Experimental investigation on the installation and loading performance of model-scale deep helical piles in very dense sand

Abstract

The use of helical piles has grown over the years owing to different advantages, such as large uplift capacity due to the anchor effect of the helix and installation torque-capacity correlation. Although the use of helical foundations is expanding worldwide, some key aspects fundamental to the design are not well understood to date. Practical experience indicates that the installation forces and loading performance of helical piles in sand are dependent on the helices characteristics and confining stresses. Therefore, for a better understanding of these dependencies, the effects of the helix-to-shaft diameter ratio (wing ratio) and vertical confining stress on the installation torque and forces, and on the uplift and compression capacities of helical piles, were evaluated from nine calibration chamber tests, conducted on instrumented single-helix piles in very dense sand. Among other findings, this study indicated that for a certain shaft diameter (i) the wing ratio influences the installation torque, but does not affect the installation vertical force; (ii) the ultimate uplift pressure mobilized on the helix decreases with the increase of the wing ratio; (iii) the growth rate of the helix bearing resistance with the confining pressure, for vertical stresses higher than 100 kPa, becomes reduced for piles with a larger wing ratio.