Evaluating Seismic Stability of Embankment Dams - Multiscale Geomechanics: From Soil to Engineering Projects - page 350

The term liquefaction includes several states and modes of deformation. These
different modes have to be distinguished in order to have a better understanding of
the liquefaction effects. Liquefaction is in theory a state of the soil corresponding to
zero effective stress. In reality, the effective stress decreases to non null but small
enough values to lead to, in the case of a loose soil, a liquefied sliding zone. In other
cases, where the dam and foundation soils are more compact, the effective stress can
increase again, leading to what is called “cyclic mobility”, which stops at the end of
the seismic loading. Therefore, two phenomena driven by liquefaction can occur:
− liquefied soil sliding, which is due to an abrupt decrease in the soil resistance
until the geometry of the soil mass produces a new stress state compatible with the
mobilized residual strength. This was the case for the San Fernando Dam whose
upstream facing slid 42 m;
− cyclic mobility, which is the consequence of the pore pressure increase during
seismic loading and leads to irreversible strains, but does not affect the post-seismic
soil strength.
To simplify, geo-structure failure is quasi-certain when liquefied sliding occurs,
whereas fissures and settlements are more probable consequences of cyclic mobility.
We, therefore, have to select the engineering works constructed on non-cohesive
loose soils. Then, we have to determine the seismic hazard for the site. Post-analyses
of geo-structure behavior give way to a first evaluation of liquefaction occurrence:
− the failure criterion expressed in terms of maximum acceleration would be
PGA = 0.3 g for a close-by earthquake (short duration and high frequencies of
seismic loading) and PGA = 0.2 g for a distant earthquake (long duration and low
frequencies of the seismic loading) [JAN 76];
− from the results of parametric calculations, the failure criterion expressed in
terms of Arias intensity would be around 0.15 m/s for fine soils. For gravels, it
would be 0.5 m/s for any initial density. These criteria remain to be confirmed;
− the sites located less than 6 km from an active fault and those located on a
gravely foundation less than 4 km from an active fault are potentially prone to being
liquified [MAT 01];
− the liquefiable sites are located at a distance smaller than the distance Re to the
epicenter or Rf of the active fault, depending on the amplitude of the moment Mw or
the surface shear waves Ms :
Mw = -0.31 + 2.65.10 -8 Re + 0.99log 10 (Re) with Re in cm [AMB 88]
Mw = 0.18 + 9.2.10 -8 Re + 0.99log 10 (Rf) with Re in cm [AMB 88]
Ms = 1.5 + 3.1log 10 (Re) with Re in km [GAL 00]