Evaluation of the existing piled foundation based on piled–raft design philosophy

Many well-documented case histories for conventionally designed large pile groups proved that this design approach is conservative and dramatically increases the cost of foundation without almost any benefit neither to geotechnical capacity of foundation nor to serviceability. Monitoring results of these case histories, especially in the case of floating piles, have shown low efficiency of pile capacity usage due to direct transfer of considerable part of the load to the supporting soil via the raft. In this paper, a proposed methodology for the combined-piled–raft design based on the conventional philosophy is applied to evaluate existing conventionally designed piled foundations of two identical residential towers located in Cairo, Egypt. The proposed methodology considers a conventional factor of safety for piles, and consequently, it does not violate the existing building codes. The three-dimensional finite-element analyses are performed to evaluate the load sharing between raft and piles. A detailed geotechnical investigation is carried out to obtain soil stratification and material parameters. The pile–soil interface parameters are obtained through back analyses of the available pile load tests. The results of the analyses show that the predicted geotechnical capacity of the combined piled–raft foundation system considering the conventional factor of safety for piles considerably exceeds the design capacity of the original conventional pile group.