Axial response and material efficiency of tapered helical piles

Different techniques have been proposed to increase the bearing capacity of open-ended piles. Welding helices to the shaft and tapering the pile shaft could be used simultaneously to enhance the static and dynamic behaviors of these piles. This paper subjects the bearing capacity, stiffness, frictional behavior, and material efficiency of the tapered helical piles to scrutiny. Tapered helical piles are introduced herein as an alternative option to improve the material efficiency of hollow piles. Based on the Taguchi method, a series of experiments was designed and conducted. The axial responses of tapered helical piles are also investigated using finite element analyses. The results derived from load–displacement curves and strain gages are used to characterize the axial compression responses of tapered helical piles. The effects of tapered angle, helices diameter and helices distance are examined using dimensionless parameters, and the degree of contribution of these factors is calculated on each of the enumerated variables individually. Experimental results show that the shaft friction resistance of tapered helical piles increases continuously with the pile head settlement. Furthermore, the effect of tapered wall on the shaft friction resistance is more tangible at low stress levels. The results showed that the relative material efficiency factor of the optimum pile could be 2.5 times that of unoptimized pile with a similar quantity of material. •Introducing tapered helical piles as an option to improve the material efficiency of hollow-section steel piles.•Measuring the bearing capacity, stiffness, frictional behavior, and material efficiency of tapered helical piles.•Obtaining the optimal geometry of tapered helical piles and its effect on material efficiency.•Performing tests designed based on the Taguchi method and examining its capability to predict the response of the system.•Introducing the affecting factors on the axial response of tapered helical piles.