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 II: Box Culverts, Manholes and Catchpits
1. Why is geotextile filter introduced below the rockfill layer of a
typical box culvert? (BC1)
In a typical box culvert, at the interface between rockfill layer and
sub-grade, a layer of geotextile filter is usually added to perform separation
function. With the addition of geotextile filter layer, it avoids the intermixing
of widely different soil granulations so as to reduce long-term settlement.
Moreover, it also prevents interpenetration of rockfill into sub-grade so that
the deformed configuration of rockfill may impair its intended function of
load spreading.
2. Can dowel bars be omitted in the joints of box culvert? (BC2)
Dowel bars in joint normally serve to maintain structural continuity by
transmitting shear forces between adjacent concrete structures. For box
culvert, the use of dowel bars in joints is essential owing to the following
reasons:
(i) Without dowel bars, differential settlement would result and it leads
to the formation of steps in the box culvert. As a result the flow
capacity of box culvert would be reduced.
(ii) The steps in the box culvert would also provide locations to trap
rubbish and debris.
3. What is the function of waterstops in joints of box culverts and
drainage channels? (BC2)
The principal function of waterstops is to prevent liquids (e.g. water),
water-borne materials and solids to pass through concrete joints. In
essence, it aims at providing watertightness to the drainage channel.
Besides, waterstops in drainage channels or box culverts can also serve
two other purposes: (i) to avoid water contacting joints’ dowel bars and
causing corrosion. (ii) to avoid water seeping in from the underside of
drainage channels or box culverts, thereby washing in soil particles and
causing voids underneath these structures and finally leading to their
failure. To serve the second purpose, obviously only one waterstop is
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