Axial Uplift of Square-Shaft Single-Helix Helical Anchor Groups in Clay

Full-scale axial uplift tests were performed on single-helix helical anchor groups in stiff and soft clay to evaluate any beneficial group effects for uplift behavior. Groups of four helical anchors in a 2 × 2 square arrangement with relative center-to-center anchor spacing of 1.5, 2, 3, and 4 times the anchor plate diameter were tested. The results of the load tests indicated that the behavior of anchor groups was different in stiff clay and in soft clay. In the stiff clay, group efficiencies (capacity of group compared to sum of capacities of individual) were less than 100% for all relative anchor spacings but efficiency increased with relative spacing. By contrast, in the soft clay, group efficiencies were all greater than 100% for all relative anchor spacings but the efficiency appears to be independent of relative spacing. The results are presented and compared with tests on a single-helix anchor of the same diameter to evaluate group efficiency and with a single-helix anchor of the same equivalent area as the four individual anchors in the group. When the behavior of the groups is compared to a single anchor of equivalent area, the group efficiency was on average approximately 200% and in some cases as much as 300%.