Armand van Wijck – TunnelTalk Europe Correspondent
- Completion of the 3.8km of twin running tunnels for Amsterdam’s new North-South Line has been a saga of station subsidence, delay, rescheduling, expense and public relations disasters. But ultimately the project has been substantially completed, thanks in no small part to technological innovation and out-of-the-box thinking. Project Leader Cornelius Schulze shares with TunnelTalk European Correspondent Armand van Wijck the story of how application of new technology and bold decision-making got the project back on track.
- Boring in soft soil, underneath historic buildings, and sometimes within millimetres of existing infrastructure and support structures, construction of the twin tunnels for Amsterdam North-South Metro has been a complex operation requiring substantial technological innovation.
Fig 1. North-South Metro Line is TBM bored between Centraal Station and Europaplein, with an immersed tunnel under the IJ
- In April 2010 the first of two 6.83m diameter Herrenknecht hydroshields started its opening drive of 730m from the working shaft in front of Amsterdam’s central railway station to the first underground station box at Rokin (Fig 1).
- Running a TBM underneath historic buildings has been a first for the Netherlands. The soil on which the old buildings of Amsterdam are founded comprises layers of highly permeable sands and soft silts with layers of compressible peat and organic materials between deposits of stiffer clays. On these soft soils, buildings, some several centuries old, are founded on wooden piles or rafts which are highly sensitive to any movement of the fine soils, or lowering of the natural water table. Despite these difficult ground conditions, construction consortium Saturn X – comprising Züblin of Germany and local partner Dura Vermeer – managed to average excavation speeds of 12.5m per day. Several innovations and out-of-the-box thinking made this possible.
Shield transfer system
- “First of all, we were working in an environment surrounded by water, 25m below ground level. We had to deal with a water pressure of up to 3 bar,” explained Cornelius Schulze, Project Leader of Saturn. “Add to this the narrow clearance between the station walls and this meant we could not use traditional sealing blocks to allow the TBMs to safely enter and leave the station boxes.”
STS assembly in place ready to receive the TBM through the station headwall
- Filling each of the three 300m-long station boxes entirely with water to counter the external pressures was not an option, since setting this up would have added too much time to a project that was already coming under overrun pressure.
Schematic of STS design and construction
- Saturn instead turned for a solution to a new technology developed on its behalf by Herrenknecht subsidiary MSD Stahlbau of Dresden – the shield transfer system (STS). Briefly, the system comprises a specially fabricated pressure pod hydraulically jammed tight against the inside of the receiving station wall. The bespoke steel container measuring 8.2m long and 7.6m in diameter, just enough to house the 6.83m diameter x 8m long Herrenknecht shield, was designed to withstand a maximum pressure of 4 bar, a tolerance level of 33%. The TBM was then carefully driven through the headwall and into the cylinder.
- “This is the first and only tunnelling project up to now that has made use of this device,” Schulze told TunnelTalk. “It is an expensive piece of equipment and complex to operate, with high physical demands on the materials used in its construction. The steel manufacturing of the cylinder has to be perfect. If the welds had not been set up properly, we would have had major problems.” The STS was used in each of the three underground stations, but each setup took four weeks, and another four weeks were needed for dismantling following successful pull-through of the TBM through each box while still encased in the device.
Pressure plate transportation
- A second innovation helped in transporting the TBMs through the station boxes themselves. With each station measuring 300m in length, project owner, Bureau North-South Line, felt that limiting the extent of ground excavation as much as possible would reap significant savings in time and overall project cost. Achieving this meant construction of a number of concrete “steps” inside each station box, some of them with height differences of 1.65m, as well as minimizing clearance on either side to as little as 25mm greater than the width of the TBM’s outer shield in some places. This, coupled with machine being encased inside the STS during pull-through, meant conventional methods of transporting the TBM to the opposite end of the station box were not feasible.
- “Instead we moved the TBMs like a hovercraft with the help of special pressure plates,” said Schulze. “These plates have holes in them, into which we inserted nitrogen to give a counterpressure of 80 bar. There were 12 plates in the whole system, meaning we could lift 960 tonne, enough to bear the weight of the 650 tonne TBMs and their STS encasements. Using these membranes the TBMs were floating about half a millimetre above ground.”
- Applying this system meant the floor had to be as smooth as possible. Saturn therefore placed thin steel plates on the ground, together with the pressure plates, resulting in a friction level of just 1%. The TBM was then pulled through the station box by a winch that required just 20 tonne of pulling force. “We used about 1.5 tonne of nitrogen per station,” explained Schulze. To negotiate the “steps” present in each of the station boxes, hydraulic jacks were fitted to the bottom of the STS encasement.
- TBM support posed another challenge. Saturn had to start operations at the heavily built-up Amsterdam Central Station area, where there was only enough room to store 20 tunnel rings at any one time. The contractor also had to contend with strict noise pollution limits, which necessitated construction of a 12m high steel isolation hall above the 60m x 20m launch shaft to keep levels within challenging parameters. The launch shaft itself was covered with sandwich panels and styrofoam to further minimise noise levels.
Slurry separation plant on the IJ River
- “Additionally,” explained Shulze, “we were not allowed to transport the tunnel segments and TBM equipment required for the northern drive by land. Everything had to arrive by water.” Saturn built extra storage space in the harbour area and transported the segment rings by ship along the River IJ, which passes directly to the north of Central Station. To add to an already challenging logistical situation, the slurry separation plant was also located on the waterfront, connected to the jobsite by a 590m-long pipeline. “I don’t think there can ever have been a project with a separation plant so far from the launch shaft,” said Schulze.
- The pipeline serving the northern operations site ran underground from the launch shaft, through Central Station, which is partly located underground, and through to the river immediately to the north of the station from where it ran underwater to the separation plant in the harbour area.
- For the southern drives at the Amsterdam RAI site Saturn built an even longer pipeline - 2.5km - which fed to a separation plant located on the Amstel River. The pipeline ran mostly underground between the launch shaft at Scheldeplein (to the north of Europaplein Station) through the metro tunnel underneath Europaplein Station and then underground to Zuid/WTC Station before emerging above ground near the city’s southern ringroad (A10), and on to the separation plant.
Fig 2. New TBM attack strategy
Dismantling the shields
- Although major subsidence issues occurred during construction of some of the station boxes, TBM-related ground movement was kept well within 10mm. Settlements along the historic buildings were negligible.
- The initial plan had been to use only two TBMs, running south towards Europaplein from a launch shaft located in the famous large square directly outside Central Station. But when subsidence affected Vijzelgracht Station it was decided that entry of the TBMs into nearby Rokin Station, where work also had to come to a halt, would have to be abandoned. Delaying the whole project while the subsidence issues were resolved would have seriously affected the tunnel construction schedule, and leaving the TBMs in the ground, idle, for a long period, was another project risk.
- So instead it was decided that the only way to continue was to change the whole excavation schedule and attack the alignment from the south using two new Herrenknecht shields to be manufactured using as many recycled parts from the now-stranded TBMs as possible (Fig 2). This strategy necessitated the dismantling of both TBM shields inside the tunnel to the north of Rokin Station, a job that took four weeks and required the use of ground freezing and demolition burners. A third and fourth TBM, manufactured using as many of the recycled parts from TBMs 1 and 2 as possible, were assembled at the southern end of the line, near Europaplein station, and it was these that excavated the remainder of the twin tunnels towards Rokin station.
- In December 2012 Saturn completed the 3.8km-long tunnel boring section of the metro line after more than two years. A month earlier Strukton successfully placed the 430m-long immersed tube crossing at the bottom of the IJ River. Tunnel construction is now in its final stage: rail tracks and tunnel technical installations are being installed and the metro stations are being completed. The line is scheduled to become operational in 2017.
Winning back trust in Amsterdam metro – TunnelTalk, September 2012
TBM completes first Amsterdam metro drive – TunnelTalk, July 2010
TBM underway for Amsterdam metro – TunnelTalk, April 2010
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