SIMPLE DESIGN TOOLS FOR PILED RAFT FOUNDATIONS

The design of piled raft foundations is hampered by the complex intera ction effects which take place between the piles and the raft. Althoug h the finite element method and the boundary element method have been used to account explicitly for the interactions, the application of th ese methods is limited to foundation systems with small pile groups. A hybrid method of analysis has been developed, by which the complete a nalysis of larger foundation systems (using quadrant symmetry) may be performed. More practical approaches for the routine design of piled r afts rely on the extrapolation of results produced for single pile-raf t units, but necessarily incorporating approximations. Little attentio n has been paid to the determination of differential settlements, whic h are often critical to the design. The paper outlines the hybrid fini te element-elastic continuum method of piled raft analysis, and descri bes an approximate method for calculating overall foundation stiffness and load distribution. The approximate method may be refined by compa rison with results from the hybrid method. Parametric studies using th e hybrid method have been used to develop approximate methods for esti mating differential settlements. An example of the application of the approximate methods is presented. Two case studies of piled raft found ations are made, which show the application of one of the approximate methods and compare the results with a complete analysis and with fiel d measurements.