A CASE FOR THE OPTIMISATION OF CFA-BORED PILE DESIGN IN UK & IRELAND CLAYEY SOILS-Paper

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The paper evaluates whether a shaft adhesion factor of 0.6 is suitable for designing continuous flight auger (CFA) bored piles in clay across the UK and Ireland. Although guidance documents generally recommend 0.5 when comparable pile-test evidence is unavailable, BS 8004 permits alternative values supported by successful tests in similar conditions.<br /><br />The authors review published research and relate empirical correlations between pile shaft resistance and standard penetration test (SPT) values to typical UK clay correlations. This analysis supports an adhesion factor of 0.6. They also examine 15 historical projects across the UK and Ireland, where CFA piles of various sizes and lengths were designed using this factor and verified through static load testing. Selected test results were back-analysed using Chin’s hyperbolic method to compare measured and predicted shaft capacities.<br /><br />Across the projects, load-test performance was reported as satisfactory. In the three detailed back-analyses, measured ultimate shaft capacities exceeded or closely matched predictions based on 0.6. At loads up to 1.5 times the design verification load, settlement was generally no more than 7 mm, and the piles were supported predominantly by shaft friction.<br /><br />The authors conclude that 0.6 can be a safe and economical design value for CFA piles in representative UK and Irish clay soils, rather than relying automatically on the more conservative generic value of 0.5. They recommend prioritising site-specific pile-test data—or evidence from sites with comparable ground conditions, pile geometry and installation methods—to achieve safe, efficient designs and reduce unnecessary material use and associated environmental impacts.

Keywords

continuous flight auger

CFA bored piles

shaft adhesion factor

clay soils

UK and Ireland

pile design

static load testing

BS 8004

Chin hyperbolic method

shaft resistance