The ultimate pullout capacity of helical piles usually increases with the number of helices. However, the results of some field tests have demonstrated that the amount of increase in the uplift capacity of helical piles with the increase in the number of helices is not as expected. The magnitude of uplift capacity increase, caused by the inclusion of one more helix, is influenced by the damaging effect of soil disturbance due to the pile installation. Helical piles are installed into soil by applying a torque to the upper end of the shaft by mechanical means, and during installation the soil traversed by the helical blades are sheared and displaced laterally and vertically, and consequently some soil properties and arrangement are modified. Centrifuge model experiments were carried out on scaled models of helical piles, with different dimensions, in sand to investigate the variability of the rate of capacity gain due to the addition of one more helix to a helical pile. Uplift loading tests were carried out on twelve different types of model piles installed in containers of dry sand prepared with two different densities. The model tests performed on double and triple-helix piles showed that the contribution of the second plate to the total pile uplift capacity decreased with the increase of sand relative density and the helix diameter.
Evaluation of the Effect of Installation on the Helical Pile Performance in Sand by Centrifuge Tests