1420
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
ground improvement and by the difference in the dates of
reclamation work. Such causes of extensive damage are, no
doubt, correct. However, sufficient explanations have not been
provided yet concerning the mechanism of liquefaction
occurrence in ground with large fine fraction content and the
reason why the liquefaction damage was nonuniform.
Figure 2 shows the geological profile of Urayasu City along
survey line A-B-C. Starting from the ground surface, the
stratum organization broadly consists of reclaimed soil, alluvial
sand, alluvial clay, and diluvial deposits, in that order. The
reclaimed soil layer is nonhomogeneous, consisting of a
complex mixture of sandy and clayey soils. The alluvial sand
layer contains silty sand mainly made up of fine particles, the
N-value being about 10 to 20. The alluvial clay layer is very
weak with an N-value of approximately 0 to 1. Looking at the
boundaries of the strata, it can be seen that the boundary
between the alluvial sand and alluvial clay layers is almost
horizontal, whereas that between the alluvial clay layer and the
diluvial layer slopes downwards from Location A (land side,
older reclaimed land) towards Location C (sea side, newly
reclaimed land). Thus, the alluvial clay deposit is thicker
towards Location C. The alluvial clay is about 10 m thick at
Location A but extremely thick (more than 40 m) at Location C.
Considering the liquefaction damage distribution shown in Fig.
1, it can be said that liquefaction damage was light at the land
side locations, where the weak clay layer is relatively thin
(about 10 m). Progressively heavier damage occurred towards
the side of the sea along with the increase in thickness of the
weak clay layer with an N-value of nearly zero.
This paper examines the cause of the extensive and nonuniform
liquefaction damage that occurred in Urayasu City by focusing
attention on the weak clay layer and its inclination in the deep
part of the liquefied ground and carrying out elasto-plastic
seismic response analysis of the multi-layer ground. The
analysis code used was the soil-water coupled finite
deformation analysis code GEOASIA (Noda et al. 2008), which
incorporates an elasto-plastic constitutive model (SYS Cam-
clay model; Asaoka et al. 2002) that allows description of the
behavior of soils ranging from sand to intermediate soils and
clay under the same theoretical framework.
2 DEPENDENCY OF SEISMIC BEHAVIOR OF GROUND
ON THE ORGANIZATION OF DEEP STRATA
The effect of the weak clay layer in the deeper part of the
ground on the reclaimed soil (silty sand containing fine fraction)
was investigated using a one-dimensional model of locations A,
B, and C in Fig. 2. Location B is midway between locations A
and C. The finite element mesh used in the analysis and the
stratum organization at these three locations are shown in Fig. 3.
The water pressure at the hydraulic boundary was made to be
zero so as to make the ground coincide with the water level, and
allowing for the existence of an impermeable layer with low
hydraulic conductivity, the bottom face was assumed to be an
undrained boundary. The two side faces, too, were assumed to
be undrained boundaries. In addition, for defining the cyclic
boundary (Noda et al. 2010) on the assumption that the same
ground extends infinitely to the left and right sides, equal
displacements were assigned as the constraint condition to each
nodal element at the same height on both side faces. Table 1
shows the material constants and the initial values used in the
analysis. Detailed soil surveys are still ongoing in Urayasu City.
Therefore, the material constants used in this study were those
of soils studied in the past at Nagoya University, which had
physical properties relatively similar to the soils at the site. The
reclaimed layer, which is assumed to be intermediate soil that is
a mixture of sand and clay, is a material that is less prone to
liquefaction than sandy soil. With respect to the initial values, it
was assumed that the specific volume, degree of structure, stress
ratio, and degree of anisotropy were uniform in the direction of
depth. The overconsolidation ratio was distributed based on the
overburden pressure. In locations A, B, and C, the conditions of
5m
5m
40m
10m
5m
5m
25m
25m
5m
5m
10m
40m
Reclaimed soil
Alluvial sand
Alluvial clay
Diluvial soil
A B C
1m
×
1m
×
60
0
100
200
300
-50
0
50
Time (sec)
Acceleration (gal)
10
-1
10
0
10
1
0
50
0
100
200
300
-50
0
50
Period (sec)
Time (sec)
FourierAmplitude (gal*s)
Acceleration (gal)
Input seismic wave
Figure 3. Finite element mesh and stratum organization at points A to C
and input seismic wave
Table 1 Material constants and initial values used in the analysis
Dilluvial
deposit
Alluvial
Clay
Alluvial
Sand
Reclaimed
Sand
Elasto-plastic parameters
Critical state index M
1.00
1.60
1.00
1.70
NCL intercept N
2.10
2.50
2.00
2.50
Compression index
~
0.20
0.40
0.20
0.15
標高
TP(m)
0.00
0.00
0.00
0.00
0.00
0.00
0.00
-10.
-20.
-30.
-40.
-50.
-60.
0.
B
地点
A
地点
沖積粘土
0 25 50
沖積砂
埋土
0 25 50
0 25 50
0 25 50
0 25 50
標高
TP(m)
A
B
C
No-liquefaction
Liquefaction
Diluvial layer
Alluvial
clay
Alluvial
sand
Reclaimed
soil
Land side (northwest)
Sea side (southeast)
0
- 0
0
-30
0
0
0
-1
-2
-3
-4
-5
-6
Figure 2. Geological profile along line of measurement A-B-C in
Urayasu City
Swelling indes
~
0.0001
0.010
0.020
0.008
Poisson’s ratio
0.10
0.10
0.35
0.10
Evolution parameters
Degradation index of structure
a
0.001
0.4
1.5
3.0
Ratio of
p
v
D
to
p
s
D
s
c
1.0
0.3
0.3
0.1
Degradation index of OC
m
50.0
20.0
0.7
2.0
Rotational hardening index
br
0.0001
0.001
0.5
0.01
Limit of rotational hardening
b
m
1.0
1.0
0.7
1.0
Initial conditions
Specific volume v
1.70
3.30
2.30
2.90
Stress ratio
0
0.545
0.545
0.545
0.545
Degree of structure
0
*
/1
R
10.0
20.0
15.0
10.0
Degree of anisotropy
0
0.545
0.545
0.545
0.545
Soil particle density
s
(g/cm3)
2.65
2.65
2.65
2.65
Mass permeability index
k
(cm/s)
1.0
×
10
-6
5.0
×
10
-7
5.0
×
10
-5
1.0
×
10
-5
