Evaluating Seismic Stability of Embankment Dams - Multiscale Geomechanics: From Soil to Engineering Projects - page 359

In some cases, the soil used in the foundation is not common enough to be
present in the published abacus, such as the silts. Laboratory testing is then
indispensable and a new abacus has to be set up. The evaluation of the seismic
behavior of gravels and silts requires joint laboratory and in situ testing. The quality
of the test results is an essential factor in preventing designs that are too
conservative. For example, the strength of silts measured in triaxial apparatus can be
lowered by more than 50%, if the samples are completely remolded. Moreover, it is
not possible to obtain undisturbed gravelly samples. To overcome this problem, we
have to determine the material's in situ state and link it to the liquefaction resistance.
In the United States, the Becker penetrometer supplants the SPT. In France, EDF
uses the CPT, whose results have been validated within a calibration chamber. We
can also find correlations between shear wave velocity and liquefaction resistance,
which allow us to overcome the difficulty of reliable measurements.
A potentially liquefiable zone is not likely to liquefy under certain given
boundary conditions and more specifically under the influence of drainage.
Therefore, if a potentially liquefiable zone is detected, the next step is to perform
coupled or semi-coupled non-linear calculations in order to quantify the drainage
influence.
10.5.7. Coupled non-linearanalysis
Coupled non-linear dynamic calculation is based on the integration of a
constitutive law for the continuous medium and is applied on the weakest
embankment section for stability analysis. The finite element method or the finite
difference method can both be used.
In the last part of this chapter, a non-linear constitutive model is presented. The
results are studied in terms of liquefaction factor (ratio between pore pressure rise
and initial effective stress), shear strains and final displacements.
The gravel permeability plays a fundamental role. Whereas the undrained triaxial
tests show a liquefaction potential of gravels similar to the one obtained on sands at
the same relative density, the calculations show that the pore pressure generation
during cyclic loading is dissipated in few seconds in the gravels (see Figure 10.10)
due to high permeability.