CONTRACTORS ARE reducing the number of columns in Unilever's new headquarters in London by transferring loads to 14 concrete caissons.
The landmark 1930s building overlooking the Thames at Blackfriars Bridge is being demolished but a new one will rise behind its grade II listed art deco stone facade.
Enormous concrete caissons made up of 600mm deep, 2.74m diameter pre-cast concrete rings, were jacked 20m into the ground from a fabricated steel reaction frame.
Ground conditions feature water bearing sand, gravels and soft clays above London Clay.
Groups of four concrete caissons in the south west, north east and north west of the site are being filled with concrete and capped.
Resting on the caissons, three concrete cores rising up 10 storeys will give the building lateral strength.
The three cores will reduce the need for internal columns, increasing floor space.
Meanwhile, the original 1930s step-taper steel shell concrete filled Raymond piles will remain to underpin the building. About 1,800 piles will be reused because Unilever has detailed records of their positions.
Demolition of most of the building by McGhee Construction is due to complete by the end of this month, then Bovis will start on the main construction contract.
Dinesh Patel, project director for structural engineer Arup, said it was very rare for jacked caissons to be filled with concrete and used as piles. Usually they are made as access shafts for tunnel construction.
The technique was chosen because there was not enough headroom to do large bored piling through the basement levels.
He added: 'This is the first time anybody has done a hand-dug caisson where there are foundations and while demolition is taking place.'
The caissons were driven during demolition of the interior. This shaved an estimated four months off construction time and reduced environmental and noise impact of the site.
Jim Mackey, managing director of McGhee Construction, said: 'The demolition lends itself to a concurrent operation of the foundations. We were taking the structure down very carefully on a floor by floor basis which made the environment safe to execute these works.'