Application of X-ray computed tomography to investigate pile penetration mechanisms in chalk

Abstract

The installation of displacement piles in chalk involves the crushing, remoulding and densifying of the material under the pile tip. This remoulded material forms an annulus around the installed pile. Shaft friction can be expected to be significantly affected by the characteristics of this annulus. However, research on its attributes and the mechanisms behind its origin is scarce. This paper explores the use of non-destructive 3D X-ray computed tomography (XCT) imaging to determine the changes in bulk density associated with chalk crushing and remoulding during the penetration of a mini-pile into an intact chalk specimen. The use of XCT allowed for the estimation of the thickness and general geometry of the remoulded chalk annulus, as well as its 3D visualisation for the first time. Preliminary results suggest that pile installation in the radially confined chalk specimen took place, primarily, by crushing and densification of the chalk under the pile tip. This led to the formation of an annulus of variable thickness and density. By implementing a mass-balance analysis, the post-pile installation XCT-derived mass was found to differ by less than 0.4% from the pre-installation mass, which gives confidence in the accuracy of XCT imaging in detecting local changes in bulk density. In general, the results show that this technology has potential for contributing to the understanding of the development of pile loading capacity in chalk.