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
9. What are the major causes of rockfall? (S6)
The causes of rockfall can be broadly classified into the following two
reasons:
(i) Freezing and thawing and the subsequent development of vegetation
root pressure in slopes is one of the major causes of rockfall in some
countries like Europe. Moreover, rockfall can also be triggered by heavy
rainstorms which bring about surface erosion and generate water
pressure in rock joints.
(ii) Rockfall can also be induced by poor joint patterns and low strength
and water pressure in joints.
10. What are the functions of drainage system and protective covers
of slopes? (SF1)
In Hong Kong the angle of fill slope is about 30 o – 40 o to the horizontal
while the angle of cut slope is about 50 o – 60 o to the horizontal. To protect
the slope from surface erosion and water infiltration, it is covered with
impermeable hard cover like chunam and shotcrete. Chunam is a mixture
of soil, cement and lime and is usually applied in two layers with the
thickness of each layer of 25mm. For shotcrete, mesh reinforcement may
be provided inside it to enhance tensile strength of cover, thereby reducing
the risk of tensile cracking of slope cover.
Weepholes are normally provided in slopes to prevent building up of water
pressure in the slope and subsequently this causes cracking and
disintegration of slope cover. For gentle slope, hydroseeding may be used
and geofabric may be introduced on slope surface to guard against
possible surface erosion.
U-channels are provided on the crest and toe of slope to divert and collect
the rain falling on slope surface. Catchpits are provided at the intersection
or junctions of drains to avoid possible splashing of water.
11. What is the purpose of installation of erosion control mat in
slopes? (SF2)
Steep slopes are prone to intermittent high velocity flows during rainstorm
and this causes erosion at slope surface which prevents the growth of
vegetation.
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