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turbed by the earthquake. If the clay is contractive in nature, use undrained strengths.
If it is dilative on shearing, use effective stress strengths c
. Usually there will be
some cracking and loosening, and if so fully softened strengths, would apply (e.g.
c
,
0,
peak ). Some apply an arbitrary 10% or 15% loss of strength;
(e)
For well compacted free draining rockfill filters and dense sands and gravels adopt the
effective stress strengths c
, with no change in the pore pressures. If large deformations
are expected in the earthquake the dense granular materials may have loosened and will
have a strength approaching the critical state strength, rather than the peak strength. In
practice this can be accommodated by a small reduction from the expected peak strengths.
,
The analysis is done with conventional limit equilibrium analysis methods. No loading
from the earthquake is applied, since this is a post earthquake analysis.
If the liquefied zone is subject to flow liquefaction and the post earthquake factor of
safety is significantly less than 1.0, large, rapid deformations and flow sliding can be
expected. If the factor of safety is only marginally less than 1.0, deformations may not be
so large as to lose freeboard between the dam crest and the reservoir level. An approximate
estimate of the deformations can be obtained using the Khalili et al. (1996) method which
is detailed in Chapter 11. For more accurate methods of assessing the deformations
numerical analysis using the post earthquake strengths can be used.
Indicative estimates of deformations can be obtained by performing a static deformation
analysis which incorporates the earthquake induced pore pressures and the residual
strength of the liquefied soils (Finn, 1993). The analysis is often performed in two stages.
In the first stage, the numerical model is initialised to the pre-earthquake conditions of the
dam by simulating the current in-situ stresses. Then, in the second stage, the earthquake
induced pore pressures and residual strengths of the liquefied soils are incorporated into
the model to simulate post-liquefaction conditions.
This type of analysis is also referred to as uncoupled deformation analysis and gener-
ally leads to conservative estimates of post liquefaction deformations, as it does not allow
for dissipation of earthquake induced pore pressures with time. More accurate estimates of
post liquefaction deformations can be obtained using fully and semi-coupled methods of
analysis, as discussed in the following sections.
12.6
SEISMIC STABILITY ANALYSIS OF EMBANKMENTS
12.6.1
Preamble
The following discussion on seismic stability analysis of embankments has been adapted
from ANCOLD (1998). The contribution of N. Khalili and other members of the
ANCOLD working group is acknowledged.
The methods of analysis currently used in practice to evaluate seismic stability of
embankment dams vary widely, ranging from simple limit equilibrium type analyses to
highly sophisticated numerical modelling techniques. These include:
- Pseudo-static analysis;
-Simplified methods of deformation analysis;
- Numerical modelling techniques:
(i) total stress;
(ii) effective stress.
The simplified methods of analyses, including pseudo-static and post liquefaction, rely
heavily on the lessons learnt from the performance of dams during past earthquakes.
Major reviews of past performance have been conducted by Sherard (1967), Sherard et al.
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