Insights into the Driveability of Large Diameter Piles-Paper

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The study examines the driveability of 7.8 m diameter offshore monopiles driven through stiff, overconsolidated clay and into very dense sand. Three piles were instrumented during installation with an IHC S-4000 hammer; detailed signal matching and calibrated GRLWEAP modelling were used to estimate soil resistance to driving (SRD).<br /><br />Despite cone resistance reaching about 100 MPa, the piles passed through the sand without exceptionally high resistance. The analysis suggests that annulus end bearing develops over approximately 1.5 m of penetration—about 20 pile-wall thicknesses—and begins to diminish at a similar distance before a weaker layer. Estimated working undrained shear strength in the sand was 800–1,000 kPa, corresponding to end-bearing pressure of only 6–7.5 MPa. Pore-pressure accumulation, cavitation, and rapid hydraulic gradients around the pile tip may limit resistance during driving.<br /><br />In the clay, shaft friction decreased as the pile advanced, with degradation occurring faster than predicted by the standard Alm and Hamre method. That method underestimated initial shaft friction, however, so its overall resistance estimate was reasonably consistent when higher shaft damping was used to account for effects not represented in the 1D model.<br /><br />The findings highlight the need to consider embedment and nearby weaker layers when estimating end bearing, and to improve modelling of friction degradation for large-diameter piles. Because 1D wave-equation analysis cannot fully represent large-pile dynamics and signal matching has uncertainty in separating shaft and toe resistance, the interpreted values should be treated as indicative rather than definitive.

Keywords

offshore monopiles

driveability

stiff overconsolidated clay

very dense sand

soil resistance to driving

signal matching

GRLWEAP modelling

end bearing

shaft friction degradation

large-diameter piles