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 (s_u of 250 kPa) to very hard clay (s_u 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.