Environmental Engineering Reference
In-Depth Information
2.10.1.2
First-time slides
First time slides are much more difficult to predict. Reliable prediction is not possible with-
out considerable knowledge of the geotechnical conditions in the slope, and of the nature
of the possible disturbing activities. For example, there is not enough subsurface data on
Figure 2.28
for one to say definitely that Wedge A B D will slide, with or without some exca-
vation above Point A. The prediction of first time slides is discussed further in Section 2.11.
2.10.2
Importance of early recognition of evidence of past slope instability at dam sites
It is very important to recognize any evidence of past instability at a dam site, during the
Prefeasibility or Feasibility and Site selection Stage of the project planning (see Chapter 4).
The reasons for this are as follows:
1. Landslide debris, comprising broken rock, soil or mixtures of soil and rock in an
uncompacted state is usually not acceptable anywhere in the foundation of a dam. This
is because the compressibility and permeability of such materials are potentially high,
variable and often unpredictable (see Sections 3.10.1.3 and 3.10.2.3). If these materi-
als occur in only in a relatively shallow deposit, it might be possible to remove them
and expose a satisfactory foundation. However, the presence of a small deposit of land-
slide debris may indicate the presence of a much larger volume of unstable or poten-
tially unstable material nearby or upslope.
2. Also, as mentioned above in Section 2.10.1.1, experience shows that where landslipped
materials on slopes are affected by excavation or inundation, renewed landsliding often
occurs either in the materials themselves or from the source areas of the deposits. Some
potential consequences of landsliding within dam storage areas are listed and discussed
in Section 2.11.
In the light of the above, if all or a large part of one bank at a potential dam site is found
to be underlain by landslipped materials, it is usually best to adopt, or look for, an alter-
native site. If it is decided to persevere with the original site, a very thorough site investi-
gation will be needed, the results of which may well cause the site to be abandoned.
Comprehensive site studies would be necessary to determine the slide models and mech-
anisms and to assess the risks and hazards involved.
It should be clear that the presence of old landslides in any of situations above raises
questions which ideally should be answered during the feasibility stage and certainly by
the end of the design stage of the project planning (see Chapter 4, Section 4.3).
Many old landslides are easily recognised during stereoscopic examination of aerial
photographs, by their characteristic surface form and by anomalies in the local vegeta-
tion, soils or rocks (see Chapter 5).
2.10.3
Dams and landslides: Some experiences
This section describes some experiences with dams at sites showing evidence of past land-
sliding.
2.10.3.1
Talbingo Dam
At the original site for Talbingo Dam, in the Snowy Mountains of New South Wales
(
Figure 2.29
)
, the right bank was formed almost entirely by the remnants of a mass of
variably weathered basalt which had slipped more than a kilometre downslope and
buried the valley of a tributary creek.
The geological picture in Figure 2.29 was deduced almost entirely from air photos and geo-
logical outcrop mapping on 1:2400 scale. It was confirmed by refraction seismic traverses
