Axial capacity ageing trends of large diameter tubular piles driven in sand

The paper examines dynamic pile test data from 25 high-quality offshore cases, where end-of-initial driving (EoID) and beginning-of-restrike (BoR) instrumented dynamic monitoring was undertaken on tubular piles driven in sands at well-characterised sites after known setup periods. The static resistances derived from signal matching by two independent specialist teams using different software are compared with CPT-based pile capacity calculations, providing the first axial capacity and setup dataset for large offshore piles driven in sand. Complementary re-analyses are made from three onshore/nearshore sites where dynamic and static testing was conducted on comparable piles. Open-ended tubular steel piles with 0.3–3.5 m diameters driven in (mainly dense) sands are all shown to develop marked setup, which is most active over the first 2–10 days. All piles show similar outcomes 20–30 days after installation. However, the larger diameter offshore piles’ dynamic tests indicate no further setup after 30 days, while smaller diameter piles at onshore/nearshore sites continue to display further marked capacity growth. Comparisons of the axial shaft capacities inferred from signal matching with CPT-based design methods provides insights into the performance of the design methods. A trend for long-term pile shaft set-up to decrease with increasing diameter is identified and ascribed principally to the diameter-dependent constrained dilatancy that develops under axial loading at the pile-sand interface.