Evaluation and modification of analytical models for the peak uplift soil resistance of buried pipelines in unsaturated soils

Buried pipelines may undergo significant vertical displacement induced by permanent ground deformation associated with geohazards. Consequently, the peak uplift soil resistance (PUSR) is a key parameter for pipeline integrity assessment. Existing analytical models primarily concern ideal dry sand or saturated clay conditions; therefore, they cannot adequately characterize the resistance mobilization mechanism in unsaturated soils governed by the combined effects of frictional strength and apparent cohesion. To this end, this study conducts vertical uplift experiments of buried pipelines in unsaturated soils and develops a three-dimensional large-deformation finite element (FE) model based on the coupled Eulerian–Lagrangian method. The results indicate that setting the soil tensile strength to 0.2 times the theoretical value obtained by extrapolating the Mohr–Coulomb (M–C) criterion to the uniaxial tensile state enables the FE model to accurately reproduce both the experimentally measured PUSR and the observed macroscopic failure range. Based on the validated FE model, a parametric database comprising 180 cases is generated, covering six soil types, five embedment ratios, and six pipe diameters. The applicability of the ALA-2001, PRCI-2009, and GB/T 50470-2017 guideline models is then systematically evaluated. The results indicate that these existing models exhibit pronounced systematic deviations when applied to unsaturated soils. Accordingly, a modified analytical model is established within the resistance-component framework of ALA-2001, reducing the mean absolute percentage deviation to 5.5 %, which is approximately 15 % lower than that of the existing mainstream models. Furthermore, a limit-equilibrium model considering both compression-shear failure and tensile failure (CST-LEM) is proposed to elucidate the soil failure modes and the mobilization mechanism of PUSR during pipeline uplift. For the experiments conducted in this study, the near-pipe tensile-cracking-affected zone accounts for approximately 60 %–66 % of the pipe-center embedment depth. This finding indicates that the uplift failure in unsaturated cohesive soil cannot be accurately represented by an ideal fully closed compression-shear mechanism. A normal-fault crossing case further demonstrates that differences among uplift soil-spring models can lead to significant variations in the predicted axial strain response of pipelines. The findings provide a theoretical basis and engineering reference for estimating the PUSR of buried pipelines in unsaturated soils, with important implications for pipeline design and structural assessment.