An experimental investigation of the time dependence of shaft friction for displacement piles in lightly over-consolidated clay

The time dependence of pile shaft friction in clay is well known. However, our understanding of the mechanisms causing capacity increase after consolidation is limited and there is considerable uncertainty on the magnitude of long-term gains in friction that can be relied upon. Design for cases involving delayed loading or re-use of foundations would benefit from increased certainty, as would estimation of loads for decommissioning of offshore piled jackets. This paper adds new insights into the ageing process of shaft friction for displacement piles using results obtained from a systematic experimental study involving model pile tests performed in a range of reconstituted clays. These laboratory-based results are reviewed in the context of the key physical properties of each clay, with results compared to field test data from three sites. Favourable comparisons with the field data suggest that the laboratory-based, clay-specific approach employed in the study can be used to estimate pile ageing factors for design. The results supported by existing numerical studies enable identification of important factors controlling pile ageing for displacement piles in clay, allowing greater certainty in the estimation of capacity gains.