Centrifuge modeling of helical piles in stiff clay: installation torque and pore pressure response

ABSTRACT: Installation torque is commonly used to predict the axial capacity of helical piles. Theoretical models for calculating the torque resistance have been developed for piles in cohesive and cohesionless soils. However, uncertainty induced by excess pore pressure during the installation and soil sensitivity still exists. The present research investigated the torque mechanism of helical piles and the pore pressure response to pile installation. A series of centrifuge model tests of helical piles were conducted at the University of Alberta. Helical piles were installed into a stiff and saturated clay at a high g level and then loaded in the axial directions. Pore pressure transducers measured the pore pressure change surrounding the four helical piles. The results show that the excess pore pressure developed instantly during installation but dissipated over approximately 6 days. The long term ultimate axial capacity of the piles was not affected by the pile installation. The installation torque and axial loading resistance were measured by strain gauges installed in the pile shaft. A theoretical model for the torque resistance was proposed based on the clay-pile interaction implied by the excavation after the load tests. The torque measurements verified the proposed model subject to the assumption that the residual undrained shear strength governed the clay-pile adhesion.