A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
pipe invert and the bedding material. The angle of contact between pipe
invert and the bedding material increases with the ratio of bending moment
at invert (for the case of three-edge bearing test) to the angle under
consideration [67].
13. What is the difference between narrow trench condition and
embankment (wide trench) condition for drainage design? (DD3)
In considering the loads on buried pipeline, there are normally two
scenarios: narrow trench condition and embankment (wide trench)
condition [23].
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.
14. Why is it preferable to minimize the width of pipe trenches? (ED3)
From the design point of view, it is preferred to minimize the width of pipe
trenches because of the following reasons [29]:
(i) Higher cost of excavation is associated with wider pipe trenches.
(ii) The width of trench affects the loads on installed pipelines in
consideration of embankment condition and wide trench condition. For
minimum pipe trench width, the loads on pipelines can be reduced.
However, sufficient space has to be provided to allow for proper
compaction. This is helpful to reduce the reaction at critical locations of
pipelines under traffic and fill loads. Moreover, consideration should be
given to accommodate temporary works for deep trenches where shoring
has to be provided during construction.
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