Fundamentals of Differential Settlement of Pipelines

Pipelines undergo differential settlement when traversing hard to soft foundations at pumping station/pipeline interfaces, for example. Settlements occur as a result of varying soil conditions that may not be obvious without a soils investigation and expert geotechnical knowledge. Compaction of soils can reduce the amount of settlement, however compaction around structures, where pipelines are entering/exiting, is often difficult for contractors to achieve. In an earthquake, some soils may liquefy, causing local loss of soil strength, resulting in settlements. Proper attention to potential settlement and design is necessary to make pipelines function reliably as a safe, integral part of a conveyance system, as intended by the designer. It is common to have a pipeline enter/exit an underground structure; however, the pipeline may settle more than the structure and pipeline joint flexibility is needed. Flexibility is increasingly important with design of large diameter pipelines because they are much stiffer and less tolerant to movements than smaller diameter pipelines. Flexibility can be achieved with the use of couplings. However, pairs of couplings are needed to provide rotational movements and a single coupling does little to accommodate differential settlements, a typical structural design consideration at pumping stations or valve vaults where the piping must transition between relatively hard building foundations and soft trench foundations adjacent to the buildings. Also, since couplings do not normally provide restraint from thrust due to internal pressure forces, external harnesses are often needed. AWWA Manual M11 suggests usage of the familiar bolted sleeve-type couplings (BSTCs) per AWWA C219 and guidance for the design of harnessed joints where the focus is to determine the proportions of tie rods and harness assembly necessary to resist internal pressure forces.