A 9.96M Diameter hard rock TBM began grinding its way out of its big assembly chamber south of Oslo in September heading northwards to the capital. Next month its partner will begin a second parallel bore through the hard gneiss. In November and December respectively, two more machines start out southwards. The clutch of four Herrenknecht machines have been assembled in one place, a complex of caverns blasted out at the Åsland construction worksite site halfway along the 18.5km drive through the highland area south of the city.
It runs parallel to the inner part of the Oslo fjord which curves south from the city waterfront. The three year excavation programme for the machines is the main part of the largest infrastructure scheme in Norway, a new NOK 25bn (USD 3bn) high speed rail link to the rapidly expanding town of Ski and its large and growing commuter population. Potentially and eventually the line will go onwards as a high speed link into central Europe. Trains will run at 250km/hour in the tunnels.
Every type of tunnelling
As well as the main TBM section, unusual in a country that more often works with conventional excavation methods (more on these methods below), there is plenty more going on in a project which includes virtually every type of tunnelling on its overall 22km length. Planning and design has been underway for four years and construction began last year on the conventionally bored sections and site works.
This work is mainly for a final 1.5km of the main tunnel section. It cannot be done by TBM because of difficulties threading the bores through the steep Ekeberg hillside close to the capital. A whole spaghetti of other tunnels has to be passed through and awkward widenings and transitions are needed too. There is an additional short tunnel for another local line to be added in as well. Some 64km of new track will be created for the whole project.
Some of this conventional work has been using the first European application of drill and split wedge rock excavation. An equally complex set of approach tunnels, branches to access points and starter caverns was also completed earlier this year for the important central Åsland site, again requiring conventional means. There will also be a complicated cut and cover tunnel running a length of 700m from the Oslo main station through the soft ground around the old harbour. This will take a series of new lines from the remodelled main station underneath the former medieval city area which will be restored around new parkland.
“You can see we have all kinds of challenges on the project" says Anne Katherine Kalager, overall project manager for the client, Norway's state railway company Jernbaneverkert.
The cut and cover is a major project in itself, carrying up to six parallel lines which draw together the tracks and points which interface with the nineteen tracks between platforms in main station. For the moment this cut and cover work is on hold because of major archaeological works which have run over four summers so far, making critically important discoveries about the city's past. “It has meant re-writing much of previously understood history" says Kalager.
Quick clay
When it is finally built, the excavation on low ground close to the waterfront will have to handle difficult unstable ground, so called “quick clay". Formed during a marine period this was originally laid down as a clay-salt matrix, she explains, “but then as the sea receded the soluble salt was washed out." That leaves voids between the remaining clay particles "which are stacked like a house of cards and just as collapsible" she says. A huge amount of ground stabilising soil mixing will be needed using lime and cement before building the concrete box tunnel structures.
The same is true near to Ski for the cutting approaches and works on the new three platform six track station. “Like the Oslo end, it all has to be done close to live tracks, keeping the existing trains running."
Existing infrastructure
Perhaps the most complex works so far have been the tunnels into the Ekeberg, a steep hillside escarpment rising on the south side of the city. Its side slopes are prime residential areas with houses offering an excellent view across the stunning fjord vista and the city centre, with its newly developed waterfront area including the gleaming white wedge of the new opera house. The housing is obviously sensitive to vibration and damage.
Just as significantly Ekeberg has roads and incoming rail tracks running along the narrow fjord edge and a network of existing tunnels inside the hill itself. Some of these date back decades and some are relatively new, including three parallel road tunnels built in the 1990s and two more added in the last decade for the interchange and underground approach works to the new cross harbour immersed tube tunnel.
The latter tunnels had their own issues when they crossed near to the existing road tunnels and at one point include an underground bridging structure over an existing bore.
There is also a river diversion tunnel dating to the 1920s which runs across the alignment carrying water from the Alna river out of the city centre and, a bit further south, a complex of underground storage caverns for oil and their access approaches. These caverns are used by a variety of oil companies and at one time were supplying approximately 50 per cent of Norway's oil consumption. They include storage for aviation fuel feeding the main airport north of the capital. They also include an access tunnel with services and electrics.
Completing the picture are two major sewer tunnels which carry city waste to a big underground processing plant.
“You can see why we call it a spaghetti," says Adler Enoksen the client side project manager for this section of the work. “And there is more we think, military tunnels though we are not told where those are; the state forces have looked at the plans however and say they are not affected."
Through the tangle the project has to thread the incoming main bores and an additional bore heading out towards the fjord side; this latter connection will carry one of the tracks for the existing Østfold Line to Ski, which has to be realigned to link into the new tracks into the station.
As well as the main bores a whole series of access adits for the work have to be made as well. Also some of the main tunnels have to be larger caverns, widening out to accommodate track junctions and divergences.
“And there is possibly a new 500m long bypass tunnel section needed to take the river tunnel past the new complex" says Enoksen "which then also requires an extra adit. But that is not decided yet and we may do a relining and strengthening of the existing river tunnel instead."
If the latter is chosen the work can be done in winter when river flow is low.
Challenges for the project include the logistics and sequencing of the work. The contractor for this section, Italy's Condotte D'Acqua has to operate from a single narrow and highly constrained site on a narrow strip of land between the fjord edge road and railway and the steep hill slope. Spoil is taken out from here by trucks which then have only a short journey to a harbourside quay where barges take most of it away.
For the actual excavation there are further challenges. One is a thin layer of alum shale on the mountain slope, about 50m thick. A frequently met geology in Scandinavia this is a variety of black shale containing pyrite which can form sulphuric acid, when weathered and which acts on other constituents to form alum.
But it is problematic because it also contains algae organics and “is rich in aromatic hydrocarbon attributed to post-depositional irradiation damage induced by uranium concentration. It also can contain radium as gas a result of uranium decay".
“We have experience in mitigating its effects in various Norwegian projects and probably the main issue will be to ensure sufficient ventilation" says Enoksen, adding that the shale has not yet been reached. He does not think protective suits will be needed.
It will require special disposal however, probably at a specialist site 100km southwards. "There is only some 30 000t likely to need removal but it is an expensive business."
Soft touch
That is to come. Work so far, begun last year, has other problems to contend with, namely vibration noise or damage. As well as not disrupting the well-off neighbourhoods above, this is important because of possible damage to existing tunnels and the underground bridge structure. Rock cover between tunnels in a few areas is just a few metres.
Consultations are needed with concerned operators says Enoksen and limits have to be set very carefully for blasting, with restricted round lengths in many areas.
The project is also trying out for the first time the rock splitting technique, developed initially in Japan. In this hydraulic wedges are used literally to split the rock. They work much like the ancient “feathers and wedge" used in quarries from Egyptian times onwards where two inclined “feathers" are driven apart by a hammered central wedge.
The technique requires a relatively precision drilled rockface, to ensure straightness of the boreholes and they have to be placed much more closely than for blasting, down to 400mm apart. Usually, according to a research paper done for the Norwegian University of Science and Technology in Trondheim by Jens Anders Brenne Volden, the drill hole length will be no more than 1m.
A drill pattern is needed which creates an initial starter space for the rock to split into, usually a short line of adjacent boreholes. The wedges used on the project are so-called Superwedges, from Italian firm Ripamonti. They are excavator mounted and driven by hydraulic takeoff.
The system is proving good at reducing vibration but it is obviously very slow compared to even restricted blasting says Enoksen. He says the length of tunnel to be done this way has been shortened. "In the beginning it was about 1km of tunnel but we have been able to reduce it over 50 per cent by agreement with some of the neighbours for slightly different vibration limits.
“The contractor has been keeping detailed blasting records of actual vibrations which allow renewed discussions." Condotte is highly experienced in the difficult and unstable tunnelling conditions often found in Italy which helps.
Once the limits are determined for blasting, mostly around 1m to 2.3m rounds with a maximum of 4.2m, the actual work is relatively straightforward in the hard granitic gneiss which prevails for most of the route. There are some dykes and igneous intrusions to look for but mostly the rock is stable and hard.
“Support is shotcrete and there is some rock bolting" says Enoksen. In some places the rock is pregrouted if water is detected by regular advance boreholes.
These are done as 24m long holes every 15m of tunnel advance," says client site engineering geologist Marcus Fritzøe Lawton “and we also log parameters like penetration rate, feed pressure and rotation speed to get an idea of rock quality ahead.
Occasionally there is more complex pressure test drilling involving core drilling. This is needed to determine rock stress patterns for 3D modelling purposes in areas of particular difficulty. “We have done this in three locations” says Lawton “using the firm Sintef”.
Spoil is loaded by Caterpillar loaders onto standard trucks. It is high quality rock and has been sold to local suppliers who use it for concrete.
While this work has been underway, major drill and blast excavation was also underway at the main central site. This is a point halfway along the main drives in an area of the highlands where the housing density is reduced.
“Overall the district is quite well populated" says Kalager. It was for that reason most of all that the TBM method was selected for the main tunnels she says.
Usually in Norway with its stable hard rock, conventional drill and blast would be used, especially as the local tunnelling industry is probably one of the most experienced worldwide in these conditions. “TBMs were used for hydro works in the 1980s and one northern project recently but have not seen much use for infrastructure" she says.
“But conventional methods would have needed half a dozen access points for a project this size meaning disruption of residential areas. And they would have had access only onto local roads" she explains. TBMs can also install a precast lining as they go, saving on follow on works.
The lining is not needed for support but for preventing water ingress, particularly important in a cold climate. “We are aiming for a watertight tunnel as a success criterion" says Kalager.
At Åsland a big enough area was available for a compact but large main site of 250,000m2, with the added advantage it was virtually alongside the main northsouth E5 road route to bring in equipment and remove spoil. Enough space was there to build segment factories and an additional 160,000m2 area to stockpile some of the eventually 11Mt of spoil the bores will produce.
But conventional methods have been used to prepare what Kalager describes as a “whole complex of access tunnels and caverns". In a pre-contract Norwegian firm AS has worked on that, driving two 1km long access tunnels and end branches, a half kilometre section of the main tunnels and two large assembly chambers for the TBMs. The underground complex is slightly different to the single chamber assembly area envisaged by the project designers she says. Local consultants Multiconsult and Swiss firm Amberg make up the design team and put forwards a proposed design but the five main contracts on the project are all design and construct "and it is up to the contractor to choose the detail."
The issue is an important one she says as it is part of a series of complex decisions needed for efficient logistics on a site which has to provide for the entire 18.5km drive from one place. Her client side team has worked closely with the contractor on this.
An additional question is for the tunnels' final operation. The site will become the location for a safety refuge and evacuation system for the tunnels, similar to the emergency stations on the Gotthard, Koralm and other long high speed rail tunnels.
“At 'just' 20km compared to say Koralm's 32km or Gotthard's 57km we do not legally require a safety station" says Kalager. "But it makes sense to use these points since they have been excavated, and we are considering a design at present."
Permanent safety measures will also include cross passages between the bores where passengers can evacuate to the opposite tunnel in the event of fire or other accidents.
For the main drives, the Spanish-Italian JV of Acciona Infraestructuras and Ghella, opted for two assembly caverns for the TBMs. They are each 22m high, 24m wide and 54m long and are linked by a 450m section of enlarged main drive tunnel between them where the backup trains are put together.
It was in these caverns that the TBMs were put together once the components started to arrive in the Spring from Herrenknecht's factory in southern Germany. Dutch lifting firm Mammoet has worked with the contractor and the TBM maker on the assembly, using a big 500t capacity strand jacking lift rig inside the cavern.
Work in progress
The first machine was taking shape at midsummer when Tunnels and Tunnelling was able to visit the project and the second was being readied. All four have progressed well since.
Elsewhere on the site a mass of facilities were being readied too, most particularly a factory for three segment production lines using forms from CBE Group. The tunnels use a seven segment ring for a lining and additionally the factories will make an invert section to provide a flat road surface in the tunnels.
The 400mm thick gasketed segments will be delivered to the machines using rubber tyred multi-service vehicles, says Kalager. "The team examined a rail system but the access tunnel gradients are too steep for that to run to the outside and it would have meant carrying segments into the tunnel for re-loading."
For safety and logistical reasons it made more sense to use the segment carriers. Additionally there is no need for spoil trains as, like most modern tunnels, a conveyor system is being fitted.
Long conveyors from Swiss supplier Marti for the tunnels will discharge onto a shorter 500m conveyor to the surface where a big stockpiling area makes up part of the site.
Spoil will be partly re-used for the segment concrete batching plant, perhaps around 15 per cent and the project has drawn on experience from other recent tunnels to work out the best methodology. In particular it visited Gotthard in Switzerland and the Koralm tunnels in Austria.
Some 15 per cent of the spoil will be used this way and much of the remainder is likely to be removed by truck to be used for reclamation and land creation for port works. There are also on-site living facilities at Åsland for some of the up to 800 workers who be on the project at its peak point once the TBMs start their drives in earnest. First of the TBMs began its drive at the beginning of September, initially testing systems and bedding in the crews, and full production should be underway on all the four drives in January with a hoped 12m to 15m daily progress.
