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.