Seismic and Surge-Induced Responses of Large-Diameter Buried Gas Pipeline Bends in Marshy Soil: A Multi-Hazard Framework

Large-diameter buried gas pipelines traversing marshy or potentially liquefiable soils may be exposed simultaneously to transient internal surge pressure, seismic ground motion and reduced soil restraint. Pipeline bends are particularly vulnerable because changes in direction generate local thrust, bending and non-uniform soil–pipe interaction. This technical paper presents a conceptual and parametric finite-element framework for evaluating these interacting effects at a 90° buried pipeline bend. Surge pressure is estimated using the Joukowsky relation and applied with seismic excitation within a three-dimensional soil–pipe interaction model. Three progressively severe multi-hazard scenarios—Cases A, B and C—are considered by varying soil stiffness, burial depth, surge pressure rise and peak ground acceleration. Within each scenario, surge-only, seismic-only and simultaneous surge–seismic analyses are compared. The assessment indicates increasing stress concentration, ovalisation and relative soil–pipe displacement as soil restraint decreases and loading severity increases; under the severe scenario, partial uplift may also become significant. Combined loading produces greater demand than either single-hazard analysis and changes the location and distribution of critical response at the bend. The framework is intended primarily as a bid- and preliminary-design screening methodology for identifying vulnerable bends that warrant site-specific geotechnical investigation and detailed nonlinear analysis before final design. This approach provides a transparent basis for subsequent detailed engineering assessment.