Reliable design against upheaval buckling of offshore pipelines requires the uplift response to be predicted. This paper describes a model-scale investigation into the mechanisms by which uplift resistance is mobilized in silica sand, and illustrates how the observed mechanisms are captured in prediction models. A novel image-based deformation measurement technique has been used. The results show that peak uplift resistance is mobilized through the formation of an inverted trapezoidal block, bounded by a pair of distributed shear zones. The inclination of the shear zone is dependent on the soil density, and therefore dilatancy. After peak resistance, shear bands form and softening behavior is observed. At large pipe displacements, either a combination of a vertical sliding block mechanism and a flow-around mechanism near the pipe or a localized flow-around mechanism without surface heave is observed, depending on the soil density and particle size.

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The design of buried anchors and pipelines requires assessment of the peak uplift resistance. This paper describes a limit equilibrium solution for the uplift resistance of pipes and plate anchors buried in sand. The geometry of this solution reflects observations from model tests. Peak angles of friction and dilation are found using established correlations that capture the influence of stress level and density. These angles govern the geometry of the failure mechanism and the mobilised resistance. The solution is validated using a database of 115 model tests on pipes and strip anchors assembled from the published literature. Good agreement with the overall database is shown, without optimisation of any input parameters. The method overpredicts the uplift resistance of smooth model pipes by $10%, highlighting the influence of pipe roughness. In contrast, it is shown that the solution for uplift resistance based on the limit theorems of plasticity is generally unconservative. The assumption of normality, which is required by the limit theorems, leads to an unrealistic failure mechanism involving uplift of a far wider zone of soil than is seen in model tests. Plasticity theory, with normality, is inappropriate for modelling this class of kinematically restrained problem in drained conditions, as normality is not observed. As finite element analysis is not routinely used in practicepartly owing to the difficulty in selecting appropriate input parameters to describe dilatancy and plastic flowthe simple analytical idealisation described in this paper provides a useful tool for uplift resistance prediction. Simple charts for the prediction of peak uplift resistance from critical state friction angle, relative density and normalised burial depth are presented, to aid the design of buried pipes and anchors.