1650
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
The β is a parameter depends on site condition which is
classified into three groups using characteristic period. For site
with longer period, which also means soft soil, β is larger.
The d
s
is depth of soil layer for liquefaction evaluation in
meters.
Where I
lE
is liquefaction index, N
i
is SPT values of layer i,
N
cri
is critical SPT value for liquefaction evaluation, d
i
is
thickness of soil layer I, Wi is weight factor of thickness.
If 0<I
lE
≤6, the site has slight liquefaction potential;
The d
w
is depth of ground water table.
If 6<I
lE
≤18, the site has moderate liquefaction potential;
The ρ
c
is clay content in percentage.
If I
lE
>18, the site has high liquefaction potential.
Using formula (4), N
cr
for all the loess ground was calculated.
Under most cases, N
cr
is larger than SPT value of the loess site.
As a result, 94% of the cases will be evaluated as liquefiable.
This creates significant exaggeration of loess liquefaction.
Hence, the loess ground has liquefaction potential should be
relatively loose and SPT value should be lower than average,
the evaluation based o approached recommend in Chinese
seismic design code is contradictive and cannot be apply for
loess ground without modification.
6 CONCLUSIONS
From this study, the following conclusions can be drawn:
1) Loess liquefaction has its unique characteristics, which is
larged determined by porous and weak cementation of loess
microstruce.
2) To discriminate liquefaction of loess liquefaction based on
triaxial test, the purpose is to make sure that loess show “sand-
like” behavior. Since the microstruce of loess collpase and pore
water could fill the enclosed pores, the pore water pressure
increase during loess liquefaction often below than confining
pressure.
5.3 Modified approach for detailed evaluation of Loess
liquefaction
To reflect the fact that SPT value of saturated loess is generally
less than that of sand, a simple modification is to reduce N
0
for
loess site. After consultation with engineers, the recommended
reference SPT for liquefaction of loess ground adopted by
Specification for Seismic design of Buildings of Gansu
Province is given as Table 2.
3) Consider the effection of isotropic and anisotropic
consolidation, certiera for discrimianting of loee liquefaction is
given by combined a residual strain and a minimum pore
pressure ratio.
Table 2. Reference SPT value for liquefaction evaluation of loess
ground (Adopted by Specification of Seismic Design of Buildings of
Gansu Province, 2012).
4) For preliminary evaluation of loess liquefaction, the age and
clay content should be used. For loess, the range of clay content
is higher, clay content limit for loess liquefaction is raised by
2%.
5) The approach for detailed liquefaction evluation
recommended in Chinese seismic design code is still useful for
loess liquefaction evaluation. But since the SPT value of
saturated loess is less than that of sand, N
0
is reduced to reflect
the fact and avoid exaggeration of loess liquefaction.
PGA(g)
0.10
0.15
0.20
0.30
0.40
N
0
7
8
9
11
13
7 ACKNOWLEDGEMENTS
Except for PGA of 0.10g, N0 for loess liquefaction evaluation is
reduced by 2 to 6 to avoid exaggeration of loess liquefaction
potential.
This study is supported by Chinese National Science
Foundation (No. 50978239).
Incorporate the modification of N0, loess liquefaction
evaluation results shows that the number of loess ground
evaluated as liquefiable decreased, the percentage of loess site
evaluated as liquefiable and mean N
cr
is given as Table 3.
8 REFERENCES
Ishihara, K., Okusa, S.,. Oyagi, N. and Ischuk, A. (1990): Liquefaction-
induced flow slide in the collapsible loess in Soviet Tajik, Soils and
Foundations, 30(4), 73-89.
Table 3. Percentage of liquefiable loess site and mean Ncr using the
modified approach for liquefaction evaluation of loess ground (Adopted
by Specification of Seismic Design of Buildings of Gansu Province,
2012).
Wang L. M., Liu H. M., et al. (2000): Mechanism and characteristics of
liquefaction of saturated loess, Chinese Journal of Geotechnical
Engineering, 22(1), 89-94.
Wang L. M., et al. (2003): Loess dynamics, China Seismological Press,
Beijing.
Hwang H., Wang L., Yuan Z. (2000): Comparison of liquefaction
potential of loess in Lanzhou, China and Memphis, Soil Dynamics
and Earthquake Engineering, 20(5-8), 389-395.
PGA(g)
0.10 0.15
0.20
0.30
0.40
Percentage of
liquefiable site
19
25.8
48
71
84
Mean of N
cr
4.5
5.2
5.8
7.1
9.1
Bai M.X., Zhang S.M., (1990): Movement of liquefied loess ground
under high seismic intensity, Geotechnical Investigation and
Surveying, 22(6), 1-5.
With the modification , the N
cr
calculated is more reasonable.
The percentage of loess ground regarded as liquefiable is only
19% under ground motion of 0.10g, and 84% under ground
motion of 0.4g.
Liao S.X., Cheng J.H., (2007): Some instances of vibration liquefaction
of loess field, Northwest Journal of Seismology, 29(1), 54-57.
Yuan Z. X., Wang L. M., Yasuda S., Wang J. (2004): Further study on
mechanism and discrimination criterion of loess liquefaction,
Earthquake Engineering and Engineering Vibration, 24, 164-169.
Prakash S., Guo T., Kumar S. (1998): Liquefaction of silts and silt-clay
mixtures, Geotechnical Earthquake Engineering and Soil
Dynamics, 1, 337-348.
5.4 Calculating of Liquefaction Index
Liquefaction index is an indicator of possibility of loess
liquefaction under different ground motion effect. The
liquefaction index is determined by formula (5)
Seismic Design Code for Buildings for Lanzhou Urban Area (2007):
Gansu Province Construction Department, Gansu Province
Quality and Technology Supervision Bureau, 8-15.
Specification for Seismic Design of Gansu Province (2012): Gansu
Provincial Department for Housing and Construction, Gansu
Province Quality and Technology Supervision Bureau, 31-32
1
[1 ]
n
i
lE
i i
i
cri
N
I
dW
N
(5)
