Breaking the ground at Crossrail
London’s £15.9bn Crossrail project hits the ground running with piling for the new Canary Wharf station. By Ruby Kitching
It was fanfares all round when Prime Minister Gordon Brown and London Mayor Boris Johnson gathered around the big red button to lower the first of 310 colossal 18.5m long, 1.2m diameter steel tubular Giken piles into the ground for the new Crossrail station at Canary Wharf.
It was a welcome distraction in a week when MPs’ excessive expenses claims and redundancies were still making headlines. “This is a great day for London, for transport, for new engineering and the partnership of the public and private sector,” declared Brown. “We’re sending out two signals today – that the project, which was mooted in the ’80s and postponed in the ’90s, will have no barriers now and [that we have]faith in the future as 14,000 will be employed in construction on Crossrail.”
Crossrail is, of course, the 118km long east-west London rail link that is due to be operational in 2017. The route involves tunnelling 21.5km under central London.
The £500M Canary Wharf Crossrail station will be the first to be completed and, up until now, had been referred to as the “Isle of Dogs Station”. The new name was requested by developer Canary Wharf Group (CWG), which is contributing £150M towards the cost of the station. The remaining £350M will come from Crossrail’s £15.9bn pot.
The risk associated with keeping to the £500M fixed cost price sits with CWG. Canary Wharf Contractors Limited, a subsidiary of CWG, is the main contractor and project manager for the project. Expanded Piling, part of Laing O’Rourke, is the preferred bidder for the enabling and civil engineering work, while structural and geotechnical engineering is being carried out by consultant Arup.
The 256m long and up to 30m wide station will sit in the north dock of West India Quay with the base slab 16m below dock bed and 25m below water level. “One of the biggest challenges is that we have little land to work on. We’re surrounded by water and have to use barges to transport material,” says Arup associate director Peter Rutty.
Akin to Eden
The structure comprises four basement levels below water level, with two levels of station concourse above water, topped off by a spectacular rooftop park that will be covered with a laminated timber structure and clad with ETFE pillows akin to the Eden Project in Cornwall. Gaps in the cladding will be left out to allow trees to grow beyond the roof canopy.
Tubular steel piles are being driven into the dock bed to create a cofferdam to allow the basement excavation to begin. The maximum length of tubular pile, which is of varying wall thickness, is 18.5m and 10 storey high Giken rigs are required to install them. Before Mayor Johnson pressed the red button last month, three reaction piles had to be installed to support the piling rig.
The toe of the Giken piles will sit in the Lambeth Group Clay layer. After about five tubular piles have been installed, and the Giken rig shifted along, one of Expanded Piling’s rigs will drill down each tube to lengthen the pile and create a cast insitu extension to 25m below dock bed.
Of course, this isn’t really the start of construction on Crossrail or the Canary Wharf station site.
Silt curtains have been in place underwater to prevent the dock bed sediments shifting around and dewatering activity has been prominent on the site since January.
Extensive site investigation and unexploded ordnance surveys were also undertaken prior to piling, as well as the construction of two survey piles.
Ground conditions on the site consist of chalk overlaid by 15m of Thanet Sands, 4m of Upnor formation, 8m of Lambeth Group Clay, 3m of Harwich Formation, 1m of River Terrace Gravels and 1m of Silt, which has accumulated over years on the dock bed, and 9m head of water in the dock.
Prior to piling rigs appearing on the site, the water level below the clay layer had to be lowered using a network of 14 dewatering wells that are embedded in the chalk layer.
“Water is pumped out of the chalk and water in the Thanet Sands drains down into the chalk,” says Rutty.
Dewatering was considered essential to allow piling to be carried out without using bentonite as a support fluid, which could have entered the lower aquifer or dock.
It also speeds up piling progress by two to three times. Similar to steel sheet piles, the Giken tubular piles have a clutch connection along their length to connect with adjacent piles. A mixture of cementitious grout and flexible polymer will fill the three chambers of these clutch connections to make them watertight.
The area around the site is dominated by skyscrapers, with one of HSBC’s towers on the south side, a road on the north and Billingsgate market nearby. The Giken piling method was chosen because it would be quiet, with negligible levels of vibration.
“Noise monitoring was carried out during a trial in February 2008 and the noise from adjacent construction sites and airplanes from London City airport were both noisier,” says Rutty. Two rigs will be used on the site. However, all buildings are being monitored for movement, as is the ground around Billingsgate.
Tubular piles are installed along three sides of the station box footprint. Anchor piles will also be constructed along the outside edge of the Giken piles and connected by tie bars to hold back the top of the Giken pile cofferdam wall.
The fourth, southern side of the cofferdam is made up of the remains of an existing cofferdam from previous developments. To the east of the site is a road bridge that will eventually be closed to traffic as a chunk of it is removed to allow piling to continue beneath.
Once the cofferdam around the station footprint is complete, the water will be pumped out (after first removing the fish).
A row of secant piles will be installed to form the south side of the station box approximately 25m to 30m inside the cofferdam north wall. These approximately 1,200mm diameter cast insitu piles form the walls of the station box and will be installed using piling rigs sitting on the dock bed.
A central spine of 30, 2.1m large diameter bored cast insitu piles will also be installed down to chalk to support plunge columns. These permanent columns assist in top down construction of the two lower basement levels. These too will be installed from the base of the dry dock, inside the cofferdam. Some 105, 2.1m diameter bored cast insitu tension piles will also be installed under the footprint of the station to hold the building down in the long term.
Some 156,000m3 of ground will be excavated on the project. The procedure involves excavating down to the level of the “-3 slab”, then casting the slab and connecting it to the plunge column below.
A pocket is left in the slab for an excavator to remove the dock bed material below to construct the “-4 slab level”, then further excavation to reach the lowest level and the basement slab is cast.
Excavated material will be used to build up about 5m depth of dock bed to create more land for the station, in an area known as Adams Place, as well as to build up the dock bed by about 3m around the station.
The station box needs to be further embedded using this material to ensure it is sufficiently anchored in the event of the dock gates failing and water pouring
out. Surplus excavated material will be transported away by barge, while silt will be redistributed to the neighbouring dock areas. “There may be some form of contamination in the silt, so it’s easier to return it to the water [rather than move it elsewhere],” says Rutty.
Canary Wharf station will be the first to be completed along the Crossrail route because it is on the critical path for tunnelling. Tunnelling under the dock was deemed
too risky, therefore, when the tunnel boring machine (TBM) from the Limmo Peninsula in the east reaches Canary Wharf in 2010, the station is being programmed to be sufficiently complete.
When the TBM breaks through the cofferdam wall, it will “simply” be dragged through and set off again through the opposite wall where it will continue tunnelling up to Farringdon, west of Canary Wharf.
“It is important to have a watertight seal between the station box and the TBM. The best way to achieve this is to drive the TBM through a wall [in a watertight cofferdam],” says Crossrail managing director bill, property and oversite developments Keith Berryman.
He adds that the risk of the tunnels being inundated with water is greater during tunnelling, particularly through the Thanet Sands layer. So the risk is being reduced by building the station box first.
Berryman says that Paddington Station in the west faces similar issues, but that the risk of flooding is reduced as the TBM passes through a thick band of London Clay.
Tubular piles will continue to be installed until October this year, with the station box due to be handed over to Crossrail in 2012.