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Tunnelling: Deep in Valhalla

tbm sept 25m

Norway is cutting a 7.6km rail relief tunnel through rock

Norway’s largest ever tunnel boring machine is currently boring its way through solid rock to create new capacity on Europe’s busiest single track railway line.

Contractor joint venture Skanska Strabag is building the new 7.6km tunnel which will relieve congestion on the line.

ulriken map

“It’s the busiest single track railroad line because you have a lot of people going into Bergen for work and freight,” says Skanska Strabag JV project manager Torbjørn Tveit Bakketun. “It’s completely over capacity.”

Pitched as being one of the most scenic railways in Europe, the Bergen line connects Norway’s second largest city in the west with its capital Oslo in the east. The rugged and twisting journey carries freight and passengers between the two cities, but also acts as a commuter line for towns along its route. It is this blurring of priorities which has created a bottleneck on the line just outside Bergen.

Leaving Bergen, the line almost immediately hits Mount Ulriken. In 1964, the Norwegian government realised that to go around it would create a major detour, so it decided to go through it. However, the tunnel has only a single track and trains have to wait at either end to allow those travelling in the opposite direction to pass through. Now, this bottleneck between the commuter towns into Bergen of Fløen, in the south west and Arna on the other side of the mountain, is tipping the railway to breaking point.

Today, 130 passenger and freight trains travel through the Ulriken tunnel each day with more than 1.9M travellers and 125,000 containers travelling through it each year.

As a result, the Norwegian government decided to bore another single track tunnel parallel to the existing to alleviate the congestion.

The new NKr1.3bn (£123M) tunnel is around 35m from the existing one, but at its entrance at Arna, the station is being upgraded and more track is required to park and manoeuvre the trains. Because of this, a much larger 800m cavern has been constructed at this end of the tunnel to accommodate the additional track.

The cavern has a cross sectional area of up to 300m2 and JV contractors for the project, Skanska and Strabag, used the drill and blast method to create the space required. Due to the new tunnel’s proximity to the existing tunnel, the times during which the drill and blast work could be carried out were extremely limited.

“We had to do the drill and blast in the four windows of 35 minutes when there were no trains. We had to prepare, give the warning signal, blast, do safety checking in the adjacent tunnel and give the signal for the train to go. It was quite a lot to fit in and if you missed one train stop for blasting, you might have to wait another three to six hours for the next opportunity.”

With this in mind, Norwegian National Rail Administration, Jernbaneverket decided to use a tunnel boring machine (TBM) to create the remaining 6.8km of single track tunnel.

The TBM being used is colossal. It has a diameter of 9.33m and is 155m in length. The cutting head alone weighs 223t and has an optimised layout of 62, 480mm diameter cutting discs for hard rock on its face. Due to the hard rock conditions, it is an open TBM which uses grippers to push against the newly formed tunnel walls to press its way forward. At their maximum the grippers can exert a force of 7,200t on to the walls, allowing the cutting head to exert a force of 2,700t on the rock.

“We needed a couple of months to get going and learn what everyone’s job was on the machine,” says Bakketun.

“It’s a big factory which was going to be started up, so we needed time to get everything in shape.”

drilling the rock bolts

As the tunnel progresses, 3m to 4m-long rock bolts are drilled into the rock in a radial pattern around the tunnel’s circumference to stabilise it

Now, the TBM is around 2.1km through the bored section of the tunnel.

Cutting through the hard granite and gneiss rock is a slow process. Currently the machine is in a period of extremely hard rock and consequently it is only able to bore through 2m of rock a day. This compared with faster progress through softer rock, where the TBM managed to cut through 37m in a day. “We only got a penetration of 15mm per minute last week,” says Bakketun. “We want 50mm per minute.”

The reason for this slow progress is not just the hardness of the rock, but the lack of fissures within it.

“When you use a TBM, you need to have some fissures in the rock so you can use the cracks in the face to excavate the material,” says Bakketun.

“I would say roughly up to 50% of the rock so far has no fractures, meaning it’s more than 1.2m between the fissures in the rock. So that means less progress.”

Through the hard rock, each of the discs on the cutter head is capable of cutting around 1m to 2m of rock equating to around 70m3 to 140m3 of rock per cutter, before they are worn out and need repairing. Every morning a team inspects the discs to determine which need to be changed. Currently, around 15 of the discs have to be changed each day. As they weigh approximately 200kg each, it is not an easy job to carry out.

“To change the discs, you can enter the cutting head from the back, so you don’t have to be in front of it,” explains Bakketun. “That is, of course, the most dangerous place to be so there are very strict procedures – it’s a very narrow space and it’s really hot.”

When worn out discs have been removed, small cranes built into the TBM lift them onto a train behind the machine, which takes them to an on-site workshop where repairs are carried out.

Wear rates on the cutting disks vary depending on their location on the face.

To get the maximum efficiency out of each disc before it has to be repaired, they are systematically moved around to areas of lower stress before they are removed. There are stricter wear limits on the outer ring of discs which travel further and are subject to higher stresses. There is also a logic about where the discs are placed within the face.

“It’s important to have a plan so you don’t put a new one in between two old ones otherwise you’ll break the new one because it’s then getting the full force on the new one,” he says.

Perhaps one of the most surprising things about the TBM is that it is not controlled solely by a computer.

“With the rock constantly changing, it has to be controlled by a person with experience,” explains Bakketun. “He’s steering the cutting head, the pressure and the thrust force. There are mixed rock face conditions, partly hard and soft. It’s important to be focused so you don’t get damaged cutters.”

He says that the cutters break up the rock, with a combination of pressing and turning to get rock burst.

“You’re overloading the rock and then putting enough stress and so you create rockburst every 50mm to 100mm,” says Bakketun.

As the tunnel progresses, 3m to 4m-long rock bolts are drilled into the rock in a radial pattern around the tunnel’s circumference to stabilise it.

Rather like reinforcement in concrete, the rods prevent cracks from slipping and opening up further. This is supplemented by an 80mm thick layer of fibre reinforced shotcrete for frost protection.

“On the TBM there are two drilling rigs for the rock bolts,” he explains.

“We’re installing a combination bolt, a CT bolt, with the concept that you can use it for both the temporary and permanent condition.

“It’s an expanding anchor so you can push the rock bolts in and expand it against the rock so it’s good for the temporary condition.” The TBM also incorporates a grouting plant, so bolts can be grouted to make them permanent.

Although the TBM has a section dedicated to spraying the concrete lining as it progresses through the tunnel, the team has decided to reach breakthrough and then carry out the sprayed concrete lining works.

“We used the shotcrete robot a lot in the beginning,” says Bakketun. “But what we found out is that in this good rock when we had 120m to 150m a week it was better for the project to postpone this until till after breakthrough, because then we weren’t limited by the speed with which we could carry out the shotcrete works.”

What is being carried out as the tunnel progresses is the laying of a 700mm precast concrete invert section.

Two pairs of temporary rails are then installed on the top of the section. All of the materials and people needed for the tunnelling operation are then transported into the tunnel via a train.

After construction of the tunnel is complete, the rails will be removed and a 400mm thick concrete slab will be cast on top as part of the permanent works.

Initially this precast section was due to be completed insitu, but the change was made to make the process more efficient and to provide a better permanent condition.

“With this change, we will have a reinforced section at the bottom and therefore a better product,” says Bakketun.

The project has not just been about the construction of the new tunnel. As part of the preparatory works, a river inhabited by salmon had to be diverted to make way for the new tracks. It also had to be widened to prevent flooding in the area. Strict timescales and environmental boundaries were placed on the team to ensure that the salmon were not harmed.

“Most important was the concern for the fish. We couldn’t work in May when the fish were spawning, to make sure we didn’t interfere with the breeding programme,” says Bakketun.

“The fish are afraid of the fine rock chippings as it gets in their gills, so we had to wash it [off] first. We were also measured on the water quality and the pH of the water.”

Next year, when the TBM reaches the Fløen side of the mountain, it will block the exit while it is being dismantled.

To avoid disruption, an access tunnel has already been completed at the Fløen end of the tunnel route. When the TBM has passed the junction between the access tunnel and the main tunnel, the next phase of the work to fit out the tunnels can begin while the TBM is being dismantled.

This will then pave the way for 16 cross passages between the existing and the new tunnel to be excavated using drill and blast.

Despite the fact that tunnelling is due to be completed in early August next year, rock conditions permitting, the team is working hard to be able to finish before the World Tunnel Congress, which is being held in Bergen in June next year.

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