Just outside Rotterdam's Central Station, site workers are using a combination of driven piles, diaphragm walls, sheet piles and combi-walls under tricky conditions to build a new four-lane road tunnel.
Main contractor BAM Civiel Projecten is responsible for building the tunnel in a contract worth 225M. The old 1950s two-lane Weenatunnel is being replaced by two cut and cover tunnels each 7m wide, with two lanes that will accommodate double the amount of trafc.
The contract includes tunnel installation, design of temporary works and mechanical engineering for pumps to control water levels.
The work is part of a larger project commissioned by the City of Rotterdam to modernise its central railway station in preparation for the Netherland's connection to the European highspeed railway network this year.
The Central Station will be rebuilt to accommodate an expected 70,000 extra passengers who will be using the new high-speed line each day. But this has not proved easy. In places, the site has up to 5m of made ground containing bricks, stones and rubble from the bombing of the city in 1940.
There is also a high level of groundwater in the area.
Because of poor ground conditions both tunnels will be supported by 500mm diameter steel-cased piles. Each has a 320mm square precast reinforced concrete pile inside.
Rig operators are installing 700 concrete piles down to a maximum depth of about 26m.
Beneath the made ground about 4m of clay and sands overlies sands, in which the piles found.
BAM Civiel Projecten site organiser Ramon de Bruijn says the 500mm diameter steel casings help get the piles down to depth in ground that, littered with rubble, could cause damage to the precast reinforced element if it was driven straight in.
Rig operators drive down the casing that has a sacrificial steel toe, using a Hitachi 300 rig with a S90 hydraulic hammer attached, and remove spoil from inside the tube.
Site workers then insert the precast element (reinforced by six or eight steel strands) down the centre of the casing.
The grout adds tensile strength between the pile and soil, and is vital because of the high groundwater in Rotterdam.
The piles will cope with the pulling force caused by tunnels wanting to float in groundwater just 2m below the surface.
Site workers are also drilling water wells to about 25m to reduce groundwater levels to about 6m below the surface. De Bruijn says this is necessary to complete the excavation work for the tunnels. 'The problem is that if you excavate without lowering the water pressure the ground will break up, ' he says.
The site is 350m long but only 12m wide because it's bordered by the train station, industrial buildings and existing roads and tramlines serving the city.
Before concrete tunnel sections can be cast, site workers are installing temporary steel sheet piles to hold back the walls of the excavation. Where extra load bearing capacity is needed the tunnel is lined by temporary steel combi-walls.
Once the concrete sections are ready, the gap between them and the sheet piles will be filled with more concrete, before the sheets can be removed.
As well as noise, there are also concerns about vibration.
About 40 vibration monitors are in place to monitor the installation of sheet piles and then their removal when the tunnel is complete.
A 110t Sennebogen 6100 crane fitted with a PVE 2335 variable moment unit is installing 2500 sheet piles. These range in size from AZ26 to AZ50 to support the tunnel walls. For combi-wall sections, two PU32 sheet piles go in on both sides of each of the 30, 1.22m diameter steel tubes.
The Hitachi rig is also installing the tube piles. All temporary steel piles go down to a maximum depth of about 25m. They are 1.5m higher in the ground than concrete foundation piles because of concerns that vibrations from temporary piles could disturb permanent piles.
The road tunnels will be built one after the other with the first - the south tunnel - under construction now. This schedule will help with the client's requirement from BAM to keep traffic disruption to a minimum.
Traffic continues to flow through the old tunnel. When the first new one is completed by next spring, traffic will be rerouted through it so the old structure can be demolished and rebuilt. The second new tunnel will be built almost exactly on the footprint of the old.
Permanent piling work takes place before site workers install temporary piles. They then excavate for the tunnel down to a maximum depth of 8m and remove grout from the top of each permanent pile.
The same temporary steel tubes act as bracings at three levels in the excavated area to ensure a maximum 10mm vertical tolerance. Site workers can take out the lower two bracings once the concrete floor is down.
But this simple cut and cover method of building the tunnels has to be adapted in a 40m section where tramlines from Central Station cross the site.
BAM Civiel Projecten project manager Joop Postma says to create the tunnel site workers have to excavate from the side and not from the top down.
Work started on the tunnel in May 2006 and is due to be completed by the end of 2009.
It will be 350m long, 15m longer than the old Weenatunnel and will have 135m of enclosed road tunnel, with 107.5m of cutting at each entrance.