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
Level One (Core FAQs)
Part IV: Sewers
1. In designing sewer pipes, why are vitrified clay pipes commonly
used for pipe size less than 600mm while precast concrete pipes with
PVC lining is used for pipe size exceeding 600mm? (S1)
The market price of vitrified clay pipes is generally less than that of precast
concrete pipes with PVC lining. Therefore, for small size of pipes (pipe
diameter less than 600mm) it is more economical to use vitrified clay pipes.
However, vitrified clay pipes do suffer from the problem of brittleness and
its effect is even severe for larger size of pipes. Moreover, it is rather time
consuming to deliver clay pipes products because the majority of them are
manufactured in Europe. Hence, for larger size of sewer pipes (diameter
more than 600mm) it is customary to use precast concrete pipes with PVC
lining.
2. Why is partial PVC lining instead of full lining adopted in concrete
pipes? (S1)
The main function of PVC lining is to protect concrete surface against
hydrogen sulfide attack. Moreover, it also guards against attack by a wide
range of acids and alkalis. Hydrogen sulfide generated in sewerage by
bacterial action under anaerobic (absence of oxygen) conditions is
converted to sulphuric acid by aerobic bacteria growing on wet sewer walls.
This acid reacts with the lime in the concrete, causing breakdown of the
concrete pipes. The addition of lining could also improve the flow capacity.
Only the concrete above the line of minimum flow shall be attacked by
hydrogen sulfide and therefore PVC lining is needed above this flow line.
For medium to large pipe sizes, partial lining of 300 degree with 60 degree
pipe invert exposed proves to be effective against attack. Small diameter
pipe may warrant complete linings of pipes.
3. Is vitrified clay pipes chemically resistant against all aggressive
materials in sewage? (S2)
In the manufacturing process of vitrified clay pipe, the clay material
particles are fused into an inert and chemically stable compound. It is
capable of carrying a wide range of commercial, industrial and domestic
sewage. In fact, it is chemically resistant against sulfuric acid induced by
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