Environmental Engineering Reference
In-Depth Information
1. Do enough surface and subsurface investigations to prove whether or not the landslide
is likely to become reactivated, and is large enough to present an unacceptable risk to
the project feasibility, when reactivated;
2. If this shows that the slide is large enough and the risk unacceptable, then the follow-
ing options would have to be considered:
- Design of systems to stabilise it, or
- Abandonment of the proposed storage area.
3. If from the investigations in 1, it is concluded that the slide is not likely to be reacti-
vated, or that the reactivation will be slow moving and/or is not large enough for its
activation to present an unacceptable risk to the project, it would still be prudent to
confirm that conclusion by continuing to monitoring its behaviour through commis-
sioning and early operation.
Stabilising systems for either situation A or B would have to be based on a geotechnical
model of the landslide, capable of providing predictions of its future behaviour. As land-
slides are characterized by extreme variability in composition, structure and permeability
(see
Sections 2.10.2
, 3.10.1.3 and 3.10.2.3), to produce such a model would require a
comprehensive and time-consuming investigation. Questions to be answered, and investi-
gation methods which can be used to answer them, are discussed in ICOLD (2002) and
Stapledon (1995). However, it must be appreciated that at the start of such an investiga-
tion there could be no guarantee that its completion would answer all of the questions and
allow the design of an adequate and economically feasible stabilizing system. This is made
clear by ICOLD (2002) under Key Management Issues, as follows:
“Advanced techniques of field investigation, laboratory testing and stability analysis
are now available. However, because of the complex nature of landslides and reservoir
interaction, it is often difficult to define specific failure mechanisms and complete a mean-
ingful stability analysis. This may leave substantial uncertainty in the determination of
hazard and risk”.
2.11.1.3
Areas where first-time landsliding may be induced (Questions 3 to 7)
As pointed out in Section 2.10.1.2 first-time landslides are much more difficult to predict
than reactivated landslides. They are often more important than existing landslides
because they are more likely to travel rapidly.
a)
Rock slopes
. For rock slopes, a common type of first-time slide occurs where joints,
bedding, or weak seams dip out of the slope and form all or part of a translational slab or
wedge with kinematic freedom to move. The shaded wedges on
Figures 2.41
and
2.43
are
examples. Other possible first-time failure types include rockfalls, buckling and toppling.
See also Section
2.10.3.5
, and Section B on
Figure 2.40
which shows the suspected poten-
tial first-time slide across the storage rim near Thomson Dam.
To assess the probability of a first-time slide from any rock slope requires (at least) knowl-
edge of the pattern of defects within it. For a rock slope showing abundant outcrops, or
whose structure is otherwise well exposed (e.g. in exploratory trenches, shafts or tunnels)
the plotted and analysed results of geotechnical mapping can indicate the probability that
a kinematically unstable wedge or slab exists, and its potential size. To assess the probabil-
ity that such a kinematically unstable mass would actually fail, due to construction activi-
ties, operation of the storage, or natural processes, would require substantial further site
investigations. Some suggested questions to be answered in such investigations are set out in
Appendix B of ICOLD (2002) and in Stapledon (1995). The assessment of the likelihood
of such sliding is a technically complex matter and should be addressed by persons with
expertise in rock mechanics and engineering geology as applied to large landslides.
Where such a suspect mass is located at or near a vulnerable project feature, the site
investigation could form part of the site studies for that feature. However it would be
