Review of the Design of Driven Piles Penetrating Weak Rock for Offshore Applications-Paper

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Driven piles penetrating weak or weathered rock are increasingly used for offshore structures, where shaft resistance is important for resisting uplift and overturning. Yet conventional guidance often treats these piles as end-bearing foundations and provides little direction for estimating shaft capacity in rock.<br /><br />Current practice adapts methods developed for piles in soils. The β-method estimates resistance using effective stress and interface friction, while the α-method relates it to undrained strength. Both approaches assume pile driving severely degrades the rock. Another proposed approach adapts drilled rock-socket methods, but its reduction factors are not established for driven piles.<br /><br />The authors compiled case histories of driven piles penetrating weak rock and reassessed two well-documented projects: Noetsu Bridge and the NGL Loading Jetty. Comparisons with load-test results indicate that existing soil-based methods generally underestimate shaft resistance; the α-method gave the closer estimate in the cases examined. The findings confirm that shaft resistance in rock can make a substantial contribution to pile capacity and should not be ignored, particularly for offshore applications.<br /><br />The study concludes that current design guidance is inadequate and that more detailed case histories are needed, including measurements of shaft resistance, rock strength and mass characteristics, and penetration depth. Scaled laboratory tests and numerical modelling are also recommended to investigate how rock strength, scale, and friction fatigue affect pile resistance.

Keywords

driven piles

weak rock

weathered rock

shaft resistance

offshore structures

pile capacity

beta method

alpha method

load test results

rock socket design