In common with many other Scandinavian cities, Oslo has been experiencing a great increase in traffic, and the authorities see an efficient public transport system as the best method of reducing, or at least stalling, the problem.
Approved in December 1997, a two phase programme by the Oslo Kommune and train operator AS Oslo Sporveir will close the current U shaped metro route into a ring serving the busiest areas. The first phase from Ullevål football stadium in the north to Storo, is set for completion in 2003, and includes stations at Nydalen and Storo. Phase 2 will link Sinsen station by a 1,700m long tunnel with Carl Berners Plass in the east, and is due for completion in 2006. This will give a new route for lines 2 and 4 of the metro system, and will make a total of 14 stations on the T-baneringen.
The new route follows the existing ring road, although the underground construction, totalling 4km length, bypasses the north-eastern loop around high ground. Another new station is planned at Homansbyen, near the Bislett Stadium to the south-west of the city.
The total project cost is NKr897M ($98.8M) at 2000 prices, split 60:40 between phase 1 and phase 2, of which national funds are contributing NKr673M ($74.2M) and the Oslo Kommune NKr224M ($24.7M).
Work is divided into four main construction contracts of which Entreprise (Contract) B2 is the Ullevål to Nydalen tunnel, visited by T&T International. This was let to Norwegian contractor Veidekke in June 2000 with its inclusive bid of NKr126.7M ($14.0M) and involves a 1,400m twin-track tunnel in rock, plus an 80m long access tunnel at Ullevål. The contract links with B3 for Nydalen Station, started last October by Norwegian contractor Selmer, now part of the Skanska group.
Veidekke’s project manager, Olav Guldseth, says that Ullevål has been the scene of various construction projects for at least 10 years, and activity is likely to continue for another 10. The local residents are therefore very sensitive to disturbance, and any contractor has to very careful about noise and mess.
Construction through hard rock in an urban location is no easy matter, particularly where strict environmental controls are in place. Veidekke’s contract includes cut-and-cover sections at both portals – 115m long at Ullevål, and 75m long linking into the Nydalen station. This leaves the main tunnel, excavated by drill-and-blast, at 1,265m long.
The ground, depending on the horizon, is either slate with limestone concretions, or syenite. Syenite is much harder, but also exhibits greater jointing and fracturing which increases water leakage paths, a critical feature on this project.
Water leakage
The tunnel passes under deep glacial deposits which hold groundwater and support surface structures. Consequently, although a light covering of sprayed concrete is all that is required for tunnel support, water control demands additional ground consolidation and lining. The tunnel cover to rock-head varies down to as little as 15m, except at the portals. Above, the surface support must be preserved through strict limitations on water leakage into the tunnel. The superficial deposits are generally found in small basins which means that leakage can have a great influence on the bearing capacity of the soft ground and possible movement of structural foundations.
Permitted water leakage has been calculated at up to 14 litre/min/100m tunnel length where there is very little superficial deposits, such as near the Ullevål portal. However, where the hard rock cover is small and the superficial deposits deep, leakage is limited to as little as 7 litre/min/100m. This section, between 650m and 900m chainage, requires systematic injection, specified in the contract, over a length of 300m, plus a full permanent lining between 625m and 870m chainage. Systematic grout injection is also being carried out between the Nydalen portal and tunnel centre point (chainage 1050m and 1650m) where the water leakage is limited to 8 litre/min/100m or 10 litre/min/100m.
Systematic grouting
The initial grout pattern comprises 31 holes, each 18m long and 48m in diameter, around the tunnel perimeter. This gives an overlap of 8m across two blasting rounds of 5.2m pull each. After curing, test holes are used to check whether continued water is within the required limits. If not, a further pattern of grout holes is drilled between the existing holes, and injected. The permitted leakage rates equate to 1.5 litre/hole/min.
The grout mix is basically cement and water with Groutaid and plasticiser admixtures. The former consists mainly of microsilica which aids penetration of fissures and acts as a microcement to seal more efficiently. The grout is supplied from an Atlas Copco Craelius containerised mobile grout station comprising a mixer, agitator, pump and recording equipment mounted on a Mercedes Benz highway truck. The 2.99m wide container, is the maximum allowed on the public road.
“There has been recent progress to fully automatic operation,” says Atlas Copco Craelius’ product manager Sten-Åke Pettersson. “There is a push for better control, and we can offer 25 different grout recipes from this unit using three weigh cells to feed the mixer. Each recipe can include up to seven different materials”
The Oslo equipment has a capacity for 4.8t of cement by dry weight (depending on water-cement ratio) and carries two feed hoppers; one of a 1t capacity and the other for bags up to 25kg. There are also two agitators and three grout pumps powered by two 45kW-rated external hydraulic packs, with electric drives. Each pump can feed two lines, in turn feeding up to four grout holes. However, with automatic operation such a set-up would be very difficult for one operator to control.
Instrumentation meets the requirement for every hole to be recorded, with readings given for grout pressure, volume and time elapsed.
Limiting factor on a grouting system is now water availability. Here the plant is equipped with a 350-litre water tank which can be filled to 250 litres from the tunnel service line in 32 seconds. The whole mix cycle time is about one and a half minutes.
Pettersson commented: “About six years ago a complete grout cycle took up to 15 minutes, with hand dosing of the additive.” Now the possible delivery on each pump is 200 litre/min (or 85 litre/min at 100bar maximum pressure). He also stressed the need for skilled operators: “It is important to keep the equipment clean, and the old attitude that anyone could work on the plant led to a lot of breakdowns. But this plant was used at Drammen for two years before its two months so far in Oslo.”
The maximum grout pressure used in Norway is 40 bar to achieve greater penetration that tunnellers thought necessary for the systematic grouting. This compares with the usual maximum in Sweden of 15 bar and in central European of around 10 bar. But the higher operating pressure means that extra precautions are needed. A direct connection between the grout plant operational controls and the monitoring instruments is designed to prevent damage from high pressure, in the event of blockage.
Grout is injected in batches of 1,200 litres with decreasing water-cement ratio, until the maximum permitted leakage rate is achieved. Veidekke uses W-C mix ratios of 1.0, 0.9, 0.8, 0.7 and a normal minimum of 0.6.
The grouting programme started on 6 December last year and Gudseth reports that, to date, 1050 tonnes of grout has been injected. This is mainly because of huge problems with water leakage during March and April which delayed tunnelling progress. “In the worst case we had water leakages of 360 m3/week, and 73 tonnes of injection was needed in one week.”
Drill and blast
The large 65m² section face is excavated in a single pull with around 110 normal holes of 48mm diameter and four 102mm diameter relieving holes in the lower centre of the face. The complex pattern, with 42 peripheral holes (including the flat invert and a drainage ditch), is guided by a Bever Control system installed on the Atlas Copco Boomer 353 jumbo equipped with COP 18-38 hydraulic drills. The same jumbo is used for the grout holes. A hole length of 5.2, produces a planned pull of 5m.
Several patterns of central relieving holes have been tested using from one to six large holes, but four is the preference of the current miners. Improved drilling equipment has made larger holes faster to create, and drilling accuracy is less important with four than with one or two.
Blasting is carried out with Nonel detonators and an ANFO-based emulsion which, unlike conventional ANFO slurry, does not emit noxious gases on exploding. Since the tunnel is within an urban area and fumes extracted by the ventilation system might endanger the public, this is seen as an important benefit, as well as allowing an earlier start to mucking out. The detonators are bunched into small groups with one delay detonator.
Another environmental control is the maximum permitted vibration levels from blasting. Normally the limits are 25mm/s under softer ground and 50mm/s with a hard rock cover. A major source of vibration is the blasting of the lower corner holes, considered as a ‘bottom hole’ in Oslo, but as a ‘perimeter’ hole in Sweden. Perimeter holes are not charged as heavily as bottom holes to achieve a smooth profile.
At least two structures require additional care. Towards the end of the tunnel line, a 30-year old sewer of 1,800mm diameter lies only 2m above the crown, crossing the route at 100°.
Under the sewer the blast pattern will be changed; the current plan being to blast the lower half of the section and excavate the upper half with a hydraulic breaker to reduce potential disturbance.
At the time of T&TI’s visit the sewer has not been accurately located since there was a 300mm to 400mm variation on the meter readings. The plan is to sink shafts either side of the approximate sewer position so that it can be logged more accurately. At another location the tunnel passes under Norway’s main postal computer centre. Here blast vibration is limited to 10mm/s calling for smaller rounds.
After blasting the face, crown and walls are scaled using a hydraulic breaker mounted on a Komatsu PW170 wheeled excavator. Mucking out is by wheeled loader into highway trucks.
A primary support layer of a minimum of 60mm of sprayed concrete is applied using an MBT wet-mix system. This serves to stabilise the rock walls and crown following scaling. Next is a layer polyethylene membrane followed by polyethylene foam, 60mm thick, for insulation against frost action. A final layer of sprayed concrete will be applied once the excavation is complete.
Work patterns
The site usually works 20h a day – longer for grouting – using two shifts of 06:00-16:00 and 16:00-02:00. To limit noise, drilling stops at 22:00.
The workforce is made up of small, highly skilled teams; a pattern common throughout Scandinavia. Three ‘miners’ plus an electrician work at the face, exhibiting a broad range of skills with no demarcation. Activities include hole drilling (for blasting and grouting), grouting including leakage testing, blasting, mucking out and scaling. In a practice said to be unique to Norway, the gangs work a ‘North Sea Cycle’ – two full weeks on site followed by one week off.
Progress
In late June the tunnel had progressed 561m as the earlier high water leakage, plus hard syenite porphyry had reduced progress rates, although they had been up to 50m/week. Now, Guldseth reports: “Fortunately we are now back to normal circumstances and up to date with our project. We are now doing an average of 30m of progress in the tunnel every week, which is quite satisfactory.” Contract B2 tunnel excavation is now expected to be completed by August this year, and all construction on this section by November.
Related Files
Inner Oslo metro system
Ullevål to Nydalen tunnel
