Challenges in pile foundation design in Northland Allochthon-Paper

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The paper examines the challenges of designing driven piles in the Northland Allochthon, a highly sheared, fractured and weathered geological formation in northern Auckland. Its weak rock can rapidly degrade when disturbed or exposed, making engineering properties difficult to establish and potentially causing conventional methods to overestimate pile shaft resistance.<br /><br />The study compares pile design predictions with dynamic load tests at two three-span bridges in Milldale: Weiti and Highgate. Investigations included boreholes, laboratory testing and Standard Penetration Tests (SPT). Because intact core suitable for unconfined compressive strength (UCS) testing was often unavailable, UCS was also estimated using an SPT-based penetrability correlation. These estimates averaged about three times the laboratory-measured UCS, highlighting substantial uncertainty.<br /><br />Pile Driving Analyzer tests and CAPWAP analyses showed that shaft resistance was strongly mobilized. However, toe resistance was only partially mobilized in the Highgate piles, which recorded much lower end-bearing values than the Weiti piles. The study evaluated several established approaches for estimating shaft friction. Their predictions varied and could either overestimate or underestimate resistance; most should therefore be treated as supplementary rather than standalone design methods. Methods based on inferred UCS were especially sensitive to the quality of the estimates.<br /><br />An empirical method developed by Barrett and Prendergast for weak rock performed more reliably when calibrated with the bridge test data, and showed good agreement when laboratory-measured UCS was used. The authors recommend further case studies combining pile load tests and laboratory testing to improve design guidance for weak rock. Overall, the findings emphasize site-specific assessment and field validation when designing piles in Northland Allochthon.
Keywords
driven piles
Northland Allochthon
weak rock
pile design
shaft resistance
end-bearing
dynamic load tests
SPT
UCS
CAPWAP