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 I: Pipelines
1. Which one is better, ductile iron pipes and mild steel pipes as
pressurized pipelines? (P1)
For watermain pipe size less than 600mm, ductile iron is normally used
because internal welding for steel pipes below 600mm is difficult to be
carried out. Moreover, it requires only simple jointing details which allows
for a faster rate of construction. For watermain pipe size above 600mm,
steel pipes are recommended because steel pipes are lighter than ductile
iron pipes for the same material strength and therefore the cost of steel
pipes is normally less than that of ductile iron pipes. In addition, in areas of
difficult access the use of lighter mild steel pipes has an advantage over
ductile iron pipes for easy handling.
2. What is the difference between ductile iron pipes and steel pipes in
resisting external loads? (P1)
Ductile iron pipes normally possess thicker pipe walls and are generally
stiffer than steel pipes. As such, it replies less on side fill soils to support
external loads. Hence, it is not necessary to achieve highly-compacted
soils for ductile iron pipes for sustaining external loads.
For steel pipes, owing to less stiffness associated with thinner pipe walls, it
relies heavily on the stiffness of backfill soils in resisting external loads.
Hence, to enhance the external load-carrying capacity of steel pipes, the
most convenient methods are to improve the quality of backfill materials
and to increase the level of soil compaction.
3. What does the pipe thickness of ductile iron pipes generally larger
than that of steel pipes? (P1)
Both steel pipes and ductile iron pipes use hoop stress equation to model
internal pressure design. The difference in pipe thickness arises as a result
of more conservative approach in DI pipes.
For ductile iron pipes, surge pressure is considered as part of design
pressure and they are added together before applying a safety factor of 2
as follows:
360