Q. Application of embankment condition for drainage design?

In considering loads on buried pipeline, there are generally two scenarios: narrow trench condition and embankment (wide trench) condition.

For narrow trench condition, when pipe is laid in a comparatively narrow trench with backfill properly compacted, weight of fill is jointly supported by both the pipe and frictional forces along the trench walls. For embankment condition, fill directly above settles less than fill on the side. Thus loads are transferred to the pipeline and loads on pipeline are in excess of that because of the fill on pipeline. 

The narrow trench condition is used where excavation begins from the natural ground surface without any fills above the surface. On the contrary, embankment condition applies where the pipes are laid at the base of fill. For example embankment condition is generally adopted where the pipes are laid partly in trench or partly in fill or poor foundations to pipes are encountered so that trenches have to be excavated wider than the minimum requirement.