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For each method, it is necessary to test the coherence and precision of the results
with:
− all the sources (magnitude and distance to the dam);
− local earthquakes;
− local geology;
− the first natural frequencies of the dam.
For this purpose, the AFPS ( Association Française de Génie Parasismique )
intends to develop a probabilistic approach at the national scale [AFP 07]. In the
meantime, it is possible to use the probabilistic approach developed by GEOTER at
EDF's request, which is based on a Bayesian analysis of the measurements given by
the accelerograph network installed by the RAP as well as measurements made on
nuclear power plant sites [GEO 08]. This study has produced maps of the maximum
acceleration in France for a given return period.
10.5. Re-evaluation of seismic stability
10.5.1. Maximumriskassociatedwithseismicloading:liquefaction
After having reevaluated the seismic hazard, we now have to reevaluate the
seismic risk attached to the most exposed dams, that is to say the embankments
prone to liquefaction. It is possible to distinguish four deformation modes:
superficial sliding of the embankment slope, deep sliding, loss of foundation bearing
capacity, and embankment settlement. For each of them, liquefaction appears to be
the driving force of the deformation. Therefore, it has to be carefully studied.
Liquefaction develops in saturated undrained soils. Its well-known appearances
are the quicksands in the zones of lake and shore sedimentation. Less well-known
are the failures during construction, as for example during the construction the
Calaveras Dam. Finally, liquefaction-induced seismic loading has been highlighted
by the failure of the Van Norman Dam. The back analysis of this failure has given
us the means to predict the most destructive effects of liquefaction [SEE 73].
What is liquefaction? A non-cohesive material subjected to cyclic shear stresses
develops contractive volumetric strains. In undrained conditions (or if the loading is
fast enough to prevent any drainage), the volume of a saturated material remains
constant. The contractancy manifests itself by an increase in the pore water pressure,
which induces a decrease in the mean effective stress, counterbalancing the tendency
of pore volume decrease during cyclic shearing. When the pore water pressure
reaches the value of the total stress, the soil is liquefied.
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