A new grouting procedure to enhance axial response of helical piles in sand

Helical (or screw) piles are deep foundation elements consisting of a steel shaft with single or multiple helical plates (helices) attached near the base of the shaft. The helices and shaft are rotated into the ground using a torque motor located at the head of the shaft. The axial resistance of the piles is provided through bearing on the helices with some additional axial resistance derived from skin friction on the shaft. This paper examines the benefits of a variation of this foundation system in sandy soils, which involved post-installation injection of a cementitious grout in the vicinity of the helix. The intent is that the grout adds a bond strength to a volume of sand that has a larger diameter than that of the helix, thereby increasing the bearing resistance. A new set-up devised to facilitate easy and rapid grouting is first described. The benefits of grouting are then examined in a series of full-scale static tension and compression tests performed at sand sites in Perth, Australia involving both grouted and ungrouted single helix piles. It is demonstrated that grouting led to doubling of axial stiffness and capacity with the addition of just 80 litres of grout to a 350 mm dia. helix. The increases observed are quantifiable by combining a recently published design method with measured injected grout volumes.