Screw piles have a greater bearing capacity than straight piles due to their larger helix. However, an excessively large helix can cause bending and reduce the soil bearing capacity. This study investigates the failure pattern and mechanical performance of screw pile helices through full-scale load tests and numerical analyses. The results revealed that the helix buckled at its connection to the shaft. Additionally, the geological characteristics of the soil in which the pile was located had a negligible effect on the mechanical properties of the helix. Furthermore, the shape of the anchor plate (flat or helical) had a negligible effect on the load-bearing properties of the pile or the mechanical properties of the anchor plate itself. To simplify the analysis, the screw pile helix was assumed to be a flat circular plate. For a uniformly loaded flat circular plate with fixed inner edges, the result of Roark's formula satisfactorily agreed with the measured maximum radial normal stress in the helix. Moreover, the value given by Roark's formula for a flat circular plate with simply supported inner edges agreed well with the measured helix deformation.