Geoscience Reference
In-Depth Information
10.5.8.
Analysisofpost-seismicstability
A simplified approach to the post-seismic stability consists of calculating the
final safety factor from the residual strength
Su
r
obtained from triaxial tests on
samples prepared in the
in situ
state. This residual strength corresponds to the
critical state. In practice, its minimum value is obtained by an extension triaxial test
at the end of the undrained cyclic loading, using the two relations:
− the initial void ratio
e
0
as a linear function of the log of the critical stress
p'
c
,
giving what is called the critical state line;
− the constant value of the ratio
Su
r
/ p'
c
.
Another approach uses penetrometer tests by correlating the residual strength of
a series of liquefied sites to the point resistance. This approach is more conservative
because it gives the residual strength after failure by liquefaction.
The experimental discrepancy has to be carefully examined in order to determine
a characteristic value for each layer of the foundation soil. Castro has proposed
taking the mean value minus half the standard deviation as the characteristic value
of the residual strength, according to the analysis of the San Fernando Dam failure
[CAS 89].
Comparison of the results obtained by the above analysis, the simplified analysis
of liquefaction and the non-linear calculations must show a global coherence of
these different approaches. This coherence being verified, the
in situ
tests are
calibrated for establishing the global diagnosis of safety under seismic loading.
10.5.9.
Assessment
Study of the re-evaluation is not complete with the sole calculation of the
stability under seismic loading.
In situ
results have shown that the failure is often
delayed by several minutes to several weeks (Manno-Ike). Indeed, the failure can be
induced by a mode different from sliding. Therefore, in order to undertake a
complete study, we have to gather all the possible causes that initiate each type of
failure and evaluate the likelihood of its occurrence for the case being studied
[BRU 03]. Three modes of failure are likely to happen:
− internal erosion;
− external erosion;
− sliding.
Search WWH ::
Custom Search