With two large properties within metres of the build, the primary challenge was to ensure that there was no danger of subsidence. This was overcome with the installation of 3 ground beams, spanning from side to side 4.1m below the ground level, supported by steel beams set at a 45-degree angle.
A further challenge was the very high and very active ground water on the site. All excavations required shoring and propping, even in the very short time between excavation and casting. The site was constantly de-watered by 5 automatic pumps to reduce the effect of the water on the build and the crew.
To avoid collapse a 1250mm ring beam was excavated, reinforced and cast in concrete between temporary shutters prior to the main excavation. Three deep ground beams were then cast in place to support the buildings either side.
Once the buildings were secure the rest of the excavation could take place and the ring beam underpinned to formation level below
Finally, the floor was cleared away, reinforced and cast to give one homogenous structure ready for a superstructure to be built on top.
The basement has been waterproofed with two systems to conform with latest insurance and British Standards guidelines:
- Types A&B – Barrier/Integral Protection – The walls have been treated with several coats of a waterproofing slurry and floor was cast using waterproof concrete.
- Type C – Cavity Drainage System – All walls and floor were then lined with a cavity drainage membrane linked to channels and two sump and pumping stations.