6.13 Piled raft foundation

A series of centrifuge tests of model piled raft foundations (Fig. 1, 2 and 3) was performed by Horikoshi and Randolph (1996) to examine the role of a small centred pile group in reducing the raft settlement. The concept of the centred piled raft system was described well by Randolph (1994).

Particular attention was paid to the differential settlement across the raft and the load transferred to the pile group during unload-reload cycles (Fig. 4). The results were compared with a fully piled raft designed by a conventional method, where any contribution of raft or pile cap was ignored. The test results showed a high level of consistency despite the small magnitude of settlements observed in the models. It was clearly found that even a small pile group could reduce the differential settlement of the raft significantly, in spite of rather low loads being transferred to the pile group (Fig. 5). In addition to the piled raft tests, a number of loading tests were performed on single piles with and without small pile caps. The results showed that a small cap could increase the total bearing capacity significantly, due to the transfer of load to the soil through the cap.

 

Fig. 1 General section of centrifuge package

 

Fig. 2 Instrumented model pile

 

Fig. 3 Pile arrangement for each piled raft

 

 

Fig. 4 Average settlement of raft during loading test

 

Fig. 5 Differential settlement of raft during loading test

 

References

Horikoshi, K. and Randolph, M. F. (1996) : Centrifuge Modelling of Piled Raft Foundations on Clay, Géotechnique Vol. 46, No. 4, pp. 741-752.

Randolph, M. F. (1994) : Design Methods for Pile Group and Piled Rafts, Proc. of 13th ICSMFE, Vol. 5, pp. 61-82.