Abstract: The current studies on the expanded shaft piles are concentrated on the axial resistance characteristics and very few research on the mechanism of the uplift characteristics and ultimate uplift capacity theoretical estimations are reported. Therefore, an un-intrusive testing method, Particle Image Velocimetry (PIV), was used to measure the deformation of the surrounding soils surrounding the expanded shaft pile. The influence of the embedment depth, relative density of sand and the diameter of expanded shaft piles on the uplift capacity are evaluated. The analytical results show that the relative density is a major factor that affects the uplift capacity of the expanded shaft piles. The uplift performance of the expanded shaft piles in the dense sand is more than three times that of in the loose sand. The test results also indicate that, due to the low relative density and high compressibility of the loose sand, in order to improve the uplift capacity of the single expanded shaft piles and reduce pile displacement, the diameter of the expanded shaft portion should be 2.5 times of the shaft diameter. For piles in dense sand, the performance of the expanded shaft pile is a process of shearing and dilatancy under the action of vertical uplifting load. The shear stress at the expanded plate is relatively concentrated. As the diameter of the expanded plate increases, the displacement in the upper soil influence zone of the expanded plate continues to be affected. With the increase of the diameter, the sliding range of the upper part of the plate along the bottom surface of the expanded plate increases. In order to improve the uplift capacity of the expanded shaft pile in the dense sand, the diameter of the expanded plate of the expanded shaft pile is optimized to be the three times of the shaft diameter.

Key words: particle image velocity measurement technology, expanded shaft piles, relative density, diameter of expanded shaft, axial capacity