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 III: Channels
1. Two contraction joints and one expansion joints are usually
adopted for drainage channels. Why? (C1)
In the life cycle of a concrete structure (not prestressed concrete), it will
generally undergo the following process of contraction and expansion:
Contraction: (a) Early thermal movement
(b) Seasonal contraction owing to drop in temperature
(c) Shrinkage
Expansion: (a) Seasonal expansion owing to drop in temperature
The order of magnitude for items (a) to (c) is more or less the same. Hence,
qualitatively speaking, for a given length of concrete structure, the number
of contraction joints should be more than the number of expansion joints
and they are roughly in the order of 3:1 to 2:1 based on the number of
expansion and contraction process above. Of course, the actual spacing
and number of contraction joints and expansion joints should be
determined case by case.
2. Should joints in box culverts and channels be completely
watertight? (C1)
The joints for box culverts and channels should be capable of
accommodating movements arising from temperature and moisture
changes. However, the joints are not necessarily designed as watertight
except the following conditions [15]:
(i) There is a high possibility of occurrence of high water table in the
vicinity of box culverts/channels. The high groundwater level and
rainwater seepage through embankment may cause water passing
through the joints and washing in soils. Consequently, the loss of soils
may lead to the failure of the structures.
(ii) If the box culvert/channels are designed in such a way that water flow
through joints from the structures causes the washing out of bedding
materials, the requirement of watertightness of joint has to be fulfilled.
(iii) In cold countries, road salt is sometimes applied on roads above box
culvert or at crossings of channels to prevent freezing and thawing. The
leaching of road salts into the joints may cause corrosion of joint
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