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
deeper in the soft clay, which should possess higher shear strength
because its strength generally increases with depth.
5. Which one is better, bentonite slurry and polymeric slurry? (M3)
For the construction of diaphragm walls, bentonite slurry is commonly used
to form a filter cake on walls of trenches to support earth pressure. The use
of bentonite solely is based on its thixotrophic gel viscosity to provide
support.
Though the cost of polymer is generally more expensive than bentonite,
the use of polymer is increasing because polymer is generally infinitely
re-usable and very small amount of polymer is normally required for
construction works. The disposal cost of bentonite is quite high while the
disposal of polymer can be readily conducted by adding agglomerator.
6. Where should be the direction of gunning in shotcreting? (M4)
During the construction of shotcrete, it is aimed at gunning the full
thickness in one single operation and this helps to reduce the occurrence
of possible delamination and formation of planes of weakness. Moreover,
the nozzles should be held about 0.6m to 1.8m from the surface [2] and
normal to the receiving surface. The reason of gunning perpendicular to
the receiving surface is to avoid the possible rebound and rolling resulting
from gunning at an angle deviated from the perpendicular. The rolled
shotcrete creates a non-uniform surface which serves to trap overspray
and shotcrete resulting from the rebounding action. This is undesirable
because of the wastage of materials and the generation of uneven and
rough surface.
7. What is the mechanism in the formation of frost heave? (M5)
In the past, it was believed that the formation of frost heave was related to
the volumetric expansion of soil water which changed from liquid state to
solid state. However, the increase of volume of changes in states for water
at zero degree Celsius is only about 9% and the observed heaving is far
more than this quantum.
In fact, the mechanism of frost heave is best explained by the formation of
ice lenses [52]. In cold weather, ice lenses develop in the freezing zone in
soils where there is an adequate supply of soil water. Soil particles are
surrounded by a film of water which separates the soil particles from ice
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