Mechanism of external confinement pressure in improving the shear behavior of PHC pipe pile

Brittle shear failure has been observed in numerous prestressed high-strength concrete (PHC) pipe piles during earthquakes, which is attributed to an insufficient understanding of their shear failure mechanism under lateral loading. This study focuses on the effect of external confinement pressure, such as that provided by surrounding soil, on the shear behavior of PHC pipe piles, given its demonstrated potential for significant improvement. Firstly, an analytical method using the 3D Rigid-Body-Spring Method (RBSM), applicable and reliable for simulating the shear behavior of PHC pipe piles under various structural parameters, is proposed. Its validity is established through comparison with results from monotonic loading tests. Subsequently, the shear behavior of a series of PHC pipe piles subjected to uniformly distributed confinement pressure (0.05, 0.15, and 0.30 N/mm²), equivalent to lateral soil pressure at depths of 10 to 30 meters, is investigated numerically using the 3D RBSM. Finally, based on a detailed analysis of internal crack development, the mechanism by which external confinement enhances shear performance is elucidated. A key finding is that uniformly distributed external confinement can be conceptually separated into two components: positive confinement and negative confinement. A pile under pure positive confinement exhibits better ductility than one confined by uniform pressure over its entire outer surface. Conversely, a pile under pure negative confinement shows worse ductility than an unconfined pile. These findings contribute to refining conventional methods for improving the shear behavior of PHC pipe piles using external confinement.