Constructing the complicated Hallandsås Tunnel
Posted: 18 September 2014 | | No comments yet
Following many years of troublesome construction work, both rail tunnels through the Hallandsås Ridge in Southern Sweden were completed ain 2013. Initiated back in 1992, the construction project has generated more headlines in the Swedish media than any other. Ulf Angberg – Communications Manager for the Hallandsås Project at the Swedish Transport Administration (Trafikverket) – takes a look back at what caused the project to be delayed, how problems were overcome, and what stage the project is currently at.
The tunnel through Hallandsås has attracted a lot of attention from the very beginning. The project was also plagued with difficulties. The two 8.7km-long tunnels were completed as part of the third construction contract, following problems and ultimately failure of the initial two. Difficulties mainly involved the extremely varied nature of the rock found in the ridge and the large volumes of water. The environmental restrictions imposed were also stringent and an even greater focus was placed on the environment following leakage of a sealing compound in autumn 1997. There has been a rigorous effort to review all chemicals used, from office material to injection grout. The environmental programme has more than 800 checkpoints, monitoring everything from changes in flora and fauna to groundwater levels.
The tunnelling project challenged the technical boundaries from the very beginning with the ridge’s highly varied geological conditions and extreme water pressures. The problems were evident from the outset, affecting the first tunnel boring machine (TBM) Hallbor. Kraftbyggarna was the name of the state-owned company that was tasked with drilling through the ridge. Hallborr was shown to be entirely unsuited to the job at hand in terms of its design, with its open structure being more appropriate for hard-rock conditions. Hallborr got stuck and sank in the loose material at the mouth of the northern side of the ridge.
Following efforts using conventional technology, the schedule could no longer be maintained and the first contractor gave up and had to pay damages to the client – the National Swedish Rail Administration.
The project was re-started in 1995 with the commissioning of Swedish construction giant Skanska. On this occasion, the ridge would be conquered using conventional Swedish engineering methods, namely, drilling and blasting. But the problem of stopping the inflow of leaking water was underestimated. The ruling of the Water Rights Court stipulating an inflow of water of 33 litres a second was exceeded almost immediately.
In consultation with the client, the ill-fated decision to deploy chemical sealing was made. Rhoca gil, a two-component thermosetting plastic containing acrylamide was to be injected into the softest sections of the rock. Much of the solution injected returned in the seepage water, covering tunnel workers, and was subsequently directed into various small streams and a river.
The scandal broke in late-summer 1997. Cows that drank from the polluted streams were paralysed and crops from the region had to be destroyed. Tunnel workers were diagnosed with high blood acrylamide levels and suffered nerve damage in their hands and feet.
When the injection of Rhoca gil was halted, some 1,400 tonnes of the sealant had been used. Construction on the tunnel was stopped and work stood still for seven years.
The initial part of this period was spent performing remediation work, tests and sealing the sections of tunnel already completed. Approximately one-third of the tunnel was completed when work drew to a halt. A 900m work tunnel between the two tunnels had also been constructed to increase the pace of the work.
The scandal involving the poisonous compound had undermined public and political confidence in the tunnel project. For a long time there was a great deal of uncertainty concerning whether the project would actually be completed. Several new investigations were performed, concluding that it would be possible to complete the tunnels without seriously damaging the environment. But now the price and the technology to be deployed would be of an entirely different scale. Boring the tunnels and lining them with concrete was deemed to be the only reasonable method.
Since the signing of the first contract in 1992, major advancements in terms of the operation of TBMs had also taken place.
However, it was still not possible to find an operator that could meet all of the requirements imposed. At its height, the Hallandsås tunnel would need to support a water column of 150m, or 15 bar. To prevent all leakage, a TBM would need to withstand and be able to drill through such pressure. On top of this was the problem of the highly varied nature of the ridge’s geology. By now, much more information had been gathered concerning the challenges the rock presented in the two-thirds of the tunnel that remained.
In conjunction with the third tendering process and construction contract, an application was also filed to increase the volume of water that could be released during the construction period. The court also gave its approval to triple the volume of groundwater released to 100 litres a second.
The National Swedish Rail Administration selected the construction consortium Skanska-Vinci, and now the TBM would once again be the tool of choice. However, there were few other similarities with the scrapped Hallborr TBM. With the new TBM from Herrenknecht, nicknamed Åsa, it would be possible to operate in both an open position and a closed, pressurised position. An extra-strong lining tube was installed behind the cutterhead and inside the robust shield to withstand the pressure of the water and surrounding rock.
The TBM was a unique compromise to cope with both soft and hard rock while being able to withstand higher water pressure than any other TBM. Another special aspect of the design was the ability to conduct test excavations and strengthen the rock in front of the cutterhead by injecting grout.
In conjunction with the re-start, it was also decided to switch sides; the new TBM Åsa would instead begin from the southern end. The aim was to allow more time for work on the most difficult section – the Mölleback zone, which is at the northern end.
The tactic deployed was to blast a pilot tunnel from the northern side between the two future rail tunnels to reach the Mölleback zone. The extremely poor rock would be stabilised using a unique method involving the drilling of long horizontal freeze holes.
To ensure that it would be possible to drill holes of such lengths with sufficient precision, a test was carried out at ground level in summer 2003 near the Lyabäcken stream. Once again, the entire project was almost derailed on account of this test. Cement and bentonite slurry seeped up to ground level and killed fish in the little stream. While this was a relatively minor incident, it generated a great deal of anger and added new fuel to groups who were already critical of the project. Following remediation measures to the channel, the lasting damage to the stream was minimal.
Despite all of the adaptations, Åsa encountered problems when work on the tunnel commenced in 2005. The starting point in a cavern immediately prior to a challenging section almost ended badly. A collapse already occurred at ring 16 of the southeast tunnel. It took time to clean up the area, reinforce the surrounding rock and start up the TBM again.
It also soon became apparent that it would not be possible to use Åsa as it was originally intended. The concept of boring the tunnel with the TBM in the closed position worked much worse than expected. However, the option of closing the machine so that it functioned like a large cork proved valuable, enabling the inflow of water to be controlled so that the ruling of the Environmental Court was not exceeded.
Due to the extremely large volumes of water involved, the bentonite suspension that was to be used to transport the crushed rock out was not sufficiently thick enough. This caused enormous wear on the pipe system running to the rear of the TBM, which resulted in constant breakdowns.
There were also major difficulties when backfilling the areas between the rock and the lining built by Åsa. A modified cutterhead fitted midway through the mid-audit improved the speed of the advance. And with a steady flow of improvements to the methods used to inject the rock and stop the inflow of water along the tunnel, the TBM moved slowly forward. But the first, southernmost part of the Mölleback zone that was pre-treated from the pilot tunnel in the north caused major problems. Injecting the rock from the TBM to further strengthen the strata dominated activities.
If boring the first tunnel was beset with problems, the Swedish Transport Administration and Skanska-Vinci could instead celebrate the first major success that involved the difficult freezing procedure. The process of freezing a section measuring approximately 130m in two stages was more successful and quicker than expected. When the TBM Åsa reached the frozen rock, the rest of the work was swift and nearly problem-free.
The major breakthrough came on 25 August 2010 with the completion of the eastern tunnel. Finally there was proof that it was actually possible to build tunnels using the methods chosen by the National Swedish Rail Administration and Skanska-Vinci. Up until that point, many people believed that there would never be any tunnels through Hallandsås.
The next phase, the relocation of the 200m-long TBM from north to south, and the subsequent re-start in the western tunnel, was also trouble-free. The large front part of Åsa was dismantled and transported by truck over the ridge, while the rear section was pulled back through the finished tunnel.
Another new and modified cutterhead had also been ordered for the re-start. A better-designed cutterhead, experience and continuously improved methods meant that the second journey through the ridge went significantly smoother and without any major mishaps.
Boring of the western tunnel started in March 2011 and the final major breakthrough was made on 4 September 2013. Tunnelling time had been cut by one-third compared with the eastern tunnel.
Three major changes were made at the re-start of the second tunnel. The National Swedish Rail Administration applied for and was granted a new ruling from the Water Rights Court. The limit was raised from 100 to 150 litres per second. Relatively soon it could be seen that, in reality, the higher limit entailed lower water inflow. The TBM could bore faster and thus seal the rock quicker, and more rapid advance through the ridge meant a lower impact on the environment.
Following the good results from the freezing project, the section to which the method would be applied was extended in the western tunnel to 200m. Another section of the very poorest rock was easy for the TBM to penetrate.
Other difficult sections were strengthened by injecting grout before the TBM reached them. This was done from some of the 19 cross passages, which will be used for evacuation of the rail tunnels when they are operational.
Before the actual tunnelling work was completed, preparations for the construction of the rail lines had already reached an advanced stage. The concrete plant in Åstorp, 50km to the south of the tunnels, which had manufactured the slightly more than 40,000 lining segments, was given another major manufacturing assignment for Hallandsås. This time it was for cable ducting, which was installed in the track ballast at an early stage. Walkways would subsequently be installed over these for use in the event of a train evacuation. Once the cable reels were brought on-site, it only took three days per tunnel to roll-out the heavy-duty high-voltage cables.
Work is currently under way on lighting and power supply. Signal boxes and other technical equipment are being installed in the cross passages. Some 750km of fibre optic cable will also be installed in the tunnels. The train tracks will be laid next summer and 2015 will be used to test all of the technology, including the surveillance equipment. Safety standards have been raised significantly since the tunnel project was initiated at the beginning of the 1990s. As a result, there has been a major investment in the installation of surveillance cameras in addition to other measures, such as an inner layer of fire protection mortar that can withstand extreme heat and permanent pipe installations for water used to extinguish fires.
Aside from the work inside the tunnels, three small-scale stations are also being constructed that also form part of the project: two in the south and one new station for Båstad located 1km to the north of the mouths of the tunnels.
Following the most recent re-write of the construction contract, the project has been on schedule and budget. The price tag is still SEK 10.5 billion calculated in terms of the monetary value of 2008.
And even the challenging environmental goals were met. Currently, the inflow of water is about 12 litres per second, less than half the permissible volume, and better than what the National Rail Administration dared to hope for. Following the initial scandals, the project has been subject to a far-reaching ecological control programme. The conclusion to date is that the tunnels have resulted in hardly any noticeable permanent damage on the unique nature of the Hallandsås Ridge.
When the first decisions regarding the tunnels were made, the main reasons given were to increase capacity for freight services on the West Coast Line, one of Sweden’s most important rail lines. At that time, the volume of passenger traffic was minimal. Over the course of the long period of construction, there has been an enormous boom in passenger volumes on trains in the southwest of Sweden. Now no one is questioning the need to replace the twisty and steep rail line from the 1800s over the Hallandsås ridge with a modern railway line that can handle six times the number of trains per hour. Instead of 80km/h, it will be permitted to travel at 200km/h, and shipment weights can be doubled.
Hallandsås Tunnel project facts
What: Two 8.7km-long parallel railway tunnels through the Hallandsås Ridge; the longest railway tunnel in Sweden.
Why: To increase the capacity through Hallandsås from four trains per hour to 24 and double potential freight weight. Part of the West Coast Line.
Present situation: Tunnel works finished and railway installations have begun.
Construction began: 1992 but stopped due to ground water problems in 1997. Construction recommenced in 2005.
Traffic to commence: 2015.
Cost: A total of SEK 10.5 billion in 2008 monetary value.
Ulf Angberg started to work in the communications field during the 1990s and has worked in many different roles. Ulf began as a Communications Officer at the City of Malmö then as a PR-Consultant at Wirtén PR & Communication, In 2001 Ulf was employed by the Swedish Rail Administration at Hallandsås and then in 2007 he become the Communications Manager for the Hallandsås Project.