Chapter 22: Behaviour of single piles under vertical loads

This chapter concentrates on the prediction of the axial capacity of piles in clay and granular materials. It is stressed that piling is a geotechnical process, controlled by an operator, the outcome of which depends on numerous factors. It is therefore not reasonable to expect to be able to predict pile behaviour with great precision. The fundamental mechanisms of load–settlement behaviour are described. The traditional approach to pile design in clay is presented using undrained strength as the controlling variable. The limitations of this approach are stressed and particular attention is drawn to the wide scatter of results on which design guidance is based. Because of these limitations, much of the chapter is devoted to the application of the effective stress method for pile shaft design. It is shown to give reasonable predictions of the shaft resistance of piles in soft clays with negative friction. The effective stress approach is also used in correlating the results of pile tests on overconsolidated clays. It is shown that these correlations are much better defined than by using undrained strength. The traditional approaches to the design of driven piles in granular soils are described.