Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Reduction in Liquefaction Induced Settlement of River Levee by Enhancing
Horizontal Stress with Sheet Piles
Mitsu OKAMURA
Professor, Graduate School of Science and Engineering, Ehime University, Matsuyama, Japan
Email: okamura@cee.ehime-u.c.jp

Tadahiro INOUE
Graduate student, Graduate School of Science and Engineering, Ehime University, Matsuyama, Japan

Naoko JIN-NOUCHI
Research assistant, Graduate School of Science and Engineering, Ehime University, Matsuyama, Japan

Kohei ONO
Research associate, Graduate School of Science and Engineering, Ehime University, Matsuyama, Japan

ABSTRACT
In order to reduce liquefaction damage to river levees, remedial stiff zones are constructed below toes of levees to restrain
lateral flow deformation of liquefied foundation soil under the levee. Sheet piling, among other ground improving
techniques, is preferably employed especially in urban areas where houses are existing close to levees and workspace islimited. Settlement of levee becomes smaller if sheet piles with a higher flexural stiffness are used. However, it is often
the case that settlement does not satisfy the design criteria even if steel pipe sheet-piles with very high stiffness are used.
In this study, an innovative countermeasure method using sheet piles is proposed. In the proposed method, the horizontal
earth pressure is enhanced with the help of elastic nature of sheet piles. This is achieved by inserting a wedge along the
sheet piles to the levee side which deforms laterally the soil below the levee, increases horizontal earth pressures andreduce the liquefaction susceptibility. The effectiveness of introducing additional horizontal stress, hereafter termed as
pre-stress in this study, was examined through centrifuge tests. In the tests, the amount of the horizontal stress increase
and the affected zone were confirmed by conducting cone penetration tests at multiple locations before and after
application of the pre-stress. Dynamic centrifuge tests were carried out on models with and without the pre-stress. It was
found that the cone tip resistance increased not only in the vicinity of the sheet pile but at almost everywhere in the soil
beneath a levee. Settlement of the levee decreased with increasing the introduced pre-stress, confirming the effectivenessof the method.
Key words: Sheet pile, River levee, Liquefaction countermeasure, Pre-stress1. Introduction
River levees in Japan have been repeatedly damaged

damaged levee. The Ministry of Construction has started

by earthquakes and occasionally, hinterlands of thecause of damage to levees in earthquakes is soil

damaged levees were in danger of overflow of tsunami runliquefaction of the foundation soil or levee soil itself.

up or flooding water due to the preceding heavy rain.

Liquefied soil beneath levees may be squeezed verticallyOsaka city was the typical case just after Hyogoken

and flow laterally to the direction away from the levee

Nambu earthquake in 1995, where a 6 m high Yodo River

centerline. Countermeasures in the remediation program,

levee subsided more than 3 m due to the foundation soil

therefore, aim at mitigating crest settlements by providing

liquefaction and river water almost reached the crest of thecontainment for the deformation of the liquefiable

remediation program to vulnerable levees. The major- 243 -

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

foundation soils by forming remediated zones inconstraint was imposed by the countermeasures, resulting

liquefiable soil layer under embankment toes (Sasaki andin formation of a low strength exit path near the toes where

Ishihara, 2016). Centrifuge tests (e.g. Okamura andthe excess pore pressure ratio was high. The liquefied sandMatsuo, 2002) and 1g shaking table tests (e.g. Ibi et al.,below the embankment was squeezed out from the2000; Sasaki et al., 1992) have been conducted to
explore effectiveness of remedial countermeasuresembankment centerline towards free field exited throughbeneath embankment toes for reducing crest settlement. In

observation suggests that prevention of heaving near thethe above referenced studies, the remedial measures used

toe somehow will add further beneficial retrofitting effects.

in practice, including sheet pile enclosure, solidification

The authors have come up with an idea of introducing

and densification, were set beneath embankment toes or inhorizontal stress (hereafter in this paper, pre-stress) to a

the free fields just outside the toes. It has been shown thatsoil below the levee using sheet piles, in particular

sheet piling is less effective for reducing embankmentliquefiable soil below the embankment toes in the vicinity

crest settlement as compared with other techniques. It isof sheet piles. The schematic illustration of the idea isoften the case especially in populated urban areas thatshown in Fig. 2. Introducing the pre-stress to the soil canavailable land area is not sufficient to execute groundbe achieved by inserting wedge along with the sheet piles

improvement at around levee toe in the hinterland side andin the embankment sides and the pre-stress will bethus sheet piling is preferably employed. It is, therefore,

maintained by the elastic nature of the sheet piles, which

needed to develop a novel sheet piling method to reduce

are installed into the stiff bearing layer through the

crest settlement more effectively.

liquefiable layers. The merits of the pre-stress inthe path and heaved up the embankment toe. This

Fig. 1 shows a model of such centrifuge test before

combination with sheet piles are; (1) the introduced

and after shaking events (Okamura and Matsuo, 2002).

horizontal forces to the soil below the embankment will be

Sheet piles were installed vertically at both toes of the

proportional to the horizontal deformation of pile headembankment with the tips sufficiently embedded to a non-which is visibly identified, and (2) a creep deformation ofliquefiable bearing layer. Subsidence of the embankment

soil may reduce horizontal stress of the soil with time butbase line was attained its maximum under the crest and it

as far as the sheet piles are deflected, the pre-stress will

became very small or even turned about upward heave inkeep acting.

the vicinity of toes. The horizontal displacement

As liquefaction resistance of a soil, that is cyclic
stress ratio to liquefy the soil, is proportional to meaneffective stress, the increase in the horizontal stress by xenhances the liquefaction resistance accordingly. For
instance, for a soil with K0 initially equals to 0.5 and after
introducing pre-stress to 1.0 (K0= 0.5), the ratio ofliquefaction resistance of soil from before to afterFig. 1Model with sheet pile enclosure at both toes after

centrifuge shaking test (Okamura and Matsuo, 2002)

introducing pre-stress is (1+2K0+K0)/(1+2K0)= 1.25.
In this paper, a series of dynamic centrifuge tests on
the effectiveness of pre-stress in reducing embankment
settlement due to foundation liquefaction is reported.
2. Centrifuge test procedure
A series of centrifuge tests was carried out on model
embankments resting on a liquefiable loose saturated sandwith several different pre-stresses introduced by sheet
piles.
Cone penetration tests were also conducted at

Fig. 2 Pre-stress introduced by wedge penetration

locations in the models in order to better understand the- 244 -

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

extent of impact of the pre-stress introduced by the sheet

test, respectively.

piles on liquefaction resistance in the model.

Table 1.

Test conditions

Thickness of model

2.1. In-flight CPT

Induced sheet pile head

sheet pile

A medium dense uniform dry sand layer was built as

deflectionmm

mm

C0, E02.0

0C1, E12.0

5mm and fixed with an electronic actuator. Dry Toyoura

C2, E22.0

10sand was rained to a relative density of 50% in the

C3, E33.0

10

shown in Fig. 3. A sheet pile was set in a model container
with its tip rigidly fixed to the container base. The head of
the sheet pile was deformed horizontally to 5 mm or 10

container with internal dimensions of 500 mm wide, 230
mm deep and 120 mm long. The model was set on ageotechnical centrifuge in Ehime University and brought

2.2. Shaking tests on foundation soil withembankment

up to 30g. A miniature cone penetrometer with a 6 mmdiameter set on the two axes loading table was used tomeasure tip resistance profile. The cone was penetrated
vertically at a constant rate of 6 mm/s at locations P1, P2
and P3. The fixation of sheet pile head was released which
introduced increase in horizontal stress in the soil left handside of the sheet pile. The cone penetration was
successively conducted without stopping the centrifuge atanother locations P4, P5, P6 and P7. The test conditions
are shown in Table 1. In this study, two types of model
sheet piles and two head deformations were tested. The
model sheet piles were aluminum plates of 2mm or 3mm
thick instrumented with strain gauges. The flexural
rigidity of the model sheet piles scaled to that of a
prototype sheet pile of 1.3 MPa m4/m and 4.3 MPa m4/m.
The letter C and E refer to the CPT and the embankment

Embank models shown in Fig. 4 were prepared in thesame rigid container. The procedure of building thefoundation soil was the same as that for CPT tests. A setof sheet pile was fixed to the bottom of the container andthe heads are horizontally displaced to 5 mm or 10 mm
and locked with actuators before staring sand raining.
During the sample preparation, accelerometers and porepressure cells were installed at the proper locations andorientations. Potentiometers to measure settlement ofembankment crest and base were also set after theembankment was set on the foundation soil.
The soil used to construct embankments was a
mixture of Toyoura sand and Kaolin clay at 3:1 by dry
weight. The soil was prepared at water content of 10% andcompacted in a wooden mold of trapezoidal shape to a dry
density of 90% the maximum dry density. The
embankment in the mold was frozen in the refrigerator, seton the foundation layer in the container box and thawed in
the room temperature. On completion of thawing theembankment,

both

the

foundation

soil

and

the

embankment was fully saturated with viscous fluid in a
vacuum chamber, to the degree of saturation higher than
99.5 %. The viscous fluid was prepared by dissolving
1.6% Metolose by weight in water, so as to achieve aviscosity of 30 times the viscosity of water (30cst
Sheet pile

kinematic viscosity).
The model was set on the centrifuge and thecentrifugal acceleration was increased gradually to 30g.
The viscous fluid on the ground and in the pore of theembankment soil was drained through a stand pipe

Fig. 3

Schematic illustration of model for CPT

connected to the bottom drainage holes until the ground- 245 -

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

water table stabilized at 10 mm below the ground surface.

prototype scale otherwise mentioned.

The actuators unlocked the displacement of the sheet piles
head to apply pre-stress to the ground. It should be

3.1. CPT

mentioned that the procedure to displace the sheet pileFig. 6 depicts profiles of CPT tip resistance observed

head at 1g and unlock at 30 g closely simulates application

at several locations in the test C3. The tip resistanceof pre-stress to the soil below the embankment in the field

increased approximately linearly

condition. However, the procedure reduced horizontal

resistance at P1 and P3 before unlocking the sheet pilestress of the soil in the free field and this is not the case forhead was lowest and increased as the distance from sheet

the field condition. In this study, therefore, effects of pre-pile decreased after unlocking the head.

with depth. Tip

stressed sheet pile on the impact of mitigating soil

It has been reported that cone tip resistance, qc, isliquefaction and embankment settlement are modelledsensitive to not only vertical stress but also horizontal

conservatively.

stress and empirical relationships between qc and mean

The model was finally shaken horizontally with aeffective stress were proposed (e.g. Houlsby 1998,

mechanical shaker. The acceleration time histories of the

Jamiolkowski et al. 2003). Since liquefaction resistance of

event have the basic shape indicated in Fig. 5, with peaksand is proportional to mean effective stress (Ishihara,

accelerations of approximately Ah = 1.6 m/s2 and lasted

1985), in-situ soil resistance to liquefaction is considered

30 seconds in the prototype scale. Predominant frequency

Tip resistance, qc (MPa)0of the input motions was 0.7 Hz. The input motion in terms

1

2

3

4

5

of shape of the waves and each peak acceleration were
P1
P3
P5
P6
P7

repeated quite well from test to test.
All the models tested in this study had the foundation

1

Depth (m)soil depth of 120 mm (3.6m in prototype) and theembankment height of 50 mm (1.5m).

2

3. Results and discussion
All results, hereafter in this paper, are presented in

0Fig. 6

CPT tip resistance observed for C3

C1
C2
C3

2
Depth 0.6 m

Fig. 4

Schematic illustration of model with embankment for

Acceleration (m/s2)

shaking test

160gal2
1

Liquefaction resistance ratio

1.5

P5
P1

1

P2

P4

P3

0.5
2
Depth 1.2 m
1.5

P1

1

P2

P7
P4

P5

P6

P5

P6

P3

0.5
2
Depth 2.4 m
1.5

0

P7
P6

P1

1

P2

P7
P4

P3

-1

0.5

-2
0
3
2
1
0
-1
-2
-3
0

10

Fig. 50.2

20Time (s)30

40Time history of input acceleration 300gal

0.4

0.6
0.8
time(s)1

1.2- 246 1.4

10

5
Horizontal distance from sheet pile (m)Fig. 70

Liquefaction resistance ratio

2
1.8 Depth 60mm
1.6
1.4
P1
1.2
P2
P4
1
0.8
0.6

P7
P3

P5

P6

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

1.800

actuators as well as change in earth pressures on the sheet

Horizontal distance from sheet pile (m)Liq. Res. Ratio

20
6.0

30
9.0

10
3.0

0

0

pile. Change in earth pressure was estimated based on the

0.0

1.675

observed strain distribution of the sheet piles and found

1.550
33.3
1.0

1.300
1.175
1.050

that the coefficient of horizontal earth pressure at rest

Depth (m)1.425increased in the order of 0.2 and 0.4 for E1 and E2,
respectively. Strain of the sheet pile was unfortunately not

66.6
2.0

observed in E3.

0.9250

Excess pore pressures during the shaking event for

0.8000E0 and E3 are compared in Fig. 10. The soil at threeFig. 8

Contours of liquefaction resistance ratio of C3

locations of pore pressure cells seems to have liquefied
during shaking and continued to liquefy after certain

to be well estimated based on qc. Robertson and Weide

duration before the pressure started to dissipate. This is not(1998) proposed the following equation for clean sand;only for test E0 without pre-stress but also for E3 with the

𝑞𝑐

𝑃𝑎

𝑛

largest pre-stress. Knowing the liquefaction resistance of3( )( )CRR = 93 [ 𝑃𝑎 𝜎′𝑣0 ⁄1000] + 0.08

medium dense Toyoura sand to be approximately 0.15, the(1)factor of safety against liquefaction, FL, for the shaking
acceleration of 1.6 m/s2 is 0.47 for the level groundwhere CRR= cyclic resistance ratio to liquefaction, Pa=-1

atmospheric pressure, v0’= effective vertical stress andn= exponent that varies between 0.5 and 1.0 (Olsen, 1997).
Eq. (1) with the exponent n= 1. The liquefaction resistancein the vertical axis of the Fig. 7 are normalized with

Depth (m)Distribution of liquefaction resistance are estimated with

GL

0
1
2

respect to that before introducing pre-stress, i.e. average at

3

Before releasing
After releasing

increase at lateral distance from the sheet pile L= 4.5m.
0

The ratio increased as L decreases, with the ratio being

200

400

Fig. 9liquefaction resistance at P7 may also be influenced by theright boundary condition (i.e. the existence of the sheetpile) as well as by the horizontal confinement due to theelastic reaction of the sheet pile.
3.2. Dynamic tests of model with embankment
In the centrifuge at 30g, the fixed sheet piles head
was released before the shaking events. The strain
measured with strain gauges attached on the sheet pile forE1 and E2 are indicated in Fig. 9. The change in strain was
due to releasing the force at the pile head with the- 247 -

Acceleration (m/s2)

reflected in the liquefaction resistance ratio. Therefore, the1000

fixingExcess pore pressure (kPa)

the left boundary on CPT tip resistance at greater depth is

800

Strain of sheet piles before and after releasing theenhanced approximately 1.8 times. Distribution of theratio of C3 are also shown in Fig. 8. Note that the effect of600

Strain ()higher for shallower depth. For test C3 the liquefaction
resistance of the soil near the sheet pile is believed to have

E2

E1

P1, P2 and P3. Liquefaction resistance ratio begins to

2
1
0
-1
-2
40

(a) Input acc.

20

E0
E3

(b) Below crest

04020(c) Below toe040(d) Free field

20
0

50

Time (s)100150Fig. 10 Excess pore pressure responses for tests E0 and E3

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

representing pre-stress. The crest settlement reduced in the(a) E0model E3.
Since the sheet piles used in this study were rather
flexible, the soil liquefied and significant settlement
occurred even for the tests with pre-stress. But further
reduction of crest settlement can be expected if sheet piles
with higher rigidity are used.

(b) E3

4. Conclusions
In order to improve the effectiveness of sheet pile
enclosure technique on mitigation of levee settlement, prestress sheet pile method was proposed. The effects of themethod were verified with a series of centrifuge tests. It
was confirmed that by introducing horizontal stressincrement in the soil below embankments, extent ofliquefaction and crest settlement were mitigated.

Fig. 11 Photos of model E0 and E3 after first shaking event

Nomalized settlement

(%)

40
35
30
25
20

Fig. 12

Potentio meter
Photo
0

0.5
1
1.5
EI・S (N mm3)2
[108]

Reduction in crest settlement with pre-stresscondition. Although the liquefaction resistance enhanced
approximately 1.8 times in the vicinity of the sheet plie,
FL value was 0.85, and thus still blow the unity. The
consequences that all the models have liquefied isbelieved to be reasonable. Excess pore pressures below the
crest and toe indicated in Fig. 10(b) and (c) for the E3 aresomewhat lower and duration of liquefaction lasted after
the shaking ceased was shorter as compared to those forE0. This difference is believed to be associated with thepre-stress introduced to the model E3.
Fig. 11 presents photos of E0 and E3 after shaking.
Deformation of the foundation soil, especially in the zone
near the sheet piles below embankment toes are reduced
for the case of E3. Crest settlement normalized with regard
to the initial embankment height are shown in Fig. 12. The
abscissas, product of flexural rigidity of the sheet pile andthe initial imposed deformation, indicates an index

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