Helical piles in unsaturated structured soil: a case study

The use of helical piles as tower foundations in Brazil has increased considerably during the last 5 years. A number of these piles are installed in unsaturated structured soils that cover a significant part of the Brazilian territory. However, the installation of helical piles in such soils produces a breakdown of the natural soil structure, which affects the pile performance for tension applications. This scenario motivates the present work, in which a comprehensive pile load-test program was carried out on helical piles composed of a single helix or multi-helices, installed in an unsaturated tropical residual soil. Eleven full-scale pile axial load tests were carried out, including two compression and nine tension tests. In addition, cone penetration tests were performed close to the piles after installation, and undisturbed soil samples were collected at the depth of the helices. The aim of these additional tests was to contribute to the understanding of the effect of helical pile installation on soil structure. The results of the tension load tests showed that the changes in the structure of the porous tested soil result in particularly low pile uplift capacities. In contrast, the load-settlement curves of the pile compression tests indicate a peculiar failure mechanism due to the sensitive soil structure associated with the high void ratio of the intact soil beneath the bottom plate.