Abstract: Threaded-group piles are widely used in clay, silt, and sand formations owing to their high bearing capacity. However, current research on threaded piles mainly focuses on single piles, and the group pile effect and variable cross-section effect have not yet been sufficiently investigated. In this study, the group pile effect of threaded group piles with different pile numbers, spacings, and lengths and the variable section effect of threaded single piles were investigated using a model test combined with the digital image correlation (DIC) technique. This study shows that: (1) the maximum vertical and horizontal displacements of sandy soil around the threaded ring occur at the threaded ring and its lower notch, respectively. The greater the number of threaded rings, the greater is the value-added bearing capacity and the smaller is the benefit of the increase, and the mechanism of the variable section effect of a threaded pile is closely related to the development of its damage surface. (2) The horizontal displacement between group piles increases and then decreases in the process of pile compression, and the vertical displacement is transferred from the top to the bottom of the arch. At the end of pile pressing, the vertical displacement vectors of the sand between the group piles are superimposed on each other, and the horizontal displacement vectors are weakened by each other. (3) The group pile effect increases with an increase in the pile number and pile length. As the pile distance increased, the pile side and pile end resistances gradually became stronger, and the group-pile effect decreased. The larger the settlement, the smaller is the group pile effect, which reveals the interaction mechanism between the soil and threaded pile and provides a theoretical basis for improving the bearing performance of threaded group piles.

Key words: threaded piles, digital image correlation (DIC) technique, group pile effect, variable section effect, pile-soil interface