Effect of trench backfill on lateral pipe-soil interaction in sand

Current industry practice for the pipe–soil interaction analysis widely uses either American Lifeline Alliance(ALA)or Pipeline Research Council International(PRCI)guidelines to model the soil load exerted on the pipeline. The maximum lateral force for the lateral soil springs recommended by these guidelines is based on small-scale laboratory tests, where the pipe, buried in either homogeneous sand or clay and was pulled in the lateral direction. However, in the pipeline construction practice, pipelines are usually trenched and then backfilled with backfill soil. The soil outside the trench of backfilled soil is called native soil, and a significant difference in the strength properties of these two soils might be expected in the field. When the pipeline is pulled laterally, the interaction of pipe with such trench backfilled soil along with native soil significantly affects the lateral soil resistance and therefore, the effect of the trench backfill on the lateral soil resistance needs to be properly analyzed. Two-dimensional finite element(FE)analyses of pipe–soil interaction for pipelines buried in medium dense trench backfill sand, supported by native dense sand, subjected to lateral ground displacements are presented in this paper. The analysis is performed in the plane strain condition using the Arbitrary Lagrangian–Eulerian method available in Abaqus/Explicit FE software. Both the classical Mohr-Coulomb(MC)model and an advanced soil constitutive model, named modified Mohr-Coulomb(MMC)model, that can capture the dependency of friction and dilation angles on the mean effective stress and plastic shear strain, are used. The pipe was modelled as a rigid body. Three trench wall slopes, 64°, 45° and34°, were used to see the effect of the trench dimension on the pipe lateral resistance. Results show that the MMC model can better simulate the trench effect than the MC model. The failure mechanism of backfill soil changes due to the presence of a stronger native soil. For steeper slope, e.g.64°, trench dimension cannot contain the ‘free-field’ failure surface as observed in the uniform backfill sand with no trench. However, for shallower trench slope, e.g.45° and 34°, the failure surfaces tend to develop completely within the backfill soil and the lateral resistances for these trench dimensions are comparable with the uniform backfill soil condition with no trench.