Failure Mechanism of Helical Anchors in Sand by Centrifuge Modeling and PIV

Helical anchors are receiving more attention these days due to their economic and environmental advantages. However, the behavior of these structures requires further investigation. One of the critical areas requiring further attention is their failure mechanism under monotonic loading conditions. This study presents the results of centrifuge testing of half-models of helical anchors behind a Perspex window, performed to visualize the failure mechanism of screw anchors using particle image velocimetry analysis. The density change that is a result of installation disturbance was simulated by a novel technique. The results showed that embedment depth had a great influence on the failure mechanism of the anchors. The critical embedment depth ratio determined was H/D = 4-5 that separated the shallow and deep behavior of the anchors. Soil was mobilized as a reverse truncated cone in the shallow mode, while a flow-around mechanism was formed in the deep mode. The inclination of the mobilized zone on both sides of the helix was close to the critical state friction angle (29 degrees) in shallow mode. The inclination of the failure surface was related to the dilation angle of the sand in the deep mode. A minimum distance of 3D was recommended to minimize the interaction of adjacent helical plates in a multihelix anchor. In addition, a distance of 4D was suggested to minimize the interaction of adjacent helical anchors in a group.