In-Situ Experiments on Cyclic Uplift Bearing Characteristics of Helical Piles in Silt

Helical piles are widely used in foundations of towers, wind turbine generators and other structures under cyclic loads because of their larger uplift bearing capacities. In recent years, with the increase of extreme strong wind disasters, helical piles have been widely used as anchorage facilities for anti galloping cables of transimission wires in transmission lines. However, there are few studies on the bearing characteristics of helical piles under cyclic loading, and there is no introduction on the bearing capacity calculation method in the relevant regulations and codes. Thus, the monotonic and cyclic in-situ loading tests of helical piles in silt were conducted, the cyclic uplift mechanical characteristics of helical piles were explorated. The results showed that the cumulative displacement of the helical pile was no more than 25 mm when the soil deformation was stable under cyclic action of 50% of static ultimate bearing capacities. The cumulative displacement of helical piles with double helix plates under cyclic uplift loads was smallest, which was about half the cumulative displacement of other helical piles with a single helix plate. When the bearing capacity of helical piles was insufficient, the foundation deformation increased sharply, and a radial crack centered on helical piles appeared on the ground. In the whole test process, the load-displacement skeleton curves of helical piles under cyclic loads were always lower than that of helical piles under static loads. After the cycles, both the load-displacement curves basically coincided under static loads. The uplift bearing capacity of helical piles was mainly composed of side resistance of anchor rods and end resistance of helix plates. Under cyclic uplift loads, the cumulative shrinkage of soil in shear zones of anchor bolt interfaces led to reduction of normal stress, and the uplift bearing capacity of helical piles was provided by helix plates mainly.