Centrifuge Modeling of the Cyclic Lateral Response of a Rigid Pile in Soft Clay

Abstract

A series of centrifuge model tests of the lateral response of a fixed-head single pile in soft clay is reported. Both monotonic and cyclic episodes of loading are described, with varying amplitude and with intervening periods of reconsolidation. The soil conditions are characterized by cyclic T-bar penetrometer tests. The ultimate capacity under monotonic load for virgin and for postcyclic conditions was found to be comparable with calculations based on existing design methods, including theoretical plasticity solutions and empirical methods. The lateral stiffness was observed to degrade with cycles, with the rate of degradation being greater for larger cycles. The degradation pattern has been tentatively linked to the cyclic T-bar response, by considering the ‘damage’ associated with the cumulative displacement and remolding, in each case. This approach provides a consistent interpretation of the tests. Although episodes of pile movement and soil remolding led to a reduction in lateral resistance, intervening periods of reconsolidation led to a similar magnitude of recovery and a reduction in the level of softening in subsequent cyclic episodes. During an initial episode of cyclic lateral movement, the stiffness degraded by a factor of 2.3, which is comparable with the strength sensitivity derived from a cyclic T-bar test. In contrast, after five episodes of reconsolidation, the stiffness had recovered back to within 25% of the stiffness observed in the first cycle of the first episode, and it showed negligible degradation during subsequent cycling. This observation implies that, over a long period of cyclic loading, the lateral stiffness of a pile may tend towards a value that is independent of cycle number, and that represents a balance between the damaging effects of remolding and pore pressure generation and the healing effects of time and reconsolidation.