Geology Reference
In-Depth Information
These installations were complemented by an evacuation network or
the collected water and by a monitoring system for the extracted discharge
and the piezometry. The evolution of the slide was totally stopped, and
monitoring confi rmed the performance of the setup and the size of the
drained discharge.
2.6 Example of the Quiaus landslide (Gorbio, Alpes-
Maritimes)
The Quiaus embankment was built in 1967 and 1968 in the context of
construction work for the A8 highway linking France and Italy. It enables
the highway's passage over the confl uence zone of several small valleys
making up a left bank tributary of the Gorbio river. With a length of 240
meters and a maximum height of 25 meters, it covers the greater part
of the slope, which consists of a nummulitic series with an upslope dip
(Figure 125):
• in the lower part of the slope, relatively impermeable gray Priabonian
(Upper Eocene) marl;
• in the upper part of the slope, an Oligocene marly-sandy fl ysch, which
acts like a multilayered aquifer with fracture permeability.
In November, 1969, spectacular settling affected the southern pavement
of the highway. The problems then evolved slowly until they appeared
in August, 1971 as a landslide in the embankment limited by continuous
fractures. They fi nally brutally accelerated in May, 1972, after a very rainy
period (Amar et al. , 1975).
The studies undertaken led to the conclusion that the movements were
a result of signifi cant hydraulic loading, generating abnormal piezometric
rises and strong increases in the quantity of water in the embankment. It
is most likely the later emplacement of an embankment in a nearby valley
that, by obstructing the aquifer's natural outlet, favored critical increases
in hydrostatic pressure in the rock mass.
The remediation methods consisted of an emergency rocky buttress
at the foot of the slide, preventing a deep slide, followed by further study
to fi nd a drainage solution for the fl ysch, in order to reduce the risk of
superfi cial slides.
The characteristics of the site led to the drainage of the embankment's
foundation with the help of a 145 meter long gallery, within which were
drilled radiating drains. Subsequently, the setup was complemented by
vertically drilled drains equipped with submerged pumps, located along
the length of the highway platform.
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