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
required at any depth location.
To serve the first purpose, a waterstop has to be installed on top of dowel
bars to prevent water from drainage channels from leaking through. On the
other hand, a waterstop has to be provided below dowel bars to avoid
underground water from surging upwards.
In fact, the other way out to serve the first purpose is by using corrosion
resistant bars.
4. What is the difference between inlet control and outlet control in
hydraulic design of box culvert? (BC4)
In the hydraulic design of box culvert, there are two flow controls, namely
inlet control and outlet control. In inlet control, the entrance characteristics
of the box culvert (e.g. headwater depth and entrance configuration)
determine the hydraulic capacity of the box culvert, and the culvert is
actually capable of conveying a greater flow than the inlet would allow.
Barrel shapes and tailwater depth are of no significance in determining the
hydraulic capacity. Inlet control usually takes place for culverts lying on
steep slopes.
For outlet control, the inlet could accept more flow than the box culvert
could carry and the hydraulic capacity of the box culvert is dependent on all
hydraulic factors upstream from outlet tailwater.
5. Which one is better, single-cell box culvert and double-cell box
culvert? (BC3)
The use of double-cell box culverts is preferred to single-cell box culverts
for cross-sectional area larger than about 5m 2 owing to the following
reasons:
(i) Where there is tight headroom requirement, the use of double-cell box
culvert can shorten the height of culverts by having a wider base so that
the same design flow can be accommodated.
(ii) The invert of one cell can be designed at a lower level to cater for low
flow condition so that it reduces the occurrence of sediment deposition
and avoid the presence of standing waters.
(iii) The provision of temporary flow diversion can be easily provided for
inspection and maintenance of each cell. During routine maintenance
operation, water flow can be diverted to one cell and the other one is
133