A bounding-surface-based cyclic "p-y plus M-θ" model for unified description of laterally loaded piles with different failure modes in clay

The increasing turbine sizes have necessitated monopiles in soft clay to have larger diameter and rigidity, from early design of flexible piles to recent semi-rigid piles, with future anticipating rigid piles. Existing few cyclic soil-pile interaction models are developed for flexible pile associated with full-flow failure (above the rotation point, RP), with little attention paid to semi-rigid and rigid piles involving rotational-shear failure (below RP). This study aims to unify the description of piles with varying rigidity by proposing a cyclic two-spring model, where lateral resistances above and below RP are described with cyclic p-y and M-theta springs, respectively. It naturally recovers to a cyclic p-y model for flexible piles. The cyclic p-y and M-theta formulations are developed within the bounding-surface plasticity framework, based on numerical results of cyclic soil-pile interaction concerning full-flow and rotational-shear mechanisms, respectively. These numerical analyses are performed using a cyclic plasticity clay model developed and implemented numerically in this study. The cyclic "p-y+M-theta" model quantitatively reproduces experimental results of cyclic shakedown and ratcheting for flexible, semi-rigid, and rigid piles. Ignorance of the M-theta spring could underestimate cyclic resistance of rigid piles by 25%, suggesting the model's merit in reducing conservatism for monopiles in feature designs.