Based on the 1g model tests

the uplift behavior of the double-blade screw anchor in loose sand was studied

where the influences of the embedment ratio and blade spacing were focused. Digital image correlation (DIC) was used in the half-model tests to analyze the deformation and failure mechanism of soils around the anchor during the uplift process. A theoretical formula for predicting the ultimate bearing capacity of double-blade screw anchors in loose sand was proposed based on the results of the half-model tests. The results show that the embedment ratio and blade spacing affect both the ultimate uplift bearing capacity and the displacement corresponding to the peak uplift force. For the double-blade screw anchors in loose sand

the embedment ratio H/D=3 can be regarded as the demarcation between shallow and deep anchors

where H is the embedded depth of the upper anchor blade

D is the diameter of the blade. When embedment ratio H/D3 is applied

the failure mode of soils above the upper blade gradually changes from overall shear failure to local shear failure. The spacing ratio S/D=2.5

where S is the blade spacing

can be regarded as the critical point that makes the failure of soils between two blades from the cylindrical shear failure to independent bearing failure. By comparing with the full-model tests and the experimental results in the literature

it is found that the prediction errors of the proposed theoretical formula for the ultimate uplift bearing capacity are generally within 25%. Thus

the reliability of the theoretical formula is verified.