Development of Design Practice for Piles in Stiff Glacial Till-Article

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The paper reviews instrumented load tests and design methods for piles installed in Dublin Boulder Clay (DBC), a stiff glacial till. It addresses the shift from driven piles to Continuous Flight Auger (CFA) piles, which cause less noise and vibration and pose less risk to nearby structures.<br /><br />Tests indicate that CFA piles can develop greater unit shaft resistance than driven piles: a shaft adhesion factor of about 0.75 is suggested for CFA piles, compared with roughly 0.55 for driven piles. However, CFA shaft resistance may require movements of 3–5% of pile diameter to mobilize, which can exceed acceptable settlement limits. CFA piles also have lower base stiffness and end-bearing resistance than driven piles. For CFA piles, a bearing-capacity factor of 9 is recommended; driven-pile end resistance in very stiff to hard DBC should be capped at 15 MPa or 400 times the SPT N-value, whichever is lower.<br /><br />Nonlinear elastic models provided reasonable estimates of initial load–settlement behaviour, including for a CFA pile in UK till. But tests on large CFA piles in Irish till revealed substantial creep at higher loads, with some piles failing to meet settlement-rate criteria. Plate-load tests suggest creep becomes important when bearing pressure approaches or exceeds the till’s pre-consolidation pressure, potentially preventing full theoretical base resistance from being mobilized.<br /><br />The paper concludes that pile design should distinguish between CFA and driven piles, account for the displacement needed to mobilize shaft resistance, and consider creep when estimating CFA base capacity. Further field research is needed to quantify creep effects.

Keywords

Dublin Boulder Clay

pile load tests

CFA piles

driven piles

shaft resistance

end-bearing resistance

pile settlement

creep effects

bearing capacity

glacial till