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
of infiltrating water when the slope of underlying low-permeability soil layer
is mild. As such, there is a localized zone of high transient pore water
pressure induced within the fill material. Flowslides normally start with a
local slip caused by transient pore water pressure by soil layering or flow
restriction. Then, the nature of slow-moving soil debris and the geometry of
slip result in a fast landslide.
4. What is the difference in failure slip surface between slopes with
cohesive and granular materials?
When cohesive strength is zero (i.e. slopes of granular types), the slip
surface is of shallow failure type and is parallel to the slope surface.
When friction angle is zero (i.e. slopes of clayey types), the slip surface is if
deep seated failure. The factor of safety of slopes is nearly independent of
the angle of slopes because the weight of deep seated failure regime is
much greater than the slope.
Normally, non-circular failure surface is always more critical than circular
one for two dimensional analysis.
5. Why are filled slopes vulnerable to slope failure?
Filled slopes constructed in many decades ago are mostly sub-standard.
The relative density of filled slopes may be below 85% and is readily
subjected to liquefaction. To rectify the situation, the sloped are
reconstructed by excavation of 3m measured vertically from slope surface.
Then, compaction should be carried out in thin layers to achieve in-situ
density of not less than 95% of maximum dry density. After compaction, the
compacted layer would not vulnerable to liquefaction failure. Moreover, it is
less permeable than loose fill upon compaction and prevents water entry
into underlying soils inside the slope.
For the case of Hong Kong, most fill slopes constructed before 1977 were
formed by end-tipping so that they are in a loose state and poses hazard to
developments nearby.
6. Does cutting slope cause slope deformation or slope failure? (S4)
Slope cutting causes stress relief in slopes which may cause slope
movement. For instance, for weathered rocks the horizontal stresses would
be relatively low when compared with normally consolidated soils.
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