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
15. How does lateral pressure of soils affect the drain performance?
(ED3)
The presence of lateral pressure of backfilling sidesoils induces bending
moments in the opposite direction from those produced by vertical loads
and bedding support reaction. Such bending moment reduces the flexure
in pipe wall and as a result this causes an equivalent increase in supporting
strength of the pipe.
The lateral pressure of backfilling soils on drains is affected by the
deflection of drains. With no occurrence of deflection, lateral pressure
induced is in the form of active pressure. If pipe deflection occurs, the drain
increases its horizontal dimension so that passive pressure is developed.
16. Why are coupling system usually used for drains constructed in
reclamation areas?
Pipes used in drainage works are normally of spigot and socket type which
is flexibly jointed. For this kind of pipe connections, it allows small amount
of rotation and hence it could withstand certain degree of uneven ground
settlement. Owing to uncertain nature of settlement in reclamation area
where differential settlement is anticipated to occur, coupling system is
usually provided for drains installed in reclamation area which provides a
higher degree of resistance to differential settlement. Otherwise, it may
cause cracking of pipelines leading to washing in of soil inside the pipelines.
In the worst scenario, significant loss of soil may result in ground
subsidence.
17. What are the major areas to be paid attention when designing
drainage pipes in reclamation areas?
In reclamation areas drainage pipes are usually laid at flatter gradients
when compared with upstream stormwater pipes. The fact that the nature
of flow in stormwater drain is by gravity makes the downstream pipes in
reclamation areas relatively deep below ground surface. It is preferable to
have outfall of drains above the tidal influence level and this accounts for
the relative flatter gradient of drain pipes in reclamation area.
Attention has to be paid to the possible occurrence of differential
settlement in reclamation area. For pavement design, flexible pavement is
preferred to rigid pavement to cater for settlement problems. Similarly, in
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