Actes du colloque - Volume 2 - page 768

1648
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Figure 1. Development of excess pore pressure, deviatoric stress, axial
train of saturated undisturbed loess under the anisotropic consolidation.
s
In Fig.1, 3% axial strain occurs before the biggest value of
excess pore pressure. Before 3% axial strain, the axial strain and
excess pore pressure increase slowly; after 3% axial strain, the
excess pore pressure still increases slowly and the axial strain
increases largely.
Under anisotropic consolidation, the excess pore pressure
cannot reach the consolidation pressure when the loess liquefied
after anisotropic consolidation. For a cyclic loading, the
changes of the excess pore pressure can increase or decrease.
Only when one cycle finishes, the excess pore pressure can have
a pure increase. When the excess pore pressure accumulates to a
certain level, it can reach the effective consolidation pressure,
i.e., instant liquefaction. Under isotropic consolidation, the
instant liquefaction occurs when the deviatoric stress reaches
zero. While under anisotropic consolidation, the deviatoric
stress can be divides into two parts: one is deviatoric stress
applied during consolidation; another is axial stress applied
during vibration. The vibration causes the volume of voids
decrease. The water in the voids cannot discharge in time,
which leads to the increase of the excess pore pressure, sharp
decrease of the effective stress applied on the soil skeleton. The
loss of the soil strength makes the application of the outer axial
stress impossible. While another part stress, deviatoric stress
applied during consolidation has been applied to the sample all
the time. When the deviatoric stress reaches the deviatoric
consolidation stress, the soil has been liquefied. It means that
the excess pore pressure can never reach the effective
consolidation pressure because of the presence of the deviatoric
consolidation stress.
3.2 Criteria for loess liquefaction based on triaxial test
As test results shows that for saturated loess, when the axial
strain increases to reach 3%, the development of residual strain
increase sharply, which indicates a substantial change of loess
structure, or to say that the loess structure collapses at this
juncture. Because the collapse of the loess structure, pore water
enters some of the previously enclosed pores and prevent the
excess pore pressure from reach the effective consolidation
pressure if the loess structure collapse created enough pores to
be filled with pore water. Wang L. M. et al. pointed out that 3%
is the value of the axial strain of the structural damage for loess
(Wang, 2003).
Based on the triaxial test study on the behavior of the
undisturbed and remolded loess, the criteria for the
discrimination loess liquefaction using triaxial test data are
given as follows:
i) Under the isotropic consolidation,
%3
d
&
4.0
f
d
u
u
(1)
1
f
d
u
u
(2)
Of the two criteria, the one which firstly reaches, would be
adopted.
ii) Under the anisotropic consolidation,
%3
d
&
2.0
f
d
u
u
(3)
As discussed above, 3% axial strain is the structural damage
value of the loess. And the increase of the pore pressure ratio of
loess during liquefaction is mostly determined by two factors:
one is degree of saturation and the other is structure strength of
loess microstructure. To reach the status of liquefaction, pore
pressure ratio is generally lager than 0.4 for isotropic
consolidation and 0.2 for anisotropic consolidation. The latter
has a lower pore water pressure because the structure collapse is
more significant for anisotropic consolidated samples. The
higher strength of loess microstructure, the longer the
continuous buildup of pore water pressure, usually, loess with
more sand content will reach higher pore pressure ratio during
liquefaction test.
When residual strain is used to evaluate liquefaction
potential of loess, pore pressure ratio must be used at the same
time. Only there is clearly indication of pore water buildup,
loess shows a “sand-like” characteristics of liquefaction, if there
is no significant buildup of pore water pressure, the
development of larger amount of residual strain can only be
regarded as “clay-like” cyclic soften and cannot be classified as
liquefaction .
4 PRILIMINARY EVALUATION OF LIQUEFACTION
POTENTIAL OF LOESS
4.1 Loess Liquefaction evaluation in Code for Seismic design of
buildings in Lanzhou Urban area
To minimize the effect of loess liquefaction, evaluation of loess
liquefaction is the first step for treatment of loess ground to
reduce or eliminate liquefaction potential. Lanzhou is located in
Loess Plateau and in recent decades, the urban area of Lanzhou
expands very quickly. Since the national “Code for Seismic
Design of buildings (GB50011-2001)” does not reflect
adequately all problems concerning seismic safety in Lanzhou.
In 2006, the Provincial Department of Construction entrusted
Provincial Committee on Architecture Sciences and Lanzhou
Institute of Seismology to draft new code for seismic design of
buildings for Lanzhou Urban Area (Lanzhou Code). Wang L.M
et al. applied the research results into practices through drafted
Lanzhou Code, which is the first local engineering specification
for the discrimination and treatment of loess liquefaction.
In Lanzhou Code, the concept of performance-based design
was incorporated in a cautious way. That is, although
performance based recommendations are made for ground
treatment for buildings of different importance under different
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