adit to one side, and also clear of the existing tunnel and the continuing train operations.

Once it is ready the machine is moved forwards with a special ‘leapfrogging’ mechanism he says. Herrenknecht has supplied the device in consultation with the JV, and particularly its TBM-savvy Strabag side. The manufacturer has its own personnel on site for the operation.

The system involves building the heavy cutter head section within a steel cradle in which there is a two-way jacking system. Two very large vertical jacks are the main component; these lift the whole head clear of the ground and the frame below. Smaller horizontal jacks then move the head forwards for 2m when it is lowered again onto the cradle. The big jacks themselves then move forwards and the cycle is repeated.

The back part of the central beam section of the TBM is supported with a wheeled steel frame and behind that the backup train runs on the usual wheels. Assembly of the machine has been underway since August and the inching forward began in September as sections were added to the train. In November the main move will take place over a two-week period along the completed drill and blast section to the 30m starting stub where the machine will be able to extend its grippers and start boring.

To prepare for the move, a concrete slab has also been cast within the 800m-long section to support the jacking frame as it moves along in 2m increments.

This will later serve to support a rail access system for the TBM to carry personnel and equipment for the machine. It will also bring up precast invert segments, which will continue the tunnel working floor along the entire drive.

The segments are being made by local firm Ølen Beton sited some 3.5km away and will be stored at the entrance of the access adit and loaded by a portal crane.

Alongside the track ventilation and spoil removal is being installed. For the TBM drive a conveyor system is being used from Agir.

"We are just concreting the 150m long belt extension cartridge area," says Bakketun, "and sorting out anchor points along the tunnel walls.

The belt will run the length of the drive and by the end of the drive in summer 2017 should have removed some 470,000m3 of spoil.

It discharges to a temporary spoil heap some 150m outside from where it will be loaded onto trucks with the wheeled loaders and onto the quarry.

"But the TBM spoil will not be crushed, the chippings are not the right size and shape for that," says Bakketun. Instead they will be stockpiled and are to be used eventually for harbour reclamation work by Bergen city authority. "They will seal and area of partly polluted seabed," says Larsen.

For the drive itself the rock is expected to be very good and the major support will be from rock anchors where necessary. These will be installed with two Atlas Copco drills mounted on the machine.

"The only possible issue is a small risk of some squeezing which was experienced in the first tunnel" says Larson. That could occur at the point of highest overburden, which is 600m. But a major advantage of boring a tunnel only 30m away from an existing tunnel is that the geologists have an excellent way to examine the rock and predict the condition ahead says Larsen.

"The existing tunnel also means that there is unlikely to be any water," says Bakketun "because the drainage of the rock mass has been underway for decades."

The TBM is fitted with 62 disc cutters, using the bigger 19in diameter for maximum pressure. Typically for Strabag there will be a cutter disc store and workshop on site for re-edging worn discs.

The start of TBM work does not see the finish of drill and blast. Each of the cross passages has to be made. Like the rail crossover diagonals, these will be excavated with blind ends to be connected later. "Some nine of the cross passages will also have equipment rooms in them," he says, which the JV will do the concreting for.

For all the drill and blast work, operations have frequently involved reduced round lengths and half face blasting, measures needed to reduce vibration effects close to the live tunnel. That is obviously important as the blind ends are approached.

"Final breakthrough will not be done until the renovation of the existing tunnel is underway," explains Bakketun. The plan for the project is to divert trains into the new tunnel when it is complete in 2020 says Larsen. Renovation will then be done on the 1964 tunnel, track re-laid and the cross connections broken through to establish rail links and finish the safety tunnels. Before that of course there is lining work, which will use a "traditional Scandinavian system of polyethylene foam and shotcrete," he says. This is partly for insulation purposes in a northern climate, though the Bergen area, close to the North Sea and the tail end of the warm Gulf Stream, rarely suffers the deep cold of the rest of Scandinavia. Famously it just rains all the time.

The JV will do the lining and finishing works once the drive is complete, finishing in 2019. Track and signalling contracts will be underway at that point making the line ready for early 2020.

"We also have a lot of outside contracts for double track into Bergen and for stations," says Larsen "which must all be coordinated."

Renovation and connection work is expected to take until 2021 when the project will finally achieve its goal of double track running.