Field tests of micro screw anchor piles under different loading conditions at three soil sites

Micro screw anchor piles are ideal as the foundation of solar power plants and have been widely used in recent years. In this study, test apparatuses for the uplift loading test, compression test, and lateral loading test of the micro screw anchor pile were designed. Comprehensive field tests (totally 72 piles) for six types of micro screw anchor pile were conducted in clayey, loess, and sandy sites, and some conclusions can be drawn as follows. (1) The typical ranges of bearing capacity in three soil sites with the micro screw anchor pile were obtained under uplift, compressive, and lateral loading conditions; the lateral bearing capacity varied over a small range (8.1 to 11.7 kN) for various site conditions and pile configurations and was much smaller than the uplift and compressive bearing capacities. (2) The screw anchor pile with more helical blades had better performance under uplift, compressive, and lateral loading conditions; increasing the embedded depth of blade substantially enhanced the uplift and compressive bearing capacities but only slightly influenced the lateral bearing capacity. (3) The clayey site had larger uplift and compressive bearing capacities than the loess and sandy sites; the uplift bearing capacity was larger than the compressive bearing capacity in clay but smaller than the compressive bearing capacity in sand. (4) The classic cylindrical shear method was improved by taking into account the shaft resistance above and below the blade(s), which can better predict the uplift and compressive bearing capacities; as for the lateral bearing capacity, a simple but practical prediction method was also proposed. This study will contribute to a better understanding of the failure mechanism and bearing capacity of micro screw anchor piles. The site information and test results are promising to provide data for further analytical and numerical works.