Geology Reference
In-Depth Information
ashorttime(typicallyafractionof a second), the exciting accel-
eration will decrease and then change direction so that the inertial
force is back into the slope. This will stop themovement unless the
slope is metastable, as discussed later. Sliding friction can be
lower than residual (Hencher, 1977, 1981a; Crawford &
Curran, 1982; Tika
et al
., 1990), and by employing pessimisti-
cally low shear strength, total displacement can be calculated for
a series of acceleration pulses and this used as part of a design
decision. Generally, even for a very large earthquake, the perma-
nent displacement in a slope directly attributable to inertial load-
ing will be small, of the order of millimetres or centimetres
(Newmark, 1965; Ambraseys & Srbulov, 1995). Nevertheless,
small permanent displacements will make the slope prone to
accelerated weathering and deterioration if not protected or
repaired.
3. Failed slopes: catastrophic landslides during earthquakes can be
the result of four different conditions, viz:
-
Low residual strength
. The inertial displacement during the
earthquake reduces shear strength to a residual value so that
even after the earthquake shaking, the slope continues to
move. Examples of large-scale failures involving sliding on
bedding planes with reducing strength are described for the
Chi-Chi earthquake (1999) by Chen
et al
. (2003) and Chigira
et al
. (2003), and for the Niigata earthquake (2004) by
Chigira
et al
. (2006).
-
Deteriorated state
. The structure of rock or soil mass is
disturbed so that it collapses and a
flow can develop.
-
Geometrically unstable equilibrium
. The initial displacement
caused by the earthquake shaking results in unstable equili-
brium. A typical example is rockfall from exposed rock cliffs.
Once displaced, the rock will fall, sometimes as a progressive
failure several days after the earthquake. Rockfalls may
become entrained and develop into debris avalanches.
-
Water-induced failure
. Firstly, loose saturated soil can col-
lapse and liquefy down to depths of about 15m on slopes
inclined at only a few degrees. The collapsed material can
spread and
flow. As a second mechanism, the general ground-
water
flow paths can be affected by earthquake loading and
this can trigger slope failures.
Keefer (1984, 2002) identi
es 14 individual types of earthquake-
induced landslide. The three main categories are:
