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
to the low permeability of decomposed dolerite dykes, perched water table
would be developed above the dykes and this essentially increase the
positive pore water pressure which reduces the shear strength of soils.
The presence of decomposed dolerite dykes in a direction perpendicular to
slope surface may also cause slope failure after severe rainstorm. The low
permeability of the material tends to act as a dam building up the local
groundwater level for water coming down from upper slopes. The increase
of water level behind the “dam” causes an increase in pore water pressure
so as to reduce the strength of soils in slope, leading to subsequent failure.
26. Does the presence of colluvium beneficial to slopes? (S8)
Colluvium refers to common surface deposit rolling down hillsides under
the action of gravity. It differs from residual soils which, upon
decomposition the discontinuity, joints and textures are preserved. The
rolling action of colluvium destroys these features.
Colluvium displays some important features which deserves attention:
(i) It appears to be more resistant than residual soil to erosion and tends to
offer protection to the soils beneath.
(ii) Its relatively lower permeability when compared with underlying residual
soils may form a perched water table at its base during heavy
rainstorm.
(iii) For residual soils, when slope cutting is carried out the presence of
relict joints may affect the slope’s stability. However, the base layer of
colluvium is also a plane of potential failure location.
27. How do horizontal drains help to stabilize slopes? (SF9)
The use of horizontal drains to enhance slope stability in the following
ways:
(i) It enhances an increase in soil strength by lowering the degree of
saturation of soils.
(ii) It reduces the pore water pressure in the region of potential slip surface.
(iii) It hinders the development of seepage forces.
28. Is bleeding test an essential requirement for grout?
Bleeding is a form of segregation in which a layer of water migrates to the
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