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
10. Why is gabion apron necessary for gabion retaining wall to retain
river embankment?
Gabion aprons are provided at gabion walls to protect its toe from scouring
due to river flow. The scouring would eventually lead to undermining of the
gabion structure and affects its structural stability. The length of gabion
apron should be long enough such that it reaches beyond the limit where
scouring may form. A layer of getextile filter is normally placed at the base
of gabion structures to prevent leaching out of foundation soils.
11. How to cater for energy dissipation at drainage outlets? (C5)
Flow velocity at outlets is usually high. Without proper control of this energy,
the subsequent bank erosion may result in failure of the banks. Therefore,
some energy dissipating structures are designed to cope with this problem.
Impact energy dissipaters may be provided at outlets by making use of
impact walls to dissipate energy. Alternatively, the flows at outlet are
dispersed artificially to achieve a significant loss of energy. However, the
problem of cavitation may occur in this type of energy dissipating
structures.
Fig. A typical drainage outlet.
12. In selection of dams in drainage channels, what are the
advantages of using rubber dams instead of steel-gate dams? (C6)
The advantages of rubber dams are as follows:
(i) Since rubber is flexible in nature it is capable of performing deflation
even in the presence of dirt and sedimentation on the downstream
side. However, for rigid steel-gate dams, it may not be possible to
open when there is excessive sediment.
(ii) Since the foundation of rubber dams is comparatively lighter than that
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