Ageing of pile shaft friction in medium plasticity clay

Long-term gains in the shaft capacity of displacement piles in clay are an important consideration for end-of-life decommissioning and platform life extension assessments. However, predicting ageing effects is a challenge as the underlying mechanisms are not fully understood, and there is clear variability in ageing rates observed at different sites. This paper presents results from a series of model-scale first-time axial pile tests performed at various times after installation in reconstituted medium plasticity clay. It is shown that, after consolidation due to pile installation is completed, peak frictions continue to increase and vary approximately with the logarithm of time. The post-consolidation gains in capacity are shown to be comparable to gains in shaft friction with time observed in a parallel field study involving driven piles in a natural deposit of the same clay. The agreement between the model scale and field scale tests suggests that ageing of pile shaft friction in clays can be estimated in model-scale investigations.