Axial testing and numerical modeling of square shaft helical piles under compressive and tensile loading

TLDR

Helical piles are increasingly used to support and rehabilitate structures subjected to both tensile and compressive axial loads. The study investigates the axial performance of helical piles, examines the link between installation torque and capacity to predict pile strength, and proposes defining ultimate capacity by head movement of 8 % of the largest helix diameter plus elastic deflection. The authors conducted 19 full‑scale load tests across various soils and performed finite element analysis on square‑shaft helical piles composed of three circular pitched bearing plates welded to a solid‑square slender steel shaft. The study identified ultimate load criteria and load‑transfer mechanisms, established torque correlation factors (K_T = 33 m⁻¹ for compression and 24 m⁻¹ for uplift) linking capacity to installation torque, and found that load transfer occurs mainly via a cylindrical shear failure surface following the tapered inter‑helix soil profile and the lead helix bearing capacity.

Abstract

Helical piles are increasingly used to support and rehabilitate structures subjected to both tensile and compressive axial loads. This paper presents a detailed investigation into the axial performance of helical piles. The study encompasses 19 full-scale load tests in different soils and numerical modeling using finite element analysis. The ultimate load criteria and load transfer mechanisms for helical piles were examined. In addition, the relationship between the installation effort (torque) and pile capacity was explored to determine its suitability for predicting pile capacity. The piles tested were made of three circular pitched bearing plates welded at a spacing of three helical diameters to a solid-square, slender steel shaft. It is proposed to determine the ultimate pile capacity as the load corresponding to pile head movement equal to 8% of the largest helix diameter plus the pile elastic deflection. A torque correlation factor, K T = 33 m –1 for compression and K T = 24 m –1 for uplift, was established to relate the ultimate pile capacity to the installation torque. It was found that load transfer to the soil is predominantly through a cylindrical shear failure surface that follows the tapered profile of the interhelices soils and the bearing capacity of the lead helix in the direction of loading.