Experimental and theoretical studies on installation torque of screw anchors

Abstract

Experimental and theoretical studies on the torque required to install screw anchors in sand are presented. Tests were conducted on five models of screw anchors with different geometry to study the effect of the shape of the screw element on the performance of the anchor during installation. Anchors were installed into prepared layers of dense, medium, and loose sand. An experimental setup was instrumented to allow the measurement of the total pullout load, the upward displacement, the sand surface deflection, installation torque value, and the stress development in the sand layer during all phases of the testing procedure. Special tests were conducted on coloured-layered sand to examine the effect of the installation procedure on the sand deposit and to define the sand wedge involved in resisting the installation of the screw anchor. Based on the results of the experimental investigation, the factors affecting the value of the installation torque were identified. A theoretical model was developed, from which the required installation torque value can be predicted. A torque factor was established in terms of the parameters affecting the torque value, and a correlation between this factor and the uplift capacity factor was proposed. From this correlation, the uplift capacity of a screw anchor can be determined from the measured installation torque value. A comparison between theoretical and experimental results showed good agreement. Also, reasonable agreement was observed when the present theoretical results were compared with the available field results from other investigations. Key words: anchorages, anchors, bearing capacity, helical anchors, models, sand, screw anchors, screw helical anchors, soil mechanics, torque, uplift capacity.