A unified framework for predicting vertical bearing capacity of fully embedded pipes/tunnels in clay seabed based on the undrained failure mechanisms

Understanding failure mechanism and predicting bearing capacity for the seabed soil-structure interaction is crucial in the design of offshore tunnel/pipeline. This study employs mesh-adaptive finite element limit analysis (FELA) to investigate the uplift/penetration failure mechanisms and bearing capacity of a tunnel/pipe segment in undrained clay seabed. Most of the critical parameters were considered in the FELA simulation, including unit weight of soil (γ′), undrained shear strength (s_u) profiles of uniform s_u or heterogeneous s_u with gradient k, embedment ratio (H/D), soil-structure interface tensile capacity, and interface roughness (μ). The criteria for the transition state between breakaway and no breakaway were quantified. When no breakaway between the structure and surrounding soil occurs, the uplift and penetration failure mode is basically the same, regardless of embedment ratio, and interface tension capacity. Dimensionless bearing capacity was quantified as functions of dimensionless parameters H/D, γ′D/s_u, γ′/k, and μ, for no-tension and full-tension conditions. Then, a unified predictive framework for both uplift and penetration bearing capacity of tunnel/pipe structures under arbitrary geotechnical conditions was proposed and validated. This work provides design framework for predicting uplift or penetration resistance, with particular relevance to upheaval buckling stability of offshore pipelines and anti-buoyancy stability of submerged tunnels.