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This paper describes a new approach to represent the effects of time on the shaft friction of displacement piles in sand. The proposal is based on the findings from pile test programmes performed at a test bed site in Western Australia in addition to data from well-documented full-scale pile tests. The investigation reveals a significant influence of installation disturbance on the subsequent gain in capacity (set-up) of shaft friction. Set-up is viewed as a recovery process, rather than a capacity gain, following the disturbance induced by pile installation. Gains in capacity in the medium and longer term tend to vary with the logarithm of time, while capacity changes shortly after installation are seen to be relatively small. A new expression is proposed to represent this ageing characteristic, which also incorporates a limiting (or maximum available) shaft friction.

This paper examines the effects of time on the shaft capacity of jacked piles in sand through an experimental program performed at three separate test sites in Western Australia. A total of 18 instrumented pile tests were conducted to track the changes of radial stress on the pile shafts, as well as shaft friction, up to a maximum of 72 days after installation. The results confirm that the gains in shaft capacity of jacked piles are small after a period of about 1 day. This observation is in keeping with limited available case history data, but is in stark contrast to the significant capacity gains reported for driven piles. Further examination reveals that set-up effects for jacked piles in sand are relatively significant within one day of installation, and that the increased friction is associated primarily with larger increases in radial effective stress during shearing.

Although field evidence of increases in pile shaft friction over time (i.e. set-up) in sand is compelling, the observations are highly scattered and the mechanisms leading to the increases are poorly understood. Several attempts have been made to model the phenomenon using small-scale physical models in a controlled environment, but the investigations were hampered by numerous restrictions. This paper describes an improved small-scale model pile experiment designed to investigate the ageing effects of pile shaft friction in sand. Details of the experimental set-up adopted and procedures employed are presented. Results show that the phenomenon of pile set-up is successfully modelled and that load-displacement characteristics are in keeping with those observed in centrifuge tests. Although the ageing effects observed in this laboratory-scale physical model may not be directly applicable to field scale piles, comparative studies involving various influential parameters provide qualitative indications of the relative influence of a variety of parameters on the phenomenon.