Structures: Buoyancy Raft.

A four-storey block of flats was to be built on a site where part of the site was liable to ground heave due to removal  of trees. The sub-soil was of low bearing capacity overlying dense gravel. The building plan was amended to incorporate two sections of flats interconnected by staircase and lift shafts, see Fig. 1.4. A basement was required beneath the staircase section and the removal of overburden enabled the soil to sustain structural loading. To have piled this area would have added unnecessary expense.

The final design was piling for the two, four-storey sections of the flats, and a buoyancy raft for the basement.

It is hoped that these five simple examples illustrate the importance of considering the soil/structure interaction and encourage young designers not to consider the foundation design in isolation.

Fig. 1.4 Buoyancy raft.