The axial compression performance of large-capacity helical piles is of significant interest because they can offer an efficient alternative to conventional piling systems in many applications such as in oil processing facilities, transmission towers, and industrial buildings. This paper presents the results of seven full-scale axial compression load tests conducted on 6.0 and 9.0 m large-capacity helical piles and a 6.0 m driven steel pile. The results are considered essential to qualify and quantify the performance characteristics of large-capacity helical piles in cohesive soils. The test piles were close-ended steel shafts with an outer diameter of 324 mm. The test helical piles were either single or double helix, with a helix diameter of 610 mm and interhelix spacing that varied between 1.5 and 4.5 times the helix diameter. The subsurface soil properties at the test site were determined using field and laboratory testing methods. The 6.0 m piles were tested 2 weeks after installation, while the 9.0 m piles were tested 9 months after installation. The load–settlement curves were presented to better understand the behaviour of test piles. An ultimate capacity criterion was proposed to estimate the ultimate load of large-capacity helical piles. The test helical piles developed ultimate resistances up to 1.2–1.8 times that of the driven pile. The load-transfer mechanisms of large-capacity helical piles were studied, and it was found that soil disturbance during pile installation had a significant effect on the pile failure mechanism regardless the value of the interhelix spacing to helix diameter ratio. The mobilized soil strength parameters were back-calculated and compared with the estimated intact soil strength parameters.

The dynamic performance of helical piles is of significant interest because such piles can offer an efficient alternative to conventional piling systems in many applications where the foundation is subjected to dynamic loads. This paper presents the results of full-scale dynamic vertical load tests on a 9.0 m double-helix, large-capacity helical pile and a driven steel pile of the same length and shaft geometry. Comparing the results is considered necessary to evaluate, qualitatively and quantitatively, the dynamic performance characteristics of large-capacity helical piles. The test piles were closed-ended steel shafts with an outer diameter of 324 mm. The piles were subjected to harmonic (quadratic) loading of different force intensities acting within a frequency range that covered the resonant frequencies of the tested pile-soil-cap systems. The dynamic and static properties of the subsurface soil adjacent to the test piles were determined using the seismic cone penetration technique and the conventional soil boring and testing methods. In addition, field observations are compared with calculated responses using the program DYNA 6 to better understand the pile-soil interaction for the case of helical piles. The effects of soil nonlinearity and pile-soil separation were accounted for in the analysis by employing a weak boundary zone around the piles in the analytical model. The experimental results show that the dynamic behaviour of helical piles is essentially the same as that of driven steel piles with the same geometric properties (without the helix plates). In addition, it was demonstrated that the program DYNA 6 can accurately simulate the behaviour of both helical and driven piles.Résumé : La performance dynamique de pieux hélicoïdaux est d'intérêt significatif puisqu'ils peuvent offrir une alternative efficace aux systèmes de pieux conventionnels dans plusieurs applications où la fondation est soumise à des sollicitations dynamiques. Cet article présente les résultats d'essais de chargement dynamique vertical à l'échelle réelle sur un pieu hélicoïdal à double hélice, de grande capacité, de 9,0 m, et d'un pieu foncé en acier de même longueur et de même géométrie de l'arbre. La comparaison des résultats s'avère nécessaire pour évaluer, qualitativement et quantitativement, les caractéristiques de la performance dynamique de pieux hélicoïdaux de grande capacité. Les pieux d'essai étaient faits d'un arbre d'acier à embouts fermés avec un diamètre externe de 324 mm. Les pieux ont été soumis à des sollicitations harmoniques (quadratiques) de différentes intensités de force agissant à l'intérieur d'une gamme de fréquences qui couvre les fréquences résonnantes des systèmes pieu-sol-cap testés. Les propriétés dynamiques et statiques du sol de fondation adjacent aux pieux d'essai ont été déterminées à l'aide de la technique sismique de pénétration du cône ainsi que par des méthodes conventionnelles de forage et d'essais de sol. De plus, les observations de terrain sont comparées aux résultats calculés avec le progr...