A comparison of the performance of different types of raft foundation in soft ground

The results from two experiments on full-scale raft foundations built at the EPSRC Bothkennar soft clay test site in Scotland are described. One raft incorporated a downstand edge-beam, the other was a simpler plane-slab raft. Both rafts were loaded around their perimeters. The edge beam raft was loaded in two stages: to design load and then to 33% overstress. The plane-slab raft was loaded in one stage to the same maximum load as the edge-beam raft. Results obtained from settlement points and extensometers on and under the rafts are described and compared. Comparing both rafts immediately after loading showed a significant stress redistribution of the load across the plan area of the rafts. At design loading, the edge-beam raft's performance was comfortably within criteria for acceptable performance. However, some surface cracking of the concrete on the plane slab raft, together with relatively large differential settlements, indicate that the edge-beam raft outperformed the plane slab raft. Nevertheless, it is concluded that the plane slab raft's performance was only marginally outside acceptability criteria and that under appropriate circumstances, such rafts could form an economic alternative to deeper or more complex foundations.