A Self-Learning Manual - Institution of Engineers Mauritius

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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 142
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU reinforcement. 3. Should the outer side of drainage channel at a bend be elevated to cater for superelevation? Flow around a bend results in a rise of water surface on the outside of the bend and it is natural to consider that extra height of channel wall on the outside of bend to prevent overflow of water. However, for supercritical flow in channel, owing to the effect of superelevation extra height of channel wall should be provided on both sides of the bend. This is because supercritical flow around a bend would make water level go up alternatively on the outside and inside of the bend owing to cross waves. This cross wave pattern may continue for some distance downstream. As such, both sides of bend shall be lengthened to cater for this effect. 4. Should the same freeboard be maintained along a channel? (C2) The freeboard is defined as the vertical distance from water surface to the top of channel bank. The selection of freeboard is dependent on the consequence should overflow out of channel bank occurs. Other than that, consideration should also be given to prevent waves, superelevation and fluctuations in water surface from overflowing the channel banks. Generally, a 300mm freeboard is generally considered acceptable. For steep channels, it is preferably to have the height of freeboard equal to the flow depth to account for high variations in swift flow induced by waves, surges and splashes. 5. In designing of access ramps for drainage channels, why should the direction of access ramps be sloping down towards downstream? (C3) In the design of access ramps, the direction is normally specified to be sloping down towards downstream so as to avoid the occurrence of over-shooting of flowing water for supercritical flow in case of aligning the ramps in the reverse direction of channel flow. Note: Access ramps refer to ramps used for maintenance vehicles during routine maintenance of channels. 143
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A Self-Learning Manual - Institution of Engineers Mauritius

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