Observed increases in offshore pile driving resistance

This paper presents 53 well-documented cases of the short-term set-up of open-ended steel pipe piles driven at two fields in the North Sea. Increases in soil resistance during driving (set-up) were observed when pile driving was re-started after delays typically ranging between 24 h and 100 h. The soil encountered at the sites consisted of overconsolidated sands and clays, where the clays varied from very stiff clay (su of 250 kPa) to very hard clay (su of 800 kPa). The field records were analysed to derive correlations between the length of the delay and the increase in soil resistance, or set-up. The available data have been classified in terms of pile diameter, pile tip penetration and the undrained shear strength of the clay strata. The data indicate significant set-up at such sites, even in the short term, with the rate of increase following a logarithmic curve. The data relate to practical issues associated with installation of piles, and do not address the long-term load-carrying capacity of the piles.