The world's longest and deepest road tunnel is underway in western Norway - Adrian Greeman reports
For a small country, with less than 5 million people, Norway has bold ambitions for its highways. And few come bolder than the giant US$47 billion E39 "ferry-free" project along the western side of the country. This 1,100km route will link the major city of Trondheim in the mid-north to Kristiansand on the south coast and aims to develop industry, commerce and social community existence for the entire west side of the country. Around half a dozen significant cities are included in the project, most notably the infamously rain-soaked Bergen, important for the oil industry and fishing and Stavanger, another major North Sea oil service centre.
There is already a highway along the coast but it is not yet a major motorway for much of the length though some southern sections are being improved. More significantly, it confronts major obstacles along the route where it has to traverse many of the great fjords for which the country is famous. Currently, this means drivers must use multi-kilometre ferry crossings at seven points along the whole route. As a result, the travel time for the full journey is 21 hours minimum.
The government wants to cut this effectively by half, to 11 hours, by building fixed crossings for all the fjords; a staggering task even for 21st century civil engineering technology.
Fjords can be wide and are always deep, due to the way they were carved into the rock by Ice Age glaciers, with most at least 400m down to the seafloor, and one on the route reaching 1300m. They are also busy with shipping, especially tall multideck cruise ships visiting the stunning landscape scenery.
A variety of bold and innovative solutions have been under study for the last decade for these crossings, some never used before, and some stretching existing techniques to the limit (see box). There are suspension bridges twice the world record in length, or with floating bases for the pylons, using North Sea oil techniques. There are other types of floating bridge and immersed buoyant tunnels below the water surface, anchored to the seabed or hanging from pontoons.
Most of these schemes remain in the feasibility stage, "…and as yet have not received funding go-ahead from the government," said a spokesman for Norway's road authority Statens Vegwesen. But two projects, using better known conventional tunnel technology, are underway.
The first of these is actually quite small, a 4km-long tunnel spur northwards from the nearly complete Ryfast tunnel project (itself a world record-breaking sea tunnel - see World Highways Jan/Feb 2015). This spur was a first stage in upgrading and widening the E39, passing it underneath central Stavanger. It will open next year.
A few kilometres onwards however the road reaches the Randaberg area, a small peninsula at the entrance to the Bokna Fjord, one of the widest crossings on the route. From here, there is to be a 26.7km-long tunnel, running up to 392m below sea level, a world record length and depth for a subsea road tunnel and a major challenge even for conventional tunnelling methodology.
The tunnel will have twin 10.5m diameter bores, which will each carry two lanes of traffic when finishedfinished; There are also three interchanges. Two of these are above ground at each end of the tunnel and one is wholly underground formed by a complex of tunnels approximately halfway along.
At this point the tunnel line passes and connects to a small group of islands, the Kvitsøy where there is a small community of around 500 people, which will be linked into the road with a 4km-long access connection. The spiral ramp from the tunnel also provides a major central access point for the seven years of construction work, and will also provide emergency access for both the finished tunnel and during the excavation phase. It will also provide space for disposal of some 2 million m3 of spoil forming a reclamation area.
Ventilation is obviously crucial for such a long tunnel, especially one carrying motor vehicles, and is one of a number of major challenges for the project. So the Kvitsøy half- way point will have two huge 10m diameter air shafts, starting at the tunnel ramp entrance point and dropping 250m down to service and pump machinery caverns at the level of the main tunnel below. One will intake fresh air and the other will expel exhaust.
Two other pairs of ventilation shafts are part of the project, one at each end portal. These are slightly smaller at 7m and 8m in diameter for intake and exhaust.
Detailed design work on the tunnel began some years back, primarily by engineering firm Norconsult working with the client's own engineers, and full go-ahead was given in 2017 by Norway's Storting (parliament). Construction on the ground began in January this year, with the excavation of an access tunnel at the southern end with northern access beginning in March.
"The access in the south is just 700m long," said engineering manager with Statens Vegwesen, Sveinung Brude, "…but the northern access is longer at 1,900m and has two bores, one for site access and the other for construction ventilation." Both were well advanced this autumn and the excavations should be complete by the New Year.
"The two starter contracts include reclamation works," said Brude. That allows for disposal of the mainly hard rock spoil, building up useful flat areas in this hilly coastal landscape. "They will provide for industrial development but in the meantime also give some additional space for the main contracts."
Both portals are in relatively unpopulated areas, the southern one at the small Harestad community, just north of Stavanger and the other at Arsvågen, a small community around the point where the existing ferry link makes landfall on the northern side. The ferry follows a shorter route across the fjord from Mortavika but this alignment was ruled out for the tunnel as the water is even deeper at this point.
The three big contracts are pending, the first for the Kvitsøy to be let shortly, and with tenders for the other two following at five month intervals.
"Actually we were going to put out the first tender in May but have been a little delayed," said Brude. The contract letting is involved, he says, because of the engineering challenges of the project, and contractors' bids need to assessed on engineering competence and experience as well as price. "Even the pre-qualification is quite complex."
"We are expecting worldwide bidding as well as from Norway," said Brude "and there has been interest expressed from as far away as China."
The last is less surprising than on first impressions, given such giant projects as the Hong Kong-Macau tunnel link, currently one of the world's longest subsea tunnel projects.
Common challenges on all the sections include the sheer length of the twin bores, two sections at 9km and one 8.5km long. That means complexities in ventilation and construction safety, in the logistical challenges of rock disposal – some 8.5 million m3 going mostly to reclamation areas on exposed coastline – and most of all in the rock excavation below deep sea depths with the danger of water ingress at high pressure. At the two deepest points of 392m on the northern side and a "mere" 290m on the south, that could be very high.
Like much of Norway some the rock is extremely hard granite, but the geology is complex across the fjord mouth with some sedimentary and softer rocks to pass through. But Brude said that he expects that well-understood drill and blast methods will be used on all three contracts. "But there will have to be significant probing ahead constantly to test the ground and the precise geology." Substantial geological investigation has been done from floating rigs but there is a lot of the detail of the rock that remains unknown. Particularly important will be discovering fractures and faults and it is certain that constant grouting will be required to seal the ground ahead. Perhaps surprisingly it is in the harder rock
that more significant cracks are likely to be found.
Pre-drilling and pre-grouting will probably run 25m ahead of the tunnel faces. A minimum 50m of rock cover to the seabed will be maintained all along the tunnel. The work on all three lots also includes construction of short cross passages, between 8m to 12m long, set between the tunnels at 250m intervals. Tunnels will have a side widening every 125m as well, to accommodate emergency layby areas.
The most difficult of the contracts is the central section running either side of the Kvitsøy community. Here the contractor must begin with construction of a spiral tunnel arm to the main tunnel line, providing for his own access and as the on-off ramp for the finished tunnel later on. First works will also include the two giant ventilation shafts dropping 250m from either side of the portal.
"These will be done with the kind of shaft sinking technology used in mines or for example on the Alptransit tunnel link," said Brude. Probably that will mean bringing in specialist subcontractors from somewhere like South Africa where kilometre-deep shafts are often required for the goldmines.
"Of course just 250m is nothing to them," said Brude. "But they will need to be in the contract offer unless the contractor can demonstrate his own skills for that."
The shafts at either end of the project are a little smaller in diameter and not so deep at 150m. For these he expects raise boring techniques will be used, a kind of vertical tunnel boring machine pulling itself up on an initially bored drillstring. "That will make a 2.5m hole which can be further widened out by conventional drill and blast."
Once the access tunnel and the two shafts are in place on the central contract there are further complications involving some 1.5km of connecting tunnels for the ramps and two roundabouts in the rock above the main tunnels. "There are another 1.2km of maintenance tunnels and chambers for the eventual permanent tunnel ventilation equipment at the base of the shafts," said Brude.
The remainder of the contract will involve main drives both north and south to link up with those coming from the southern portal and the northern.
As far as technique goes, it will probably be straightforward with jumbo rigs drilling the face and the rock loaded and removed by conventional machines and trucks. For one contract on the 14km-long Ryfast, now nearing its opening, Swiss contractor Marti used a conveyor system and this is not ruled out, but Brude said that he thinks it unlikely.
The excavated tunnel will have an inner concrete shell fitted said Brude, again in line with Norwegian practice. This will be formed from precast concrete panels bolted to the rock walls. "It has a double purpose to deflect vehicles if there is an accident firstly and then in conjunction with a membrane to help drain away any water."
Unlike many tunnels in Scandinavia, protection from icing and very cold temperatures is not a significant issue. The west coast is warmed by the tail-end of the Gulf Stream carrying warm water across the Atlantic and only occasionally sees snowfall and greatly subzero temperatures. But there will be some tunnel wall insulation at the tunnel mouths, and water heating pipes installed in the final road surface in the tunnels, close to the portals. These elements also have a safety function, a significant issue for a tunnel of this length.
The finished tunnel will feature video monitoring throughout combined with the latest radar detection systems to spot slowdowns and incidents in the tunnels. There will be variable lane arrows at 200m intervals and lighting control. Water supply lines and foam pipes run the length of the tunnel.
"The radar will activate the appropriate video cameras and alert the control centre which will be at the city of Bergen where they have banks of screens and other instrumentation," said a spokesman for the road authority.
If there is fire or other emergency the tunnel will use modern principles of isolating the area with signage and ventilation changes, and evacuating passengers to the opposite bore. Its 250m cross-passage placing, less than the usual 330m, recognises the great length of the tunnel. An electronic fire detection system using a temperature sensitive wire is also being considered. "That is not always used but the tunnel is the highest of five categories of tunnel," he added. "The Norwegian system is based on traffic loads, in this case on the over-30,000 vehicles daily we expect when it opens in 2026. But we have a lot to decide yet and will draw on experience from the Ryfast tunnel operations once it is open next year."
A particular feature will be the modified tunnel appearance to cope with driver monotony. Ryfast has a space halfway along which widens slightly and features changed lighting. "But you have to be careful how you do this," explained the spokesman. "You need to choose lighting installations which will perk up the driver but not something so interesting it becomes a distraction in itself."
There is time yet for these decisions, while current attention is on the tunnelling itself which will last for the next five years at least, but the E39 project is underway.
E39 project
The seven crossings to be built for the E39 in the next decades include some of the world's most challenging. Among them are:-
Rogfast:
The first underway across the Bokna Fjord, it stretches conventional subsea tunnelling technology to new lengths. Its 26.7km length and maximum 392m depth below sea level are world records.
Romdals Fjord:
Much further north, this is also using a conventional tunnel, a "mere" 16km long to cross beneath a complex of inlets and underneath the mountainside on the north shore. It emerges at another 400m-deep channel which will require a 2km-long suspension bridge, with a 1,625m central span, among the longest five in the world. Piers would be founded on the shore.
Bjørn Fjord:
Just north of Rogfast, is 5km across and 600m deep which begins to stretch engineering imagination. A floating bridge is the currently envisaged solution with concrete pontoons anchored to the seafloor. A short section at one end would use a cable-stayed bridge with a longer span to provide a shipping channel with the required 70m clearance.
Sula Fjord:
This 4km wide crossing with a depth of 400m has two possible solutions, both pushing technological limits. The first is to use a suspension bridge of two spans, with side piers on the shallow shoreline and a third to link them centrally. This would be on a concrete pier plunging 400m down. The alternative is a floating tunnel. The buoyant tunnel would be held over 20m below the water surface by giant cables looping over the top, with concrete foundations anchored into the deep sea floor to tether the cables. Small diagonal side tunnels would criss-cross between the two main tubes to stabilise them and to provide emergency escape routes.
Sogne Fjord:
About halfway along the route, this is Norway's largest with a 200km inlet length and is known as the King of the Fjords. It sees considerable traffic from big tour ships and would require a shipping channel of 400m width, 20m depth and 70m height to accommodate them. The most suitable crossing point is 3.7km wide between mountainsides, with a sea floor plunging a stunning 1,300m down. Four solutions are proposed, each stretching the technology. First is a conventional suspension bridge but on twice the scale of any ever built, with a central span of 3,700m and towers on the shore reaching 450m in height, a world record. Second is a floating tunnel similar to Sula but this time suspended from a string of floating hollow concrete box pontoons, avoiding the need to drop cables to the deep sea floor. A third proposal is for a multispan suspension bridge with steel towers supported on floating bases, using North Sea tension leg oil platform techniques. Cables would hold the bases underwater where the upward buoyancy forces would keep them steady. Another floating bridge solution is for a cable-stayed structure also using floating pontoons of a different type, like vertical cylinders.
