Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 243

2.2 Causes of landslides
There are multiple factors governing slope stability (geology, topography,
climatology, hydrology, the rocks' state of disintegration), and it is often very
diffi cult to specifi cally identify the causes of a landslide. There is generally
a complex combination of a number of factors leading to the rupture, but
it is usually set off by changes in the geometry of a talus slope and in the
role of water.
The equilibrium of natural slopes varies noticeably as a function of
the characteristics of their constituent rock, and can evolve over time
due to weathering of this rock and to the infl uence of external factors.
Disequilibrium can be reached at any moment through a decrease in
resistant forces, following the elimination of a basal abutment. This is
classically the case when a large-scale excavation generates a brutal
instability in the overlying talus, but also when natural erosional processes
slowly destabilize the foot of a slope (basal undercutting by the sea or by
a river, melting of a glacier). The disequilibrium can also be a result of
the creation of a point surcharge which increases destabilizing forces (the
emplacement of an embankment on a slope, for example).
The role of water is always highly important, even if it is not the
only factor, and there exists a strong correlation between the setting off
of landslides and the intensity of rainfall. The accumulation of water in a
sensitive slope, either by direct or nearby infi ltration, or by subterranean
infl uxes of more distant origin, directly infl uences the characteristics of the
component materials, by reducing its resistance to shearing through an
increase in interstitial pressure and by reducing its cohesion. These effects
can infl uence the entirety of a sensitive site, and can translate as marked
amplitudes in the level of the water table, with periods of instability often
in phase with piezometric rises. The effects of the increased presence of
water can also concern only certain permeable areas within an impermeable
formation, or a deep permeable horizon likely to be fi lled with water below
impermeable formations. Such permanent or episodic upwards pressure
facilitates the setting off of landslides.
2.3 Treatment of landslides
Landslides being essentially provoked by gravitational and hydraulic
factors, it is normal that stabilization techniques focus preferentially on
geometry and mass distribution as well as on drainage.
Changes in geometry and masses tend to restore an equilibrium profi le
to unstable areas, either through excavation (substitution of high-friction
materials for the slipped mass, basal buttress, unloading at the head of the