<p>In this study, in-situ loading tests and numerical analyses were carried out to investigate the relationship between pull-out resistance characteristics of helical piles in sand and the effect on the ground when the helical wings pass through (pile installation effect). The tested piles had four helical wing plates and were installed into a sand layer for pull-out tests. An analysis of the bearing capacity characteristics and the load shared by each wing showed that the lower the wings, the greater the pull-out resistance of the wing, and the pull-out resistance of the wing yielded sooner. The results confirmed that the bearing capacity factor at each wing tended to decrease by a factor of approximately 0.5 in the upper wings. In numerical analysis, to consider the pile installation effect, the soil parameters were adjusted by simply representing the ground as being disturbed according to the number of times the wing passes over it. The results of the analysis agreed well with the test results. The failure of the ground around the wing propagates upwards from the lower wing.</p>