Geology Reference
In-Depth Information
1. Disrupted slides and falls: these include highly disrupted landslides
that move down slope by falling, bouncing or rolling, or by transla-
tional sliding, or by complexmechanisms involving both sliding and
flow. They typically originate on steep slopes, travel fast
and can transport material far beyond the slope in which they
originate. Other than large rock avalanches, failures in this
category are thin with initial failure depths less than 3m.
2. Coherent landslides: these include translational slides and rotational
slides. Such failures are typically relatively deep seated (greater than
3m), slow moving and displace material less than 100m.
3. Lateral spreads and
fluid-like
flow is the dominant mechanism
and this mode of failure is typical of lique
ows:
uid
ed soils.
The most common failures, according to Keefer, are rockfalls, rockslides
and disrupted soil slides. This follows from the analysis of
Table 6.4
where it can be seen that signi
cant landslides will only occur where
there are predisposing factors such as a topographic setting that is in
unstable equilibrium or strain softening (due to collapsing structure or
low residual strength, for example, through the loss of rock bridge-
cohesion during the earthquake shaking). Keefer compiled data from
many earthquakes and plotted the area affected by earthquakes vs.
magnitude of the earthquake. The upper bound is rather well de
ned.
For a magnitude M5, the affected area might be about 100 km
2
, 1,000
km
2
for M6, and 10,000 km
2
for M7. Keefer also presents data on the
maximum distance of landslides triggered by earthquakes of given
magnitude. He provides separate upper bound curves for disrupted,
coherent and
flow-type failures. Disrupted landslides such as rockfalls,
which are the most common type of earthquake-triggered landslides, are
also shown as themost likely to occur at far distances from the epicentre.
Rodriguez (2001) has carried out a further review of data, including
more recent data from Japan, and his data demonstrate the considerable
scatter that can be expected and therefore the dif
culties in prediction on
c basis. For example, some M7 earthquakes only cause
landslides within an epicentral distance of 10 km whereas others of
the same magnitude cause landslides 200 km away. This might be
attributed in part to resonance effects associated with ground frequency
spectra and duration, as for buildings (Hencher & Acar, 1995).
asite-speci
Traditionally, and in most software packages, there are two main
approaches to slope design to withstand dynamic loads (mostly earth-
quakes). The options for landslide prevention are essentially the same
as for the static condition (change geometry, reinforce, reduce water
pressure, protect the site below or move the facility at risk).
