Norway's convoluted coastline of fjords and high mountains is famously scenic but also a major problem for transport and connections. The country has long experience of constructing tunnels as a result.
Now a series of tunnels underway, or in design, around the oil industry city of Stavanger will stretch its skills more than usual. A first scheme, the Ryfast will feature three tunnels including a 14.3km long link that plunges to 290m, deeper beneath the sea than any tunnel before. It is required for local city expansion to the east. Ryfast will include a branch running northwards through the city, which will be the first stage of a national coastal highway all the way to the far North of this long, narrow country. This second branch, forming a Y-shape on plan, will link to an even deeper tunnel which starts excavation in 2017, the Rogfast. That will be 25.5km long and will dive 390m deep beneath the sea.
It is in turn only a beginning, comprising the first main section of a €150 billion scheme to link together the entire western coastline. Known as the “No-Ferries” project this will include some half dozen extraordinary crossings of the large fjords along the way including floating bridges, suspension spans of record lengths and underwater “floating” tunnels (see box). Over the years Norway’s oil revenues have made the country’s economy strong and much-needed investment in road infrastructure is now being seen.
The Ryfast project is the first in construction. The total €700 million (NOK 6 billion) scheme comprises three separate tunnels, two making up the eastern link to the city's outlying suburbs on islands and across a large fjord inlet. The larger of these is the 14.3km long Solbakk which crosses the main fjord to the island of Hundvåg. A surface interchange there will then dip into another 5.5km tunnel carrying the road into the city centre, its depth dropping to a mere 90m.
Though significant in length the Solbakk will carry the least traffic of the tunnels; surprisingly for a tunnel this size it will initially see less than 10,000 cars/day and will have only two lanes in each bore, one for either direction. Such traffic levels are not unusual in Norway, which after all is a fairly small country in population terms, with 4.5 million people scattered along its vast length. Stavanger itself is a large city for Norway although small by international standards. Commuters currently use a 45-minute ferry crossing into the city.
Its economic importance is greater however because the port city is the main service base for the huge North Sea oil industry which powers much of the Norwegian economy. There are other economic activities too, particularly fishing and forestry, but like Aberdeen in Scotland it stands out in terms of local prosperity, even with a low oil price.
The third tunnel, the Eigenes at 3.7km long, is the shortest of the three and shallower, but is more complex to construct as it will include a series of interchange ramps and must pass beneath the urban centre. It will help bypass the city's increasingly congested streets, taking through traffic from the capital Oslo and the south, on to the Rogfast.
All three dual bore tunnels in the project are under construction, under four main contracts, one for each of the smaller tunnels and two on the long one, divided into roughly 7km halves. Interchanges in the city between the Eigenes and Hundvåg are included in the Eigenes work, which therefore excavates the first kilometre of the Hundvåg too.
The geology is fairly straightforward, according to project director for the overall scheme Gunnar Eiterjord from client Statens Vegvesen, the Norwegian Public Roads Administration. Like most of Norway and the Scandinavian block, it is hard rock. Two main types are present, hard gneiss on the eastern side, and a softer phyllite claystone on the west. The transition boundary is approximately halfway along the big Solbakk tunnel.
Substantial ground investigation was done along the tunnel line to check the condition of the rock, working from one of the many drilling barges available in the area and usually used for North Sea work. The important factor was the possibility of fractures running down to the tunnel depth; so deep would mean considerable pressures.
"But in the main the rock was found to be very good for the tunnel work," said Eiterjord.
A feasibility study on using TBMs for the project ruled them out because of the hardness of the rock and local constraints. He explained that time issues suggested traditional drill and blast would be more effective and more versatile for the application.
All the tunnels, designed by consultant Nordconsult, will be relatively small in cross section at 8.5m width after lining. That will accommodate two 3.25m wide traffic lanes with a separate bore for each direction. There will also be cross passages between the bores which in most places will be 12m apart.Even with a relatively small cross section the length of the tunnels means a huge amount of spoil for the city to cope with, an estimated 3.8 million tonnes or more. But work has been synchronised with a number of urban developments that require reclamation and virtually all of the spoil is going into coastal fill sites, either close by the tunnel portals or easily accessed by barge from loading points.
The excavation of the three tunnels follows a common pattern of using shotcreting and bolting for support. An initial safety layer of shotcrete is applied first, after blasting and mucking out. Depending on rock conditions there may be two or three blasting rounds before it is applied. Bolting is done once the rock condition is assessed by inspection by the engineers from the Vegvesen. Bolts are usually 3m long but in bad conditions and wider spanning sections can be 4m or even 5m.
"Rather than used fixed categories this decision is made each time for maximum flexibility and economy," said Eiterjord. For the whole project he estimates that there will be total of 250,000 bolts installed and perhaps around 100,000m of shotcreting.
When the basic excavation is complete the tunnels will show some differences in fitting out. The long tunnel which will have a relatively low traffic volume of around 8,000 vehicles/day, is a class five tunnel in the Norwegian system. It needs only a waterproof lining with a mesh reinforced shotcrete protection on the face of 80mm thickness. The membrane is held on bolts leaving a gap with the tunnel wall. The road itself will have a Jersey concrete barrier along the side.
The other two tunnels have higher traffic levels, the Hundvåg a maximum of 25,000/day and the Eiganes city tunnel as much as 35,000/day. For these tunnels a more robust tunnel wall is required, usually made in Norway using precast tunnel elements. These are 300mm thick curved units 3.7m high and 5m long. They are held slightly away from the tunnel wall with rock bolt fixings. The crown is done with the same kind of membrane and shotcreting as the full lining on the long tunnel. In places, especially near the tunnel entrances where cold air can enter, the lining is polyethylene panels. “In fact AF Gruppen has chosen to use the polyethylene foam for the whole tunnel section while Marti uses membrane,” said Brekke.
Within the basic parameters there are some variations in the work methods. Some of the tunnels are being excavated by Norwegian firms using traditional lean methods with small workcrews and truck spoil removal.
But there are differences too, most notably with the first of the two contracts for the big Solbakk tunnel itself. This is a 7.9km long section running from the eastern side and was let to Swiss owned Marti which has decided to use a conveyor system for the spoil removal, one of the first seen in Norway and being watched with some interest.
This eastern contract was the first let in May 2013 for €150.5 million (NOK1.3 billion) with tunnelling commencing in that August. The other half of the tunnel, setting out from Hundvåg island has also been underway for some time, this time with the local firm AF Gruppen under a €127.4 million (NOK1.1 billion) contract let in December last year. It is using standard methods and a truck spoil removal system, thus offering at least a partial comparison.
The conveyor system makes sense for many reasons said André Pas, project manager for Marti IAV Solbakk, with safety and economy the main ones. "The tunnel width is firstly quite narrow compared to many road tunnels in Europe which puts constraints on turning ability and passing for vehicles. Secondly we have the deepest point along the tunnel route and our section has the steepest gradients with a slope of up to 7.2% which is quite a lot." It makes manoeuvring and power demands more difficult for vehicles, especially loaded trucks needing to climb back out.
There are also two sharp bends in this section he said, because the tunnel turns southwards before making landfall. He added, "The other contract also has a bend but it is a longer and shallower one."
At the tunnel face itself Marti is using Sandvik 1120i computerised triple boom rigs, which it bought partly because it likes them and partly because they were available for delivery. Two other
"We have to stay at least 8m ahead with the probes," said Pas. The work is particularly important undersea because of the danger of hitting a surface running fracture. "At these depths you are talking of between 15 and 30 bar water inrush," he said.
If water is detected over a certain minimum then an "umbrella" must be grouted ahead. Excavation work is underway from both ends for the resulting 7.5km section, which will eventually meet Marti; the tunnelling began early this year. The Norwegian approach is much more obvious here with a spartan and pared-down site and just three workers inside the tunnel while drilling is underway. "In Norway the drill operator is usually in control within the tunnel and making the decisions directly," explained one of the Vegvesen engineers. Lighting levels too are noticeably less.
The contractor is currently using one
Work on the Hundvåg began only in September last year. The contractor group of Kruse Smith and Risa began with a long side adit to the main tunnel line which it must excavate from the centre, because its contract overlaps the other two contracts' end. Portals for both ends fall into the other contracts.
The Eiganes tunnel also began only last year under a €2.55 billion (NOK22 billion) contract let in June to a joint venture of Germany's Bilfinger & Berger and Norwegian local firm Stangeland. It includes a 3.7km twin bore tunnel but also complex interchanges on the south end, with wider sections for slip lanes.
Bilfinger's job also involves tunnelling much closer to the surface and for this reason it has to use "slow detonation" methods for the blasting with the sequence staggered to reduce vibration levels. Forward probing is needed too, this time to prevent groundwater disruption beneath city buildings.
A complex element for the Bilfinger JV is traffic routeing during the works and some difficult logistics in taking possession of above ground sites. Tunnel work completes in 2016 and the traffic should flow in 2018 by which time the Rogfast will be underway.
Norwegian plans
The long Rogfast tunnel is only the first of a series of extraordinary crossings required for the whole E39 coastal highway “No Ferries” project, which runs from Kristiansand in the south, through Stavanger and Bergen and finishes in Trondheim in the north.
The route means it must pass over a number of major fjord entrances, up to 5km in width and with water depths between 500m and, in one case, the Sognefjorden some 1250m. Event the “shallower” depths rule out conventional tunnels or bridge foundations. Eight major crossings are needed.
“We are hoping to stimulate innovation and imagination for the technology to cross these depths and lengths,” said Statens Vegvesen fjord crossings project manager, Mathias Egeland Eidem.
In particular it is hoped to tap the four decades of engineering that has gone into the North Sea and its floating oil platforms in both steel and concrete. Floating suspension bridge piers anchored to the seafloor, tensioned buoyant tunnel tubes, again anchored to the seafloor by cables, floating pontoon bridges and suspension bridges with spans up to 5km are all being studied. Another idea is a tunnel suspended beneath floating pontoons.
Schemes might also include undersea tidal power generation, in fjords with high in and out flows.
Many of the major Scandinavian consultants and contractors such as Denmark's Cowi and Sweden's Skanska have been participating in concept studies.
