An experimental study on factors affecting the ageing of shaft friction on steel displacement piles in sand

Ageing of shaft friction is now an accepted characteristic of displacement piles in sand. However, there is little guidance available on how the associated gains in pile capacity can be incorporated in design. This paper provides an overview of findings from a number of previous high-quality investigations before presenting results from a recent pile testing campaign in the field that targeted aspects of pile ageing to improve understanding of the ageing mechanism. The field testing involved 52 tension tests on pipe piles and specifically examined the influence of installation method (driving, vibration, vibration + driving), pile diameter, steel type and loading history (static and cyclic). These results, taken together with other experimental investigations, show that the development of a welded crust on the pile shaft is a necessary part of the ageing process for steel piles and that the increase with time in the level of constrained dilation under shear on this crust is a primary contributor to ageing of shaft friction. Creep within the sand mass following the disturbance induced by installation can explain the increase in the level of this constraint, while stress redistribution and breakdown of arching may also contribute to the phenomenon. Ageing leads to the inference of instances of long-term aged shaft friction (in tension) of 500 mm and 2 m dia. pipe piles that are, respectively, at least 50% and 30% higher than given by the design method in ISO 19901-4 (2024). Additional observations relating to the effect of the installation method, steel type and load history on the ageing of shaft friction, which could assist designers.