Journalof Applied
Applied Mechanics Vol.13
Vol.13, (August
pp.453-462
Journal of2010) （August 2010）

JSCE
JSCE

Simulation of reclaimed ground composed of dredged soil for two kinds ofearthquake ground motions with different spectral characteristics
Kai Sun*, Eiji Yamada**, Masaki Nakano*** and Akira Asaoka***** Member M. of Eng. Doctoral student Dept. of Civil Eng. Nagoya Univ. (Chikusa, Nagoya 464-8603)**Member Dr. of Eng. Research Engineer, Chubu Electric Power Company, Incorporated(20-1,Kitasekiyama, Ootaka, Midori, Nagoya, 459-8522)***Member Dr. of Eng. Prof. Nagoya Univ. (Chikusa, Nagoya 464-8603)****Member Dr. of Eng. Senior Research Advisor, Association for the Development of Earthquake Prediction(1-5-8, Sarugaku, Chiyoda, Tokyo, 101-0064)
In this research, the co-seismic and post-seismic behaviors of an offshore artificial island
composed of dredged soil under two kinds of earthquake ground motions, which areshort-period (Type 1) and long-period (Type 2), are investigated by using a soil-water coupledfinite element analysis codeʊGEOASIA. The parameters used in the calculation are obtained by
the simulation of a series of mechanical tests using the SYS Cam-clay model. In the analysis,
the construction history was considered by way of simulating the construction of embankment
and reclamation of ground. Compared with the short-period motion, different results are
obtained under the long-period motion: 1) larger deformations during the earthquake, bothvertical settlement and horizontal displacement, are caused by long-period motion due to thelarger shear strain, 2) in some part of the ground, larger post-seismic settlement caused bylong-period motion due to the decay of soil structure, and 3) in the soil element with significant
shear strain, volume expansion is observed during earthquake, which is usually regarded as an
undrained simple shear process. For clay ground, the settlement after earthquake lasts for longtime and the amount of settlement is large.
Key words: numerical analysis, seismic response, reclaimed ground, construction history

1.

Introduction

The Port of Nagoya is a shallow-depth artificial harborthat has been constructed through repeated dredging andlandfilling. The dredged soil in the Port of Nagoya of
about 1.3 million m3 is generated per year. An offshore
artificial island that serves as a temporary disposal for
dredged soil is located at the entrance of the port.
However, the predicted Tokai-Tonankai Earthquake will
occur around the Chubu Districtof Japan in the near
future. It is very important to know whether the functionof the Port of Nagoya can be maintained after the
Earthquake, and to evaluate the co-seismic andpost-seismic responses of an artificial island due to thehigh possibility of major earthquake in Nagoya region.
Recently, elaborate finite element approaches1)-4) havebeen used to analyse the seismic behaviors of thereclaimed ground. In the past analysis, the constructionof structure, which greatly affects the loading history andthe current state distribution of the soil, was seldom
considered. Moreover, for the reclaimed groundcomposed of dredged soil, it is difficult to describe thesoil properties by a constitutive model as well as tocalculate the behavior of the soil ground during /after theearthquake.

In this study, seismic response analysis on an offshore
artificial island mainly composed of dredged soil, iscarried out with the aim of extracting the problems that
may arise in the case that subduction zone earthquakes
occur. Two types of ground motions with different
spectral characteristics are then applied on the rigid
engineering bedrock respectively. Subsequently, thebehaviors of the reclaimed ground during and after theearthquake under plane strain condition are investigatedby using the static/dynamic soil-water coupled finite
element analysis code GEOASIA5), 6). In the analysis code,
the SYS Cam-clay model7)-9) is used as a constitutive
equation for the soil skeleton. It is available in a wide
range, from clay through intermediate soil to sand within
a single theory system merely by controlling the rate ofloss of overconsolidation, decay of structure anddevelopment of induced anisotropy. The validity of
performance of GEOASIA has been verified through thedeformation and failure analysis not only under dynamic
loading but also under static loading. More details are inreferences10), 11).
In order to obtain the current state distribution of thesoils by considering the stress history, the construction
process, including the construction of embankment andreclamation of dredged soil, is reproduced according to- 453 -

As shown in Fig. 1, in order to accelerate theconsolidation of clay layer, an area in AcU1 layer and
AcU2 layer was improved by sand drain, on which asand-mound of 6 m, a rubble-mound of 4 m and aconcrete retaining wall of 6 m were then built in order.
The sand drain area was 68 m long and 12 m deep. All of
three embankments built on the reclaimed ground were 2
m high. The sea level was set to be 10 m above the top
surface of the seabed ground. The bottom of the groundwas set to be rigid engineering bedrock in the analysis.

2. Determination of soil parameters of seabedground and reclaimed ground
2.1 Soil layers distribution
This research focuses on the mechanical behaviors of
the artificial island composed of a reclaimed ground of
dredged soil and a seabed ground. The seabed groundmainly consists of naturally deposited clay and silty clay.
The distribution of soil layers and the structures on theisland are shown in Fig. 1. Based on the groundinvestigation, the initial ground is assumed to be
horizontally homogeneous and can be divided into six
layers, from bottom to top, which are an alluvial clay
layer (AcL layer), an alluvial sand layer (As layer), three
alluvial clay layers (AcU3 layer, AcU2 layer and AcU1
layer) and a reclaimed layer of dredged soil (Bc layer),respectively. The thickness of them is 7.5 m, 2.5 m, 10 m,
10 m, 6 m and 20 m, respectively and the total is 56 m.
Embankment

Retaining wall
Sea level

Bc
AcU1

Sand drain

AcU2
AcU3
As
AcL

Engineering bedrock

AcU3

200

Bc
AcU1

100
Bc
0

10
Axial strain Ha(%)

20

300
200

q = Mpc

Specific volume v (=1+e)300

Deviator stress q (kPa)

Deviator stress q (kPa)

Fig. 1 Distribution of soil layers and structureson the artificial island

AcU3

Bc
AcU1

100
Bc
0
100
200
Mean effective stress300
pc(kPa)

2.2 Determination of soil parameters
The soils in seabed ground and reclaimed ground are
regarded as natural deposited soils and exhibit
complicated mechanical behaviors that differ from that ofremolded soil. For example, naturally deposited clay in anormally consolidated state commonly exhibits softening
in undrained shear tests or “rewinding” in a heavily
overconsolidated state12)-14). For this reason, in this study,a series of laboratory tests were carried out on undisturbed
clay, and then the behaviors were simulated by using the
SYS Cam-clay model.
Firstly, the material constants (elasto-plastic parametersand evolutional parameters) for each layer are determinedby undrained triaxial compression tests and oedometer
tests. Figure 2 shows the simulation results of triaxial test
and oedometer test by using the SYS Cam-clay model. In
this figure, the bold lines and points are the tests results,
and the thin lines the simulation results. In the triaxial test,
as shown in Fig. 2(a), each specimen was subjected to an
undrained compression under a controlled axial strain rateof 0.0875%/min after 24 hours isotropic consolidation. As
shown in Fig. 2(a), the simulation of triaxial test also
started from isotropic consolidation and then reproduced
shear process. In Fig. 2(b), the specimens were applied onstaged load in accordance with the experiment standard of
JIS Designation A 1217: 2000. In the simulation ofoedometer tests, for each kinds of soil, same material
constants (elasto-plastic parameters and evolution
parameters) were used as same as those of triaxial tests14).
It can be found that the simulation results can basicallydescribe the mechanical properties of the soils by using
the SYS Cam-clay model. Due to the similarity ofphysical properties, all of the clay layers (AcL, AcU1,
AcU2 and AcU3) and dredged soil layer (Bc) are regarded
as same material and has same material constants,
although the initial values are different. The material

3.2
2.8

Bc
AcU1

2.4

NCL
Bc

2.0
AcU3

1.6

0
100
200
Mean effective stress(a) Undrained triaxial compression test

CSL
300
pc(kPa)Specific volume v (=1+e)

the actual construction history by using the analysis code
GEOASIA5), 6). A series of laboratory tests results andground survey data are used to determine the material
constants and the initial values (before construction) byusing the SYS Cam-clay model7)-9).
For the ground mainly consists of naturally deposited
clayey soil, attention should be paid to large settlementof clay layer during/after earthquake and large horizontal
displacement near the retaining wall. The analysis resultsof the island, including the pre-seismic (construction andconsolidation processes), co-seismic and post-seismic
mechanical behaviors, are interpreted in the theory frameof the SYS Cam-clay model based on the concept of soil
skeleton structure (structure, overconsolidation andanisotropy).

3.2

Bc

2.8

AcU1

2.4

NCL

Bc
2.0
CSL

1.6
1

10

2

(b) Oedometer test

Fig. 2 Simulation results of triaxial test and oedometer test by using the SYS Cam-clay model- 454 -

3

4

10
10
10
Vertical stress ǻv (kPa)Material constants
Clay

Sand

Compression index O

0.18

0.05

Swelling index N~

0.019

0.0002~

Critical state constant M

1.6

1.1

Intercept of NCL at p c =98.1kPa N

2.22

1.985

Poisson’s ratio Q

0.3

0.3

Evolution parameters

Clay

Sand

Degradation parameter of overconsolidation m

3.0

0.12

Degradation parameter of structure a

0.3

5.0

Degradation parameter of structure b

1.1

1.0

Degradation parameter of structure c

1.0

1.0

 Dvp

D sp

cs

0.4

1.0

Rotational hardening index br

0.001

3.0

Rotational hardening limit constant mb

1.0

0.9

Ratio of

and

400
332
242
194
153

200
73

AcL
AcU3
AcU2
AcU1

Bc
Bc
Bc
Bc
10
Axial strain Ha(%)

57

51
46

0
0

Fig. 3[Initial state]

–15m
–20m

2.4 2.4
2.2

2.45 AcU2
2.36
AcU3
2.3
2.27
AcL
2.2
2.18

CSL

NCL

0
200
400
Mean effective stress pc(kPa)[After constructionof reclaimed island]

AcU1

AcU2

–25m
–30m

AcU3

–35m

Measurment
Calculation

As
–40m

D sp : Derivative component of plastic sketching tensor
Dvp : Mean component of plastic sketching tensor

20

2.8 2.82.78
2.75 Bc
2.7
2.6
2.5 AcU1

Simulation results of unconfinedcompression tests–10m

Elevation (m)Elasto-plastic parameters

Specific volume v ( =1+e )Table 1from the initial state to consolidation of the clay layer inthe ground by landfilling (one-dimensional consolidation)
ψ sampling (unloading with constant volume) ψunconfined compression test.
Based on the results of previous ground surveys, layer
Ac was divided into 4 different layers (AcU1, AcU2,
AcU3, and AcL). The simulation results of unconfinedcompression tests are shown in Fig. 3 and the comparison
results with boring data in 2005㨪2007 are given in Figs.
4 and 5. Considering that the measured data in Figs. 4 and5 obtained from many different areas in 2005̚2007, it
indicates that the initial conditions have been determined
appropriately and is available for the following
calculation. In addition, the initial values of sand layer(As) were obtained from the past study15). The initialvalues are shown in Table 2. For clay layers, the initial
specific volume decrease from upper layer (AcU) to lower
layer (AcL). And due to one dimensional consolidation,
Compression stress V (kPa)

constants of sand layer (As) were obtained from the past
study15). The material constants of clay and sand arelisted in Table 1. According to the relation between
specific volume and mean effective stress (in Fig. 2(a))
or vertical stress (in Fig. 2(b)), all of initial specificvolume of the clay lied beyond of the normal
consolidation line, which correspond to the so-called“impossible stress state” with high degree of structure.
In the SYS Cam-clay model, the evolution parameters
are used to describe the loss of overconsolidation, decayof structure and development of induced anisotropy. As
shown in Table 1, the degradation parameter ofoverconsolidation m for clay is larger than that of sand,which means that clay is easier to lose overconsolidation.
And decay of structure is described by degradation
parameters of structure a, b, and c. For sand, it is easier

AcL

–45m
1.5 2.0 2.5 3.0 3.5 1.5 2.0 2.5 3.0 3.5 0 100 200 300 400v0 ( =1+e0 )v ( =1+e )qu (kPa)- 455 -

Fig. 4 Simulation and survey results of thesoil in seabed ground[Initial state]
15m
10m
Elevation (m)to decay structure compared with clay. Details about the
SYS Cam-clay model are in the references7)-9).
Secondly, the initial conditions for each layer are
determined by simulating unconfined compression tests
with the target specific volume and unconfinedcompression strength from ground survey. Since theprocess of landfilling was taken into account in this study,
the initial condition of the ground was set to be the state
before the reclamation work. Because the results ofunconfined compression tests of the clay layers sample
after reclamation were available, the initial conditions
were configured so as to reproduce the unconfinedcompression strengths. It was done by assuming theinitial conditions of the ground and using the SYS
Cam-clay model to calculate the following processes

5m
0m
–5m

[After constructionof reclaimed island]
Measurment
Calculation

Bc
Bc
Bc
Bc

–10m
1.5 2.0 2.5 3.0 3.5 1.5 2.0 2.5 3.0 3.5 0 100 200 300 400v0 ( =1+e0 )v ( =1+e )qu (kPa)

Fig. 5 Simulation and survey results of thesoil in reclaimed groundTable 2
Initial value

Initial conditions of the soils

AcU1

AcU2

AcU3

AcL

Bc

As

Degree of structure 1 / R0

9.0

7.0

5.0

4.0

20.0

2.0

Overconsolidation ratio 1 / R0

10.8

3.8

2.4

2.2

5.7

12.0

Specific volume v0(=1+e0, e0: Initial void ratio)

2.50

2.40

2.30

2.20

2.80

1.84

Stress ratio K 0

0.23

0.23

0.23

0.23

0.0

0.23

Degree of anisotropy 9 0

0.23

0.23

0.23

0.23

0.0

0.23

*

Permeable boundary
10m

Sea level

36m

Viscous boundary (Vs=300 m/sec)
Impermeableboundary

Permeable boundary

Fig. 6020

Vcv(kPa)

300 600 0

1650 mImpermeableboundary

Finite element mesh for initial seabed ground and boundary conditions

3

u (kPa)pc (kPa)K = q / pc
v ( =1+e )Jt(kN/m )300 600 0 300 600 0 0.5 1 1.5 2.0 2.5 3.0 0 10 20 30 0w (%)
50 100 0

R
0.5

1 0

R*
0.5

]
1 0

0.5

1

Elevation (m)Sea level
0
–20
–40

Fig. 7 Initial conditions of the soils in seabed ground

Sea level

Sand drain

Reclaimed ground

Initial state

After 12 years

(d)

(a)
Sand mound

After 21 years

Rubble mound

(e)

(b)
Retaining wall

After 34 years

After 2 years

(c)

(f)

Fig.8

Specific volume distribution of ground near the retaining wall in the construction and reclamation process

the lower layer has smaller initial degree of structure( 1 / R0* ) and overconsolidation ratio ( 1 / R0 ). For the

newly reclaimed layer (Bc), the initial degree of structureis higher. Compared with clay, the sand (As) with smaller- 456 -

1 / R0* and larger 1 / R0 is in low structured and heavily
overconsolidated state.

3. Calculation of the landfilling process of thereclaimed ground before the earthquake

Fourier amplitude (gal s)

4.1 Two kinds of earthquake ground motions
In the calculation, two kinds of earthquake groundmotions, which are short-period ground motion22) (Type 1)
and long-period ground motion23) (Type 2), arehorizontally applied on the nodes of ground base to
analyze the seismic responses of the island. The
information about the input ground motions is shown in
Fig. 9, in which the upper figures are the acceleration andthe lower figures are the corresponding relation between
period and Fourier spectrum. Compared with Type 1
motion, the duration of Type 2 is longer and its maximum
acceleration is relative higher. Moreover, the prominent

150 s Max. 122 gal

0
–200
0
10

4

10

3

10

2

10

1

10

0

10

60

120
Time (s)

180

–1

0

10
Period (s)

200

10

90 s

(a) Type 1

10

4

10

3

10

2

10

1

10

0

10

60

120
Time (s)

180

240

–1

10

–1

0

10
Period (s)(b) Type 2

Fig. 9 Two kinds of input ground motions with different spectral characteristics- 457 -

Max. 156 gal

–200
0

1

150 s

0

240

–1

10

Co-seismic and post-seismic analysis

Acceleration (gal)

90 s

200

4.

Fourier amplitude (gal s)

Acceleration (gal)Figure 6 shows the finite element mesh and boundary
conditions of the initial seabed ground before
construction and reclamation. The ground is assumed tobe horizontally homogenous. And the sea level was set
10 m higher than the top surface of the initial seabedground, which means that the soil in seabed ground is in
fully saturated state. The surface of the seabed ground isconsidered to be drained condition. And the bottom
boundary is also a drained boundary, taking into accountof the high permeability of bedrock. The boundary
condition which gives simple shear deformation to thefinite elements of a side boundary16) was set to bothlateral sides of the ground. The bedrock was made as theengineering base with the shear wave velocity of Vs=300
m/s, which is defined as the standard value in the port
and harbor construction. The bedrock is dealt withviscous boundary17), 18).
The initial conditions of the soils in the seabed groundare shown in Fig. 7. Based on the initial conditions listed
in Table 2, it was assumed that the stress ratio K , specificvolume v, degree of structure 1 / R * and degree ofanisotropy ] of the elements in each layer were
uniformly distributed along the depth. The
overconsolidation ratio 1/R remained constant withineach layer, the initial values remain constant.
According to the history of Port of Nagoya19), theconstruction and reclamation processes were reproduced
and then the current state of the soils in the island wereobtained. Figure 8 shows the change of specific volume
during the processes of construction of retaining wall andground reclamation. Assuming saturated conditions, thereclaimed ground made up of dredged soil was
considered to be a two-phase elasto-plastic material, andthe simulation was performed by applying the method offinite element addition to the initial ground20), 21). As

shown in Fig. 8(a), a part of clay in AcU3 layer and AcU2
layer was replaced by sand drain to improve the drainage
condition before the construction of upper structure. And
then in the following two years, as shown in Figs. 8 (b)and (c), a sand mound, a rubble mound and a concrete
retaining wall were built on the replaced area in order. The
following works were the embankment construction andground reclamation. The dredged soil was firstly
reclaimed onto the seabed ground. When the height ofreclaimed land reached the top surface of embankment, a
new embankment was built on the surface of newly
reclaimed ground to accommodate more dredged soil. As
shown in Figs. 8 (d)-(f), the processes of embankment
making and landfilling repeated in the past 34 years anduntil now the total height of the ground is 20 m from the
top surface of the seabed ground. The construction andreclamation processes were carried out exactly as same as
actual schedule of the artificial island. By this means, thecurrent state of the soils in the island was obtained andthen was set as the referenced initial condition for thefollowing seismic analysis. In the calculation, all of thematerials, like clay, sand, sand drain and rubble wereregarded as elasto-plastic materials except the concrete
retaining wall, which was modeled using linear elastic
solid element with Young’s modulus E=23.5 MPa andPoisson’s ratio v=0.2.

101

motions in Type 2 have periods of around 2 seconds,
which show different spectral characteristic as Type 1.
The seismic response analysis was carried out 5 years
after the completion of the ground reclamation.
4.2 Co-seismic and post-seismic analysis results
In this section, special attention was firstly paid to theground near the retaining wall, where large shear strain
was observed. Figure 10 shows the shear strain
distribution of this area with time during and after theearthquake. The state of the artificial island at five years
after the final construction and reclamation was set as theinitial state in the seismic analysis. As shown in Fig.
10(a), small shear deformation is found at the bottom ofreclaimed ground layer due to the development of one
dimensional consolidation, which suggests that the soil
in same layer were in different condition. It is therefore
necessary to reproduce the construction process andobtain the current state distribution of the soils after
construction.
The evolution of shear strain with time at 90 seconds,
150 seconds and 16 years after the occurrence ofearthquake is shown in Figs. 10 (b)-(d) respectively. In
Fig. 10, in both Type 1 and Type 2, it shows that large
shear deformation occurred in the reclaimed ground (Bclayer) compared with the sea bed ground. The shear
deformation areas firstly appeared under the embankment
1 (Emb. 1 for short) and embankment 3 (Emb. 3 for short),
and then increased with earthquake shaking and extended
to each other. In Fig. 10(b), at 90 seconds after the start ofearthquake, larger shear strain occurred under Type 1, of
which the mainshock is earlier than Type 2. However, inother stages, larger shear strains were caused by Type 2,
because of the longer mainshock duration and larger
maximum acceleration of Type 2. In Type 1, the shear
strain distribution changed smoothly with depth. By
contrast, in Type 2, especially in the Bc layer, a large
shear strained areas (from Area D to E in Fig. 10(d)) anda slide surface were observed. It suggests that the
distribution of shear strain is highly inhomogeneous in Bc
layer.
The displacement at two sections (Section A and B in
Fig. 10(a)) and the mechanical behavior of the soil
elements in three areas (Area C, D and E in Fig. 10(d))

150 mSection A

Emb.3 Emb.2

Section B
Emb.1

Retaining wall

Shear deformation(a) Before earthquake (5 years after ground reclamation completion)

Type 1

Type 2(b) 90 seconds after occurrence of earthquake
Shear band

Type 1

Type 2(c) 150 seconds after occurrence of earthquake
Area E

Type 1

Area D
Area CType 2(d) 16 years after occurrence of earthquake(All values larger than 0.5 are contoured as 0.5)

Fig. 10Distribution of shear strain of the ground near the retaining wall

- 458 -

are illustrated in the following.
(1) Horizontal displacement
The horizontal displacement at two sections (Section A
and Section B) is shown in Fig. 11. It can be found thatlarger horizontal displacements were caused by Type 2.
In Fig. 11(a), the ground surface displacement of Type 1
and Type 2 was 1.9 m and 5.1 m at Section A,
respectively. While in Fig. 11(b), the displacement ofType 1 and Type 2 was 3.9 m and 7.4 m at Section B,
respectively. The displacement at the retaining wallcaused by Type 2 is much larger than that of Type 1. In
addition, as shown in Fig. 11(a), the increase rate ofdisplacement in reclaimed ground was faster than that inseabed ground. It is because the soil in reclaimed groundis in a relative loose state (specific volume v: 2.8)
compared with the soil in seabed ground (v: 2.2~2.5). It
also can be found in Fig. 11(b), though it is not as
obvious as that in Fig. 11(a) because of the constraint
effect of the retaining wall.
(2) Settlement of the retaining wall
Figure 12 shows the settlement curve of the retaining
wall during and after earthquake. The settlement process
can be mainly divided into two stages: (1) immediate
settlement during the earthquake and (2) large settlement
after the earthquake. The first stage is from the beginningto the end of earthquake, in which the settlement during
earthquake is 0.49 m (Type 1) and 0.81 m (Type 2). The
following settlement after the earthquake is 0.58 m (TypeInitial state

40

Type 1 Type 2

40

5.1 The settlement after the earthquake
In Fig. 12, the simulation result of the settlement curve
seems to be similar to the settlement after the earthquake
often observed in clay21), 25). The responses of the elementin Area C under Type 2 motion are shown in Fig. 13. The
response of this element under Type 2 is similar to that

ReclaimedgroundType 2
20

0

0

0

2

4

6

Seabed ground

150
0

2

4

6

8

Displacment (m)Displacement (m)(a) Section A(b) Section B

Fig. 11

10

Deviator stress q (kPa)20

In this section, the macroscopical responses of theisland are explained by the mechanical behaviors of soil
elements. The elements of the seabed ground in Area C,
which is directly under the retaining wall shown in Fig.
10(d), are chosen to interpret the settlement of theretaining wall. In Fig. 10(d), the shear strain of thereclaimed ground in Areas D and E in Type 2 are muchlager than that of Type 1. Thus, the mechanical behaviorsof the elements in these three areas are investigated in thefollowing sub-sections.
In the SYS Cam-clay model, the degree of structure andoverconsolidation ratio are expressed as 1/R* and 1/R,
respectively. Both R* and R vary in the range from 0 to 1.
The closer R* to 0 the higher the degree of structure while
the soil changes into remodeled state (no structure) whenR* equals to 1. Similarly, the closer R is to 0 the higher
overconsolidation ratio of soil; and when R approaches to
1, the soil changes to be normally consolidated state. The
details of the SYS Cam-clay model can be found in thereferences7)-9).

Horizontal displacement at two sections(b)

16 years after
earthquake

0.5

50

(c)

(a)

0.5

R*

End of earthquake
0

10

2

0

10

4

10

6

10

(b)
(d)

(c)
15

(a)

0
50
100
150
Mean effective stress pc(kPa)

R

1.5

10

50

10
20
Shear strain H s(%)

Type 22 –2
10

q = Mpc
100

1Type 1

1

150

(d)

100

0

R, R*

Settlement (m)0During Earthquake
Deviator stress q (kPa)Elevation (m)60

5. Interpretation of seismic mechanical behaviors

8

10

10
20
Shear strain H s(%)

10

Fig. 13- 459 -

2.6

(a)
(b)

2.5
2.4

(c)

(d)

2.3
0
50
100
150
Mean effective stress pc(kPa)(a)-(b): Construction(b)-(c): During earthquake(c)-(d): After earthquake

Time (s)Fig.12 Settlement of theretaining wall during andafter earthquake

Specific volume v (=1+e)

Type 1601) and 0.83 m (Type 2). It can be found that the settlement
after earthquake lasts for many years and is even larger
than that during earthquake. This kind of “delayed
settlement” after earthquake is observed in Mexico
earthquake of 1957. The clay layer of 28 m settled about
1.4 m in the following 1500 days after the earthquake24).

Responses of the soil directly below theretaining wall (in Area C)

During earthquake

During earthquake

(d)

20

(c)

(a) 0

20
Shear strain Hs(%)

(b)

40

(d)

20

(c)

(b)
30

20
40
60
Mean effective stress pc(kPa)0

20
Shear strain Hs(%)

(d)

(c)

20

20
40
60
Mean effective stress pc(kPa)20
Shear strain Hs(%)

40

(b)

R

0.5

(d)
2.6

R*
0

0
20
40
60
Mean effective stress pc(kPa)20
Shear strain Hs(%)

60

(d)
(c)

50
100
Shear strain Hs(%)

40

20Specific volume v (=1+e)

Decay ofstructure

0

50
100
Shear strain Hs(%)

150

(d)

(c)

(d)

2.6

(a)
2.8

40

(c)
(d)

20

Earthquake-induced
volume change

(c)

60

(b)

(a)

20
40
60
Mean effective stress pc(kPa)

1

R*

(b)

(a) 0

150

R

60

q = Mpc

0200
400
Shear strain Hs(%)

(b)
0.5
R*

(d)
0

0
20
40
60
Mean effective stress pc(kPa)

200
400
Shear strain Hs(%)

600

(b)

40

(d)

(c)
20

(a)

600

R

2.7

2.6

q = Mpc

020
4060
Mean effective stress pc(kPa)

Expansion during
earthquake

1

R, R*

40

0.5

2.7

0
20
40
60
Mean effective stress pc(kPa)

Deviator stress q (kPa)

Deviator stress q (kPa)(b)

Deviator stress q (kPa)

60

0

(b)

(c)

During earthquake

During earthquake

(a)

(a)
2.8

(a) Type 1(a) Type 1

20

2.9

40

Specific volume v (=1+e)

0

(c)

2.7

Specific volume v (=1+e)

0.5

R*

Deviator stress q (kPa)

(b)

40

(a) 0

40

(a)

2.8

R, R*

Specific volume v (=1+e)

R

R, R*

q = Mpc

1

1

R, R*

(d)

(c)
(a)

(a) 0

40

Deviator stress q (kPa)40

q = Mpc

Deviator stress q (kPa)

(b)

60

60

60
Deviator stress q (kPa)

Deviator stress q (kPa)

60

2.9

(a)
2.8
2.7

(c)

(b)
(d)

2.6
0
20
40
60
Mean effective stress pc(kPa)(b) Type 2(b) Type 2(a)-(b): Construction(b)-(c): During earthquake(c)-(d): After earthquake(a)-(b): Construction(b)-(c): During earthquake(c)-(d): After earthquake

Fig. 14 Responses of high-structured overconsolidated
clay during and after earthquake (in Area E)

Fig. 15

under Type 1 except the value of the shear strain.
Therefore, the response of the element directly below theretaining wall under the Type 2 motion will be observed
to interpret the “delayed settlement” of the retainingwall.
As shown in Fig. 13, the behaviors of the soil elementin Area C during construction, during earthquake andafter earthquake are indicated by (a)-(b), (b)-(c) and(c)-(d), respectively. It can be found that the shear strain
mainly produced during earthquake shaking ((b)ĺ(c)).
However, during the earthquake, there was hardly any
volume strain in a short time, and consequently, the shear
deformation under undrained condition is prominent andthe excess pore pressure increases. After earthquake, due- 460 -

Different responses of the soil element during
earthquake (in Area D)

to the low permeability of reclaimed ground, it took longtime for the excess pore water pressure to dissipate andthe settlement was resulted from consolidation.
5.2 Earthquake-induced settlement
The responses of the elements in Area E in Fig. 10(d)are shown in Fig. 14. The response of the soil in Type 1
seems to be similar to that in Area C and the mechanismof the settlement after the earthquake is the same as thatin Area C. However, the response in Type 1 is different
from that in Type 2 in the Area E. It is need to consider
an additional mechanism about the volume change. Thatmeans the volume change due to loss ofoverconsolidation and decay of structure. The volume
change of the highly structured and heavily
overconsolidated soil is much different after earthquake
motion Type 1 and Type 2.
After earthquake, the soil in Type 2 is in a relative
lightly overconsolidated and low structured statecompared with the soil in Type 1. Moreover, after
earthquake Type 2, as shown in Fig. 14(b), there is
obvious to decay of structure in Type 2, while the decayof structure in Type 1 in Fig. 14(a) is very limited. Bothelement response of soil in Type 1 and Type 2 exhibit
loss of overconsolidation (R increased) after theearthquake. However, it is difficult to compare thedegree of loss of overconsolidation in Type 1 with that in
Type 2. It is clear that the degree of the decay ofstructure in Type 2 is prominent compared with that in
Type 1. Therefore, larger degree of decay of structure in
Type 2 cause much larger volume change than Type 1,
which can be found in the relation of specific volume
and mean effective stress in Fig. 14. It is hard for clay todecay the structure, in other words, the increase of R* isvery limited, even in Type 1 with the shear strain over
20%. However, when the shear strain is large enough as
much as that in Type 2, very large volume change can beproduced due to decay of structure. And it can be
regarded as the reason of earthquake-induced settlement
in clay layer.
5.3 Expansion during earthquake
The responses of the soil element in Area D in Fig.
10(d) are shown in Fig. 15. According to the calculated
results of specific volume change, as shown in Figs. 13
and 14, the earthquake is generally regarded as an
undrained shear process. However, as shown in Fig. 15(b), the expansion of soil element is observed under theearthquake Type 2. This element located in the large
shear strain area between the first soil embankment andthe retaining wall, and the shear strain was over 300%
during earthquake and achieved 400% after earthquake.

by unconfined test respectively. The construction andreclamation process are reproduced to obtain the current
state of the ground. Based on the study presented, thefollowing conclusions can be made:(1) It is possible to determine the initial conditions of
the soils in the ground through triaxial test and oedometer
test, by computing the process of building up thereclaimed ground with reproducing the unconfinedcompressive strength and specific volume distribution that
had been obtained from ground survey data. It can befound that the state of soil changed with the construction
and reclamation process. Small shear deformation isobserved at the bottom of the reclaimed ground. It willaffect the seismic response of soil and it is thus necessary
to consider the construction in calculation.
(2) In the case of earthquake Type 1, the horizontal
displacement of the retaining wall is about 3.9m, and in
the case of earthquake Type 2, the value is 7.4 m. The
horizontal displacement of the reclaimed ground surface
150 m from the retaining wall is 1.9 m and 5.1 m,
respectively.
(3) In earthquake Type 1, the settlement of the retaining
wall during and after earthquake is 0.49 m and 0.81 m.
And in Type 2, the value is 0.58 m and 0.83 m
respectively. The “delayed settlement” after earthquake
lasts for many years and is even larger than that during
earthquake.
(4) In some area with large shear strain (˚75%), forthe dredged soil with high structure, additional volumetricstrain is produced by the decay of structure and therefore
earthquake induced settlement is observed, while thevolume expansion of soil element during earthquake isobserved in the localization of the shear strain (˚300%)
between the embankment 1 and the retaining wall.
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6. Conclusions
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(Received: March 9, 2010)