Effects of inter-helix spacing and short-term soil setup on the behaviour of axially loaded helical piles in cohesive soil

Abstract

Axial compressive load tests performed on piles instrumented with strain gauges were completed in order to investigate the effects of inter-helix spacing on the behaviour of helical piles. The test piles had two helices with varied values of helix spacing. The helix-bearing soil layer consisted of a homogeneous clay with an average undrained shear strength of 65 kPa. The test pile failure mechanisms were determined by comparing the measured load distributions to the distributions predicted by the individual bearing and the cylindrical shear models. The results suggest that the individual bearing model dominates the pile behaviour for piles with an inter-helix spacing to helix diameter ratio greater than or equal to 1.5. The helix bearing capacity factor and the shaft adhesion factor were evaluated by comparing the measured pile component resistances to the theoretical estimations. The back-calculated bearing capacity and the adhesion factors were below the values traditionally used in helical pile design. The effects of the soil setup on the pile behaviour were evaluated by comparing the load-settlement response of a pile tested immediately after the pile installation to equivalent piles tested many days after the installation. A piezometer installed near the upper helix edge was used to measure the excess pore pressure generation and dissipation induced by the installation. The results suggest that the pore pressure generation induced by the pile installation was minimal and had little influence on the short-term ultimate capacity.