The steel pipe sheet pile (SPSP) foundation has been developed and applied as a dual-use structure for temporary coffering and bridge foundations in Japan since the 1960s. Mechanical complexity occurs essentially in the joint part for which each steel pipe is a coupled structure. This paper tries to experimentally explain how the pipe-joint-soil interaction (PJSI) influences the lateral load-displacement curve of the SPSP foundation. PJSI has mainly been discussed through finite element (FE) analyses due to the difficulty of creating experimental models that realize both geometric and mechanical similarities to prototypes. This study introduces a centrifugal model of SPSPs in which the joint filling material is modeled using gypsum plaster, which exhibits quasi-brittle behavior similar to that of the mortar used in practice. The mechanical and geometrical similarities between the joint model and prototypes are evaluated through a series of loading tests. Additionally, lateral loading tests on SPSP models with 1 pipe, 1×2, and 2×1 arrangements are conducted under both 1 G and 50 G conditions. Based on the pipe-joint interaction (PJI) and the PJSI evaluated experimentally, it is found that the arrangement direction of the piles causes a significant variation in the load-displacement relationship. Specifically, in the 1×2 arrangement, the yielding of the pipes and the vertical shear failure of the joint occur compositely, resulting in a diverse variation in the load-displacement relationship. It is also observed that the subgrade reaction from the soil leads to the restoration of lateral bearing stiffness by causing the compressive failure of the joint in this arrangement. On the other hand, with the ground model, the compressive and vertical shear failure of the joint occur sequentially, dominating the lateral load curve of the 1×2 arrangement together with the subgrade reaction. Without the ground model, the lateral load-displacement curve of the 1×2 arrangement is mainly dominated by only the vertical shear failure of the joint.
