Science and empiricism in pile foundation design

Scientific approaches to pile design have advanced enormously in recent decades and yet, still, the most fundamental aspect of pile design—that of estimating the axial capacity—relies heavily upon empirical correlations. Improvements have been made in identifying the processes that occur within the critical zone of soil immediately surrounding the pile, but quantification of the changes in stress and fabric is not straightforward. This paper addresses the degree of confidence we can now place (a) on the conceptual and analytical frameworks for estimating pile capacity, and (b) on the quantitative parameters required to achieve a design. The discussion is restricted to driven piles in clays and siliceous sands, with particular attention given to extrapolating from design approaches derived for closed-ended piles of relatively small diameter to the large-diameter open-ended piles that are used routinely in the offshore industry. From a practical viewpoint, we need design approaches that minimise sensitivity to the estimated pile capacity. This may be achieved partly through a greater reliance on pile load testing, where significant advances have been made in the last decade, but also by adopting design approaches that are focused more on guarding against unacceptable deformation of the complete foundation. Example applications in the paper are drawn both from offshore applications, where current challenges include estimating the axial capacity of ultra-thin-walled, large-diameter caissons, and from onshore applications such as bridge piers and piled raft foundations, where inelastic displacement of the piles is not only acceptable, but often essential for efficient design.