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
the design of drains flexible joints like spigot and socket joints and
movement joints in box culverts have to be provided to guard against the
effect of differential settlement.
18. Is it preferable to compact bedding for concrete pipes? (ED2)
In the middle third of the base of precast concrete pipes, the bedding layers
are recommended to be left uncompacted because it helps to reduce the
reaction force at the invert of the pipes and intensifies the effect of shear
forces. Moreover, the bending moment at pipe invert is increased by the
compaction of bedding layer. The general rule for this region of bedding
layer is that it should be firm enough for the pipes to rest on.
The sides of haunch and bedding directly under the haunch should be
compacted because this will reduce the bending moment at the invert
which is the critical failure location for pipes. The compacted haunch helps
to resist the pipe load and maintain level and alignment.
19. When branch pipelines are connected to main pipelines,
sometimes Y-junctions or fitting branched pipelines to main pipelines
by formation of holes in main pipelines are used. Which one is a
better choice?
By using standard precast units of Y-junction branch pipelines, it is beyond
doubt that joints between branched pipelines and main pipelines are
properly formed and the quality of joints is relatively less dependent on
workmanship. However, it suffers from the problem that with fixed precast
units of Y-junctions, sometimes it may be difficult for contractors to
determine the precise orientation of specific angles of Y-junctions with
respect to gullies. (e.g. gullies are connected through side branches to
carrier drains)
By forming elliptical holes in main pipelines and fitting the side branches
into them with cement mortar, the quality of pipe joints is highly dependent
on workmanship. It is commonly found that in subsequent CCTV
inspections side branches are projected inside main pipes. This is
undesirable because the projected side branches reduce the cross
sectional area of main pipes locally and affect their hydraulic performance.
Moreover, the projected side pipes may trap rubbish and dirt in the vicinity.
On the other hand, cement mortar may not be properly applied at
connection joints because these areas are hidden from view and are
difficult to be inspected by engineers. Therefore, in selecting between the
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