Hallendsås breakthrough - TunnelTalk
Hallandsås milestone achievement Jun 2010
Shani Wallis, TunnelTalk
After years and many attempts of battling extreme conditions, efforts on the famed Hallandsås rail tunnel in Sweden are winning. On Tuesday of this week (June 8), a mid-tunnel breakthrough completed a north-south connection through the mountain. Not a full TBM diameter breakthrough after slow cycles of pre-excavation grouting followed by a short advances of tunnel, but connection with a pilot heading that marks safe passage through the most complex and risk-filled zone of the alignment.

Measuring 250-300m, the Mölleback Zone (MBZ) of the 8.6km long tunnel is a tectonically faulted zone of highly variable and altered rock with a high ground water content. "The zone is made up of thin layers of different types of weathered rock and soils all lifted vertically," explained Dr Robert Sturk Technical Manager for the Skanska-Vinci HB construction joint venture. "A high water table in the 130-140m of cover produces water pressures of about 11-13 bar at the tunnel horizon."
The 10.6m o.d. Herrenknecht TBM being used to tackle this attempt to conquer the Hallandsås challenge is designed to withstand 15 bar in the static mode, 8 bar in continuous dynamic mode, and 13 bar for short stretches of a few meters in dynamic mode. "Almost 95% of the tunnel is in hard abrasive rock but it is highly fractured and the high water content results in high volume water ingress at high pressure in the tunnel," said Sturk. "We used the machine in the closed slurry mode for a time at the beginning but the wear on the cutterhead and the demand on the systems was very high, so we have instead progressed mostly in the open mode with extensive pre-grouting ahead of the machine to control the water; much like the process used at Arrowhead in California. Our Herrenknecht TBM is fitted also with the ability for extensive grouting ahead of the tunnel face to provide a treated zone into which an advance of five to ten 2.2m wide rings of precast lining can progress before the grouting cycle starts again."
This method has served TBM progress well through the 5km it has advanced from its start at the south end of the tunnel, but it was not considered capable of controlling the water content in the essentially flowing materials of the MBZ. Here the more effective pretreatment of ground freezing installed horizontally from a pilot tunnel was adopted.

Surrounded by frozen ground in the pilot tunnel

The various phases of installing the freezing through the MBZ started in January 2006 (Fig 1). By the time the TBM arrived, a collar of frozen ground about 4-6m thick around the 8m diameter circle of freeze pipes was in place. The freezing circuit was discontinued in mid-May and it took 14 days, working around the clock, to get through the zone and achieve breakthrough with the pilot heading on Tuesday.
"This was a truly significant achievement for us and the project," said Sturk in an exclusive interview with TunnelTalk. "We now have about 600m to go to breakthrough completely in about mid-September, when there will be major celebrations. Following that we will dismantle the machine, take it back to the south portal and repeat the same process for the second tube. That should be an easier affair, having learned and experienced much on this first drive."
Only one TBM could be used on the twin tube project. Tight restrictions imposed on the project by the Swedish government's environmental protection agencies to control ground water flows from the headings limited excavation to only one tube at a time.

Frost on the freezing equipment

Much like the completed Arrowhead tunnels before it, controlling ground water and working through exceptionally difficult geological and logistical conditions has charted construction of the Hallandsås project. Work started back in the early 1990s when an open hard rock machine was procured and assembled but abandoned after only some 10m as being unsuitable for the job. In 1996 a new contract was awarded to Skanska for a drill+blast operation on four headings. This advanced for about 1.5km from the north portals and about 2km from the south before it too was terminated over an environmental issue. The toxic catalyst of a two-part chemical grout being used to control water ingress ran freely from the tunnel heading and contaminated local water courses, poisoning wildlife and cattle drinking from and living in the rivers and creeks.
A remediation contract was then let for the years 1998-2000 to line the drill+blast lengths of tunnel and hold all secure while the client, the Swedish National Railways, and the government decided the fate of the project. In 2003 it was decided to let another contract, which Sweden's international construction company Skanska together with French company Vinci Grand Projects won again for a new TBM attempt, this time using a closed pressurized system with a precast concrete segmental lining.
The 10.6m o.d. Herrenknecht TBM stated from the end of the drill+blast heading at the south portal in early 2006 and has worked pretty much 24 hours/day, 7 days/week since then to connect last Tuesday with the pilot tunnel and drive on to hole through into the 1.5km of drill+blast tunnel from the north portal in September.

TBM connection with the pilot tunnel after driving through the freeze zone

Knowing the risks of the MBZ ahead of them, the construction team decided to drive an in-line pilot heading of about 600m from the face of the north portal drill+blast headings to the start of the MBZ and from there install the 100m of full-circle ground freezing. With the ring of freeze pipes set at an 8m diameter profile within the 10.6m o.d. profile of the TBM, a new system of glass-fibre freeze pipes was developed by the joint venture and specialist Austrian drilling company Insond so that the TBM could grind through these as it progressed.
"Getting through this MBZ zone was one of the high risk elements on the project," confided Dr Terry Mellors, adviser and monitor of the project for the insurers. "To have come through successfully is a benchmark achievement technologically and an outstanding milestone for the project."
Driving through the frozen zone was said to have been highly successful according to Sturk. "Of course there was still considerable risk involved - the effectiveness of a freeze can never be guaranteed - but it proved to be very secure. There were no incidences of serious concern. We will now repeat, basically, the whole routine to complete the second tube."
Hallandsås is among the benchmark projects in the development of TBM technology to work under high ground water content and high ground water pressures. The Herrenknecht machines designed for the Arrowhead project in California began this cycle of development and the Herrenknecht system procured for the Lake Mead water intake tunnel project in Nevada is the next in the evolution of the technology.
Arrowhead: Clawing success from the extreme - TunnelTalk, Dec 2007
Final breakthrough for Arrowhead - TunnelTalk, Aug 2008
Lake Mead TBM designed for the extreme - TunnelTalk, Nov 2009

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