Experimental study on uplift capacity of multi-helix anchors in sand

Helical anchors are widely used to resist uplift loading for a variety of applications involving a wide range of soil conditions. The performance and ultimate capacity of multi-helix anchors depends on the depth of embedment, spacing and number of helixes and soil conditions. The interaction of adjacent helixes can make the overlapping of soil failure zone, which thus will affect the failure mode and ultimate uplift resistance. However, there is very limited experimental study on the problem. The behavior of multi-helix anchors in medium and dense sand is investigated by performing a series of small-scale pullout tests. The embedment ratios for deep anchors in medium and dense sand are regarded as 6 and 10.5 respectively. The critical spacing of helixes of deep anchors in medium sand that makes individual bearing capacity occur is between 3.0D and 4.5D. The critical spacing of helixes of shallow anchors in dense sand is greater than 6.0D, which can reach the efficiency of more than 90%. The uplift resistance will increase with the increasing number of helixes. However, cylindrical failure mode will happen when the spacing decreases to some value due to the increase in number of helixes, which can no longer produce higher uplift resistance. The critical values of spacing are about 1.5D and 2.0D for the medium and dense sand respectively.