The plugging effect on the design of open-ended steel pipe piles

Abstract

This research looks at the workings of three design methods for piles loaded in compression and tension by modelling capacity for different soil profiles, varying parameters and load tests. Compared methods are from the NEN 9997-1 (NEN), CUR 2001-8 report (CUR) and a newly developed international standard (ISO). It is generally accepted that the NEN is overly simplistic and research shows the CUR to be generally over-predicting capacity, but the implications of these flaws are not in detail investigated. Per method, the behaviour of shaft and base resistances/capacities and total axial capacity were modelled for constant cone resistance, real CPT's and applied to case studies. Case studies, where possible, are focused on Dutch soil condition to see how the ISO would apply. The capacities were modelled over the whole domain of the CPT to get insight in general behaviour of the methods and to see where potential weaknesses lie. Also an evaluation on installation effects as residual stresses and pile ageing was done.

The CUR and NEN both have demonstrable flaws that negatively impact their capacity predictions, and most notable are their approach on shaft friction that translates to non-optimal shaft capacity profiles. Also, their response on increasing diameters of concern, as design methods are in enormous disagreement on capacities for piles with diameters larger than 1 m. Predictions compared to each other can vary threefold and differ with 8.5 MN.

The CUR generally over-predicts capacities in looser sands but under-predicts for deeper tests. The NEN generally under-predicts for denser sands but is sometimes largest of three for looser sands. General under-prediction does not necessarily mean the method is conservative. The ISO performs in most cases best, and uses a predictive plug length to influence both shaft and base capacity. The NEN is poorly suited for design, the CUR is in particular unsuited for longer piles or diameters larger than 1 m. The ISO is an improvement on current design guidelines for the Netherlands and a good step in incorporating plugging in the design of open-ended piles.