Objective Under high-temperature and high-pressure service conditions, subsea buried pipelines may experience vertical heave. The vertical restraint capacity offered by cover soil directly governs cover soil design and vertical stability evaluation. However, existing research and standard methods generally treat global failure and local failure during pipeline uplift as two independent modes, and insufficient attention is paid to the continuous evolution of cover soil failure mechanisms with changing embedment depths in soft clay.

Methods To clarify how uplift bearing capacity evolves at different embedment depths, a two-dimensional, plane-strain finite element model of homogeneous soft clay is established using PLAXIS 2D. The pipeline uplift process is simulated via gradual application of vertical uplift displacement to the pipeline. Variations in the uplift bearing capacity factor with the embedment depth ratio H/D are calculated. The evolution of failure mechanisms is identified through contour plots of plastic zones. Finally, calculation results from ASCE standards and Pipe-soil Interaction for Submarine Pipelines (DNV-RP-F114-2017) are compared with numerical data from published literature to analyze the applicability of predictions from both standards across different embedment depth ranges.

Results The failure mechanism of the cover soil exhibits a distinct staged evolution with increasing embedment depth. In the shallow embedment stage (H/D ＜ 2), the cover soil primarily undergoes global heave. Plastic zones extend from the pipeline crown to the seabed surface, enabling failure zones to readily breach the seabed surface. In the medium embedment stage (H/D = 2–4), characteristics of both global and local failure emerge simultaneously. While plastic zones retain upward global heave features, concentrated local shear develops around the pipeline perimeter. This stage is the critical interval for the continuous transition of uplift bearing capacity failure mechanisms from global failure to local failure; it also represents the range where the uplift bearing capacity is most sensitive to depth variations and where the differences in standard calculation results are concentrated. In the deep embedment stage (H/D ≥ 4), plastic deformation gradually converges near the pipeline perimeter. Local shear failure dominates, and uplift bearing capacity growth levels off. Comparative analysis reveals that both the ASCE design guidelines and DNV-RP-F114-2017 capture the overall trend—where uplift bearing capacity rises with embedment depth before eventually leveling off—yet both standards oversimplify the continuous transition within the medium embedment range.

Conclusion These findings serve as references for the cover soil design of subsea buried pipelines embedded in soft clay, the selection of standard calculation methods, and the flotation and heave resistance design of pipelines in saturated low-strength cohesive soils such as onshore marshes and tidal flats. For scenarios involving long-term cyclic loads, distinctly layered soils or highly structured clays, further research combining laboratory tests and numerical analysis is still required.