This paper describes the first phase of an extensive campaign of axial cyclic loading tests performed on intensively instrumented displacement piles installed in large pressurised calibration chamber. The chamber was filled with medium dense silica sand that combined measurements of the effective stress paths developed along the pile shaft and within the sand mass at different distances from the shaft. The paper also outlines the evolution of the local interface effective stress paths during cyclic loading. Three different styles of cyclic response - stable, metastable and unstable - are identified and linked to the global degree of cyclic stability. Increases or decreases in shaft capacity induced by cycling can be related to the cyclic loading parameters and to the local mechanical behaviour of the pile-sand interface and the sand mass. The second phase of testing, which involves many more cyclic experiments, is currently underway.

1. Introduction

The pile foundations of offshore platforms, as well as wind and water turbines, experience environmental cyclic loading. These cycles can affect the stress conditions in the soil mass close to the pile and, consequently, the pile stiffness and capacity. Significant degradation of pile skin friction (reduction of static shear resistance) due to cyclic loading has been noted in field scale tests on clays (Karlsrud et al., 1986) and in sands (Jardine and Standing, 2000). Further tests with model piles installed in surcharged sand tanks confirmed that shaft capacity degradation grows markedly as the relative level of cyclic loading increases (Poulos, 1989; Al-Douri and Poulos, 1994; Chin and Poulos, 1996; Le Kouby et al., 2004; Tali, 2011). Jardine and Standing (2000) and Jardine et al. (2006) also concluded from full-scale tests at the Dunkirk site described by Chow et al. (1996) that high-level cycling damages pile shaft capacity.

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