Nonlinear pipe–soil–liner interaction of corroded steel pipelines rehabilitated with FRP liners under differential ground settlement

Differential ground settlement can produce severe localized deformation in corroded water-supply pipelines. This study develops an idealized three-dimensional nonlinear finite element model of a corroded X65 steel pipe rehabilitated with a close-fitting, unbonded fiber-reinforced polymer (FRP) liner under monotonic quasi-static settlement. The pipe–soil component of the framework is partially verified against an existing full-scale reverse-fault test. The generic liner is represented by a simplified isotropic elastic–perfectly plastic law, and 160 MPa is used as an interpretive tensile-stress threshold. Under the baseline calculation at a settlement displacement of 0.30 m, the computed steel-pipe peak strain is 22.5% lower with the liner; however, the liner reaches its stated stress threshold at approximately 0.24 m. Results beyond the first component threshold are therefore reported only as constitutive trends and are not used to claim safe capacity. Local corrosion increases the peak steel strain by approximately 155%, and the most unfavorable investigated defect location is 1.1 m from the settlement interface. Increasing soil stiffness intensifies strain localization, whereas increasing liner stiffness reduces steel strain while increasing liner stress and reducing the settlement required to reach the liner threshold. The present study quantifies conditional quasi-static resistance indicators, not time-dependent system resilience. Orthotropic progressive damage, calibrated bond–slip, gradual settlement profiles, and uncertainty quantification remain necessary for design-level conclusions.