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Performance-based Evaluation of Saturated Loess Ground Liquefaction
Évaluation des risques de liquéfaction d’un Loess saturé
Wang L.M., Yuan Z.X., Wang Q., Wu Z.J.
Lanzhou Institute of Seismology, China Earthquake Administration, Lanzhou, China, 730000;
Loess Earthquake Engineering Key Laboratory of China Earthquake Administration, Lanzhou, China, 730000
ABSTRACT: This paper reviewed the characteristics of loess liquefaction. Based on triaxial test, criteria for loess liquefaction is
proposed. Later, Liquefaction evaluation procedure for both preliminary and detailed evaluation of loess liquefaction is modified for
evluation of loess liquefaction. These modifications are based on study of liquefaction potential of loess and field test data, which now
is adopted by Specification for Seismic Design of Buildings of Gansu Province, China. This progess marks a new milestone for study
and treatment of loess liquefaction.
RÉSUMÉ :
Cet article analyse les caractéristiques de liquéfaction d’un loess. A partir de l’essai triaxial, les critères de liquéfaction
du loess est proposé. Ensuite, la procédure d'évaluation de liquéfaction pour l'évaluation préliminaire et détaillée de la liquéfaction du
loess est modifiée pour évaluer la liquéfaction du loess. Ces modifications sont fondées sur l'étude du potentiel liquéfaction du loess et
des données d’essai in situ. Cette procédure est maintenant adoptée dans les recommandations pour la conception parasismique des
bâtiments de la province du Gansu, en Chine. Cette avancée marque une nouvelle étape pour l'étude et le traitement de la liquéfaction
du loess
.
KEYWORDS: loess, liquefaction, performance-based design, seismic code.
1 INTRODUCTION
Loess is a problematic soil in terms of its property in civil
engineering. Studies revealed that saturated loess ground has
high potential of liquefaction (Ishihara 1990, Wang 2000 &
2003, Hwang and Wang 2000) under effect of earthquake
ground. During the Great Haiyuan Earthquake of 1920 in China,
a sandy layer of loess liquefied and triggered flow of nearly
horizontal loess ground with maximum displacement of nearly
1.5km (Bai & Zhang, 1990). During the 5.5 magnitude
earthquake occurred in Tajikistan in 1989, liquefied loess layer
turned into mud flow and buried one village.
Through series studies carried out by Wang L.M. et al. the
liquefaction of loess is better understood and recognized by
more and more professionals in China. The cases in China and
Tajikistan show that it surely exists and could be disastrous.
Liao S.X also reported the loess liquefaction caused by
vibration during ground treatment of loess ground (Liao 2007).
In order to properly treat liquefaction potential of loess
ground, it is important to establish proper procedures for
evaluation of loess liquefaction both in laboratory and in the
field.
2 THE CHARACTERISTICS OF LOESS LIQUEFACTION
Liquefaction of loess is a flow type of liquefaction. That is,
under the effect of cyclic loading, the pore water pressure in
loess rises, effective stress is down and large amount of residual
strain develops, finally, loess loses its strength and became mud
flow. The amount of residual strain could easily reach 10%
during liquefaction test of loess.
Loess has the low permeability; pore water movement is
quite uneven at different parts of the sample, most of the pore
water running through the channels composed with the round
holes and channels connecting or around them. For sand
liquefaction, it is the result of densification of sand structure
which forced some portion of water move out from the skeleton
and even rise up. But for loess, because of its low permeability,
water cannot move as freely as it is in sand. As a result, the pore
water pressure build up applies shear stress to loess structure
and causes it to collapse. The collapse and consequentially, the
development of larger amount of residual strain are because of
loess structure is porous with void ratio usually more than 1.
3 CRITERIA OF LOESS LIQUEFACTION BASED ON
TRIAXIAL TEST
Due to the characteristics of loess liquefaction, there is need to
establish test criteria to assess liquefaction potential. Laboratory
test using triaxial, it is found that because of the pore
microstructure, weak cementation among particles and the
hydraulic sensitivity of the loess, the air in the enclosed void is
difficulty to discharge. Usually, the degree of saturation ranges
from 0.8 to 0.95.
3.1 Loess liquefaction test under isotropic and anisotropic
consolidation
Generally, under the isotropic consolidation, the excess pore
pressure of the saturated undisturbed and remolded loess can
reaches the effective consolidation pressure. When the axial
strain is less than 2%, the structure of the loess is relatively
stable and the excess pore pressure increases continually; when
the axial strain reaches 3%, the excess pore pressure increases
obviously and the deviatoric stress decreases obviously; when
the axial strain is more than 3%, the axial strain increases
largely, the increase of the excess pore pressure is slow and can
be negative value, and the soil is destroyed.
Figure 1 is development curve of the excess pore pressure,
deviatoric stress and axial strain of undisturbed loess with
loading cycles under the anisotropic consolidation. The
effective consolidation pressure is 130kPa and the consolidation
ratio is 1.2.
