At first glance the 'Belgian method' for underground construction seems to be a remnant of the 19th rather than the 20th century. Hand digging of tunnels is commonplace and the method is now being used to form the walls of a large tunnel just 6.5m under Antwerp's central railway station.
This is considered to be the best (and only) way of forming the tunnel, which will carry high-speed trains between Brussels to the south and Amsterdam to the north.
The new line forms part of the Euro13bn ($13.3bn) high-speed railway linking Paris, Brussels, Cologne, Amsterdam and London, one of 14 priority transport projects being built under the European Union's Trans European Networks programme. The Belgian section will link the capital Brussels to the French, German and Dutch borders. Of the 312km of track, half will be new and the rest upgraded lines. The route includes about 10km of tunnels.
The high-speed tunnel is part of an extensive upgrading of Antwerp station.
Built in 1905, the imposing and highly ornate building harks back to the glory days of railways, with a 70m high central dome and vast amounts of polished stone, glass and gold. Its transition from terminus to major interchange means that capacity must be increased. As well as the new tunnel, three underground levels of platforms are being built on the south side of the station to cater for both local commuter routes and main-line services.
After passing under the station, the high-speed trains will travel another 1.5km northwards in twin bored tunnels beneath the city. Construction of this section will begin after completion of an underground car park to the north of the station.
Belgian geotechnical contractor Smet is building the tunnel for client Belgian Railways (NMBS), with its engineering arm, Eurostation, acting as consultant.
German foundation contractor Keller Grundbau has been subcontracted to carry out compensation grouting to protect the station from any ground movements caused by tunnelling.
'This is the first use of compensation grouting in Belgium, ' says Keller Grundbau project engineer Thomas PaBETAlick.
The firm is working from two construction pits: a 15m by 30m, 12m deep box in the north and a 30m by 12m, 9m deep box inside the station to the south.
The 80m long tunnel will carry two tracks and be 18m wide for most of its length, expanding to 26m at its southern end to accommodate four tracks, as well as forming part of the excavation for the new local line platforms.
Keller's work began in August with installation of the extensive instrumentation system used to monitor ground and building movements. PaBETAlick explains that accurate monitoring is essential to ensure that grout is injected exactly where it is needed.
Ground movements are monitored by 84 water cells, fixed both inside and outside the station at 3.5m and 6.5m above ground level and in the cellar. These are accurate to 0.3mm and read levels every 15 seconds.
Extensometers, inclinometers (accurate to 0.005mm) and geodetic surveying are also used, along with crack gauges fixed to the station and temperature gauges monitoring any temperature effects on measurements.
The instruments are networked to a PC in the site offices, allowing continual monitoring and analysis of data from one or a selection of the instruments over any chosen period. Software developed in-house by Keller then produces a variety of plots, including contour maps of settlement. PaBETAlick says that the client also has direct access to the raw data and that he can also access the system remotely from a laptop computer.
Despite lying 90km inland, Antwerp is a historic port thanks to the River Schelde. But the proximity of the river also means groundwater level is high and had to be reduced to below 15m before work could start. A series of wells put down across the site did not lower the water level enough, so vacuum pumping is now being used. Groundwater will be allowed to return to its natural level once the tunnel is finished.
Before tunnel construction could start, Keller had to carry out pre-grouting of the 80m by 60m wide area beneath the station, lifting the entire building by a uniform 2mm. Keller's patented Soilfrac system is being used, with 42, 50mm diameter and up to 45m long steel tube a manchette pipes installed from each of the construction pits.
The site is underlain by up to 25m of dark grey clayey fine sand (with a 20% clay content), underlain by Boomse Clay.
All the excavations are being carried out in this sand but ground conditions vary considerably across the site, PaBETAlick says, including boulders in some areas.
This has affected grout take, with vast amounts beneath the foundations but only small volumes used in the centre of the site.
Boreholes were drilled using water/bentonite flush at an inclination of 1.5degrees, in places only 1.8m below the four 4.5m thick massive brick pad foundations supporting the station. Because of the 6.5m gap between the top of the tunnel and the base of the foundations, dr i l ling tolerances were t ight .
'At the end of drilling the boreholes had to be within 300mm vertically and 500mm horizontally, ' says PaBETAlick, 'and 90% of them are within this.'
Drilling started at the beginning of December 1999, with pre-grouting completed at the end of February. In all, 316,000 litres of grout were injected - 'equivalent to 237,800kg of grout against an original estimate for the entire job of 150,000kg, ' reports PaBETAlick.
He adds that grouting was at relatively high pressure, up to 50bar - 'normally it is about 30bar, but the sand is quite dense'. The Soilfrac pipes have ports either 1m or 0.5m apart, depending on the predicted grout requirements of the areas where they are installed.
The first stage of tunnel construction involves forming roof support for the excavation. This consists of eight tubes lined with precast concrete segments.
These are being built from the north construction pit by TBM under bentonite. The four 2.5m diameter tubes in the centre are filled with reinforced concrete to provide support, while the outer four, 3m diameter tubes are used to construct the tunnel walls. Most of the excavations are from the inner two of these four, with the outer two used where the tunnel widens at its southern end.
Each tube takes about three days to construct (although the first took four and a half days). An assessment of ground movements is then made and any necessary grouting carried out.
PaBETAlick says a maximum settlement of 10mm is allowed but 'actually the maximum was 4mm'. This occurred towards the north end of the tunnel in the centre of the alignment and was mainly due to large boulders.
Excavation of the tunnel walls started at the beginning of February. These are 16.5m deep and 1.3m thick, hand excavated through 3m long slots in the concrete lining of the tubes. When foundation depth is reached, reinforcement has to be carried in by hand and fixed before concreting.
Excavation of the tunnel will begin from the north end once the walls are finished. This will be carried out by machine in 6m long drives, with casting of the finished roof and then the floor as excavation proceeds.
PaBETAlick expects that only small amounts of grouting will be needed while the walls are built, with more during the main excavation.
Tunnel construction is due to finish in January 2001 but Keller is contracted to continue to monitor ground movement for six months after that.
The high-speed line is due to open in 2005, cutting travel times between Paris and Amsterdam to 4h 15min. The first train is expected to pass through Antwerp station as the building celebrates its centenary.