Simple design tools for piled raft foundations

The design of piled raft foundations is hampered by the complex interaction effects which take place between the piles and the raft. Although the finite element method and the boundary element method have been used to account explicitly for the interactions, the application of these methods is limited to foundation systems with small pile groups. A hybrid method of analysis has been developed, by which the complete analysis of larger foundation systems (using quadrant symmetry) may be performed. More practical approaches for the routine design of piled rafts rely on the extrapolation of results produced for single pile-raft units, but necessarily incorporating approximations. Little attention has been paid to the determination of differential settlements, which are often critical to the design. The paper outlines the hybrid finite element-elastic continuum method of piled raft analysis, and describes an approximate method for calculating overall foundation stiffness and load distribution. The approximate method may be refined by comparison with results from the hybrid method. Parametric studies using the hybrid method have been used to develop approximate methods for estimating differential settlements. An example of the application of the approximate methods is presented. Two case studies of piled raft foundations are made, which show the application of one of the approximate methods and compare the results with a complete analysis and with field measurements.