A Study on Failure Surface of Helical Anchors in Sand by PIV/DIC Technique

Abstract

Helical anchors have been used to carry tension loads in different applications including transmission tower foundation, pipeline anchors, foundation repair elements, and excavation bracing. There have been numerous changes in the shape and size of helical anchors and piles since their first usage. Several researchers have studied their failure mechanism to find their pullout capacity. In this paper, the uplift capacity of helical screw anchors has been investigated through laboratory testing. Half-cut double-helix anchors were tested in a sand tank by varying helix size, helix spacing, and relative density of sand. A series of images were captured during the process of anchor pullout. The images were used to obtain displacement and strain fields by Particle Image Velocimetry (PIV). Load–displacement curves have been presented and compared to the earlier works. Afterwards, pullout capacity factors were calculated from peak load values. PIV analysis results were used to study the effects of helix spacing, helix size, and relative density of sand on the displacement fields. The results showed that the effect of helix spacing and soil density in increasing pullout load is more than that of helix size. Moreover, failure surfaces were discussed through displacement and strain fields. The findings indicated that failure surface above the top helix of deeply embedded anchors is a truncated cone with the angle of approximately $${\raise0.7ex\hbox{$\phi $} \!\mathord{\left/ {\vphantom {\emptyset 3}}\right.\kern-0pt}\!\lower0.7ex\hbox{$3$}}$$, with the vertical and the failure surface shape between the two helices dependent on helix spacing.