By incorporating a cassette raft in their design for the renovation of One Southampton Row,
Ramboll UK managed to save 1500m³ of concrete and costly excavation when compared with a conventional raft. It was constructed using traditional building methods and saved the client an estimated half million pounds.

The innovative 'hollow' foundation was made possible by utilizing the latest cutting edge computer analysis software. This shallower foundation solution raised the founding level by nearly 1m which eliminated the need for underpinning and dewatering, whilst reducing the health and safety risks and program costs associated with deeper excavations. The construction industry as whole can benefit from this as an example of lean design resulting in a 450 tonne reduction in embodied C02. That is approximately 85% of the buildings estimated annual operational carbon footprint, equivalent to roughly 1.5 million car miles.

Challenge:

A water tight single storey basement was designed to support five levels of superstructure in the heart of the West End of London. Four underground tunnels and future cross rail tunnels crisscross the building footprint, thus excluding the possibility of piles. To complicate matters further, the water table allowed a maximum raft thickness of 750mm before extensive dewatering would be required. Such a shallow section on its own would fail in both bending and punching shear under loading from the long span superstructure.

Innovation:

The simple principles governing the efficiency of I beam and box sections have been applied to the cassette raft, but in three dimensions, and on a much larger scale. The basement and ground floor slabs are connected by a grid of storey high cross walls which transmit shear between the top and bottom slabs. The result is a cellular deep beam structure with the ground and basement slabs acting as "flanges" of the cassette. Punching shear reinforcement in the raft is reduced by aligning the superstructure steel columns with the concrete shear walls below, encouraging an even stress distribution. As a result of this composite action, the basement raft thickness was halved, providing a very lean design for a typical shallow foundation problem.

Implementation:

The implementation of these principles in three dimensions was made possible by state of the art finite element analysis. This mapped the stress distributions to an accuracy which would not be possible otherwise. This provided verification of the selected geometry and stab thicknesses. It also informed an efficient reinforcement distribution that could be implemented by conventional building methods.

Conclusion:

The cassette raft at One Southampton Row is an example of high tech computational design resulting in a low cost alternative to conventional solutions. The approach has provided value by saving program time and reducing material costs whilst still applying traditional concrete construction methods and standard reinforcement detailing.