Canada Water, London

Central Piling has completed a major piling contract that is enabling King’s College London to build new high quality student accommodation in southeast London.

The mixed-use development at Canada Water is made up of four separate buildings, two of which will provide 770 bedrooms, shops, cafes, healthcare and recreational facilities for students, together with a nine storey commercial office building and an eight-storey block of affordable housing.

The £90 million scheme is being built on the site of a former business park, with Scape developing the site on behalf of Kings College London.

Beneath block C – the office building – is a two storey basement, which is currently being excavated within secant piled retaining walls installed by Central Piling. The company was also employed by main contractor HG Construction to install the bearing piles the provide the foundations for the four in situ reinforced concrete frame buildings, as well as those needed to support the tower cranes during construction.

Ground Conditions

The variable ground consists of made ground to a depth of approximately 5m above thin layers of alluvium and Kempton Park gravels. Beneath this is a layer of London clay from 12.7m to 21.5m below ground level, which sits on top of an 11.5m deep layer of Thanet sand. The underlying chalk bedrock is found at approximately 33m below ground level.

The groundwater table on the site is around 4m below ground level.

Solution

Conventional foundations could not be used for the main structures as the Jubilee Line underground tunnels run diagonally across the site at a depth of around 17m below ground. Instead, a piled solution was developed for all the main buildings, with a reinforced concrete bridging structure designed to span over the tunnels onto piled foundations either side.

Central Piling carried out the pile design for both the building foundations and the secant piled retaining wall that will enable the basement to be constructed beneath Block C.

Pile loading information was supplied by the project’s structural designer Campbell Reith, which also identified an exclusion zone around the tunnels where no piling could take place. The bearing piles have been designed for compressive loads ranging from 450kN to 3850kN, and are all embedded into the underlying Thanet sand at depths of between 23m and 25m.

In total, Central Piling installed 468 bearing piles, including 32 piles for the tower crane bases. The piles have diameters of 450mm, 600mm, 750mm and 900mm, and have been designed for compressive loads ranging from 450kN to 3850kN.

In order to install the piles with the minimum noise and vibration, Central Piling opted for continuous flight augered (CFA) piling using two Soilmec CFA rigs: the SF-65 and the SR-75, which is the largest rig in Central Piling’s fleet.

At the same time as installing the bearing piles, Central Piling used another Soilmec SF-65 CFA rig to install 267 piles to create the retaining wall that will enable HG Construction to excavate a 45m x 15m x 5,5m deep basement. The retaining wall is designed as a secant piled wall formed of alternate hard and soft 600mm diameter piles typically at 450mm centres.

The typical depth of the hard piles is 23.3m, with the longest being 24.5m. Most are embedded into the Thanet Sand, although some lightly loaded hard piles are stopped in the London clay at 15.2m and 19.6m. The soft piles are all 14m in length and stop in the cohesive London clay.
The retaining wall piles are designed for compressive loads of between 400kN and 2150kN.

Challenges

Carrying out the bearing pile and retaining wall piling simultaneously required careful management of the readymix lorries coming into the site to deliver concrete for the piles. With up to 300m3 of concrete being poured each day, and the different machines using different concrete mixes, this was quite a logistical challenge.

Central Piling also segregated the different working areas due to the quantity of plant on site.

As the work was carried out during the autumn of 2020, when Covid-19 restrictions were in place. The company made special arrangements to keep the workforce safe, including distancing during travelling, staggered break times and additional welfare accommodation.