Dynamic Load Testing of 2 Metre Steel Tubular Piles Driven into Foliated Phyllite Bedrock-Paper

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The paper examines the design, construction and dynamic testing of large driven steel tubular piles supporting the Nambucca River bridge in Australia. Piles up to 2 m in diameter were driven through variable alluvial soils and weathered rock, and founded in foliated Phyllite. Boreholes and rock-strength tests informed the design; pile-driving analysers (PDA) monitored every pile, while CAPWAP wave-matching analyses were performed on about 40% of piles to assess capacity and control over-driving.<br /><br />At restrike, tested piles generally mobilised 1.25–1.5 times their maximum design ultimate load. CAPWAP results attributed 60–70% of total capacity to end bearing and 30–40% to shaft resistance. Shaft resistance generally increased after driving, while toe resistance relaxed; these changes largely offset one another. Driving stresses remained below the specified limit.<br /><br />Rock fabric strongly influenced end bearing. For Phyllite with foliation angles below about 20° from horizontal, the end-bearing pressure-to-UCS ratio was approximately 5.5. At steeper angles, it declined, reaching about 4 at 40°, consistent with the effect of weakness along foliation planes.<br /><br />In Phyllite, mobilised shaft friction was higher in low- to medium-strength rock than in medium- to high-strength rock. The authors suggest that stronger rock may crush around the pile, reducing confinement and pile–rock contact. Gravel shaft-friction results were broadly consistent with established API sand-based recommendations. In contrast, measured shaft friction in alluvial clays was substantially below conventional API design estimates, potentially because full resistance had not mobilised and setup was still developing.<br /><br />The study demonstrates how PDA monitoring and CAPWAP testing can verify pile capacity and inform practical acceptance criteria, while highlighting the importance of rock foliation and material-specific behaviour in interpreting pile resistance.

Keywords

steel tubular piles

bridge foundations

dynamic pile testing

PDA monitoring

CAPWAP analysis

end bearing

shaft resistance

Phyllite foliation

pile driving

alluvial soils