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In considering the loads on buried pipeline, there are normally two scenarios: narrow trench condition and embankment (wide trench) condition.
For narrow trench condition, when the pipe is laid in a relatively narrow trench with backfill properly compacted, the weight of fill is jointly supported by both the pipe and the frictional forces along the trench walls. For embankment condition, the fill directly above settles less than the fill on the side. Consequently, loads are transferred to the pipeline and the loads on pipeline are in excess of that due to the fill on pipeline.
The narrow trench condition is used where excavation commences from the natural ground surface without any fills above the surface. On the contrary, the embankment condition applies where the pipes are laid at the base of fill. For instance, embankment condition is normally adopted where the pipes are laid partly in trench or partly in fill or poor foundations to pipes are encountered so that the trenches have to be excavated wider than the minimum requirement.