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

Evaluation of Effectiveness of PFS Method Using 3D Finite Element MethodKeisuke TANAKA
Engineer, Kajima Corporation, Tokyo, Japan

Makoto KIMIZU
Engineer, Takahara Wood, Yatsushiro, Japan

Jun OTANI
Professor, X-Earth Center, Kumamoto University, Japan
Email: junotani@kumamoto-u.ac.jp

Teruo NAKAI
Professor Emeritus, Nagoya Institute of Technology, Japan

ABSTRACT
In order to improve the effectiveness of steel sheet-pile method for the countermeasure on soft ground settlement underembankment load, a PFS method was developed under the research group whose chair was Prof. Ochiai of Professor
Emeritus at Kyushu University, Japan in 2005. At that moment, a series of in-situ full scale tests for this countermeasuremethod including PFS method which is the combination of end bearing sheet-pile with those of floating type wereconducted in the City of Kumamoto, Japan under the Ministry of Construction (current name of this ministry is Ministryof Land, Infrastructure, Transportation and Tourism).
In this paper, the main objective is to discuss the quantitative evaluation of PFS method using coupling finite element
analysis. Because of the geometry of PFS structures, this analysis was conducted in three dimensions. A numericalanalysis was done for the cases of the full scale tests at the site in Kumamoto City, in which the field tests with the caseof PFS method was conducted. A “tij model” developed by Nakai et al which can be considered the effect ofintermediate principal stress was used as a constitutive model for clayey soil. Here, not only displacements in theground but also the change of excess pore water pressure were compared.
Finally, based on those numerical studies, an effectiveness of PFS method as a countermeasure method was discussed.
Key words: Steel sheet-pile, Soft soils, Countermeasures, Numerical Method, Elastoplastic

reduction of subsidence is expected at the area where the1. Introduction
Steel sheet-pile has been widely used, especially for

private housings are located. When the embankment isearth retaining or excavation. In those cases, the reduction

constructed on soft ground, the ground subsidence for notof lateral displacement with its total stability has been

only the ground under the embankment but also the ones

expected and the installation of the sheet-pile is relatively

around embankment are serious problems and someeasy, so that most of those works have been realized as acountermeasures have to be considered. A steel sheet-pile

temporally ones. However, recently this sheet-pile method

method is one of the countermeasures for this problem as

is being used as a permanent method for the prevention

shown in Fig. 1. However, this type of structure has a cost

technique on slope failures, and that for the case ofproblem when the area and depth of soft ground are wider

embankment construction, the sheet-pile has beenand deeper,

constructed at the toe of embankment due to the purposeof stress shut down to the surrounding ground, so that the- 209 -Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Embankment

Soft ground
Soft ground(consolidation)
Bearing stratum

End bearing Sheet pile(a) Without countermeasures

Floating Sheet pile

Embankment

Fig. 3 PFS method

a new sheet-pile method called PFS method (Partial
Sheet pile

Floating Sheet-pile) was proposed under the activities ofthis committee. In this method, the end bearing sheet-pile

Bearing stratum

and that of floating type were combined to deal with its
effectiveness and cost as shown in Fig. 2. Fig. 3 shows(b) With sheet-pile countermeasures
Fig. 1the details of this structure (PFS Method, Technical

Sheet-pile countermeasures

Manual, 2005).

so that a development of new sheet-pile method was

In this paper, first of all, in-situ measurements at theexpected.

site is introduced as a performance of PFS method andIn 1975, a collaborative study was started between

then, a numerical modeling using 3D FEM is conducted

Kyushu University and the Ministry of Construction

for one of the PFS construction sites. Finally, based on(Ministry of Land, Infrastructure, Transportation andthe results shown in this paper, the effectiveness of PFS

Tourism at present) in Japan. Under this collaboration, amethod is convinced and the next step on this PFS

series of in-situ full scale tests were conducted in Kyushu

method is briefly discussed.

area. Based on those activities, a research committee fordeveloping a new sheet-pile method was established in

2. Case history

2003 in which the chair was Prof. Hidetoshi Ochiai,A large number of in-situ full scale tests for PFSProfessor Emeritus of Kyushu University, Japan. In 2005,

Soft ground(consolidation)(b) Floating type(a) Conventional type
Effectiveness: highEffectiveness: low

CostCost: high(c)(a)+(b)
Effectiveness: high
Cost: low
Fig. 2

Idea of PFS method- 210 -: lowProceedings of the First International Conference on Press-in Engineering 2018, Kochi

method were conducted in Kumamoto City, Japan. This

the PFS method is clearly shown. Since a large volume ofarea is well known as a region of Ariake clay which issheet-pile materials were reduced, the cost of the PFShighly sensitive clay and its depth is up to 40m. Fig. 4

method is obvious and the construction time is also highly

shows the soil profile at the site of in-situ test for PFS

reduced because of the less volume of the sheet-piles.

27.5m
Embankment
Clayey soil
Sandysoil
Clayey soil

3. Numerical analysis
4.0m

3.1. Site condition
A series of numerical modeling were conducted for

Floating

one of the site of PFS method. First of all, theSheet pile

information at the site in Kumamoto City, Japan is shown

L=19.5m

Clayey soil
Sandy
Clayey soilsoilin Fig. 7, in which the height of embankment was 4m

End bearing

with 48 m width and the total length of the sheet-pile for

Sheet pileend bearing was 30m while the floating part was 20m.

L=34.5m
One end bearing Sheet pile for five Floating
Sheet piles

Fig. 4The embankment was constructed by step by step
loadings with total of 40 days as shown in Fig. 8. Fig. 9

Ground condition at the site

shows the soil condition, which is the multi-layer of sandmethod. In this case, one end bearing sheet-pile for fiveand Ariake clay. In fact, the layers of Ac1, Ac2-U andfloating sheet piles were constructed. Fig. 5 shows theAc2-L were relatively soft clayey soils and the water

results of measurement for the settlements at the site andlevel was relative shallow as 0.8m depth. It is obviousFig. 6 shows the results of lateral displacement at the toethat total depth of the ground (H=25m) is mostly soft soilof embankment. As easily realized, the effectiveness ofwith SPT N-value of nearly zero.
3.2. FEM modeling
Based on the results of soil tests at the site andEmbankment

laboratory for the soils at the site, all the soil parameters

Settlement (mm)were determined as shown in Table 1, in which the sandyground was assumed as an elastic material while tij model(Nakai, 2013) was used for clayey ground. An
After 4 years
and 1months
Fig. 5

Measurement results of subsidence at
the ground surface

elasto-plastic consolidation analysis was conducted with
Finite Element Method (FEM) and here, because of theshape of PFS method, three dimensional analysis wasconducted. Fig. 10 shows the 3D mesh with those scaling
for this analysis in which the total number of elements is4550 for the ground. Sheet-pile was modeled by beam

048m

4.0m

盛土

20m

軟弱30m

層

支持2m8m層Fig. 6Measurement results of lateral displacement atFig. 7 PFS method at the site

the toe of embankment- 211 -

48m

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

盛土

88

盛土施工高(m)3.3. Results
and discussion
66

75Fig.
11 shows the comparison between numerical

軟弱層

66
54431

different1 clayey layers were compared. As easily realized

支持層
505000100150100150
経過時間(day)

20020000from thosefigures,
the numericalresults cansimulate
050
100
150200
250

250
250

08m

Loading step at the site for FEM

B

100

150

200

250

same type of comparison for excess pore water pressure2m

Fig. 850

real behaviors relatively well.
Fig. 12 also shows the経過時間(day)Elapsed time (day)

00.8
3.1
4.6

3settlements.
Here, total settlement and those of 3
22

2200

44

results with those of observations at the site for

30m

盛土施工高(m)height (m)Embankment

720m

88

for different soil layers (Ac1, Ac2-U, Ac2-L, and As2).
Although there are some difference between analysis

BB
Ac1
Ac1埋土
Clay
埋土
粘性土
粘性土
Clay
粘性土
粘性土
Clay

those of measurements, especially just afterB results
24m
埋土4and盛土
m2
the end of embankment, itPFS工法
can be said軟弱地盤
that over all
Am1
Am1
Ac1behavior
粘性土
8.0
seems close enough for both results. Fig. 13
As1
砂As1
Sand
砂9.4
Am
1
粘性土
Am2
粘性土
粘性土
Am2
Clay
shows the comparison between numerical results withAc2-U
As2-U 粘性土
粘性土
Ac2-U
ClayAvAs1those砂 observations for the distribution of lateral
17.2
砂Av
砂Av
Sand
Ac2-L
Ac2-L 粘性土
Ac2-L
粘性土
Clay
Am2displacement
粘性土 at the toe of the embankment after 250 days.
23.0
ZAs2
As2
砂
SandAs2砂
those comparisons,the numerical results can150m
Y
Ac2-UAs粘shown
性土
24.4
Dc1
Dc1
Dc1 粘性土
粘性土
Clay
Xreal behavior to some extent and as a result, 3D
Dg1
Dg1
砂礫
Dg1 Gravel
砂礫
Av simulate
砂30.0
0 10 20 30
FEM with elasto-plastic constitutive model by tij model
深さ(m)N値
Depth(m)Ac2-Lcan粘model
性土 the real behavior, fairly well.
N-valueFig. 9 Soil profile at the site for FEM
As2 砂elements in which joint element was not used for the Dc14. Concluding
remarks
粘性土interaction behavior. And in the analysis, the step of Dg1 砂In
礫 this paper, the numerical analysis on the PFS
34m

Ac1
Am1
As1
Am2

embankment during construction was modeled by adding

method was compared with the measurement results at

solid elements with its weight based on the loading

the site and fairy good agreements were obtained.

process as shown in Fig. 8. In the analysis, the results such

However, the location of the site of full scale test is

as total settlement at the center of the embankment withlimited, so that the performance of PFS method withthat of each soil layer and the change of excess pore water

different soil conditions have to be checked. In fact, onepressure were calculated.

of the important requirement for the use of steel pile isTable 1.

Soil parameters for FEM- 212 -

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

Boundary conditions:Y-Z plane: X-direction fix,
Drain condition

34mZ-X plane: Y-direction fix
ZUndrain condition
YBottom plane: X, Y, Z-directions fix
150m00Time step (days)
100
150
100
1505050Settlement (mm)00-200200-400
400-600
600-800
800
-1000
1000
-1200
1200

3D FEM modeling

0-200
200-400
400-600
600-800
800-1000
1000
-1200
1200

250
250

Observation
Analysis result(a) Total settlement
00

Time step (days)
100
150
100
1505050Settlement (mm)

0200
200

200
200

250
250

Observation
Analysis result(e) Ac2-U layer

Excess pore pressure (kN/m2) Excess pore pressure (kN/m2) Excess pore pressure (kN/m2) Excess pore pressure (kN/m2)

Fig. 118080050

Time step (days)
100
150

20000-200
200-400
400-600
600-800
800-1000
1000-1200
120000505000-200200-400
400-600
600-800
800-1000
1000-1200
1200005050

図6 盛土中央地盤沈下量比較

Comparison of ground settlements

250

20208080

050

050(a)100
Ac1 layer150
Time step (days)
100
150

200

250

200

250

Observation
Analysis result

6060
4040
20
20008080

050

050

100150(b) Ac2-UlayerTime step (days)
100
150

200

250200250

Observation
resultFig. 12 Analysis
Comparisonof excess pore water pressures

6060
4040
2020008080050050100150(c) Ac2-LlayerTime step (days)
100150200250200250

Observation
Analysis result

60604040- 213 -202000
050(d)100As2 layer150

図7 盛土中央直下地盤

200250250250(b) Ac1 layer

404000200200Observation
Analysis result

Observation
Analysis result

6060

Time step (days)
100150100150

Settlement (mm)Fig. 10

Undrain condition

32m

Time step (days)
100150100150

Settlement (mm)X200200250250

Observation
Analysis result(g) Ac2-L layer(d) Ac2-L layerProceedings of the First International Conference on Press-in Engineering 2018, Kochi

how much the lateral displacement at the toe ofembankment and this allowable value has to beReferences
PFS Method

TechnicalManual.

2005. Technicaldetermined to generalize this PFS method. And one more,

Committee of PFS Method, JASPP, Japan. (inas many of engineers concerned, the behavior during

Japanese)

earthquake is important, especially in Japan. Therefore,

Nakai, T. 2013. Constitutive modeling of geomaterials

the confirmation of performance under earthquake-Principals and applications-. CRC Press.

should be examined. Those should be the next target for
the PFS method.
5. Acknowledgements
As mentioned in this paper, the development of PFS

300Lateral flow (mm)
0 20 40 60 80 100120

method was done under the research group of PFS
255University, Hidetoshi Ochiai. The authors would like togive their sincere gratitude to all the members of thiscommittee for their enormous efforts to develop thismethod.

Ground depth (m)method whose chair was Professor Emeritus of Kyushu

201015151020255

Observation
Analysis result

3000Fig. 1320406080100

toe of embankment- 214 -120

Lateral displacement atthe