When King Leopold II of Belgium decided to have an impressive railway station built in the centre of Antwerp at the end of the 19th century, it was designed as a terminus, earlier links to the north having been closed.
Based on the design for the now demolished Lucerne station, it was, and remains, an impressive structure which is now officially protected as a monument as is the elevated southern approach.
It was not long before the flow of traffic to and from the Netherlands and the North demanded links in that direction. For many years this demand has been satisfied by reversing inter-city trains out of Antwerp Central Station to follow a loop around the city.
The resultant delay has long been recognised and alternatives have been in planning for some 30 years. The advent of high-speed passenger services accelerated this need and NMBS, the Belgian national railways, decided to provide Antwerp with a through route.
The need to work around protected structures in the middle of a busy city makes it difficult to provide additional capacity within the existing boundaries. The solution was to go underground with two additional sub-surface levels, and widen the south end slightly for a new entrance. Security of a parallel metro route, and of the station’s foundations, were major considerations. The existing masonry rests directly on the local dense sand.The aim is to limit settlement, by using methods described below, to 5mm, and any ’tilt’ to 1:1000.
Underground construction is split into three geographically separate main sections: the reconstruction of the southern approaches including ramps to the new underground levels; new northern approaches including twin bored tunnels; and excavations under the Central Station necessitating extensive precautions to prevent disturbance.
The first two are being managed by TUC Rail, a rail engineering consultancy which is a partially owned subsidiary of Belgian Railways. This work will be described in a future article in T&T International. The third is being designed and is project-managed by Eurostation, a similar subsidiary of Belgian Railways which specialises in major station construction and development. Andrew Watt, Eurostation’s project civil engineer, explained that the Central Station construction work is split into three contract areas: the elevated viaduct and the foundations for the new station, the two underground train levels, the tunnel directly under the monumental station building, and a tunnel under Koningen Astridplein with an underground car park above it. Work is expected to start on the latter by May 2001.
Sequenced
Preparatory work for the station project started in 1988 with the excavation of a small tunnel on the west side of the station rail approach to form a temporary link from the Antwerp by-pass rail loop.
At the start of the project there were 12 tracks and associated platforms in the terminus station on an elevated (+1) structure. The sequence of construction allows some train services to be maintained by working on one half of the station at a time so, in common with the approach works to the south, half the tracks have been closed to traffic to allow excavations beneath for new tracks to the planned lower levels. Consequently, inter-city services to central Antwerp station can be maintained while suburban services are terminated at Berchem station which is connected via the metro. The existing station building is being lengthened to 450m with the addition of a new southern entrance. Work at the southern end includes the construction of a 40m square escalator shaft, a light well, a car park on six levels, and new shops under the station floor slab.
Current construction work is concerned mainly with tunnelling under King Leopold’s main station hall, which is 80m long, and completing reconstruction of the tracks and platforms in the station on the elevated level to allow the construction of the two lower levels and associated access structures. The latter includes excavations to form new support structures, escalator passages and an atrium for a ‘light well’. The main work on the new tunnel levels should start by the end of 2002.
Support walls
The elevated platforms and floor slab are being supported by composite steel columns filled with concrete in a fire resistant structure. These penetrate 30m through the sand to the hard Boomse Clay which is some 100m thick here. An outer row of columns, supporting the station building, are sited in slurry diaphragm walls.
Where possible, diaphragm slurry walls have been used to form side supports for larger excavations, but in many cases this is deemed too inaccurate in consideration of the adjacent structures. In these cases the ‘shaft mining’ or ‘trenching’ method is used in which a one metre-wide trench is dug to stages to a total depth of 20-25m in steps 1.5m long, within a dewatered zone. As each 400mm vertical step is excavated it is strutted and the walls supported with concrete slabs.
Continued stable support of the main station hall with its huge dome is vital to the success of the project. 110 monitoring stations have been installed as part of a larger systems (see below) for computer-controlled data collection and analysis. Half are installed on the surface structure and half in the basement. The monitoring system includes provision for real time warnings to be transmitted to the main interested parties in the event of pre-set levels being exceeded. During T&TI’s visit Andrew Watt received such a warning, with the relevant measurements, on his mobile ‘phone whilst on site.
Pipe roof
On the north side of the station an access pit for works under the station building has been excavated. This will be connected to another to receive the TBM constructing the twin tunnels forming most of the link to the north. A similar pit has been excavated to the south of the main station hall which serves for compensation grouting and reception for the pipe-roof microtunnelling, now completed.
As a first step in the stabilising work beneath the station, the contractor joint venture, led in this case by Smet Tunnelling, formed a pipe-roof using eight remote-control shield-machine drives. Four outer steel pipes (two each side) are 3m in diameter separated by four concrete pipes of 2.4m diameter. Each has been filled with concrete, in some case encasing instrumentation to allow later movements to be monitored. With the completion of the pipe roof work continues above to instal the permanent reinforced concrete floor slab.
A similar structure of 1200mm id asbestos cement pipes was installed 20 years before for the construction of the Diamant metro station adjacent to the new excavations. The maximum settlement had then been limited to 8mm, but the larger excavation and pressure of the new work required additional measures.
Compensation grouting
In the first application of compensation grouting in Belgium, Keller Grundbau is employing its Soilfrac process extensively to adjust for any settlement affecting the station halls and associated structures.
Keller’s resident technical manager, Thomas Paßlick, explained how the whole area was covered by a pattern of tube-à-manchette grouting holes, each 45m long, in two fan arrays totalling 3500m of boring. Boring accuracy of 1% in all planes was required. Each hole has ports at half-metre or one metre spacing as required. These were pumped with grout in stages to prestressthe ground between the station foundations and the pipe roof. The area covered is approximately 5300m².
The process is not, of course, simply a matter of injecting grout to pre-determined needs. ‘You first need information on the station," says Paßlick, "because otherwise you can’t continue with the further planning of grouting steps." Therefore the whole area, both above and below the surface, has been covered with 480 monitoring points using different types of instrument mainly from the Glötzl range. These include 188 automatic horizontal and vertical chain inclinometer, 114 precision electrolevels, 93 hydraulic pressure and temperature gauges and 67 automatic optical crack monitors. The data from all can be handled by Keller’s computer system. Paßlick also emphasises that quick information is needed because you cannot wait to take corrective action. The computer control checks the instruments and handles data in real time, at 15s intervals if required.
The pre-excavation ground stabilising programme was planned to lift the surface structures by 2mm. Initial dewatering produced an average settlement of 1.5mm, followed by another 1.5-2.0mm on the main building. Once the pre-grouting process was completed, the instrumentation was zeroed to provide a base-reading for future measurements. During the entire excavation process, and afterwards, there is a 24-hour computerised monitoring check. If settlement is noted, compensation grouting is restarted using the appropriate tube-à-manchette valves to compensate.
Computerised data analysis is essential to present the values from all the monitoring points in a more easily understandable form. Available printouts include contour maps to show the extent of settlement or heave in relation to the major surface structures and the progress of excavating the tunnel. Monitoring points are arranged to give 13 possible logitudinal sections and 13 cross-sections (across the pipe-roof and rail tracks), easing graphical representations. Daily reports are sent to the offices of Eurostation, NMBS and the engineering auditor, Seco.
From starting grouting in December 1999, Keller has injected 450 979kg of solids to the end of October, contained within 608 061 litres of grout. Keller uses grout of its own composition with little or no cementitious content and low water. One of the main tasks of the grouting programme is to maintain the support for the four main columns of the stations hall which present the greatest load on the ground in this area. Some tube-à-manchette ports on the project have been used over 60 times.
Station hall tunnel
Support walls under the pipe roof have been built with the mining method described above in readiness for excavation of the new rail tunnel at the -2 level. This will have a final height of 7.5m, and widens from 12m adjacent to the drive pit to 20m at the other side of the station hall This gives room for the bifurcation of the rail tracks from the north-side running tunnels to the platforms to the south of the station hall.
Excavation is mainly by mini-excavator and manual methods with sprayed concrete for temporary support. The excavation is protected by a dewatering programme for which the sand is said to be ideal. The wellpoint heads for this part of the programme are situated in trenches within the station hall passages.
Final structure
Ultimately there will be three operational track levels. The current elevated level will serve national trains from Ostend, Gent and the West, with trains for Brussels, Hasselt and the east on the new first basement terminus. The through route on the -2 level will carry mainly high-speed (TGV) trains from France and the Netherlands, and also some local trains to new suburban stations.
A particular feature of the final underground structure is an open section between the platforms from the surface to the second basement level, forming a light well. It was felt that the underground platforms could be dark, forbidding places without the access of natural light.
Schedule
The immediate construction priority is to get both sides of the station on the original elevated (+1) level back in action as soon as possible, while construction carries on beneath. At the time of T&TI’s visit work at this level on the east side (Phase 2) was complete, but work on the west side was running late, with civils work expected to be completed by the New Year ready for track laying.
Most current work is on two 8-h shifts per day, including Saturdays, in order to get back on schedule. The services will then be switched from the west to the east side allowing renewal of the west tracks. By June 2003 all new tracks required on the elevated level will be complete, ending Phase 3. Phase 4 represents the underground levels.
On the current schedule the entire project, including approach tunnels, will be completed by the end of 2005, at a current cost of BFr45bn ($9.64M).
Related Files
Longitudinal section
Cross-section through the station
Example of settlement contours
