Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, Japan

SIMULATION OF SLIDING FAILURE OCCURRED
AT HIGHWAY EMBANKMENTIN 2011 GREAT EAST JAPAN EARTHQUAKE
Kouki MURAKAMI1, Ken-ichi TOKIDA2 and Aino TACHIBANA3
1

Graduate Student, Department of Civil Engineering, Osaka University,
Osaka, Japan, kkmurakami@civil.eng.osaka-u.ac.jp
2
Professor, Department of Civil Engineering, Osaka University,
Osaka, Japan, tokida@civil.eng.osaka-u.ac.jp
3
Undergraduate, Department of Civil Engineering, Osaka University,
Osaka, Japan, atachibana@civil.eng.osaka-u.ac.jp

ABSTRACT: However almost highway embankments weren’t damaged severely inthe 2011 Great East Japan Earthquake, only one damaged embankment can be shownas the typical case where the toe sliding failure was observed at the shallow softbase ground and one side slope of the embankment with the symmetric cross secti
on form was slid. In this study, the reasons why the toe sliding failure occurred at
the one side slope of the embankment are investigated and discussed through the circular sliding method and dynamic response analysis.
Key Words: Highway embankment, toe sliding failure, circular sliding method,
dynamic response analysis

INTRODUCTION
In the 2011 Off the Pacific Coast of Tohoku Earthquake, the road embankments at the expressways arealmost less damaged comparing with the ones occurred in the past earthquakes in Japan. The only one
embankment close to the Naka Interchange at the Joban Expressway was damaged a little severely andremarkable with the following two interesting phenomena from the engineering view points. One point
can be indicated that however the embankment constructed on the base ground with the soft layer of
3.6m thick, the sliding failure form of the embankment was the toe sliding one. Second one can beindicated that however the whole form of the embankment is almost symmetrical, one-sided slope ofthe embankment was slid and the opposite side slope was not slid.
In order to simulate the toe sliding failure of the embankment on the soft ground in thisearthquake, the circular sliding method with the design seismic coefficient of 0.0, 0.1, 0.2 and 0.3 isapplied to the embankment and base ground model estimated using the limited data obtained just after
the earthquake. Furthermore, to verify the reasons why the slope was slid or not at the same crosssection, both the circular sliding method and the finite element method using the earthquake groundmotion estimated from the observed ground motion in this earthquake are applied.

949

Through this study, the possibility of the toe sliding failure on the soft base ground can beclarified firstly, comparing the soil properties of the embankment body with that of the base ground.
Secondly, however the reason why the embankment slope was slid or not can’t be clarified with use of
the circular sliding method in case of the symmetrical cross section of the embankment, the possibility
and importance to consider the reduction of the soil strength of the unsaturated embankment bodyinduced by the increase of the degree of saturation is discussed and indicated to investigate the sliding
occurrence of the embankment slope. Furthermore, the direction of the seismic ground motion can beindicated to be very important to estimate the sliding failure with use of the dynamic response analysis.
As the results of this study, very important lessons on the sliding failure of the embankment can beobtained for the future.

OUTLINE ON DAMAGED EXPRESSWAY EMBANKMENT
Location of Objective Site
However the 2011 Great East Japan Earthquake was the plate boundary type with the magnitude of 9.0,
the expressway embankments were almost less damaged comparing with the ones in the pastearthquakes in Japan. Because the seismic source area expanded widely from Tohoku Region to Kanto
Region, very huge ground motion observed widely in Japan.
Fig. 1 (a) shows the location of the embankment at Joban Expressway to investigate in this study
which is about 100km far from Tokyo. Fig. 1 (b) shows the satellite photograph by Google (2011)
where the damaged embankment is covered with blue sheets and about 1km away from Naka
Interchange which locates in the right direction of the figure. It’s necessary to note that the highway
lanes from the right side to the left one in Fig.1(b) is named “Up Lane” to Tokyo where theembankment was slid and the reverse direction is named “Down Lane” from Tokyo in this paper.

Joban Expressway 92.4kp
Mito IC ～ Naka IC

国道118号
Route118
Bird Line
バードライン

Similar Deformed

類似変状地点
Site
Down
Lane
下り

Tokyo

Up Lane上りBox (Mito 26)
Box水戸26

Joban Expressway
常磐自動車道Bridge
橋梁

Slid
崩壊地点
Embankment

Naka IC那珂IC

Similar
Deformed Site
類似変状地点

400m(a) Location of investigate site(b) Bird’s eye view around damaged embankment

Fig. 1 Objective site and embankment investigatedOutline of Damaged Embankment
Photo 1(a) reported by the East Nippon Expressway Co. Ltd. just after the earthquake shows thesliding failure at the embankment crest with the maximum displacement of 1.5m along the distance ofabout 150m together with a slight sliding occurred along the boundary of the two traffic lanes.
Because the one lane of the Up Lane and two lanes of the Down Lane were not damaged, they could
be used for the recovery works after the earthquake. So the damage level at this site can be indicated tobe medium considering the performance-based design. Photo 1(b) shows the toe sliding failure of theembankment on March 13 because the side road and the base ground neighboring the embankmentaren’t deformed. Furthermore, the sand boils induced by soil liquefaction can’t be observed at the baseground surface.

950

(b) Toe sliding failure under urgent repairing work on March 13(a) Sliding failure around crest
Photo 1 Sliding failure of embankment at Up Lane
Conditions of Base Ground and Embankment
The modeling the cross section of the damaged embankment and the base ground is conducted basedon the data by the East Nippon Expressway Co. Ltd. (2011) as shown in Fig. 2. The conditions of thebase ground are assumed based on the soil classification and N-value estimated by the boring data
which were measured before the earthquake about 60m away from the slid embankment referring to
the Guideline of Retaining Wall (1977) as shown in Fig. 3. Based on Fig. 3, the depth of the stiff
diluvium ground can be estimated to be 9.45m deep and the above alluvium ground are divided into
three layers such as layer 1 of 5.8m thick, layer 2 of 1.15m thick and layer 3 of 2.5m thick as shown in
Fig. 2. Especially, the soft ground at this site is assumed to be 3.6m deep from the ground surface.
The embankment body is modeled with four layers such as layer 4, layer 5, layer 6 and layer 7
shown in Fig.2 where layers 5, 6 and 7 are the pavements except for the asphalt and modeled as the
sandy gravel in this study. The thickness of layer 7 of 0.6m is measured from Photo 2 and that oflayers 5 and 6 is assumed to be half of the layer 7. The soil properties of the embankment body are
assumed based on the results of the unconfined compression tests of the slid soils which conducted on
March 13 by the East Nippon Expressway Co. Ltd. (2011).
Based on the mentioned above, the saturated unit weight, wet unit weight, internal friction angle
and cohesion of each layer is summarized in Table 1, and Fig. 2 and Table 1 is assumed to be thefundamental model for the circular sliding method in this study.
Down Lane

7

Up Lane

6

5

4
3

2

1

Fig. 2 Fundamental model on cross section of embankment and base groundFetureThickness（m）

Depth（m）

S-wave vel.（m/s）

N-value

topsoil

0.45

0.45

86

0.64

cohesive soil

2.05

2.50

86

0.64

cohesive soil

1.15

3.65

123

1.88

sand gravel

5.80

9.45

246

29.0

rock surface

330

Fig. 3 Soil profile of base ground applied to damaged embankment

951

Table 1 Contents of embankment, pavement and base ground in Fig.2
NumberSaturated unit
weight(kN/m3)

Wet unit
weight(kN/m3)

Internal friction
angle(°)

Cohesion (kN/m2)

1
2
3
4
5
6
7

20.00
20.00
20.00
20.00
20.00
20.00
20.00

20.00
16.00
16.00
19.00
20.00
20.00
20.00

35.86
0.00
0.00
33.70
30.00
35.00
40.00

0.00
18.75
6.40
0.00
0.00
0.00
0.00

Photo 2 View of pavement and gravelly fine sand layer
SIMULATION OF TOE SLIDING FAILURE
WITH USE OF CIRCULAR SLIDING METHOD
How to conduct simulation
In order to clarify the reason why the sliding failure form takes on the toe sliding failure, thesimulation with use of the circular sliding method is conducted on the fundamental model and then thefundamental one is modified by trial and error to simulate the toe sliding failure by changing the soil
conditions of each layer at the embankment body and the soft ground.
Simulating in case of fundamental model
The minimum safety factor (Fs) with the circular sliding method in case of the fundamental model
shown in Fig. 2 is 0.56 which is smaller than 1.0 in the general condition as shown in Fig. 4. The
minimum safety factor of 0.56 should be more than 1.0 in general condition. Furthermore, the sliding
failure form takes on the base sliding failure which means that the sliding plane goes through the soft
alluvium layer 3.
The above two results indicates that the fundamental model estimated firstly is necessary to bereviewed. As for these results, it can be considered that because the fundamental model assumed in Fig.
2 and Table 3 is based on the data obtained at the site of about 60m away from the slid embankment,
the conditions of the soil properties at the slid embankment are different with the fundamental model.
7
6

0.56
4
3

5
2

1

Fig. 4 Minimum safety factor and sliding failure form in general condition952

Modification and simulation in case of modified fundamental model
The modification of the fundamental model is conducted by trial and error to secure the minimum
safety factor in general condition larger than 1.0 and simulate the toe sliding failure form at the slid
embankment as follows.
As for the soil layers of the embankment body considering the collapsed profile in detail, it can
be clarified that the upper part of the embankment body whose thickness is 5.3m seems to becomposed of the gravely fine sand and the lower part of that whose thickness is 3.3m seems to becomposed of the loam. Then the embankment body is divided into two layers such as layer 4 and layer
8 shown in Fig. 5. Additionally, the ground water level which is 0.59m below the bottom of layer 4 or
the surface of layer 6 is assumed as shown in Fig. 5. The unit weight of water is 10.0kN/m3.
7
8
6

5

4
3

2

1

Fig. 5 Modified model considering soil layers of embankment body and ground water
The trial and error to modify the soil properties of each layer in Fig. 5 to satisfy the two
conditions such as the minimum safety factor in general condition and the toe sliding failure in bothgeneral condition and seismic one applied with seismic coefficients of k = 0.1, 0.2 and 0.3. Table 2
show the final results by trial and error and Fig. 6(a) shows the example where the minimum safety
factor of 1.085 is larger than 1.0 and the toe sliding failure form is performed in general condition.
Furthermore, the sliding failure forms applied with the above seismic coefficients are also performed
based on Table 2 as shown in Fig. 6(b) which is the example of k = 0.2. Compared Table 2 of themodified model with Table 1 of the fundamental model, generally speaking, the toe sliding failure canbe assumed to occur when the strength of the base ground is relatively larger than that of theembankment body.
As mentioned above, the possibility and the conditions to perform the toe sliding failure at theexpressway embankment occurred in this earthquake can be confirmed comparing the soil propertiesof the embankment body and the soft base ground. The detail process of the trial and error can bereferred to Murakami etc. (2011b)
Table 2 Soil properties of modified model
NumberSaturated unit
weight(kN/m3)

Wet unit
weight(kN/m3)

Internal friction
angle(°)

Cohesion (kN/m2)

1
2
3
4
5
6
7
8

20.00
20.00
20.00
20.00
20.00
20.00
20.00
20.00

20.00
16.00
16.00
14.00
20.00
20.00
20.00
19.00

35.86
20.00
15.00
20.00
30.00
35.00
40.00
20.00

0.00
18.75
13.00
5.00
0.00
0.00
0.00
10.00

1.085

0.75

7
6

7

8
4
3

5

6

2

8
4
3
1

1

(a) In general condition(b) Seismic condition in case of k =0.2

Fig. 6 Minimum safety factor and sliding failure form by modified model

953

5
2

INVESTIGATION ON DIFFERENCE
BETWEEN DAMADE SLOPE AND NON-DAMADED SLOPE
Investigation on Symmetrical Cross Section with Use of Circular Sliding Method
Because the slid embankment is almost symmetry, the sliding failure can be estimated to occur at bothside slopes during earthquakes in general. However only one side slope at the Up Lane to Tokyo wasslid, the opposite side slope at the Down Lane was not slid. The reason why the difference between theslid embankment and non-slid one occurred is very interesting from the engineering view point andcan be applied for the future design of the expressway and/or other embankments.
The above reason is investigated and discussed with use of the circular sliding method by theseismic coefficient method firstly. The embankment model identified as the modified model shown in
Fig. 5 and Table 2 is applied to investigate the difference of minimum safety factors and sliding plane
positions calculated at both side slopes applying the seismic coefficients of k = 0.1, 0.2 and 0.3.
The example of results on the slope at the Down Lane in case of k = 0.1 can be shown in Fig. 7(a).
As shown in Fig. 7(a), the sliding failure form takes on the toe sliding failure one. Furthermore thesummaries of results in all cases of k = 0.1，0.2 and 0.3 are shown in Table 7(b) and the siding failure
forms in all cases take on the toe sliding failure ones. As shown in Fig. 7(a), although the minimum
safety factors at the slid slope at the Up Lane are a little larger than those at the non-slid slope, thedifferences between them seem to be very small from the engineering view point.
As mentioned above, the reason why only one-side slope was slid at the same cross section can’t
be clarified in this study. The result is clear and expected because the both slopes at the almost
symmetrical embankment with the same soil materials are estimated to be same as far as the circularsliding method is applied, in other words, it indicates the limitation to apply the circular sliding one.
0.90

Condition

7
6

8
4
3

5
2

1

(a) Critical slip circle at down lane tin case of k = 0.1

Down Lane
In General
1.094
0.902
k =0.10.758
k =0.20.645
k =0.3Fs
Up Lane
1.085
0.896
0.754
0.642

(b) Safety factor at both lanes

Fig.7 Stability at Down Lane and safety factors between Down Lane and Up Lane
Investigation with Use of Seismic Response Analysis
In order to investigate the reason why only one-side slope at the symmetrical embankment slid, theseismic response analysis with the Finite Element Method (FEM) have been conducted on themodified model establish in Fig. 5 and Table 2 to know the difference between the slid slope andnon-slid one in detail.
Model of base ground and embankment
Fig. 8(a) shows the analytical FEM model of the objective embankment and base foundation. The leftside slope didn’t slide and the right-side one slid in this earthquake. The static soil properties are samewith Table 2 applied for the circular sliding method. The dynamic soil properties of both shear
modulus ratio G/G0 and damping content h according the shear strain γ shown in Fig. 8(b) which are
estimated referring to Yasuda & Yamaguchi (1985) for the layer 1 to layer 4 of the base ground andSeismic Design Code of Railway Structure (1999) for the layer 5 to layer 9 of the embankment body,respectively are applied for the equivalent linear method. The layers 7, 8 and 9 are the pavements andmodeled as the gravel, and the layer 10 is assumed to be the engineering base ground.

954

1

0.5
2

0.8

0.4
3
6

0.6

4

8

9

5

3

0.3

7

0.4

0.2

5
9
4

0.2

7

0.1
8 6

0
1.0E-06

1.0E-05

1.0E-04

2

damping constant h

Shear Modulus Ratio G/G01

10
1.0E-03

1.0E-02

1.0E-01

shear strain γ(b) Dynamic properties of each layer(a) Analytical FEM model

Fig.8 FEM model and dynamic deformation properties
Analytical case and input seismic ground motion
Two analytical cases shown in Table 3 are considered to investigate the effects by the input earthquake
ground motion in this study. The earthquake ground motion inputted at the lower plane of the layer 10
of 10m deep in Fig.5 is estimated by Hata etc. (2011) from the earthquake ground motion recorded atNaka City Office which is about 4.5 km far away from the slid embankment. The time history of theacceleration estimated in the direction of the cross section of the slid embankment is shown in Fig. 9
where the applied time history is 100 seconds including the main large ground motion. It can be
known that the maximum acceleration amplitude is +651gal and -533gal.
The plus (+) direction of the acceleration in Fig. 9 means to be the right side direction in Fig. 8(a)and then the seismic force induced by the plus acceleration tried to drive the body at right side slope toslide toward the right side in Fig. 8(a). So the direction of the inputted acceleration becomes important.
In this study, two cases on the inputted direction of the base ground motion are considered, i.e. thedirection of the plus acceleration is set to the right-side which is named “Real Direction” and theleft-side which is named “Reversal Direction” to investigate the effects of the direction of the irregular
ground motion on the dynamic behavior of the embankment.
Table 3 Analytical cases
Case

Input Direction Max. Acc. to right Direction (gal)Max. Acc. to left Direction (gal)

1

Real

651

-533

2

Reversal

-533

651

800

Acc. (gal)

400
0
-400
Applied Time History: 100 sec
-800
0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

Time (s)Fig. 9 Inputted earthquake ground motion at engineering base ground of 10m deep
Effect of direction of seismic ground motion
Fig.10 shows the distribution of the maximum acceleration at both embankment body and base groundfor all duration time comparing the Real Direction with the Reversal one, and the maximum values
only at the embankment body are picked up in Table 4. It can be known that both the distribution of955

the maximum acceleration and the maximum values are almost same at the right-side body and theleft-side one in the Real Direction and the Reversal Direction. Therefore, the difference induced by the
direction of the inputted earthquake ground motion acting on the maximum acceleration at theembankment body can’t be clarified from the view of the maximum acceleration occurred.
Fig. 11 shows the distribution of the maximum shear strain at both embankment body and baseground for all duration time comparing the Real Direction with the Reversal one, and the maximum
values only at the embankment body are picked up in Table 4. It can be known that both the
distribution of the maximum acceleration and the maximum values are almost same between theright-side body in the Real Direction and the left-side body in the Reversal Direction. This indicatesthat referring the maximum shear strain, the slope at the Down Lane has the possibility to be slid whenthe earthquake ground motion in Fig. 9 was inputted in the Reversal Direction.
Table 5 is another discussion based on the averages of absolute maximum acceleration, shear
strain and horizontal displacement occurred in the embankment body. From this Table 5, the influenceby the direction of the earthquake ground motion can be assumed.
+17m/s2+

0-17m/s2(a) Real Direction in case 1++17m/s2
0-17m/s2(b) Reversal Direction in case 2
Fig. 10 Distribution of maximum acceleration
0.021
0.011
0

(a) Real Direction in case 1
0.021
0.011
0

(b) Reversal Direction in case 2
Fig. 11 Distribution of maximum shear strain
Table 4 Maximum of acceleration, shear strain and horizontal displacement
at embankment body above base groundMax. Acc. (gal)CaseMax. Shear Strain

Max. Disp. (m)1

Left-sided
Body
-1649

Right-sided
Body
1674

Left-sided
Body
1.31E-02

Right-sided
Body-1.58E-02

Left-sided
Body-0.077Right-sided
Body
0.100

2

-1649

1674

1.53E-02-1.28E-02-0.097

0.081

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Table 5 Average of absolute maximum of acc., shear strain and horizontal displacement
at embankment body above base ground
Case

Ave. of Max. Acc. (gal)Ave. of Max. Shear Strain

Ave. of Max. Disp. (m)Left-sided
Body

Right-sided
Body
5.76E-03

Left-sided
Body
0.051

0.069

5.39E-03

0.066

0.053

Right-sided
Body

1

542

510

Left-sided
Body
5.34E-03

2

501

553

5.71E-03

Right-sided
Body

The above results indicate that the direction of the inputted base ground motion seems to relate to
the sliding failure, which seems to occur at the slope which is forced in the direction of the absolute
maximum acceleration amplitude which is +651gal in this earthquake.
Investigation on Effects of Degree of Saturation with Use of Circular Sliding Method
Because the sliding stability of the embankment is generally affected by the water contents in theembankment body, the conditions of water contents are paid attention in this chapter to discuss thereason why the slid slope and non-slid one occurred at the same cross section in this earthquake.
Possibility of increase of degree of saturation in embankment body
Although the slid embankment with the symmetry shape locates on the flat alluvium plane, the plane
linearity of the highway route around the slid embankment can be known to be curved a little toward
the lower direction in Fig. 1(b). Then, according to the layout of the drainage system around the slid
embankment shown in Fig. 12 by the East Nippon Expressway Co. Ltd. (2012), it can be assumed thatthe rain fall and the water drained from the pavement are leaded to the lower direction in Fig. 12.
Furthermore the detail field survey by the East Nippon Expressway Co. Ltd. (2012) has reported thatthe ground water level is observed to be high just after the rainfall inside of the right-side body at the
Up Lane.
Based on the above conditions, the water oriented by the rainfall and/or drainage may be possible
to concentrate and/or stored in the embankment body at the Up Lane in Fig. 1(b) where the sliding
failure occurred in this earthquake. On the contrary, the embankment body at the Down Lane where
the sliding failure didn’t occur may be difficult to be concentrated and/or stored with the above water.
In other words, it can be indicated that the degree of saturation of the embankment body before
the earthquake is different between the slid body and the non-slid body. Then the circular sliding
method is applied to discuss the above estimation to clarify the reason why the slid and non-slid slope
at the same cross section of the embankment as follows.

Fig. 12 Drainage system of rainfall on pavement surface
Estimation of effect by change of degree of saturation
As shown in Fig. 5, the groundwater level is lower than the base ground surface. This means that thewhole embankment body is located above the groundwater level and isn’t saturated generally.
However, estimated the body water conditions concentrated and/or stored as mentioned above, it canbe assumed that the water contents above the groundwater level may be distributed irregularly andthen the strength of body soils may be different at each position.

957

For investigating the above estimation, it is necessary to know the factors affected by the change
of degree of saturation Sr which are considered to be the shear strength and the wet density in thisstudy. As for the above shear strength, Athapaththu etc. (2007) indicate that the internal friction angle
isn’t related to the Sr and on the contrary, the apparent cohesion is related to that as shown in Fig. 13(a)through the consolidated drain tests. Based on Fig. 13(a), the cohesion of soils can be assumed todecrease according to the increase of the void ratio e of 0.7 to 1.0 depending on the Sr of 40% to 80%.
Modifying Fig. 13(a), Fig. 13(b) can be obtained as the relation between the Sr and cohesion in case of
the e of 0.95.
As shown in Table 2 which shows the soil properties on the layers 4 and 8 measured by the East
Nippon Expressway Co. Ltd. (2011) just after the earthquake, the layer 4 is the loam with the void
ratio e of 1.81 and then the cohesion is assumed not to be affected by the Sr in this study. On the other
hand, because the layer 8 is the gravelly fine sand with the e of 0.95, the change of the cohesion isassumed according to Eq. (1) shown in Fig. 13(b).
c = -0.112Sr (%) + 12.93
(kPa)
(1)
14

12

Cohesion [kPa]10

8

C = -0.112Sr + 12.93

6

4

2

0
20

30

40

50

60

70

80

90

Degree of Saturation [%](a) cd, e and Sr : Athapaththu etc. (2007)(b) c and Sr in case of e = 0.95

Fig. 13 Relation between cohesion cd, degree of saturation Sr and void ratio e
Based on the Eq. (1), it can be known that the cohesion of 0.112kPa is decreased according to theincrease of Sr of 1 %. Although the Sr of the layer 8 measured just after the earthquake is 44.2 %, the Srof the layer 8 before the earthquake can’t be known shown in Table 6. In order to investigate thedifference of the slid slope and non-slid one, the layer 8 in the original cross section of Fig. 14(a) is
divided into two layers: the half layer at the right side where the sliding failure occurred and the that at
the left one where the sliding one didn’t occur shown in Fig. 14(b). Then, the degree of saturation of
the layer 9 in Fig. 14(b) is estimated to be Sr0 before the earthquake and the sliding safety factor is
calculated applying the increase of Sr from 10% to 30% with the step of 10% from the Sr0 .
On the other hand, the relation between the wet density and Sr is estimated based on that
estimated by two soil samples shown in Fig. 15(a), and then the wet density of 0.0494kN/m3 isassumed to be increased according to the increase of Sr of 1% in this study. The analytical conditionsof 3 cases where the decrease of cohesion is considered in Case-1, the increase of wet density is
considered in Case-2 and both cohesion and wet density are considered in Case-3 according to theincrease of Sr are summarized in Fig. 15(b).
Table 6 Soil properties of layer 4, layer 8 and layer 9
Layer No. Soil Classification

Water Content (%)

3

4

loam

58.4

Wet Density (ｋN/m )16.36

8 and 9gravely fine sand

15.7

16.58

958

Void Ratio

Degree of Saturation (%)

1.81

89.9

0.95

44.2

7

7
8
6

8

6

5

4
3

9

5

4
3

2

2

1

1

(a) Original cross section(b) Modified cross section to consider Sr

Wet Density (kN/m3)

Fig. 14 Modification layer 8 at original model to consider effects of Sr
16.8
16.6
16.4
16.2
16
15.8
15.6
15.4
15.2

Increase of Sr at
Layer 9 （％）

0

20

30

40

50

60

Degree of Saturation Sr (%)

Case-3Case-1Case-2Wet
Cohesion
Density2（kN/m )
3
(kN/m ）
10
19

10

8.88

19.49

20

7.76

19.99

30

6.64

20.48

(a) Relation Sr and wet density(b) Conditions of analytical cases
Fig. 15 Relation between cohesion and wet density considering Sr

The calculated minimum safety factors with the circular sliding method in case of the seismic
coefficient of k = 0.1 can be obtained in Table 7. Table 7 is indicated that according to the increase of
the Sr, the minimum sliding safety factor of the layer 9 at the right-side slope decreases compared withthat of the layer 8 at the left-side slope, and the decrease of the cohesion affects a little more largely
than the increase of the wet density on the decrease of the sliding safety factor of the embankment.
As mentioned above, the possibility to explain the difference between the slid slope and thenon-slid one can be indicated when the strength, i.e. the cohesion and the wet density of the body layer
is possible to decrease according to the increase of the degree of saturation induced by the water
concentrated and/or stored in the embankment body.
Table 7 Minimum sliding safety factors affected by Sr in case of k = 0.1
Increase of Sr at
Left-sided
right side of Layer 9
Slope
（％）
0
10
1.102
20
30

Right-side Slope
Case-1Case-2Case-3

1.089
1.068
1.046
1.023

1.089
1.078
1.067
1.056

1.089
1.057
1.025
0.994

CONCLUSIONSThe typical sliding failure of the highway embankment interested in the 2011 Great East Japan
Earthquake are investigated to simulate the toe sliding failure on the soft base ground and to clarify thereason why only one side slope at the symmetry formed embankment during the earthquake. The
following lessons can be obtained in this study.
1. The toe sliding failure at the soft base ground with the thickness of 3.6m which was observed in thisearthquake can be simulated with use of the circular sliding method considering the seismic
coefficient of 0.0, 0.1, 0.2 and 0.3, compared the soil strength of the embankment body and that of959

the soft base ground.
2. The reasons why the slid slope and non-slid one at the same cross section with the symmetry shaped
embankment can’t be clarified with use of the circular sliding method considering the seismic
coefficient. This indicates the limitation to investigate the detail reasons of the sliding failure withthe circular sliding method.
3. One of the above reasons can be indicated to be the direction of the maximum acceleration
amplitude of the earthquake motion inputted at the base ground with use of the dynamic response
analysis with use of FEM. This indicates that the direction of the earthquake ground motion shouldbe paid attention when the dynamic response analyses are applied.
4. Another reason mentioned above can be explained by the different conditions of the degree ofsaturation in the embankment body before the earthquake. In other words, when the degree ofsaturation in the embankment body is distributed un-uniformly even if the shape of the embankment
body is symmetry, the occurrence of the sliding failure can be indicated to be influenced by both thedecrease of the cohesion and the increase of the wet density in the body soils induced by theincrease of the degree of saturation before the earthquake. This indicates that the change of bothcohesion and wet density of soils above the ground water level should be considered to estimate thesliding failure of the embankments.
Further study on the decrease of the strength of the embankment body induced by the water
seepage should be investigated in the future.

ACKNOWLEDGMENTS
The authors would like to express sincere gratitude to Dr. S. Yokota of the Nippon Expressway
Research Institute Co. Ltd. and the East Nippon Expressway Co. Ltd. on the data at the slid
embankment of the Joban Expressway, and to Dr. Y. Hata of the Nippon Koei Co. Ltd. on theearthquake ground motion estimated at the slid embankment.

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