Uplift Mitigation Strategies for Pipeline Overbends

Abstract Pipeline upheaval buckling due to insufficient vertical restraint and ratcheting caused by cyclic loading are especially challenging issues, with each likely to occur at overbends. A primary factor contributing to the uplift resistance available to counteract upheaval buckling is the strength of, and the ability to mobilize the overlying soil. Overbends have the potential to require additional uplift resistance, over and above that provided by the native backfill, to resist upheaval buckling. This buckling has primarily been mitigated by relying on increased soil cover or through the use of screw anchors in competent soils. These mitigations have drawbacks, as greater than required burial depths add to construction time and cost, while screw anchors can increase localized stresses on the pipeline. To assess currently available uplift resistances and evaluate the effects of various potential mitigation measures, a series of laboratory uplift tests using scaled pipe sections and granular backfills were completed. To address insufficient uplift resistance, a series of prototype mitigation measures were chosen for proof of concept testing. The design of each measure focused on increasing the peak uplift resistance and maintaining a residual resistance closer to the peak, without significantly increasing cover depths or stresses applied to the pipeline. Three of the concepts tested, a structural cap layer, pipe stirrups and a rigid open saddle each widened the soil column mobilized during uplift, increasing the peak resistances achieved in scaled testing, while also maintaining the integrity of the soil block at larger displacements, thus reducing the decay often seen in residual resistances. These concepts showed promise and were advanced from prototype to a field deployable design.