Uplift Performance of Large-Capacity Helical Piles in a Tropical Residual Soil

Helical foundations have been frequently used in Brazil for the construction of transmission line towers to resist uplift and compressive forces. However, in recent years the height of the towers has increased and consequently the loads on the foundations are also of greater magnitude. Normally, for these higher towers the loads are supported by groups of small-capacity helical piles, which are typical in Brazil (shaft diameters from 73 to 100 mm). In these cases, an advantageous alternative to reduce the number of piles and the costs of the foundations is the use of large-capacity helical piles. However, helical foundations of large dimensions have never been investigated before in Brazilian tropical residual soils. Therefore, to address the need of examining the performance of large-capacity helical piles in typical Brazilian residual soils a field-testing program was carried out at the Experimental Site of the University of Sao Paulo in Sao Carlos City. For this study, a single-helix and a double-helix instrumented large-capacity helical piles, with shaft diameter of 168.3 mm, were installed and tested under tensile loads. The load-transfer mechanisms of the large capacity helical piles were evaluated and discussed in this paper. The results of this study showed that although the uplift bearing capacity of the second helix of a pile is more impaired by the installation procedure compared to the bottom helix, the use of a double-helix pile is still more advantageous because it leads to a shorter pile resisting the same capacity of a longer single-helix pile of same shaft diameter.