Mechanical behaviour of a ground deformation-resistant ductile iron pipeline joint under sequential pre-bending and axial pull-out

Earthquake-induced permanent ground deformation (PGD) can impose axial pull-out and deflection demands on the joints of segmented water pipelines, yet restrained pipeline joints have mainly been evaluated through independent axial-tension and bending tests. This study investigates how prior deflection modifies the subsequent pull-out behavior of a restrained bell–spigot joint in an 8-in. (203-mm) ductile iron pipeline. Two full-scale tests were conducted with different locking-segment orientations at joint to investigate the response under pre-bending and axial tension. Distributed fiber optic sensing (DFOS) provided continuous longitudinal and circumferential strain fields along the pipeline, and a three-dimensional finite element model with explicit locking segment contact was evaluated against the measured global responses and spatial strain distributions. The results show that pre-bending created a non-uniform contact state and modified the subsequent pull-out response. The specimen with a locking segment at the crown reached a peak pull-out resistance of 171 kips (760 kN), 18% higher than the 145 kips (645 kN) of the alternative orientation. During pulling, the strain fields revealed a transition from global bending to axial localization near the joint. In both specimens, cracks initiated at the slot-segment transition but followed orientation dependent crack paths. A parametric study over 0–10° of pre-bending identified a transition at approximately 3–4° from retention-dominated to contact-dominated behavior. At the most adverse angle, the pull-out capacity decreased to approximately 70% of the pure tension value. These findings demonstrate that the axial force-displacement response of a restrained joint is dependent on its prior deflection and loading path. The results provide an experimental and mechanistic basis for developing deflection-dependent joint relationships for future soil-pipeline interaction models at system level.