Field axial cyclic loading experiments on piles driven in sand

AbstractMultiple axial cyclic and static loading tests have been performed on industrial steel pipe-piles driven at Dunkerque, northern France. This paper describes the site's geotechnical characteristics and experimental arrangements before defining and describing the stable, unstable or meta-stable responses observed under various combinations of cyclic loading. The interpretation draws on numerical analyses and a parallel model study by Tsuha et al. (2012), relating the field response to the probable shaft shear stress distributions and local effective stress conditions. It is argued that cyclic degradation is controlled by: (i) contraction in the highly constrained interface shear zone and (ii) kinematic yielding within the surrounding soil mass. Finally, interaction diagrams linking shaft response to cyclic loading parameters are proposed based on the field test data and a simplified cyclic capacity predictive approach

Joint research into the behaviour of driven piles

Large driven piles are used widely in both onshore and offshore construction. Predicting their limiting capacities and load-displacement behaviour under a range of static and cyclic, axial, lateral and moment loading conditions is critical to many engineering applications. This paper reviews relevant recent joint research by groups at Imperial College London (ICL) and Zhejiang University China (ZJU). Two tracks of enquiry are outlined: (i) assembling and analysing a major and open database of high quality load tests conducted on industrial scale piles at well characterised sites; and (ii) modelling the effective stress regime developed around piles driven in sands. Both avenues of research are vital to enabling scientifically well-founded and yet industrially credible improvements to practical pile design methods. The scope of future joint research is also outlined

Effects of particle breakage and stress reversal on the behaviour of sand around displacement piles

The stresses acting in the soil mass adjacent to the tips and shafts of displacement piles during installation and loading in medium-dense sand have been simulated in triaxial stress path tests on Fontainebleau NE34 sand. The very high normal and shear stresses recorded in calibration chamber model pile tests involving the same sand were first reproduced in high-pressure triaxial tests, so changing the sand's physical properties markedly. The behaviour of the mutated sand was then examined in second, lower stress, stages of the same experiments, demonstrating important changes in the sand's mechanical behaviour, including a significant increase in the angle of shearing resistance and a relocation of the sand's critical state line in the e−log p′ plane. Image analysis confirmed changes in the sand particles' micro-characteristics. The particles' size distributions altered and grain surface roughness increased markedly, while particle sphericity was only mildly affected. Similar surface roughness changes were noted between the particulate characteristics of specimens examined after the triaxial laboratory tests and those sampled from around the shafts of the calibration chamber model piles