The US$1.2bn Bothnia Rail Line is the largest rail infrastructure project to be constructed in Sweden for over 50 years. Designed to convey both high-speed freight and passenger trains, the line stretches 190km from the northern town of Umeå down Sweden’s east coastline to the industrial town of Nyland (figures 1 and 2). Along the route are 15 single-track tunnels, totalling 25km, the construction of which is estimated to be taking up roughly 20% of the project costs.
For Sweden, this is a large amount of underground work. The Bothnia Line alone will increase the total length of the country’s rail tunnels by 50%. An ironic bonus for the international industry is that the project’s current workload is draining Sweden of its contractors to the point that the major tunnelling section of the lucrative southern stretch, soon to be let, will be the first to go out to international tender.
Added advantages
Freight currently travels through the Gulf of Bothnia on the Trunk Rail Line, built 40km inland during the late 1800s. The over-stretched and ageing low speed freight railway finally reached capacity in the 1980s, prompting the Swedish National Rail Administration (SNRA) to seriously reconsider alternatives that had been the subject of debate for many years.
Upgrading the Trunk Line to a system capable of handling the 25t axle load trains travelling at 120km/h was found to be only marginally less expensive than building a new line. A major factor that swung the decision in favour of the new line, was that the alignment could be placed nearer to the coast, thereby connecting the region’s alternating industrial and administrative cities with a 250km/h passenger train service. The current nine hour journey from Umeå to Stockholm will be nearly halved to a little over five hours.
Almost 50 years on from the initial conception, plans were cemented in November 1997 with the signing of the construction agreement between the Swedish State and the four east coast municipalities of Kramfors, Örnsköldsvik, Nordmaling and Umeå, and the two county councils of Västernorrland and Västerbotten – all included in the route. Construction started on the 14th August 1999, when the Swedish Prime Minister, Göran Persson set off symbolic charges on the first of the 130 contracts that make up the line.
The tunnels and contracts
With the exception of the 700m long Håknäskälen tunnel, all of underground works are situated on the southern half of the Bothnia Line. Construction is now underway on the Örnsköldsvik to Husum section which includes four of the line’s tunnels, ranging from 230m to 1.4km in length. Three of these are finished. The project’s remaining 11 tunnels are in the bidding stage, with only the Stranneberg tunnel currently under excavation.
All of the line is single track, hence the 70m² tunnel cross section, with passing stations situated every 10-14km. Although the use of TBMs was studied for the project, the client decided that drill and blast was more suitable for excavating the relatively short drives, running through the region’s good quality Gneiss that exhibits very limited crushed zones.
The current build-only tunnel contracts are fixed price with a certain amount of flexibility. Botniabanan AB wants to get things right first time and has gone to pains to establish contractor’s needs as a spokesperson explains, “We have done a lot of research over the last couple of years, talking to contractors. One point always raised is risk distribution. As a buyer, we don’t want to put unreasonable risk onto contractors. We want to provide low risk contracts and have done extensive ground investigations, 30%-50% more than normal, to make sure there are as few surprises as possible.”
The Stranneberg tunnel
Of the Bothnia line’s 25km of tunnels, possibly the most complex underground section is currently under construction at the 1.46km long Stranneberg Tunnel site, some 6km north of Örnsköldsvik.
Between the cut and cover approaches, 1,300m of the tunnel alignment runs at depths of up to 85m through the region’s competent Gneiss. But between ch13+522 and ch13+850 (figure 3) the alignment passes under a 300m long field where ground cover is reduced to 10m, forcing the tunnel to pass twice (for around 100m each) along the intersection of the Gneiss and overlying unconsolidated soil, composed of silty sand moraine and sandy silt moraine. Not only is the soil unconsolidated, with little to no cohesion, but it is overlaid by clay with a very high sulphide content. Strict Swedish environmental laws prohibit the freeing of sulphides from soil into the environment, forcing the client to stipulate trenchless excavation. This precluded constructing the shallow tunnel through the field by the far more economical and convenient cut and cover technique.
Charged with building the Stranneberg tunnel is Finnish company Lemminkäinen Construction, which won the US$17.3M construction contract in November 2001. The contractor is no stranger to conditions along the Bothnia Line, having already built two tunnels on the route, the 1.1km long, US$6.7M Kalldal tunnel completed in May last year and the 480m long, US$4M Öberget tunnel finished in February this year.
The complexities of the job are not lost on the contractor. “We think this is the most challenging section of tunnelling works on the entire Bothnia Line, it’s extremely interesting and technically is proving to be a king-size job,” says Vesa Vaaral, project manager with the Lemminkäinen Construction team at Stranneberg.
The Stranneberg solution
The Stranneberg Tunnel’s 9.6m high x 8m wide profile is being excavated by drill and blast from three faces. Construction started in April on the 220m long, 39m² access tunnel and was completed three months later in July. The tunnel provides entry to the two working faces at the mid point of the main tunnel and also serves as the route for transporting rock, lining equipment etc. to and from the main tunnel during construction. The third attack point is from the east portal. Construction of the running tunnel started here at the same time as the access tunnel, and was the launch site for the project’s workhorse drilling jumbo, the four boom Tamrock Axera T12. The machine is the only one of its kind in the world and was developed jointly by Sandvik Tamrock and Lemminkäinen Construction, which used it previously on both the Öberget and Kalldal tunnels. The Axera is drilling the main tunnel’s 120 x 6.3m long blast holes per round in about two hours using a pre programmed computer drilling pattern. One operator controls two booms, enabling four booms to work simultaneously. “It’s working really well, the drilling capacity is around double that of the triple boom,” said Vaaranta. The machine is working alternately between the three faces, drilling one whilst blasting and mucking out is carried out on the other two.
The round consists of ANFO packed inner holes with the outer ‘contour’ holes filled with metracord to minimise overbreak. Following detonation, the excavated rock is loaded onto a 36t Volvo L220E and removed via the access tunnel or eastern portal, depending on the drive. A back-up Caterpillar loader is on hand should the Volvo run into mechanical problems.
The Gneiss is proving good ground to work in with no rockbolting at the face required and 3m-4m long bolts at 2m spacing forming the permanent support of the profile. The lining consists of a 60m-100mm thick layer of steel fibre reinforced shotcrete that is being left as the final lining of the tunnel. Watertightness is ensured by pre-grouting with cement and water in 23m long holes, drilled every 18m or third round.
This construction method is proving successful for the rock sections with a good progress of around 50m-60m per week being achieved.
But an alternative was needed to drive and line the two shallow 100m long soil/rock interface sections. With the groundwater level situated some 2m below surface and the tunnel underwater, stabilising the soil (before excavation of the tunnel profile) was vital. A combination of ground freezing and jet grouting was chosen to achieve this. Bjarne Liljestrand, Lemminkäinen Construction’s area manager for Scandinavia explained, “The original plan was to freeze both 100m long sections, but we made an alternative to jet grout one of these. A split between ground freezing and jet grouting suited our equipment, and was found to be competitive.”
The big freeze
The ground freeze has been designed to stabilise the area where the tunnel crown and walls project through the rock into the soil, although the bottom line of the tunnel stays in rock. Starting in February, a total of 336 x 210mm diameter vertical holes were drilled on 3m centres to a maximum depth of 22m. In April the process was started by pumping freezing brine into the circulation system at a temperature of -30°C to achieve a minimum temperature of -25°C in the soil. As T&TI went to press the contractor had finished the approach from the service tunnel and had excavated some 30m into the freeze. It has reported steady, if slow, progress of around 10m per week with each blast being reduced down to 3m. Temporary waterproofing is being handled by a water and cement pre-grout. Close behind, a primary 250mm thick shotcrete lining is being applied, this will then be covered by the 650mm thick cast in place concrete lining that also serves as the permanent waterproofing, coupled with contact grouting both ends.
Jet grouting
The jet grouting is situated between ch13+740 and ch13+850 (figure 3) in very similar conditions to the ground freeze. The pattern consists of 830 x 1600mm diameter vertical holes drilled at 1300mm distance from each other. The depths range from 5m to 13m to ensure complete consolidation of the ground. Roughly 1500kg/m of cement was pumped down at the extreme pressure of 400 bar to create the 1m diameter of workable ground around each drill hole. The grouting programme has finished and the contractor is currently blasting its way from the eastern portal and anticipates reaching the zone in the middle of this month. The ground will be excavated using a 4m-6m long drill and blast round, followed by an 80mm thick steel fibre reinforced shotcrete lining. The zone will be waterproofed in the same way as the ground freeze.
Schedule
The tunnel is scheduled for completion in September 2003. Lemminkäinen is confident that it can complete the excavation by the end of this year once the difficult ground freeze and jet grout areas have been passed. The Westerly drive had progressed 200m when T&TI visited the site in September and was on course for breakthrough at the western portal before Christmas.
International involvement
The planned, 32km long, Offersjön to Bjällstaån stretch on the southern most section of the Bothnia Line promises the lengthiest tunnelling works over the entire 190km, with 11km of 70m² section running tunnel and 11km of parallel 25m² service tunnel. These contracts will be the first to go out to international tender, with procurement in the form of a design-build contract (with limited design freedom) based on general conditions ABT 94, a standard in Sweden. Within this framework, Botniabanan is trying to ensure clarification of the risk distribution between itself and the chosen contractor.
As with the previous sections of the line, Botniabanan’s intensive pre-investigations, especially with regard to soil mechanics and rock engineering surveys, form the basis of risk distribution decisions. This is reiterated in company literature that states, “Botniabanan AB are attempting to pass on only those risks that Botniabanan consider the contractor to be capable of managing in a reasonable manner and within the framework of the agreement.” Similar to the geotechnical baseline reports used in the US, this essentially means that the client could take responsibility for problems during construction arising from conditions that differ from those stated in the client’s survey report.
“We’ve carried out extensive geotechnical pilot studies and have concluded that the rock is good quality, we don’t expect to encounter technical problems of significance,” said Kenneth Nordstrand, project manager with Botniabanan. With pre-qualification already being advertised, Nordstrand concluded, “we expect to send out the invitation to tender in November 2002, with the start of construction planned for August 2003. It’s vital we have a contractor onboard by next August. We really don’t want it slipping into September.”
The tenders will be scrutinised in accordance with the Public Procurement Act (LOU) and the winner will be based on the “financially most advantageous tender,” Nordstrand was keen to point out that this does not necessarily mean the lowest price but the most economically beneficial contractor.
At 230m asl, the tunnels will run through the highest point of the entire line with the maximum overburden being 200m. The inner width has been slightly reduced to 7.5m to allow for the thicker lining needed. Excavation will be by drill and blast, again through the competent Gneiss experienced in the north. The support has been designed for the different rock qualities according to Q values ranging from Q = 0.1-1 up to Q = >10. Support classes start with class 1 for Q>10 and consists of a 40mm thick fibre reinforced shotcrete lining with extremely minimal rockbolting when required. Class 2 for Q = 4-10 features a 30mm thick lining on the walls increasing to 50mm across the roof with up to nine rockbolts being placed. When Q is reduced to class 3 at Q=1-4, the wall lining is increased to 50mm and the roof to 75mm with 11 rockbolts and finally the worst case Class 4 of Q = 0.1-1. At these points a 75mm thick wall lining will be applied with 100mm sprayed over the roof with up to 15 bolts placed across the tunnel profile.
Pre-grouting has been split into three classes ranging from 11 x 15m-20m long holes spaced 3.2m apart with a 5m overlap to 19 x 10m-20m long holes spaced every 5m. The service tunnel will be constructed in exactly the same way, except that it is 25m² in cross section.
Construction is planned to start mid next year to meet the completion inspection, planned in early 2007. It is hoped the Bothnia Line should be fully operational by 2008, providing Sweden with an east coast rail link that will serve the country’s industry and coastal population for decades to come.
Future opportunity
Already on the drawing board is a plan by the SNRA to completely upgrade the existing Ådal Line to the south of the Bothnia Line. The line will be the final section on the east coast to be brought up to high speed rail specifications and will link Umeå with the Swedish capital, Stockholm. It has already received State and Parliament consent and will include the construction of a further 11km of tunnel. Although still in planning the project has a tentative start date of 2006 with completion envisaged by 2008.
Related Files
Fig 1 – The complete alignment of the Bothnia Rail Project
Fig 3 – The Stranneberg tunnel alignment with sections of the freeze and grout zones
Fig 2 – Project position
Fig 4 – Longitudinal section through the tunnelling section of the 32km long Offersjön to Bjällstaån section
