or underground excavation will aid slope failure,
especially if such features do not reach the surface. Any
water-saturated, porous lens within a regolith will also
tend to form a zone of preferred shearing. Topographi-
cally, any cliffed or steep slope is also susceptible to land
sliding, as are areas of block faulting, previous landslides
or subsidence, and artificial excavation. Finally, any area
that has been denuded of its soil-retaining vegetation
because of deforestation, overgrazing, cultivation, fires,
or climatic change has the potential for sliding.
Initiating causes of landslides are just as plentiful.
The removal of basal support provides the easiest way
to start a landslide. Natural agents such as undercut-
ting by running water, waves, wind, and glacial ice are
prime candidates. Extrusion of material from the base
of a slope is also effective. This can take the form of
outflow of plastic material within the deposit, washout
of fines, and melting of ice. The material at the base
could also change its characteristics through water
absorption due to flooding, solution of soluble material
such as limestone or salt, or chemical alteration. In
developed areas, people have now become the major
cause of toe instability on slopes through mining, exca-
vation, quarrying, basal construction, and road, rail and
canal works. Overloading of the slope material is also a
common means of initiating failure. Overloading can
occur by saturating the slope with rainwater or water
from upslope streams or springs, or by loading the
slope with snow or debris from upslope instability.
Humans can also initiate overloading by dumping spoil
or by building foundations and structures on the slope.
Slope failure can also be induced by reducing internal
coherence in the slope material. Increased lubrication
due to heavy rainfall or runoff filtering through a
regolith is a common mechanism for triggering failure,
either immediately or some months afterwards.
Cracking of the slope material through partial failure,
desiccation, earthquakes, or internal movement can also
allow water to penetrate more easily into the slope
material. Blockage of drainage, usually by raising the
base of the watertable, can reduce shear resistance
internally. As already mentioned, the activities of
humans through drainage modification, deforestation,
overgrazing, or water discharge from septic systems and
domestic usage also reduce internal coherence through
lubrication. Earthquakes and volcanoes can either
crack slope material, or reduce coherence through the
process of liquefaction. Even thunderstorms can trigger
rockfalls and landslides because of the vibrations that
are transmitted through slopes. Similarly, humans now
perform activities that vibrate the ground and could
trigger landslides. These activities include vehicle move-
ments, blasting, pile-driving, drilling, and seismic work.
A number of processes can cause slope failure
due to prying or wedging. Water freezing in cracks,
increased pore-water pressure after heavy rain,
expansion of soil material because of hydration, oxida-
tion, carbonation or the presence of swelling clays, tree
root growth, and temperature changes are the main
mechanisms. Finally, strains in the earth due to sudden
changes in temperature, atmospheric pressure, or the
passage of earth tides are also factors that could trigger
landslides. The above conditions favoring landslides
may act in concert to trigger instability. As a result,
slope failure can happen instantaneously, irregularly
but cumulatively over time, or with considerable
Short-term oscillations in soil strength,
for example, changes in pore-water pressure.
A point where shear
strength equals shear stress.
Therefore failure occurs.
Long-term change in soil
strength, for example,
in shear stress, for
example, due to an
Progressive increase in
shear stress due to
loading, for example,
Critical shear stress.
If soil strength falls below
this level, failure occurs.
The complexity of landslide initiation over time (Finlayson & Statham, 1980 © with permission Butterworths, Sydney).