Piling work now underway at Canary Wharf marks a new era with the foundations set to support the estate’s first residential development.
London’s docklands have been a “des res” for almost 30 years but in July the area gained a new contender in the residential development market as Canary Wharf ’s first homes went on sale. Despite the sales launch, it will be another four years before buyers can take up residence at Park Drive to the east of the estate better known for its office developments.
Centre stage of the new development will be a 57-storey tower. At the moment, the area is still filled with water but piling is underway to change that. Commercial Marine Piling (CMP) is currently creating a cofferdam to reclaim the dock for development while Bachy Soletanche is working on the existing land to construct a secant pile wall.
“There are over 35 buildings planned in this latest phase of Canary Wharf, to the east of the existing estate with a mix of residential and commercial, a school and a doctors’ surgery,” says Canary Wharf Contractors (CWC) assistant project manager Fay Crawford.
“In total, the development will include 3,200 apartments, of which around a quarter will be affordable homes. The focal point of the new district of Canary Wharf is the Herzog & De Meuron-designed 57-storey residential tower which is one of eight buildings, excluding the affordable, that will be built in the first phase. The remainder of the development is split into three phases that represent 10 years of construction work for Canary Wharf Contractors.”
CWC refers to the new development as an eastward extension of the estate but the area was once known as Wood Wharf. It is believed that it gained the name because of the materials that used to be landed there, but wood imports were moved to Canada Water in the 1930s due to concerns about fire risk.
The area is owned by the Canal & Rivers Trust but Canary Wharf bought a 250-year lease for the site in 2012. Later phases of the area are still occupied by warehouses similar to the ones that have already been demolished from the phase 1 area.
The current piling work on site is for phase 1 of the development and is split into two parts with CMP building a combi wall cofferdam construction to reclaim 8,800m2 of the dock basin and Bachy constructing a new quay wall using secant piles. The site is currently a hive of activity with three Bachy Soletanche rigs on land and two CMP rigs and 15 barges on the water.
But further piling works are planned. These include the contract for the bearing piles, which is due to be awarded soon, and overwater piles for the boardwalk area that will surround the edge of the development.
“There will be two levels of basements constructed on the majority of the first phase, with the remainder having three levels, but until the single skin cofferdam is sealed against the installed secant pile wall, dewatering contractor WJ Groundwater cannot commence dewatering of the upper aquifer to facilitate bulk excavation,” says CWC project manager Eldon Cholmondeley-Smith.
Ground conditions at the site comprise made ground and Terrace Gravels over the Lambeth Formation and Thanet Sands, which are underlain by Chalk. Groundwater at the site is complicated with a perched water table within the made ground at about 1m below the ground level. There are also two natural aquifers. The first natural aquifer sits on the Lambeth clays that are located at 8.7m to 16m below ground level. The second is within the Chalk at 32m below ground level.
CWC head of structural design John Crack adds: “We have the added complication that dewatering from the Chalk and Thanet Sands for Crossrail C305 works will stop soon, so we will be working with groundwater flows as the lower aquifer recharges. The dewatering means that there are in effect two aquifers – one in the Thanet Sands and the other in the Chalk with negative pore pressures set up by the groundwater lowering. A further consequence being that we have had to recharge the aquifer locally for the test piles.”
Dewatering for the phase 1 work will only focus on the upper aquifer in the Terrace Gravels. According to CWC senior sustainability manager Martin Gettings, the project team is looking at reusing surface and upper aquifer water for on-site activities such as dust management.
CMP started work on its £10M package of work in February. According to CMP project manager Ross Edgar, the work has gone well so far. “It is a big job for CMP and we will have 35 staff on site at the peak of the work,” he says. “The logistics are challenging but the work itself is fairly straightforward.”
CMP worked with Byrne Looby on the design and build contract for the cofferdam and backfill. The solution involves constructing a 450m-long combi wall using 1,200mm diameter piles to depths of between 19.5m and 21.5m with 1.4m steel sheet piles between to create the cut off.
“The cofferdam wall has been split into three parts so that hand over can be staged, with the first section to be completed being for the footprint of the tower,” explains Edgar.
Edgar also says that the final pile in this section is due to be completed soon and the area will be handed over in mid-September. Much of the design work has focused on avoiding the need to excavate the 2m to 3m thick layer of silt from the reclaimed dock. Deep soil mixing was initially considered, but CMP will now use geotextiles to separate the silt from the imported marine ballast.
“The material is selfcompacting but there is a series of CPT tests to planned once the backfill is complete,” says Edgar. CMP is using a frame to position the piles, which are cased through the silts into the clay, and clean aggregate is placed in the annulus before the casing is extracted.
One of the challenges identified in the early stages of the project was the old dock wall – known as a banana wall not because of the cargos that used to be offloaded at the wharf but because of the shape of the wall into which the hulls of the wooden ships would “nestle” while being unloaded. “The original sections built in 1802 are listed structures but it is linked by a wall built in the 1920s that is not listed,” says Crack.
CMP will connect to the banana wall with a small cofferdam that will tie into struts in the wall at four levels to hold the sheets in position until the mass concrete infill is completed. The cofferdam will be grouted to seal it into the existing wall, if necessary.
While CMP progresses the over water work, Bachy has been working on installing the secant pile wall since January and expected to complete the final pile out of the 300 planned as this issue of GE went to press.
“We used CSP and CFA piles in the early part of the piling work with the 900mm diameter piles extending to 26m below ground level,” says Bachy Soletanche project manager Tom Lee. “We are now using LDA piling techniques to install five, 2m diameter female piles that are being installed to 35m and base grouted into the Thanet Sands with full casing to avoid the need for bentonite support.”
According to Lee, Arup specified the larger diameter piles in the corner section of the secant pile wall to provide additional support during the excavation phase. External propping will be used to keep the excavation area clear of obstructions.
Bachy’s work also includes eight 1,180mm diameter piles to cut through the large diameter female piles. “The secant wall is hard-firm with 40N concrete being used for the male piles and 20N concrete for the female piles,” explains Lee. “The maximum time between constructing the male and female piles is seven days.” Bachy is using its Bauer BG46 rig for the CSP work. It was initially brought to the UK for the piling work at Pudding Mill Lane for Crossrail.
The work being undertaken by Bachy had the added complication of piling above London Underground’s Jubilee Line tunnels which are 16m below ground level and, although the piles were shortened in this area, they still extended to within 3m of the tunnel.
Lee says that the work has gone well so far despite the challenges of completing the large diameter piles within 24 hours. “We used segmental casing to 18m to separate the work to fit within the timescale,” he says.
For the tower, there will be 1,500mm diameter base grouted piles, as well as 11, 900mm diameter piles to 25m and plunge piles for the two level basement and a 2.5m thick raft. The other buildings are 12-storey low rise complexes and 40-storey towers, which will be supported on 600 CFA piles installed to 25m into the Thanet Sands.
This is where CWC will draw on its experience from developing the main estate to the west. “As you extend 5m to 6m into the Thanet Sands it gains strength but after that it becomes weaker again, so the length is critical,” says Crack. The final part of the piling work for phase 1 involves constructing 152 1,200mm diameter steel tube piles backfilled with concrete to 19m depth to support the decking area. CWC has said that it expects the work to take 30 weeks to complete and is aiming to have a contractor appointed and on site this autumn.
The contract for bulk excavation has also yet to be let, but piling work so far has already given CWC an indication of the challenges ahead with lots of obstructions encountered in the old dock.
While there is still a lot of work to be delivered before buyers can start measuring up for curtains, it is clear that construction to extend Canary Wharf estate is well and truly underway. Given the number and scale of piling contracts already in progress and planned, this phased development presents some great opportunities for the UK foundation sector for years to come.