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
13. Why are some manhole covers made of cast iron while some are
made of ductile iron? (M2)
Traditionally, manholes covers are made of cast iron. However, in the
viewpoint of pipe maintenance, frequent opening of manhole covers has to
be carried out. Therefore, it poses potential safety hazard to the workers
during the lifting-up process of manhole covers because cast iron manhole
covers are very heavy to normal workers. Consequently, research has
been conducted and ductile iron is considered as a better choice than cast
iron because it can resist the same traffic loads with lower self-weight.
Moreover, as ductile iron is less brittle than cast iron, the traditional cast
iron manhole covers are more susceptible to damage and thus requires
higher maintenance cost.
However, ductile iron manhole covers do suffer from some demerits. For
instance, owing to their relative low self-weight, vehicles passing over
these manhole covers would lead to the movement of covers and generate
unpleasant noises. To solve this problem, instead of increasing the
self-weight of ductile iron manhole covers which similarly causes safety
problems to workers during regular maintenance, the covers can be
designed to be attached to the manhole frames which hold them in firm
position.
14. The spacing of manholes in straight sections for different pipe
sizes is stated in Stormwater Drainage Manual. How are these figures
arrived at? (M3)
For pipe size < 300mm, rodding is usually adopted in which workers place
about 1m long rods through the pipes to the location of blockage and
manually operate the rod to clear the blockage.
For pipe size < 700mm, water-jetting is normally employed in which water
is supplied from nearby fire hydrants and pressurized water jet is used for
clearing blockage.
Winching method is adopted for all sizes of pipes.
For instance, for pipe size exceeding 1050mm, it is stated in Stormwater
Drainage Manual that maximum intervals between manholes along straight
lengths should be 120m. This is because for sizes over 1050m, the main
method of pipe maintenance is by winching whose maximum length of
operation is 120m. Similarly, the maximum intervals of manholes for other
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