With a compressive strength of up to 300MPa, the gneiss in Oslo is one of the hardest rocks ever successfully excavated with mechanised tunnelling technology.
The Follo Line Project in Oslo is a revolution for tunnel-rich Norway. Previously, tunnels have mostly been built with explosives using the drill and blast method. But now, for the country’s largest ever infrastructure project, tunnel boring machines (TBMs) are being used. Four 9.9m diameter double shield TBMs operating in pairs have each been excavating over 9km of tunnel for a new rail link from the capital Oslo to the growing commuter town of Ski, 20km to the south.
The second and final breakthrough of the TBMs into a construction yard on the outskirts of Ski took place in late February to considerable fanfare in front of government ministers, the local mayor, national TV, the entire construction team and hundreds of excited onlookers who all recognised the significance of the milestone. More than 25,000 people worldwide watched the first breakthrough into a cavern beneath Oslo via live streaming.
So why is this such big news? “In Norway, tunnels have previously been driven with explosives. Now we know that TBMs are an alternative, even in our extremely hard rock,” states Anne Kathrine Kalager, project manager for Norway’s state railway company Bane Nor, speaking from the site of the breakthrough.
This simple statement is significant, and it is a significance certainly not lost on Martin Herrenknecht, founder and chairman of TBM supplier Herrenknecht who was standing with New Civil Engineer as the pair of TBMs emerged together in a carefully synchronised double breakthrough.
“It looks good doesn’t it?” he says, beaming happily, genuinely swept up in the emotion of the moment, as he moves around the assembled onlookers, picking out all in the construction team that made this moment happen. But should the chairman of one of Germany’s biggest industrial firms be this excited at a TBM breakthrough, albeit a double one?
“It’s always exciting,” he says when New Civil Engineer finally catches up with him. “But this is a big breakthrough.
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“It was the same in Austria, with the New Austrian Tunnelling Method,” he reflects, referencing another tunnel-rich nation that took many years to be persuaded away from drill and blast-based methods and sold on the benefits of TBMs, having believed that TBMs would be unable to cope with the hard Alpine rock.
“But now, here, like Austria, they have seen that it can be done,” he exclaims.
Making this minor miracle happen has been a Spanish/Italian design and build joint venture of Acciona and Ghella (AGJV). Denmark’s Cowi and Germany’s Maidl Tunnel consultants are its specialist designers and local Scandinavian firms AF Gruppen and Peab are subcontractors.
AGJV’s contract is the largest and most significant on the project, constructing the 20km long twin bore tunnel – of which 18.5km by TBM – between 21m and 160m beneath the mountain range that divides Oslo and Ski. Once complete it will be the Nordic region’s longest railway tunnel.
The NOK8.7bn (£790M) contract was signed in March 2015 and is the largest of the project’s five design and build contracts.
In addition to the 20km long tunnel, the project includes significant improvements at Oslo Central station, realignment of existing railway lines and construction of a new modern public transport hub in Ski.
Not all contracts are going as well as this one. Bane Nor had to step in and take over two of the other four contracts last year after Italian contractor Condotte ran into financial difficulties. This delayed the overall project by a year and pushing project costs up by 7.7% to £2.8bn. Despite this, the project still enjoys great public support.
And AGJV’s project director Fernando Vara is as excited as Herrenknecht. Both are excited as all good engineers should be at the technical achievement. But both are equally excited at the opportunities this project will unlock: Norway’s economy is prospering and it is planning a big investment in its road and rail network (see box). As Vara puts it: “The pipeline is amazing.”
It has not been easy. Major construction obstacles have been encountered – notably considerably worse ground conditions than expected – but the tunnel breakthrough happened on schedule.
In Norway, tunnels have previously been driven with explosives. Now we know that TBMs are an alternative
“We have been facing Norwegian hard rock; ‘extremely Norwegian’ hard rock and some challenging ground conditions, especially here in the south,” says Vara. More seriously he explains how weak, heavily fractured rock encountered in the southern bores had to be stabilised with significant amounts of advance grouting. This threatened to slow progress and put pressure on the programme.
“We had to do pre-grouting to avoid water ingress,” he explains. “We had to stop every two days to do that. In the end we did 7,600m of pre-grouting; we expected only 1,500m.”
Yet this additional work did not ultimately affect the programme. “We have bored 36km of tunnel in two years,” says Vara. “Each TBM has done 9km more or less, with production rates very similar on all four.” Rates have indeed varied little, ranging from 136m to 145m per week. “And we are still breaking through on schedule.”
Not even the ground conditions, nor the water leakages, are show stoppers for using TBMs in Norway
“They have had problems with the rock, and the water, but still they have done it,” reflects Herrenknecht.
“With the mix of quality of expertise we have brought from around the world and the good quality of the machines we have been able to build with this rock,” explains Vara. “We have probably had the best team in the world to build these tunnels,” he says.
It is quite a boast, but it has been quite an achievement. Indeed, the tunnelling strategy – to launch all four 9.9m diameter TBMs (two for each bore of the twin bore tunnel) from the same launch site – is being claimed by the project team as a world-first.
“Launching four TBMs from one place; that is unique,” says Vara. “It was a strategy that we took a long time to work on and we are very happy with how it has worked.”
The availability of the site, tucked away in a forest clearing just off the E150 motorway halfway between Oslo and Ski, is the reason TBMs won out over drill and blast. The site is broadly equidistant from the two terminuses and had enough space to enable the excavation of two giant 60m long and 20m high interconnected caverns. These formed an assembly and launch site where two TBMs were assembled and launched south towards Ski and two more were assembled and launched north towards Oslo. All four broke through within weeks of each other. Using TBMs accelerated the programme, dramatically minimised environmental impact of the work with all spoil and disposed of on site. It also ensured the project has had little or no impact on the people of Oslo and Ski.
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This was critical, explains Kalager: “If we had used conventional drill a blast we would either have used a longer time, or to do it to the same schedule we would have needed seven access points on the surface between Oslo and Ski.”
Many of them would have been in the metropolitan area, some difficult to access and all close to residents. “After detailed study we saw there would be a huge environmental benefit in four TBMs using one access point,” explains Kalager. “So this project was tailor made for TBM excavation.”
“We have tried to find other projects with a similar set up, but so far have not succeeded in finding one,” she adds, slightly dampening the TBM lobby’s spirits. “But I guess there will be some upcoming projects though,” she quickly adds.
Using double shield TBMs is unusual, but the choice was made with speed of construction in mind.
Double shield TBMs are among the most technically sophisticated tunnel boring machines (see box). They combine the functional principles of gripper and single shield TBMs. In stable geological conditions, gripper shoes in the shield brace the machine radially against the tunnel with the cutter head jacking off this section of the shield, allowing installation of the tunnel’s concrete ring segments to continue in tandem with the drive. This results in very high performance rates. In weaker areas the TBM behaves like a single shield with the cutter head jacking off the tunnel lining ring, with boring alternating with ring installation.
Clearly a dual-functioning machine is more expensive than a simpler single-functioning one, and it has more moving parts needing maintenance, but Herrenknecht predicts demand will grow quickly, especially in regions where ground conditions are uncertain. “Double shield TBMs for hard rock? It’s coming, in South America particularly,” he says. “We will see a lot more of this.”
Aside from the dual-functionality to cope with variable ground, the machines themselves had to be powerful enough to get through the “extremely Norwegian” gneiss which has a compressive strength of up to 300MPa. Each machine was equipped 13 engines, an especially hard wearing 265t cutterhead and similarly hard wearing discs. Despite this, 6,000 disc changes were needed on each TBM.
“Running four TBMs at the same time has not been easy,” reflects Vara. “But we started boring three months ahead of schedule – and that really gave the client confidence that we knew what they were doing.”
The central construction site is also noteworthy for its quiet efficiency. Laid out in four districts, it is in effect a mini-city where up to 1,200 people from 25 different nationalities work, live and play for the duration of the project.
“It is our little city; we have people working here; people living here; all in the middle of a forest so nobody is disturbed,” says Vara.
“We have everything including a canteen that is the best restaurant in Oslo and a 400-room accommodation block that is like a fully booked hotel.”
Concrete was batched and segments made on site, with 140,000 lining segments constructed at a rate of 16 to 20 rings a day.
“Specially-designed multi-surface vehicles take two rings at a time down into the tunnel via access tunnels constructed by drill and blast,” says Vara.
The little city is now quieter with focus shifting to tunnel fit-out. “We are now entering a new phase, and we are confident that we will once again do an excellent job,” says Vara.
AGJV is contracted to carry out all railway installation works, except for the signalling systems. Planning for these activities has already begun and installation of precast drainage units begins in the coming months. Slabtrack laying will start in January 2020 with the line now scheduled for commissioning in December 2022.
And while Vara is focused on that he already has his eyes on the next prize and plans to win it by yoking local knowledge.
“We are deliberately using local contractors AF Gruppen and Peab as our subcontractors; they are doing a lot of the drill and blast caverns which they are very experienced in doing. Initially they were looking at us as international contractors and saying ‘what are you doing?’ Now we are looking at working with them on future projects,” he says.
“We are here to become a Norwegian company as the pipeline is amazing.”
Vara is not alone in exploiting others in the project team to build expertise. It is the same for Bane Nor, which is getting to grips with using TBMs for the first time.
Bane Nor project director Per David Borenstein explains: “We are in a transition phase. Important lessons are being learned. For us this is a training area too and we are bringing back the lessons learned into Bane Nor; we are building up best practice and that is very important.”
“We, together with AGJV, have demonstrated that not even the ground conditions, nor the water leakages, are show stoppers for using TBMs in Norway.”
Double shield TBMs consist of two main components: a front shield with cutterhead, main bearing and drive; and a gripper shield with gripping unit, auxiliary thrust cylinders and tailskin.