Geology Reference
In-Depth Information
unstable area), or through shoring up of the base with the help of various
constructions (gravity walls, anchored supports, soil nailing).
Water plays a primordial role in all types of landslide occurrences,
and its elimination must remain a principal goal in order to move towards
stabilization. This is why drainage is almost systematically used in curbing
landslides, either as the principal method or accompanied by another
stabilizing measure. The goal is to reduce the interstitial water pressure
in the terrain, in order to increase the resistance to shearing and therefore
the security coeffi cient with respect to potential or declared ruptures. The
goal is also to maintain this pressure below a threshold deemed critical
in order to guarantee the future security of the site. A very thorough
hydrogeologic study is then essential in order to design an appropriate
drainage system. The development of the project particularly requires a solid
understanding of the nature of the concerned aquifers, of their extent and
eventual exchanges, as well as the origins of the water infl uxes maintaining
the system, in order to enable the control of disfavorable hydraulic factors
by an adapted drainage type (superfi cial, upslope, lateral, deep). Design
or construction errors can, indeed, lead to the total ineffectiveness of the
drainage system, or even to negative effects on the stability of the site, as
is also the case when the system is poorly maintained after construction,
even if the initial construction was highly effective. Several types of
interventions can be considered, as a function of the depth of the rupture,
the global geometry of the landslide, the local topography, and the water
infl ow conditions: surface drainage, drainage ditches, subhorizontal drains,
vertical drainage, or drainage galleries. The defi nitive drainage of a slide
or of an unstable slope is generally put in place progressively, in parallel
with the creation of a piezometer network which allows the monitoring of
the drainage's effectiveness. The system is rarely fi xed, and can, over time,
require complementary or restoration work, particularly when monitoring
reveals ineffectiveness due to lack of maintenance or to the clogging of
certain drainage structures.
2.3.1 Superfi cial drainage
The good management of surface water plays an important role in slope
stability, and it is undeniable that the gradual abandonment of ancient
drainage and irrigation systems is now leading to the rapid degradation of
many slopes, made fragile by the anarchic outfl ow of previously channeled
water.
Immediately after a landslide is set in motion, superfi cial interventions
are always very useful in attempting to curb its progression, and can
lead to emergency measures when the protection of the downslope area
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