Key geological issues - Geotechnical Engineering of Dams - page 64

preferable to look for an alternative, less suspect, site for the project feature, and spend
the time and money confirming its stability.
If the investigations show that the probability of landsliding is high, and its conse-
quences could affect the project feasibility, then the options for action would be:

- Design of stabilizing systems, or

- Abandonment of the proposed storage area.

b) Soil slopes . An apparently intact slope formed by soils would be suspect for first-time
landsliding during filling or operation of a storage, if the slope was known to have mar-
ginal stability and:
- It was close to areas also underlain by soils, but showing evidence of currently active
or past landsliding;
- The underlying soils were currently well drained, at low moisture contents, and the
water table in them was low (well below the proposed storage levels);
-The soils were known to contain swelling clays or soluble materials such as halite or
gypsum.
A large slope meeting any of those criteria would be best treated in the same way as a sus-
pect rock slope, that is, avoided if possible during the location of any project feature. Should
its potential size and location be such that its failure would endanger existing infrastructure
or the project feasibility, then the options would be the same as those for a rock slope.

2.11.1.4. What is the likely post failure velocity and travel distance?
For landslides into the reservoir, it is large, rapid slides which are of most concern because
they have the potential to cause waves which can overtop the dam, even breaching the dam.
Glastonbury (2002), Glastonbury and Fell (2002a, b, c) have developed a decision analysis
framework for assessment of the post failure velocity of large natural rock slope failures based
on the study of a large number of rapid and slow landslides. This shows that for a landslide
to travel rapidly after failure there has to be a significant loss of shear strength on the surface
of rupture and/or internally or on the lateral margins of the landslide; or the factor of safety
has to be maintained below 1.0 after failure by high groundwater pressures.
A slide having these characteristics is often considered as being “brittle”, with the strength
of a surface being brittle if there is a large loss of strength. Table 2.7 summarises the domi-
nant sources of brittleness for the landslides studied by Glastonbury (2002). All the “rapid”
slide cases examined in the database involved relatively high strength rock masses. Many
rupture surfaces were pre-sheared, yet brittle collapse still occurred due to either brittle-
ness on lateral margins or brittle internal deformation.
Table 2.8 and Figure 2.44 summarise the typical characteristics of the rapid landslides.
The study also showed that many of the more obvious landslides associated with reservoirs

Table 2.7. Landslide mechanism classes and dominant source of brittleness (Glastonbury, 2002;

Glastonbury & Fell, 2002c).

Brittle Buttress Brittle Internal Brittle Basal Rupture Brittle Lateral Rupture

Deformation Deformation Surface Surface

Toe Buttress Bi-Planar Compound Rough Translatio nal Large Ro ck Glide

Compound Curved Compound Rock Collapse

To e-Buckling Translation Planar

Translational

Irregular
Compound