Centrifuge tests were performed to investigate the cyclic response of piles in soft clay under lateral loads. Typical data are presented and discussed. The accumulation of pile-head displacements and maximum bending moments with number of cycles in particular are examined. Power law models fitted on experimental data are proposed to describe the effect of large numbers of cycles.

1. Introduction

Pile foundations of offshore structures used for the oil and gas or wind farm industries are subjected to large cyclic horizontal loads resulting mainly from the action of waves and wind. Pile design procedures are essentially based on the application of recognised standards or professional recommendations, among which the American Petroleum Institute (API) RP 2A (2000) is the most commonly used. It proposes a design methodology for horizontally loaded piles based on the use of local pile-soil transfer (p-y) curves. Monotonic, as well as so-called cyclic, p-y curves are provided for both sands and clays. The cyclic p-y curve concept requires some attention. It was derived in the 1970s on the basis of pile tests performed on relatively small-diameter (123/4–24-inch) piles for soft clays (Matlock, 1970), stiff clays (Reese et al., 1975; Reese and Welch, 1975) and sands (Cox et al., 1974; Reese et al, 1974). The piles were subjected to a series of cyclic loads representative of load histories imposed by Gulf of Mexico storms to jacket piles. The final result was an ‘envelope curve’, which is aimed at reproducing the response of a pile monotonically loaded at the end of the extreme event (e.g. the centennial storm). More recent laboratory tests (Craig and Kan, 1986; Kitazume and Miyajima, 1994; Jeanjean, 2009; Zhang et al., 2011) emphasise the significance of progressive accumulation of pile head displacements with applied number of load cycles.

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