Amsterdam metro rearches new depths: with its high water table, historic buildings and busy city centre streets, threading The Netherlands' first bored-tunnel metro line beneath Amsterdam represents a major engineering challenge

International Railway Journal, Oct, 2006 by Peter Badcock

THE AUTOMOBILE is loosening its grip on Amsterdam's historic city centre. Such is the success of public transport in the city that bus, tram, and metro services are often overcrowded and among the busiest in the world, carrying one million passengers per day. It's a trend that looks set to accelerate after 2012 with the opening of the new north-south metro line, which will connect two northern suburbs with the Central station and city centre, and the South/WTC commercial centre.

The construction of the south-eastern metro line in the 1970s resulted in much unacceptable demolition and chaos at street-level. This time, three different techniques are being used to ensure disruption is kept to a minimum. Most of the route runs under existing streets and canals so the need for demolition is much more limited. Secondly, advances in tunnelling have made boring a 3.2km tunnel through soft layers of clay and peat with high water-tables technically feasible. This will be the first time tunnels have been bored under a city centre in the Netherlands. Thirdly, construction of the city centre stations is in covered sites. This involves sinking the 1.2m-thick walls 40m deep into the ground, placing the roof a few metres below ground level and only then digging out the box from the inside while normal life resumes at street level.

Construction during the first phase is being concentrated on the Central-Ceintuurbaan section as this lies under the city centre and is technically the most difficult. Modern techniques, such as compensating grouting in the area of Dam Square and a "corset" around the foundations of the historic Munt tower, should prevent damage to buildings above the line of the tunnels. The wooden pile foundations of 185 buildings adjacent to the three deep city-centre stations are being replaced by steel piles and concrete decking. The earliest signs of subsidence are detected by measuring the reflections of rotating infra-red light beams, which are shone onto 5500 small prisms fixed to 1250 buildings. Underground measurements are also being made at depths of up to 60m to monitor the ground for signs of subsidence.

Major Excavation

The intermediate stations of Rokin, Vijzelgracht and Ceintuurbaan are being built first. Rokin station, serving the city centre, is expected to handle 57,000 passengers daily. Major excavation is required to build the station at a depth of 22m and as such, the planning authorities have decided to build an underground car park at this location to replace car parking at street level. Underground car parking is also being incorporated at Vijzelgracht station (27,000 passengers daily). South of Vijzelgracht, an extra 70 million [euro] has been invested to divert the route of the line away from a wide canal to run instead under a narrow street. This will also allow Ceintuurbaan station to be built in a better location, which is expected to help it attract 35,000 passengers per day. Because of the narrow site, the tracks and their platforms will be stacked at depths of 16 and 26m instead of side-by-side. Specially-designed diaphragm walls will prevent subsidence of the buildings above. It is also here that the only two buildings for the whole metro project need to be demolished.

Construction of the twin-bored tunnels, each 3.2km long and 6.5m in diameter, and at a depth of 25 to 30m, will begin in early 2008. The two tunnel boring machines (TBMs) will be capable of boring at a rate of 13m per day. Trains will not run at a constant depth: at stations they will approach on a rising gradient and depart on a falling gradient.

Meanwhile, road traffic around Central station has been diverted over a temporary road built out into the River IJ. This will later become a promenade and cycle route. Central station handles 200,000 passengers per day and impact of the new metro line on passenger movement through the station must be kept to a minimum. The north-south metro line is one of a package of projects to improve facilities around the Central station. Work started on installing new foundations under the railway station in 2003. Mini-TBMs are being used to replace the old oak piles under the main station building with 90 steel piles in two rows, which reach depths of 30 to 60m. The new metro station tunnel will then be built between these two rows; the roof of this box will form the floor of the railway station concourse.

In 2009, a 136m-long, 8m-high metro station tunnel segment will be floated from the River IJ into an excavation under the station platforms. The new metro station will span the whole width of the railway station and out into a box under the station square. Being part of a major transport hub and serving the northern part of the city centre, this new metro station is expected to handle 62,000 passengers daily.

North Of The River

Construction of the stations north of the River IJ at Buikslotermeerplein and Johan van Hasseltweg started in 2005. Here the line runs at surface level in the central reservation of a main road. The 350m dry-dock at Sixhaven, on the northern bank of the River IJ, is currently the tunnel construction site and there is the potential for a further intermediate station here. It is from Sixhaven that the three 142m curved tunnel segments will be floated into a trench dredged out of the bed of the River IJ. Measures to contain water ingress will be incorporated in the paired metro tunnels on both sides of the IJ.