‘Reliability’, or at least the Norwegian equivalent, is a big buzzword amongst infrastructure engineers at present. The matter was brought to a head following widespread public criticism of Jernbaneverket (the Norwegian rail infrastructure authority) and NBS Norwegian railways caused by weatherinduced service stoppages during 2010.

As a long-time leader in the annual length of tunnelling, Norway has many tunnels of fairly basic design, excavated by drill and blast through competent rock, with simple or no support lining. With the large investment in infrastructure made possible by North Sea oil revenues, particularly to link far-flung settlements and shorten travel times in fjord districts, tunnel design has become more sophisticated.

A major aspect of this increasingly sophisticated design approach is to improve water control. Whereas leaks could formerly be tolerated in tunnels with low traffic, frozen water leading to icicles can be dangerous. With higher levels and speeds of traffic, both in rail and road tunnels, such conditions cannot be tolerated. Safety measures in general also have to be brought up to modern expectations, including escape routes and ventilation.

With the greatly increased tunneled infrastructure network, including its upgraded facilities, there is a concern that the relatively small population of Norway cannot afford to maintain such assets, perhaps when energy revenues reduce. Therefore it is essential that all new and refurbished tunnels have to be constructed to a standard that will ensure reliability and long periods between maintenance.

Tunnel to Hell
To the north of Trondheim, about half-way along the long western coast of Norway, Jernbaneverket decided to construct a tunnel, the Gevingasen Tunnel, away from the coast, to improve the reliability of the Nordland service. In the past, landslips and snowfall have closed the route in the winter, on the narrow coastal strip of land where the route now runs. The new tunnel will allow this route to be closed.

There has been some public controversy about whether the tunnel should have been for double-track and designed for high-speed trains, defined in Norway as up to 250km/h. However, while the tunnel allows for electrification, the layout of the rail connections at each end of the tunnel will not allow trains above 160km/h for the foreseeable future. Current traffic plans are for one passenger train per hour, but with a fairly high level of freight use such as twothree trains per day in each direction. The tunnel has increased total route capacity to eight trains an hour serving Stjordal.

The tunnel is 4.4km long between Hommelvik and Hell, near Stjordal and Trondheim’s Vaernes airport. As it happens, the new tunnel’s north portal is near the village of Hell, on roughly the same route as the E6 ‘Helltunnel’ for the main south-north highway, and so is the cause of much amusement for visitors, but its name is hardly reflected by the temperatures.

The tunnel has been excavated in metasediments consisting mainly of sandstones and siltstone shales partially metamorphosed. The low presence of minerals that can be dissolved or carried away by water means that a drainage method of water control can be carried out as described below.

Tunnelling was contracted to Mika, now part of Betonmast Bygg, in 2009. The tunnel work is worth around NOK 400M (USD 74M) out of a total project costs of NOK 650M (USD 120M). Excavation of the tunnel using conventional drill and blast methods is complete with only support lining, track bed laying and services installation in progress. Three twin-boom Sandvik drill rigs were used (models DT 1120, Axera T11/12 and a DT 920). The section is horseshoe-shape 6.4 x 6.5m high to accommodate overhead power lines. In addition to the running tunnel, other excavations include access ways and escape routes, two of three of which link with the E6 Helltunnel.

There is also a possible interaction with the small-section Finatunnel that has to be allowed for in the excavation and support design. The Gevingasen Tunnel passes below the bore that carries aviation fuel lines to the nearby airport.

In addition to the installation of frostresistant waterproofing, further cold weather precautions include the use of Sundec hard foam boards within the rail track bed. Jernbanverket construction manager Christoffer Ostvik explains that these boards are designed to protect the bed against frost and likely consequent structural deterioration, in line with the recent emphasis on service reliability.

The tunnel is due for handover to Jerbanverket later this year after approximately 2.5 years of work.

Waterproofing
Masterseal 245 was first trialled in a small tunnel in Norway in 2008, but the Gavingasen Tunnel is the first full-scale installation in a Nordic country. Following its application to a short section in 2009 and Sintef tests (see below) Jernbaneverket agreed to apply the new system to a length of approximately 1850m in the central part of the tunnel along with more conventional methods of sealed draining at either end. BASF’s Karl-Gunnar Holter explains that as the main design for the tunnel had already been made, the inclusion of Masterseal together with sprayed concrete lining could be brought forward quickly, with initiation of work within six months.

The ground surrounding the Gevingasen Tunnel was only partially pre-injected with grout to consolidate the portal area, prevent settlement and to protect the aquifers serving the needs of some residences above the tunnel route. Consequently, although this had some effect on tunnel waterproofing, it was not the main purpose of grouting. Specialist waterproofing was still necessary. The expected groundwater pressure head on the lining is 0.6 bar at the tunnel crown and 0.8 bar at the walls, being much less than the designed 5-bar limit of the sprayed concrete and 3mm-thick Masterseal 345 lining. It is expected, therefore, that any groundwater approaching the finished tunnel will be diverted through to the tunnel invert and channelled to a main pumping station.

Masterseal was applied using conventional sprayed concrete equipment, with the applied thickness of 3mm being verified during application. Firstly the roughness of the first layer of sprayed concrete, including polypropylene reinforcement fibres, was measured to ascertain its suitability for Masterseal. Although Masterseal can be applied directly onto fibres it was decided to apply a smoothing layer first.

Once the Masterseal had been sprayed to a thickness of 3mm it was left to cure before applying the internal, bonded layer of sprayed concrete. As the work was carried out in the cold temperatures between mid-November and the end of March last winter, this took about a week to achieve the necessary hardness of Shore 50 before applying the sprayed concrete.

Within this ‘sandwich’ concept of sprayed concrete lining and bonded internal waterproofing, the first layer of sprayed concrete is part of the designed permanent lining. This has to be a minimum of 80mm thick. Rockbolts were also used in support of the arch as necessary, with sprayed concrete applied over these. Secondly the 3mm of Masterseal was applied and, after curing, followed by another layer of fibre-reinforced sprayed concrete of a minimum thickness of 60mm.

Holter explains that a specialist contractor was employed to apply the Masterseal 245. “As the tunnel is fairly narrow and tall it was decided to apply Masterseal manually. You need a specialist contractor rather than general tunnellers as there will be better concentration on quality rather than speed,” commented Holter.

The owner is particularly interested in the performance of the sprayed membrane during the considerable variations in ambient temperature experienced, and also the transfer of heat between the tunnel air and the surrounding ground. Holter adds that it is known that installations in very high temperature tunnels elsewhere have been subject to thermal deformation requiring slight design modifications, so checks needed to be made on any possible variations needed in cold climates.

“The client was willing to note the advantages of sprayed waterproofing membrane over any perceived disadvantages, despite some opposition,” says Holter. “And the result is a completely dry tunnel.”

Sintef tests
Sintef is the largest independent research organisation in Scandinavia, and maintains links with industry and universities to develop technological solutions that are brought to practical use.

In order to test the actual heat transfer properties between the ground rock and tunnel interior across a sprayed concrete lining with Masterseal 345 double-bonded sprayed waterproofing membrane, Sintef has been carrying out a programme, supported by Jernbaneverket, to establish the properties of the combined layers. The balance of cold in a tunnel in winter, and the relative heat from the rock, may be crucial in maintaining the integrity of the lining and the waterproofing membrane. The concern is that groundwater in rock fissures connecting with the extrados of the lining could freeze, expand, and cause damage, but no such action has been determined, it is understood, due to the minimal heat loss preventing water freezing in the rock. So far the Sintef tests and the tunnel installation have confirmed that everything is going according to design intentions.

Down in the basement of the Sintef Building & Infrastructure Institute premises on the campus of the Norwegian University of Science and Technology (NTNU) in Trondheim, Prof Eivind Grov, Sintef chief scientist in rock engineering, guided this author through a collection of large-scale, mainly water-based, experimental apparatus to small twin rooms which form the test centre for tunnel lining thermal performance, believed to be the only facility of this kind in the world.

Blocks of local Storen granite cut to uniform size and representing the ground surrounding a tunnel separate the two rooms. The blocks’ size is limited by the practicality of handling when installing. Each is 1.5m long, projecting into the ‘cold’ room representing the tunnel. Both rooms can have their temperatures maintained at require levels by air conditioning units and heaters. During testing the ‘warm’ room is kept at a constant temperature of 7-9oC to represent the ground temperature, which tends to remain stable. In the insulated ‘cold’ room the temperature is adjusted as required to represent the tunnel air temperature, using computer control, down to about -20oC. The computer controls the temperature in cycles to simulate natural conditions, and these cycles can be varied by the operator as required.

“As temperature and heat transfer fluctuations can take a considerable time,” Prof Grov says, “the whole programme is necessarily lengthy.”

Temperature sensors have been placed at seven intervals (20 to 1280mm from the cold room) through the rock mass with cable connections along the joints of the granite blocks. This allows the heat flux migration from one room to the other to be monitored to produce a series of data. There are 70 separate measurement points.

The granite blocks also accommodate four pre-drilled holes with hydraulic connections to a pump. These can be used to simulate the presence of water in the rock mass, up to the tunnel sprayed concrete lining, at pressures of up to 8 bar.

Between the granite rock mass and the ‘cold’ room, layers can be installed to represent the tunnel lining. In this case the layers consist of a layer of sprayed concrete, reinforced with polypropylene fibres, to represent the tunnel primary lining, the sprayed membrane of BASF Masterseal 345 bonded to it, and finally another layer of fibre-reinforced sprayed concrete, also bonded to the membrane. The three together are 125mm thick.

Roads
The use of sprayed waterproofing membrane has also attracted the attention of the Norwegian Public Roads Administration (Statens vegvessen) for future use in the country’s trunk-road tunnels. However the Administration’s requirements seem likely to be slightly different and discussions are currently under way on specifics.


Hell freezing over? The north Hell portal, junction with the existing railway and its narrow coastal route Figure 1, map of the new tunnel location between Trondheim and the airport Prof Eivind Grov with hydraulic connections to the test apparatus and granite blocks at Sintef Figure 2, section through Gevingasen Tunnel to show drainage design and position of membrane Figure 3, diagrammatic section through the Sintef tunnel wall test with cold ‘tunnel’ on the right Jerbaneverket project manager Christoffer Ostvik (right) with Mika contract site manager Ove Rabba at the northern Hell portal An AMV 7450 access and concrete spraying unit is used to aid application of the Masteral 245 waterproof membrane