PERFORMANCE STUDY OF ANCHOR PILES IN LOAD TESTS

Abstract

Experimental and theoretical investigations on pullout capacity of vertical and inclined helical screw anchors in sandy silt soil are presented. A brief literature review gives a consolidated view of the work done by previous investigators. Three model anchors having varying no. of helical blades (n=1, 2 & 3) of same geometrical properties have been used. Tests were conducted by varying no. of helical blades, installation depth (H/B= 4, 6 & 8) and inclination angles (0, 15 & 30° from vertical). The anchors were pre installed in test tank and prepared soil at 65% relative density was hand packed in the test tank in layers 50mm thick to achieve uniform density. Few tests were also performed with pair of helical anchors to study the group effect. In order to observe the difference in pullout capacity, if any, between pre installed and post installed anchors, tests were also conducted with post installed helical anchors and pre installed helical anchors. The results indicate that ultimate pullout load capacity of anchor increases with installation depth, no. of helical blades and decreases with angle of inclination. A sudden failure is observed as pullout approaches its ultimate value. The average pullout capacity of anchors when post installed is about 77% to that of pre installed anchors. For vertical anchors, values of pullout load obtained by using theories of Mooney, at al. (1985) are in very close proximity with experimental values for anchors installed at H/B= 4, while at deeper depths, it underestimates. Values obtained by method adopted in Wayne Roger's manual (2002) yields highly conservative value for n=1, while it overestimates the capacity for n=3. But for n=2, values obtained are reasonable. A best fitted equation has also been developed based on the test results. This equation has also been verified by some tests conducted in field.