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− the geomechanical model that characterizes the soil's mechanical properties
and the mechanical strength value required for the calculation;
− the hydraulic model that quantifies the hydraulic properties of the site,
including the permeability tensors, Skempton's coefficient B, and the boundary
conditions.
It would be dangerous to start the numerical modeling without any in-depth
assessment of the construction of these four models. The reality is unknown; it has
been replaced by the theoretical construction of the four models. It is therefore
important to judge their pertinence in the light of some elements of reality. The
strongest uncertainty concerns the seismic hazard that is part of the geological
model. It is very difficult to obtain a good knowledge of the active faults that are
hidden or remolded by the alluvial filling. For example, the geologists working on
the Shih-Kang Dam project did not localize the fault that was passing under the dam
which was responsible for its failure during the Chi-Chi earthquake.
10.5.4. Pseudo-staticanalysisofstability
Pseudo-static analysis of stability under seismic loading consists of calculating
the safety factor of a dam subjected to a horizontal force considered representative
of the earthquake action. This force is taken to be equal to the mass multiplied by a
percentage K of the gravity acceleration g , called the pseudo-static coefficient. This
coefficient is usually taken equal to 2/3 the ratio between the maximum acceleration
at the base of the dam and g.
The analysis of seismic loading on pier retaining walls has led us to consider that
the seismic load can be assimilated to a horizontal force with a pseudo-static
coefficient K = 0.33( PGA/g ) 1/3 [NOD 75]. The pseudo-static analysis is the first step
of the stability verification. It gives access to the critical pseudo-static coefficient Kc
for which the stability is lost.
An evaluation of the irreversible deformations of the geo-structure can be
achieved by comparing PGA/g and Kc in the absence of liquefaction. It is the first
step of the following method.
10.5.5. Pseudo-staticanalysisofdisplacement
During the earthquake, the geo-structure can have momentarily a safety factor
smaller than 1, without the overall security being questioned. If the duration of this
transient disequilibrium remains smaller than several milliseconds, the resulting
displacement is negligible. It is therefore better to define criteria in terms of
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