Analysis and design of pile foundations under vertical load: an overview

The paper is intended as an integration and an update of a previous State of the Art report; only selected topics concerning pile foundations under vertical load are addressed. The effects of the installation technique are first discussed, based on experimental evidence collected in different sites and with different pile types. The effect on load-settlement relationship is shown to be less significant than that on bearing capacity; this plays in favour of settlement controlled design. Further available experimental evidence on the behaviour of pile foundations under vertical loads is collected and added to the existing database. Simple empirical methods for a preliminary evaluation of the settlement are refined and further validated. Rational methods for the analysis of pile foundations under vertical load are next reported and compared to the extended database in terms of reliability and accuracy. The recent case study of the Burj Khalifa Tower is discussed with more details. Load sharing and interaction between the piles and the connecting raft is a key factor in the new criteria for an optimum design. A simple semi-empirical method for the preliminary evaluation of the load sharing at SLS is first presented; it is partially calibrated on the results of non-linear analyses carried out by Napra code. A few recent experimental investigations on the interaction between piles and connecting raft are presented and discussed; it is claimed, however, that further research on this topic is badly needed due to the rather scanty available evidence. It is finally claimed that the conventional capacity based design approach should be abandoned in favour of various existing settlement based approaches and that some codes still act as a restraint rather than a stimulus and need some revision.