Influence of Pile Diameter on the Vertical Bearing Characteristics of Railway Bridge Friction Piles in Permafrost Regions

The vertical bearing capacity of friction piles in permafrost regions is significantly influenced by pile diameter. Scaled model tests were conducted to investigate the vertical bearing characteristics of railway bridge friction piles with different diameters in permafrost, revealing the effects of diameter on failure modes, settlement response, and bearing capacity. The results demonstrated that increasing the pile diameter reduces pile foundation settlement, enhances the stiffness and recoverability of the pile–soil system, and improves the overall bearing capacity. At diameters of 10 and 20 cm, the failure mode manifested as abrupt collapse, whereas the 15-cm-diameter condition demonstrated progressive failure. Under permafrost conditions, small-diameter piles (10 cm) exhibited discontinuous vertical load transfer, preventing load distribution to the pile tip and thus compromising bearing capacity. Larger diameters (15 and 20 cm) achieved continuous load transfer with fully mobilized side friction resistance, reaching approximately four times that in unfrozen soil layers. However, excessive diameters (20 cm) suppressed end bearing capacity development (maximum contribution of only 0.34%) due to frost heave forces amplifying side friction. A hyperbolic load-transfer function model was developed to predict the bearing capacity of variable-diameter friction piles in permafrost, effectively capturing settlement trends and load-transfer mechanisms. This study provides a theoretical foundation for optimizing the design of pile foundations in permafrost regions.