PULLOUT CAPACITIES OF MULTI-PLATE ANCHORS IN SAND

Abstract

The advancement in the design and construction practices of foundations had led to cost-effective and greener solutions. The classic use of heavy concrete foundations has found its replacement in tensile foundations. The use of anchors is an advantageous venture in numerable civil engineering structures. Plate anchors, block anchors, helical anchors, shank anchors, torpedo anchors, tie-back anchors are a few kinds of anchors. Plate anchors are of particular significance due to the ease of installation in the field. Excavation of test pit, placement of plate anchor, the connection of central shaft for transferring load from the structure to the anchor, soil re-filling, placement of monitoring devices forms the procedure of installation of a plate anchor. Plate anchors are of many types: strip anchor, square anchor, circular anchor, rectangular anchor, eccentric anchor, etc. The plate anchors can be further classified into horizontal plate anchor, inclined plate anchor and vertical plate anchor according to the angle of inclination. A horizontal plate anchor is a structural member designed to resist the vertical pullout forces and ensure the stability of structures like the tower foundations, masts, solar towers and bridges. An inclined plate anchor is a structural member designed to resist the pullout forces and overturning moments, in order to ensure the stability of retaining structures onshore. A vertical anchor is a structural member designed to resist the horizontal pullout forces to ensure the stability of retaining structures on- and off-shore. The traditional practice of single-plate anchors is evolved due to continuing interest of the research community. The expanse of plate anchors can be stretched to accommodate innovative modifications to its traditional design. One such attempt is concreted by an extensive experimental study here. Multiple plates are attached to a single stem to design a multi-plate anchor. The tensile load of a structure is resisted by the multi-plate anchors using the many plates connected axially along the central shaft. The multi-plate anchors would be capable of generating higher pullout capacity in comparison to the single-plate anchors for an embedment depth considered. The pullout capacity is generated by the inherent structure of the multiple plates encompassing layers of sand. The present work focuses on the experimental study for the estimation of the ultimate pullout capacity of multi-plate anchors embedded in the sand. The study presents a comparison of the multi-plate anchors with the existing studies of single-plate anchors. The plate anchors are inclined at 0° (horizontal multi-plate anchor), 22.5°, 45° and 67.5° (inclined multi-plate anchor) and 90° (vertical multi-plate anchor) to the horizontal. In the case of vertical plate anchors, they are placed at varying horizontal distances from the side-wall of the test tank. II Results show the multi-plate anchor replacing the existing single-plate anchors efficiently. The pullout capacity of the single-plate anchors from the present study was compared with the existing studies. Few of the primary conclusions are (a) the increase in the pullout capacity of the double-plate and triple-plate anchors with the increase in the embedment ratio (for all inclination angles) and placement ratio (for the case of vertical multi-plate anchors only) (b) the critical embedment ratio being indicative of a transition of the failure mechanism developing for shallow and deep anchors (c) a decreasing pullout capacity for shallow horizontal multi-plate anchors in comparison with the horizontal single-plate anchors (d) an increasing pullout capacity for deep horizontal multi-plate anchors in comparison with the horizontal single-plate anchors. (e) the increase in the breakout factor of the multi-plate vertical anchors with the increase in the placement ratio (f) the increase in the breakout factor of the multi-plate anchors with the increase in the angle of inclination and spacing between the plates. The study provides a brief discussion on the scale effect, the effect of spacing, the effect of the shape of the anchors and the effect of anchors' placement ratio (for the case of vertical multi-plate anchors only) on the pullout capacity. The dimensionless breakout factors show the multi-plate anchor replacing the existing singleplate anchors efficiently. The pullout capacity of anchors can also be studied using numerical analysis. They reduce the time consumed by extensive experimentation in a laboratory setup. The numerical analysis provides for an increased number of parametric studies. In the present work, numerical analysis using PLAXIS 3D has been performed to understand the failure mechanism of multi-plate anchors. A pictorial representation of the steps followed is presented. The failure envelopes developed around the multiple plates of the multi-plate anchors were similar to the one rationalized in the analysis of the experimental results. The study restricts to a spacing of 3d between the plates of multi-plate anchors. The failure envelopes generated along each plate can be seen to overlap with the adjacent ones. An increased spacing could possibly generate a nonoverlapping stress zone. The extensive study thus provides scope for further studies on multiplate anchors and subsequent application in the field. Keywords: Horizontal plate anchors; Inclined plate anchors; Vertical plate anchors; Multi-plate anchors; Square anchors; Circular anchors; Sand; Pullout capacity; Failure mechanism; PLAXIS 3D