Behavior of Grouted Shaft Helical Anchors in Clay-Article

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The study investigates the tensile capacity contributed by grouted shafts on helical anchors installed in clay. Grouted shafts are formed as displacement plates advance with the anchor, creating a cavity around the steel shaft that is filled with cement grout. Besides improving buckling resistance under compression, the grout may provide substantial shaft resistance in tension and compression.<br /><br />Four anchors with grouted shaft lengths of 10, 20, and 30 ft were installed at the University of Massachusetts test site, which comprises stiff clay fill and crust over softer varved clay. Wooden dowel connections on three anchors sheared during tension tests, isolating the grouted shaft’s contribution; a 10-ft anchor with a steel connection and an ungrouted anchor provided comparison tests. Initial tests took place about six months after installation, and repeat tests were conducted nearly four years later.<br /><br />The grouted shafts developed considerable tensile resistance, with ultimate capacity generally reached at displacements of 10–20% of shaft diameter. Capacity increased with shaft length, though the incremental resistance varied across the soil layers. Back-calculated adhesion values were broadly consistent with published values for grouted piles and micropiles. Results also showed that shaft resistance can account for a substantial share of total anchor capacity, particularly at working loads, depending on shaft slenderness, soil conditions, and the lead helix’s contribution.<br /><br />Repeat tests showed higher plunging capacities for all anchors, but gains varied by depth: the largest increase occurred in the shortest anchor in the stiff upper clay, while capacity for the longest anchor was unchanged. Because the anchors had previously been loaded to failure, the effects of soil aging could not be separated from those of preshearing. Overall, the findings demonstrate that grouted shafts can be an important component of helical-anchor capacity in clay.

Keywords

helical anchors

grouted shafts

tensile capacity

clay soil

shaft resistance

cement grout

buckling resistance

anchor testing

soil adhesion

load capacity