pullout behaviour of belled piles under axial and oblique pull in soil-rock composite ground: an experimental study

Transmission tower foundation is an important facility to ensure the stability of transmission tower structural systems under severe environment (such as strong winds). these piles are usually subjected to oblique pullout forces under the horizontal wind loads. however, the research on the pullout behaviour of rock-socketed pile under oblique load is insufficient, especially for the belled pile. in this study, the failure model tests of belled piles in soil-rock composite ground under axial and oblique pull are conducted, and the load–displacement behaviour, ground displacement curve, failure mode, and load transfer mechanism are investigated. these results show that the weakening effect of oblique load (an angle of 14° deviates the vertical direction) on the ultimate uplift capacity of belled pile is 16.3%, and the deformation of belled pile enhances (compared to the axial load). moreover, under an axial load, the failure mode of the bedrock is an inverted cone with a failure angle between 0.8φr and 1.0φr. under oblique loading, a ring crack appears on the bedrock surface in the tensile side region, while there is no obvious crack propagation in the compression side region, which is about a fan-shaped area with a central angle of 120°. it is also observed that the pile bending moment increases initially and then decreases with the increase of depth, and the position of the maximum bending moment occurs below the ground surface (2.3–2.8d depth). these findings play an important role in pullout design of transmission tower belled pile foundation in soil-rock composite ground. © 2022, iran university of science and technology.