The technical challenge was to restabilise the building. This followed the discovery that the brick corbels had been sheared off the bottom of the party walls in an earlier enlargement of the basement - and the same walls had been loaded with an additional storey above.
A 'holding' foundation was cast at an early stage, in the form of a triangular fillet of concrete, which directly replaced the corbelling. This was overlaid with a separating dpm, before a stepped rc slab was cast across the full width of the basement. Structurally, this transferred some of the load away from the party walls and down onto six mini-pile foundations. It also allowed the removal of the internal cross walls, which had previously braced the party walls. The pile cap was cast in waterproof concrete that ensured a direct connection between concrete slab and piles.
The in situ spine wall was cast in two pours within Douglas fi r shuttering. This was selected for the high sugar content of the timber, which prevented the fine particles on the outer surface of the concrete from curing, and revealed the grain upon release of the shuttering. The concrete is a standard London concrete mix, except for the use of a 10mm aggregate and an admixture.
This optimised concrete flow within the formwork and minimised the requirement for vibration.