Since its inception, Hallandsas has become a by-word for how nature can throw nasty surprises at tunnellers who have the temerity to believe that a project is going to be relatively easy. Now the project’s third contractor is making relatively good progress with the Hallansas Tunnel against undeniably difficult ground conditions.

The missing link
The high-speed (up to 200 km/h) rail Scandinavian Link route, which the Hallandsas Ridge lies across, will join Oslo to Sweden, and, through the Oresund Link, Denmark, Hamburg and the rest of northern and central Europe.

Approximately 85 per cent of the West Coast Line has been expanded to double-track operation, leaving Hallandsas truly as the ‘missing link’.

The route of the twin-bore tunnel of 8.7km length runs between the town of Bastad on the left, north side of the Hallandsas Ridge, and Forslov on the right at the south portal. It is a long-awaited link in the upgrading of the Gothenburg-Malmo high-speed line, the main advantage of which is to by-pass a circuitous singletrack route over the ridge with steep grades that greatly limit freight train loads and speeds. The new tunnel route should allow at least twice the payload and virtually eliminate winter delays due to snow, track icing and leaf-fall.

The tunnel will also improve road safety by eliminating 20 level crossings as well as keeping traffic flows down.

Geology
At 10km wide and tens of kilometres long, the Hallandsas Ridge is prominent above the flat landscape of southwest Sweden. It was thrust up as a ‘horst’ during mountain-building period at the end of the Cretaceous period, and so has complex disturbed geology. Fracturing of the rock mass is frequented with several zones virtually disintegrated and weathered to clay. The fractures give free passage for high potential flows of groundwater. When massive, however, the rock quality is hard and good, therefore creating challenging conditions for tunnelling. The most common rock type is gneiss, with amphibolite also common, and diabase.

History
Construction first began in 1992, but the contractor Kraftbyggarna only achieved 13m advance with an open, hard-rock TBM called Hallborr. A major problem was inadequate operation of the grippers against broken ground that offered no resistance for the forward thrust. Drill and blast was used instead from 1993 but progress was again slow, achieving a total of 3km before 1995 when the contractor abandoned the project.

In 1996 Skanska Sweden was awarded a second contract and chose drill-and-blast tunnelling. Despite excessive overbreak in poor rock sections, and high flows of water progress continued on eight faces. Those at the centre were gained by driving a central access tunnel, the Mid Adit.

Poison grout incident
One of the major obstacles in the development of the Hallandsas tunnel was actually a potential solution to another problem; the use of a chemical grout to stem the water flow through fractured rock, as the cementitious grouts then used did not penetrate the water flow paths sufficiently, or were washed away by high flows.

Much has been written about the incident in 1997 without repeating the details here, but to summarise, the grout used, Rhoca-Gil manufactured by Rhone-Poulenc, was found to have entered the surface waters above the construction, paralysing cows and fish. It was also found that workers coming into contact with the material suffered nerve damage and sickness, although fortunately no permanent effects have been reported. The cause was later found to be toxic acylamide in the grout product acting as a nerve poison.

The supplier and the users were found to be liable by the Swedish courts, resulting in heavy fines for Skanska and two employees.

As far as tunnelling is concerned the lasting effects of the incident were a delay in construction whilst investigations and negotiations on continued construction took place. At one time it was thought that the much needed tunnel would not proceed if an environmentally acceptable means of progress could not be agreed, but the Swedish government requested that Banverket did just that.

The non-construction cost penalties, including fines and compensation, were also great. As it happened the fact that the project had cost so much already until the construction hiatus was an incentive to complete the tunnels with a new contract.

Skanska introduced a new environmental management system including a control system for chemicals to prevent use of dangerous substances. From 1997 Skanska undertook an extensive quality and environmental control programme throughout its organisation, resulting in it becoming the first international construction group to be awarded certification according to the ISO 14001 standard.

Hiatus ends
Following an examination of the project by the Swedish Environmental Supreme Court, the project was allowed to restart in October 2003. The court set limits to the amount of water that could be drained from the total excavation over set periods, with both average and maximum flows. The chief aim was to preserve the wetlands habitat and agriculture at the surface over the tunnels route.

Originally the laws on water extraction, whether deliberate or ‘accidental’, were also to preserve the water supply to wells in rural communities although there is now a piped fresh water supply in Bastad and Forslov. Another requirement of the Court was that the ground had to be sealed before excavation, thereby requiring grouting of the ground in front of the tunnel face.

Tunnelling restarted in the winter of 2003, following the award of a design-build contractor to a joint venture of Skanska Sweden and Vinci Construction Grands Projets to complete the remaining twothirds of the tunnelling. Skanska joined with Vinci due to the latter’s extensive experience of TBM operation as it was decided that, in consideration of previous experiences, only a TBM could achieve reasonable progress in the difficult ground.

Under the contract, split 60:40 between the joint venture partners Skanska and Vinci, work is paid per metre of tunnel completed, and per treatment carried out, according to costs incurred relative to a bill of quantities. This was last negotiated with the client in 2008. According to the jointventure’s production director Francois Dudouit, since then everything has been working well.

With a full geophysical survey over the length of the tunnel and other site investigation, plus Skanska’ extensive knowledge of the geology from previous experience, the Skanska-Vinci could be better prepared for the conditions. Even so nothing could be certain. Especially difficult to predict are the location and geometry of amphibolite dykes.

Water controls
The permit granted for tunnelling specifies an average of 100 litre/s over a 30 day (one-month) period, plus maximum flows. “The limit takes into consideration water leaking through all parts of the construction site and not just where tunnelling is taking place,” says Dudouit.

In figure 1 the dotted blue line shows the extent of surface effects of tunnel dewatering. If the limits were not complied with it is likely that the effects would extend over a bigger area.

The TBM
The TBM chosen was a special form of Herrenknecht Mixshield named ‘Asa’ that can operate in open mode or closed slurryshield mode. A key design is the openings in the cutterhead. Asa has openings (albeit small) within the face of the cutterhead, as in standard soft-ground machines to cope with the clay and crushed rock found in highly disturbed zones, but also it has peripheral openings so that hard rock can be scooped up for passing to a belt conveyor.

Before TBM work could start a launch chamber had to be excavated some 1800m under the Hallandsas from the south portal, roughly where the previous tunnelling had halted. This was because the previous drilland- blast drives were excavated with an arch profile only 7.2m wide, whereas the TBM is over 10m in diameter.

“Operating in open mode, as is usual, is the most efficient,” explains Francois Dudouit. “Also, in closed mode progress is slower primarily due to significant maintenance on the slurry circuit, and additional time for cutterhead maintenance is required. Rock blocks from broken ground are difficult to transport in the slurry pipes.” There is also the additional operational requirements of a slurry treatment plant, and the time taken to switch between modes. Although the specified changeover time is 24 hours, other considerations can mean it takes up to five days before the TBM can restart. However, as so often happens at Hallandsas, the decisive factor is water control. If it appears that the dewatering limits might be breached, then the face can be closed off and pressurised to hold back the water coming through the face. Then grouting has to take place before deciding in which mode to advance.

Mucking out in closed mode is by pumped slurry to the surface treatment plant, but in the open mode the rock is loaded onto a 150m belt conveyor to pass it through the back-up system, and thence onto another 7000m long, one metre wide belt. The maximum removal rate is 1000t/h. With the open system, any water make is pumped to the surface for treatment in a separate plant.

It has been found that cutter replacement rate is also affected by the size of the backloaded cutters themselves. The original cutterhead used 17 inch (432mm) diameter cutters. “Our big problem was blocks falling from the face and roof in fractured zones,” reports Dudouit. “Although broken, they are hard, with impact loading on the cutters, breaking bearings and mountings, the cutter rings themselves, or flattening of the ring edge due to stuck bearings.”

From the ‘Mid Adit’ access tunnel, 900m long, 40m of running tunnel had been excavated in both bores. When the TBM reached here the opportunity was taken to carry out intense maintenance and to replace the cutterhead with one that could accommodate 19 inches. (483mm) diameter cutters. “The new cutterhead with bigger cutters is better for impact loads,” said Dudouit, “and gives something more robust. We now have 50 per cent less cutter replacement needed, saving costs (despite increased cutter unit cost) and time on cutter changes. Advance rates are also about 50 per cent better.”

Cutter replacement rates were around one cutter per lining ring before the refit.

The closed-mode TBM can cope with groundwater pressures of up to 13 bar when operating, and and a rarely used 15 bar when static. It is regularly pressurised to 9-11 bar when static.

As the main water problem is flow, a development of the TBM by the contractors has provided considerable operational advantages. The water passes through the annular gap, normally 250mm wide and previously filled with gravel or crushed rock, to be subsequently extracted by pumps. At regular intervals barriers are constructed by injecting backfill mortar and grout to fill the allulus of four rings (the shield being pressurised to balance the water table) so detering water flow. When the TBM is in open mode the water carried out on the belt is separated from the rock within a flat conveyor section.

The TBM cutterhead permits drilling of the face, which is where most of the grout holes are placed, at the same time performing a probing function. Three drills are mounted inside the shield machine for this.

Grouting Programme
Grouting is necessary in many parts of the tunnels for water control, but only infrequently for the purposes of general ground consolidation, despite the broken rock. Since the poisoning incident there is a natural bias amongst all parties against chemical grouting. Consequently most grouting uses Rheocem 650 microfine cement grout, supplied by BASF Meyco and manufactured by Lafarge.

“Rheocem 650 has a particularly quick setting time (less than three hours),” says Dudouit, “which is quite important to minimise the duration of the grouting cycle. The results with the microfine material have been successful.”

Grouting activities tend to delay excavation progress. Dudouit has reported two thirds of the team’s time taken up by grouting, resulting in monthly progress of only 120m at times, compared to up to 250m in good months. However this is preferable to the progress achieved by previous contracts, and gives full compliancewith the environmental requirements regarding control of water flow.

Despite the overwhelming use of cement grout, some chemical grouts have been tried and used in special circumstances. Minova has developed some polyurethane (PU) products for use at Hallandsas in order to meet environmental demands. Francois Dudouit reports, “We have limitations concerning the quantities of polyurethane. We have used them mainly for tests of Carbopur WF and WFA, in very limited quantities. CarboStop (Minova specially developed a new version – CarboStop E) has been approved, but again we do not intend to use large quantities. In total we have only used about 5t of polyurethane from Minova. Another difficulty with chemical grouts is that we have to use a specially qualified sub-contractor. ”

So far around 3000m3 of the Rheocem 650 grout has be injected from the TBM. A typical grouting cycle swings into action after about 20m of TBM advance. To seal water flow and probe ahead the drill pattern involves 10-15 holes in the face up to 40m long. These are injected with grout at up to 30bar to overcome water pressure and penetrate the fissures. Sometimes a fan pattern of holes may be drilled around the tunnel profile but usually only for weak ground consolidation or overbreak filling. “The grout pressure is monitored and we also limit the grout quantity to 5m3 to avoid any chance of the grout reaching the surface,” explains Dudouit.

Backfill grouting also aids water control. This is injected through holes cast in the segments, from the back-up system. Normally this is carried out in two stages – bottom and then top – to avoid washout.

Freezing Molleback
The zone forecast to be the worst for rock condition and water flow and ground instability was the 300m-long Molleback Zone (MBZ). In the MBZ there is a mixture of weathered material with highly fractured and crushed rock. Where the rock is less weathered the fracturing permits major water flows backed by high pressure. It was decided that freezing was the only practical answer, but set-up was far from simple.

Dudouit explains, “Access was a problem but we could get near the zone from the north, and an access tunnel was driven from the old face. We needed 130m horizontal of ground freezing cover, but had to do it underground, and to achieve the necessary accuracy we had to limit the freeze-holes to 100m in length. Using oilfield directional drilling technology we bored a pilot hole surveyed by a gyroscopic and inclinometer device. Then, using electro-magnetic field guidance we drilled a ring of 16 holes referenced to the pilot hole. This ensures that the freeze-zone does not have any breaks in it once the freeze tubes had been inserted and connected to the refrigeration plant.”

Brine refrigeration was used to withdraw heat from the ground. Direct freezing with liquid nitrogen was discounted due to the costs and possible safety hazards in a remote working area.

Once the first freeze-zone had been completed and the tunnel excavated, a second, shorter zone 30m long was generated by repeating the process. Finally, in a third phase, intensive use of cement grout from three holes formed another seal over a further length of 135m.

Lining
The TBM is equipped with an erector to install pre-cast concrete segmental lining of eight segments per ring. The segments are manufactured in Skanska-Vinci’s own plant at Astorp, some 20km south of the south portals construction site near Forslov, and transported to the portal site by rail. There was insufficient space for a manufacturing plant nearer to the tunnel. Together with the segment gaskets and annular grouting through the lining, this ensures that water leakage with the tunnel is limited, allowing the overall drainage limits to be met.

The design life for the lining is 120 years, which, in consideration of high ground pressures, places high demands on concrete quality, and the casting tolerance is only +0.5mm.

The mix employed for the 540mm thick, 2.2m long segments employs synthetic microfibres to improve fire resistance in the tunnel. A steel reinforcement cage is used in each mould to provide tensile strength against ground pressures and also the TBM thrust of up to18,850t. After casting each segment is transported to a steam-curing chamber to prevent surface drying and cracking, and then a Phoenix hdpe sealing gasket is fitted. The segments are stored in ring assemblies for at least 28 days before transport to Forslov. In winter at least four of these days storage are indoors.

The segments are attached to each other by cone-shape dowels that give a strong connection and ensure an accurate fit without steeping between rings before bolting. Each has a 15t shear capacity.

In all, the pre-cast plant will manufacture 41,000 segments to line the entire tunnel in the current contract, as specified according to the new water-control measures.

Progress & completion
At the time of writing there remains only 790m to complete excavation of the east bore to join with the 1149m already excavated under the previous contract, thus forming a welcome milestone with completion of one of the bores. The ‘Asa’ TBM will them be disassembled and transported through the south portal of the west bore to be reassembled in an erection chamber within the Southern Marginal Zone (SRZ) where rock conditions are slightly better. Here 1712m of west bore excavation had been completed under the previous contract, and 1649m on the east bore. The TBM erection chambers were excavated to 16m wide x 18m high and 30m long.

Tunnelling is currently progressing according to a revised schedule due to the geological difficulties. The first trains should run through the tunnels in 2015, three years later than planned when the joint venture contract was awarded.


Map of project area with TBM position at 21 January 2010. Dashed blue line indicates the surface zone influenced by dewatering from the tunnels The Asa Herrenknecht TBM with the shield. Note the grouting/probe drill near to crown Thrust rams within the Herrenknecht TBM shield Pre-cast segmental lining seen from the TBM erection chamber