Lateral Behaviour and Failure Mechanism of Stiff Monopile in Clay-Paper

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The study investigates the lateral response and failure mechanisms of large-diameter, stiff monopiles embedded in clay, for which no widely accepted design method exists. Three-dimensional finite-element analyses using an advanced hypoplastic clay model were conducted and checked against centrifuge-test data.<br /><br />The analyses identified two soil-flow mechanisms: a wedge-shaped failure zone near the ground surface and rotational soil flow at greater depths. These mechanisms differ from assumptions underlying the API p-y method, which was developed for slender piles. For the stiff monopile studied, API curves were substantially softer than the finite-element predictions near the surface and did not capture the bilinear distribution of ultimate soil resistance at depth. Overall, the API method significantly underestimated the monopile’s lateral stiffness and capacity, although its assumed resistance factors also misrepresented resistance in the deeper rotational zone.<br /><br />The results show that p-y response depends on pile diameter: larger piles exhibit greater stiffness, while API stiffness parameters become unsuitable as diameter increases. The authors propose a new p-y model that accounts for soil failure mechanisms and pile-diameter effects. Comparisons with finite-element results and centrifuge tests on piles of different sizes show that the model satisfactorily predicts lateral load–deflection behavior, offering a more suitable design approach for stiff monopiles in clay.

Keywords

stiff monopile

lateral response

clay soil

finite element analysis

hypoplastic clay model

centrifuge test

soil failure mechanisms

API p-y method

pile diameter effects

lateral load-deflection