Behavior of soil anchors under dynamic loads

Helical anchors placed in a cohesionless soil in a laboratory setting were tested under static and dynamic loads. The dynamic tests were performed after subjecting the anchors to a seating load. The dynamic load had an intensity that changed in sinusoidal fashion and was superimposed to the static seating loads. Although, the anchors have a static pull-out capacity, when the dynamic loads are applied the anchor experiences additional deformations for each load cycle. The deformations per cycle are initially high but then decrease to a nearly constant rate. Eventually, the constant rate increases suddenly accelerating until failure. This failure can take place even at small fractions of the static pull-out capacity. The rate of deformation per load cycle is found to increase for larger seating loads and for larger dynamic pulsating loads. The results of this study shows that the designer can only adjust loads to decrease the deformation rate to suit the design life of the structure.