Piled Raft Foundations for Tall Buildings

Piled raft foundations are increasingly being recognised as an economical and effective foundation system for tall buildings. This paper sets out some principles of design for such foundations, including design for the geotechnical ultimate limit state, the structural ultimate limit state and the serviceability limit state. The advantages of using a piled raft will then be described with respect to two cases: a small pile group subjected to lateral loading, and then the design of the Incheon Tower in South Korea. Attention will be focussed on the improvement in the foundation performance due to the raft being in contact with, and embedded within, the soil. High rise buildings are usually founded on some form of piled foundation which is subjected to a combination of vertical, lateral and overturning forces. Combined pile-raft foundations can be a particularly effective form of foundation system for tall buildings because the raft is able to provide a reasonable measure of both stiffness and load resistance. This paper sets out a limit state design approach for tall building foundation systems, with attention being focused on piled raft foundation systems. Some of the advantages of piled rafts are outlined, and then the principles of the design approach are set out. A published case history involving a 9-pile group is then analysed to compare the performances of a pile group and a piled raft. An example of the application of the design approach is then described for a proposed tower on reclaimed land in Incheon, South Korea.