Tensile monotonic capacity of helical anchors in sand: interaction between helices

This note examines the interaction between the helices of a multi-helix anchor in terms of the mobilized drained capacity response in tension. Assessments are made on the basis of centrifuge tests in dense silica sand, supplemented with data from existing studies. The centrifuge tests were designed to isolate potential anchor installation effects from those due to the interactions between helices. The data show that additional helices will only contribute to anchor capacity if they are located outside the region of soil mobilized in the failure mechanism of the lower helices. In the dense sand considered in these centrifuge tests, this required helices to be separated by greater than nine diameters, and hence for the lowermost helix to be located at a depth greater than nine diameters. This separation distance is much higher than suggested in previous studies, which tended to attribute the low or nil contribution of additional helices to the soil disturbance generated during anchor installation.