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
(iv) It does not provide any structural data on the integrity of pipe wall.
7. What is the application of inverted siphons? What are the
disadvantages of using inverted siphons?
Inverted siphons are designed at locations in which a sewer system is
blocked by underground utilities or stormwater drains. They are sometimes
called depressed sewers because it is claimed that there is no actual
siphon action. They connect the upstream and downstream sewers with
U-shaped vertical alignment such that they are always running full.
The drawbacks of inverted siphons are:
(i) They induce additional head loss to the sewer system which is
undesirable in hydraulic performance;
(ii) U-shaped siphons create sediment accumulation problem and
previous experience showed that inverted siphons were easily
blocked due to siltation;
(iii) Maintenance of invert siphons is difficult due to its inaccessibility.
8. Does pipe deflection affect its flow capacity? (DS1)
When excessive pipe deformation occurs, it may impair the joint
performance and affect the strain in pipes. Based on the information by
PIPA, there is a 5% reduction of flow capacity when the pipe is deflected by
15%. Hence, pipe deflection has impact on flow capacity but its effect is not
significant.
9. What is the difference of movement of solids in large sewers and
small sewers? (DS1)
For solids in large sewers, forces on solids position them at different flow
heights depending on their specific gravity.
There are generally two modes of movement of solids in small sewers,
namely, floating and sliding dam. The floating mechanism operates when
the size of solid is small when compared with the diameter of sewer. Solids
move with the wave of sewage. On the other hand, the sliding mechanism
functions when the size of solid is large when compared with the diameter
of sewer. The sewage waves build up behind the solids which act as a
barrier at the base of sewer. When the waves store sufficient energy to
overcome the friction between solids and sewer invert, the solid would
move along the sewer.
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