Construction Pipe Settlement: Causes and Mitigations

Abstract With pipelines, as with any new construction some settlement of the foundation is expected. An allowance is made during design for pipe settlement associated with trenching and backfilling construction activities. However, additional unexplained settlements can occur and become problematic, potentially overstressing a pipeline. These issues are exaggerated at pipeline facilities. An evaluation of current construction guidelines and practices along with additional case study and literature reviews was utilized to determine potential factors contributing to this settlement. Several factors were identified, revolving primarily around trench movements and backfilling quality. Various factors can contribute to trench movements, including elastic and consolidation movements of the surrounding soil due to overburden removal, trench wall movements due to frost heave or water entry while the trench is open. The pattern and magnitude of these movements will depend on soil types, trench support and trench geometry. Winter construction can create frost heaving in both the trench base and walls, followed by settlements during thawing. While consolidation of trench soils due to the increased loading from the new pipeline is considered, the combined result of this swelling is often not accounted for in design. Beyond this, the backfill and its handling can alter the settlement response during and immediately post construction. Controlled granular backfills, compacted with near optimum water contents may provide the highest level of predictability post construction, but will still settle under water wetting. Controlling backfill quality, material, water contents and compaction assurance, and trench swelling prior to pipeline construction, may provide the best path to controlled, predictable and quantifiable pipeline construction settlements. More economic pipeline designs will result if pipeline movements due to external effects can be reduced.