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

Estimation of External Force Acting on Steel Pile of Steel Pile Reinforced
Breakwater
Koyo HIKICHI
Graduate Student, Department of Civil Engineering, Graduate School of Tokyo University of Science, Noda, Japan.

Yoshiaki KIKUCHI
Professor, Department of Civil Engineering, Tokyo University of Science, Noda, Japan.

Taichi HYODO
Associate Professor, Department of Civil Engineering, Tokyo University of Science, Noda, Japan.

Atsushi MOHRI
Graduate Student, Department of Civil Engineering, Graduate School of Tokyo University of Science, Noda, Japan.
Email: 7617618@ed.tus.ac.jp

Keiichi AKITA
University Student, Department of Civil Engineering, Tokyo University of Science, Noda, Japan.

Naoki SHOJI
University Student, Department of Civil Engineering, Tokyo University of Science, Noda, Japan.

Shinji TAENAKA
Senior Researcher, Department of Steel Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, Futtsu,
Japan.

Shunsuke MORIYASU
Senior Researcher, Department of Steel Research Laboratories, Nippon Steel & Sumitomo Metal Corporation, Futtsu,
Japan.

Shin OIKAWA
Manager, Department of Construction Products Development Div, Nippon Steel & Sumitomo Metal Corporation,
Chiyoda, Japan.

ABSTRACTThe coastal areas of the Pacific Ocean suffered extensive damage by the tsunami of the 2011 off the Pacific Coast of
Tohoku Earthquake. Then, there are needs to develop reinforcement methods of existing caisson type breakwaters against
tsunami force. Installing a row of piles behind the caisson with backfilling, the space between the caisson and piles withrubbles is proposed as one of the reinforcing methods against tsunami. In order to establish the design method of thisreinforcing method, the design of the steel pile is the most important. For designing steel piles, it is necessary to estimate
the load acting on them from the caisson side. Here, a series of model loading experiments were conducted. The
experimental results suggested that the external force acts on the pile only in the ground at the initial stage of loading andit was found that the external forces acting on the piles dominate in the vicinity of the backfilling part by graduallycompressing the backfilling part by displacement of the caisson.
Key words: break water, steel pile, reinforcement, subgrade reaction, tsunami- 281 -

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

1. Introduction
The coastal areas of the Pacific Ocean sufferedload distribution received from the caisson side, and theextensive damage by the tsunami of the 2011 off thepile when receiving external loads were experimentally

Pacific Coast of Tohoku Earthquake. In particular, many

investigated with different number of steel piles.

effect of the maximum bending moment generated in the

breakwaters that had kept the port facilities safe weredamaged. Under these circumstances, rebuilding design

2．Experiment outline

concept is required. Moreover, it has been predicted withIn this study, a series of model loading experimentsa high probability that the Tonankai, the Nankai Troughwere conducted by changing the spacing between the steel

earthquake might occur in near future. Therefore, the port

piles behind the caisson. This schematic diagram of thestructures are strongly required to be strengthened againstmodel set up is shown in Fig. 2.

large tsunamis, by reinforcing the existing caisson
breakwaters more than ever. Therefore, by constructing
breakwaters that "have toughness" against the tsunami, it
is conceivable to make it possible to protect as many
people as possible when a huge tsunami occurs.
In order to improve performance of breakwaters
against tsunami, Moriyasu et al. proposed a method toreinforce a breakwater by installing steel pipe piles behindthe breakwater and then filling the space between themusing aggregates as shown in Fig. 1 (Kikuchi et al., 2015).
This reinforcement method intends to effectively use theresistant behavior of the ground by using steel piles. It has
been confirmed that this reinforcing method has highFig. 2

Schematic diagram of the experimental model set up

resistance against horizontal force during horizontal
loading and hydraulic flume experiments (Arikawa et al.,The experiment model was designed at 1/60 scale2015). In order to improve the accuracy of design method

according to similitude shown in Table 1 (Iai, 1988). Aof this structure, it is necessary to estimate the external

steel sand box of 1580 mm long, 800 mm high and 400

load received from the caisson side of the steel pipe pile in

mm wide was used to prepare the model ground as showndetail, and to check the maximum bending momentin Fig. 2. Dry silica sand #5 (D50 = 0.591 mm, Gs = 2.647,

responding to actions of external force.

emax = 1.072, emin = 0.689) was used to prepare the model

Here, a series of model loading experiments wereground. The model ground was prepared with 80% ofconducted. The influence of the steel pile on the external

relative density by the air pluviation method. The steel
piles (800 mm long, 30 mm wide, 2.3 mm thick, flexural
rigidity EI = 6.24 × 103 kNmm2, 463 mm penetration
depth) modelling a steel pipe pile were set at a position
separated 50 mm from the back of the caisson (380 mm
long, 300 mm high, 300 mm wide, γt = 22 kN/m3). Here,experiments were carried on with changing a center tocenter distance of the piles. A center to center distance ofthe piles was selected from 120 mm, 60 mm, 33 mm, andthe number of piles corresponding to 3, 6, and 12,
respectively. Table 2 shows the experiment cases. A
backfill of 50 mm height was installed between theFig. 1

Schematic diagram of a reinforced gravity typecaisson and the steel pile. The material for the backfill was

breakwater

gravel of D50 of about 10 mm (Gs was about 2.65, γd was- 282 -

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

12.1 kN/m3). A Teflon sheet was affixed to the loading

points on the upper surface. For the piles at the center andsurface of the caisson model to reduce friction. On theside end, as shown in Fig. 3, 20 strain gauges were

bottom of the caisson, sandpaper #150 was attached.

attached to both sides of the pile in the depth direction, andFriction was reduced by using a Teflon sheet and a

the bending strains were measured.

membrane on the contact surface between the ground andthe sand box. In the loading experiments, a horizontal load

3．Experimental results

assuming a tsunami wave force was applied. The

Fig. 4 shows the relationship between horizontal load

horizontal load was applied under displacement control atand displacement at the center of the caisson in each case.

the height of 150 mm, and the monotonous loading speed

It is understood that the resistance of the caisson decreases

was at 1 mm / min.

by decreasing the number of piles. However, until the

Horizontal displacement of the caisson was measured

horizontal displacement of the caisson gravity centerusing LVDTs, fixed at 4 points on the load surface and 3

reached 25 mm, there was almost no difference inresistance between 6 and 12 piles.

Similarity ratio (Iai,1988)
Prototype/model1
1
1/λ1/2
1/λ3/41/λ1/λ1/λ3/2
1/λ3
1/λ7/2Items(λ=60)
Density
Acceleration
Ground strain
Time
LengthEffective stress
Ground displacement
Bending moment
Flexural rigidity

1000
12 Piles
750
630

Load(N)

Table 1.

500

250
Table 2.
Numberof piles3
6
12

3 Piles

6 Piles

The details of test conditions.

Center to center
distance of piles,l (mm)
120
60
33

Embeddedlength of pile
L (mm)
463

Total flexural
rigidity, EI(×104kNmm2)
1.87
3.74
7.48

0

0
15
30
45
Displacement of the center of the casson (mm)Fig. 4

Relationships of load and the displacement at the centerof the caisson under various number of the steel piles.

Fig. 5 shows the bending moment distribution of thecenter pile in each case at the load of 630N. The bending
moment distribution was obtained by approximation using
the spline function from the bending moment calculated
by strain gauges. As shown in Fig. 5, as the number ofpiles decreases, there is a difference in the load handled by
one pile, so the smaller the number of piles is, the larger
the maximum bending moment becomes.
Fig. 6 shows the deflection distribution obtained bysecond-orderintegrationof thebending

momentdistribution in Fig. 5 by the bending stiffness EI of the pile.
Fig. 7 shows the load distribution acting on one pile foundby second-order differentiation of the bending moment
distribution in Fig. 5. This load distribution is theFig. 3

Schematic diagram of the experimental model setup.(the view from the rear of the caisson.)difference of the loads acting on both sides of the pile, andthey cannot be separated. Therefore, with respect to thesubgrade reaction acting on the steel piles acting on the- 283 -

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

passive side of the pile, assuming the PHRI method of typewere set similarly to those of the caisson experiment andS model shown in Eq. (1), and the coefficient of subgrade

statically loaded with a loading height of 60 mm.

reaction called ks required at this time was separately
estimated by pile loading experiments (Kubo, 1964).

Fig. 8 shows the relationship between the pile headload acting per pile and the ground surface displacementin the pile head horizontal loading experiment. The pile0.5

p = 𝑘𝑠 ・𝑥・ｙ

(1)
3.5

head load acting per pile was determined from the slope

where ks: coefficient of subgrade reaction (kN/m ), x:of the bending moment above the ground by measuring

depth(m), y: deflection of steel pile at depth x (m), p:

the bending moment by the strain gauge attached to the2subgrade reaction per unit width at depth x (kN/m ).

aerial part and the ground surface. As you can see from

Since ks is influenced by pile width and groundthis figure, the pile head load acting per pile turns large

rigidity, the conditions of the pile (rigidity, number of piles,

when the ground surface displacement is equal bycenter pile spacing) and the ground preparation method

reducing the number of piles. However, until the groundsurface displacement reached 3 mm, there was almost no
difference in the load acting on 3 piles and 6 piles.

400

630N
40

6 Piles
3 Piles

0-200

12 Piles

-400

-600

Fig. 5

Pile head load acting per pile (N)Depth(mm)

200

0.000
0.003
0.006
Bending moment(kNm)

30
24
20

12 Piles
10

0

0

5

10

15

Ground surface displacement (mm)Bending moment distribution under various number ofthe steel piles at the horizontal load of 630N

6 Piles

3 PilesFig. 8Relationship between the pile head load acting per pileand the ground surface displacement in the pile head

400

400

12 Piles6 Piles200

200

3 Piles

6 Piles

Depth(mm)3 Piles

Depth(mm)horizontal loading test.

630N

0

-200

From the relationship between the pile head load andthe ground surface displacement shown in Fig. 8, ks as the0whole ground was obtained based on the PHRI method of
12 Piles-200

type S model. The relationship between ks and the groundsurface displacement is shown in Fig. 9. From Fig. 9, it is-400

observed that the initial ks matches for 3 and 6 piles but

-400

630N

the ks with 6 piles at the ground surface displacement of 3

-600
0

20

40

60

-600
-40

80

Deflection(mm)

-20

0

20

40

Load(kN/m/m)

mm is slightly smaller. The reason is inferred as theinfluence of the group pile effect in the case of 6 piles,

Fig. 7 Load distribution

because the receiving area of the adjacent pile interfered.

under various number of theunder various number of the

In the case of 12 piles, ks has a small value from thesteel piles at the horizontal

steel piles at the horizontal

beginning. This is because the influence of the group pileFig. 6 Deflection distribution

load of 630N

load of 630N

works immediately after loading as the pile center interval- 284 -

is very narrow.

24N

0

In Fig. 9, ks represents the overall behavior of the pile.
-100

Therefore, ks was estimated from the subgrade reaction

-200

3.5

Coefficient of subgrade reaction ks (kN/m )and deflection based on the measured values at each depth.

Depth (mm)However, ks is not constant in the depth direction.

-300

Maximum bending moment
generation depth lm,max

PHRI method of type S model
3.5
ks=2500(kN/m )

-400

7500
-500

Actual measurement
value

relation to the normalized depth, x / lm, max

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi-600-15 -10 -505
10Subgrade reaction (kN/m/m)

3 Piles

0
-1
-2
-3
-4
-5
-6

0.0 0.2 0.4 0.6 0.8 1.0

ks/ks0

5000

Fig. 106 Piles

2500

Subgrade reaction

Fig. 11

distribution in the pilethe coefficient of subgrade

head horizontal loading

reaction for each depth

test

12 Piles

100

100

0

630N
0

5

10

15

0

6 Piles

6 Piles0

Ground surface displacement (mm)displacement

-100

-200

-300

Fig. 10 shows the subgrade reaction distribution

3 Piles

-200

-300

-400

630N

value when the pile head load acting on one pile is 24N

-500
-40

under 12 piles and the subgrade reaction distribution by
the PHRI method of type S model when ks is constant

3 Piles-400

based on the bending moment of the actual measurement

12 Piles-100

12 PilesDepth (mm)Relationship between ks and ground surface

Depth (mm)Fig. 9Change model of

-20

0

20

Subgrade reaction (kN/m/m)

Fig. 12along the depth. In addition, ks in the PHRI method of type

Subgrade reaction

-500
-10

0

10

20

30

40

External force (kN/m/m)

Fig. 13

The external force

distribution under various

distribution under various) from the result in Fig.

number of the steel piles.

number of the steel piles.

9. At this time, the maximum bending moment generated(Horizontal load of 630N)(Horizontal load of 630N)

S model was set to 2500 (kN/m

3.5

at depth lm, max was -93 mm. In order to be consistent withthe actually measured subgrade reaction distribution, ks

reaction shown in Figs. 9 and 11 and the deflection

from the ground surface to lm, max is ks0 and ks / ks0 is shown

distribution shown in Fig. 6. Here, from the results in Fig.

in relation to the normalized depth x / lm, max in the Fig. 11.

9, the values of ks0 for 3, 6, and 12 piles are set to 5300,

The conditions of other piles (number of piles, pile center

5000, and 2500 (kN / m 3.5), respectively. Fig. 13 shows

spacing), was also examined load conditions, regardless ofthe external force distribution from the caisson side acting

the load level in PHRI method of type S subgrade reaction

on one pile at a loaded load of 630 N, which is obtainedmodel, at a depth from the ground surface lm, max, subgradeby subtracting the load distribution in Fig. 7 and thereaction can express with high accuracy with constant ks.

subgrade reaction distribution in Fig. 12. Looking at the

However, for depths beyond lm, max, unless ks is varied

external force distribution, it is understood that in eachaccording to Fig. 11, the result is not consistent with thecase the external force acting in the vicinity of the depth

estimate.

of 50 mm to -100 mm is larger than the depth of -100 mm

The distribution of external forces at 3, 6, and 12

or more. In addition, it can be seen that the magnitude ofpiles in the series of model loading experiments was

the external force distribution in each case does not change

examined. Fig. 12 shows the subgrade reaction

very much at a depth of more than -100 mm. Comparing

distribution calculated from the coefficient of subgradeeach case with a depth of 50 mm to -100 mm, it can be- 285 -

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

seen that the smaller the number of piles is, the larger the100

external force acting from the vicinity of the back-in

0100

12 Piles630N

0

portion becomes.
-100

acting on all the piles estimated as the loading load wascompared (Table 3). Since the external force distribution

A
A
A

630N

-100

Depth (mm)Depth (mm)The resultant force of the external force distribution

12 Piles

396N
-200

897N
-300

396N
-200

897N
-300

shown in Fig. 13 is the external force distribution per pile-400

in the center part, the resultant force of the external force
multiplied by the number of each pile was used. In the case

-400

-500
-30

of 12 piles with a very narrow pile center spacing, the

-20

-10

0

-500
-30

10

Subgrade reaction (kN/m/m)

-15

0

15

30

Load (kN/m/m)

resultant force of the estimated external force was 715 N

Fig. 14 Subgrade reaction

Fig. 15

against the loaded load 630 N. In the case of three piles

distribution for the load level.

for the load level. (12 piles.)and six piles, the resultant force of the estimated external(12 piles.)Load distribution

force was 295 N and 480 N, respectively, compared to the100

loaded load 630 N, resulting in a large difference between

630N

them. In addition, the resultant force of the estimated0external force obviously became smaller than the loading
force. From these results, in the case where the pile center

Depth (mm)distance is wide, not all external forces from the caisson
act on the pile, but the external force acting between thepile and the pile, passes to the inside of the port andresisted by the ground. Looking at the external force acting
on one pile, the resultant force of the estimated external
force was 98.3, 80.0, and 59.6 N at 3, 6, and 12 piles,

897N

-100
-200
-300
-400

respectively. From this, it was found that the external force

12 Pilesacting per pile is increased by widening the center to

-500
-10

center of piles.

0

10

20

30

External force (kN/m/m)

In the case of 12 piles, the estimation accuracy was
also confirmed for some load levels. Fig. 14 shows the

396N

Fig. 16

External force distribution for the load level. (12 piles.)subgrade reaction distribution acting on one pile at someload levels with 12 piles. This is based on the deflection

external force does not act in the backfilling portion of 50

distribution

momentmm to 0 mm at the loading level of 396 N, which is the

distribution obtained from an experiment by the second

early stage of loading, and it acts in the ground. The reason

order integration and dividing by the flexural rigidity EI

may be the influence of the lateral acting force generatedof the pile and the coefficient of subgrade reaction as

by the weight of the caisson and the friction of theshown in Fig. 11 by applying the PHRI method of type S

caisson's bottom. At the loading level of 630 N, anmodel. Fig. 15 shows the load distribution acting on one

external force acted on at the depth of 50 to -100 mm. It ispile at some load levels with 12 piles, which is obtained

considered that this external force was generated as thefrom second-order differentiation of the bending moment

caissondistribution obtained in the experiment.

compressed. When the loading load is 897 N, the external

obtained

from

the

bending

moved

and

the

backfilling

portion

was

The external force distribution from the caisson sideforce generated in the ground was almost the same as 630

acting on one pile found by subtracting the groundN, but the external force generated in the vicinity of thereaction force distribution in Fig. 14 and the load

backfilling portion dominated.
In order to investigate the validity of the estimateddistribution in Fig. 15 is shown in Fig. 16. Looking at the
distribution of external force, it is understood that theexternal force distribution, the resultant force of the- 286 -

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

Table 4.

Ratio of the resultant force of external force and

100

100

0

0

Vicinity ofthe backfillingis increased

loading force. (12 piles.)②/①×100
(%)
104
113
105

Vicinity ofthe backfillingis increased-100external force distribution acting on all the piles estimatedas the loading levels were compared (Table 4). Since theNot applied at
a depth of morethan -60 mm

-200

-200

Standard-300

Not applied at
a depth of morethan -60 mm

-400

-100

Depth (mm)②Resultant force of theexternal force (N)
413
715
945

Depth (mm)①Loading force
(N)
396
630
897

-300

Standard-400

external force distribution shown in Fig. 15 is the external
-500
-10

force distribution of a pile in the center, the resultant force

10 20 30 40 50

Fig. 170.0000 0.0025 0.0050 0.0075

Bending moment (kNm)

External force (kN/m/m)of the external force multiplied by the number of piles wasused. As can be seen from Table 3, since the difference-5000Changing the external

Fig. 18

force distribution

between the load force and the resultant force of external

Bending moment

distribution

forces estimated was error of 4~13%, it can be concluded
400

that the external force can be estimated with relatively
good accuracy even at the load level. The bending moment

200

400
PHRI method
Actualmeasurement value

200

external force distribution used for calculation. It is
indicated by a solid black line with reference to the

Depth (mm)force distribution shape was considered. Fig. 17 shows the

Depth (mm)distribution acting in the pile when changing the external
0

-200

0

-200

external force distribution of three piles in Fig. 13. The-400

blue solid line is the case where the external force

PHRI method
Actualmeasurement value-400

distribution is not applied at a depth of more than -60 mm.
-600

The red solid line is the case where the external force in

0.00000.00250.0050

Bending moment (kNm)the vicinity of the backfilling part (depth 50 mm to - 50

Fig. 19mm) is increased.

Comparison of

-600
0

20

40

60

Deflection (mm)Fig. 20

Comparison ofbending moment distribution

deflection distributions ofin Fig. 18. Looking at the black solid line and the blueof actual measurement

actual measurement value

solid line, it can be seen that the bending moment

value and calculated value.

The bending moment distribution obtained is shownand calculated value.

generated in the pile hardly changed, although the external
force at the depth of -60 mm or more was not applied at

S model. However, when using this model, it is necessary

the blue solid line. From this, it can be seen that theto confirm whether the behavior of the actual pile can be

external force occurring at the deeper point hardly

expressed. Therefore, the inverse analysis was carried out

influences the bending moment distribution shapeon the validity of the PHRI method of type S model using

generated in the pile. Next, looking at the black solid line

the calculation program similar to the previous

and the red solid line, it can be seen that the bending

examination. Fig. 19 shows the bending moment

moment generated in the pile is larger in the red solid line

distribution of three piles in Fig. 5 and the bending

with the larger external force acting near the back filled

moment distribution obtained by inputting the reference

portion. From this, it was found that the bending moment

external force in Fig. 17 in the calculation program. Fig.

distribution generated in the pile can be estimated with20 shows the deflection distribution by the same

relatively high accuracy by considering the external force

comparison. Looking at these, it is understood that thein the vicinity of the backfilling part.

bending moment and the deflection distribution coincide

Above all, the subgrade reaction acting on the steel

very well between the two. In the case of other pilepiles acting on the passive side of the pile has been

conditions and loading levels, calculation of above

assumed and investigated using the PHRI method of typementioned procedure was conducted and the results- 287 -

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

showed good agreement with this conclusion. From this,model, the behavior of the pile I could confirm that I

it was confirmed that the actual behavior of the pile can be

could express it.

represented by using the PHRI method of type S model.

This series of experiments was conducted under
extremely limited experimental conditions. The authors
consider it is necessary to investigate the effects of

4．Conclusions
In this study, the authors examined the difference ofimportant parameters such as ground condition, the filledexternal forces received from the caisson side whengravel between the pile and the caisson, and so on. Andchanging the number of piles to be placed on the groundthe authors hope to construct design procedures for thisbehind the caissons to 3, 6, and 12, and following

kind of improvement.

conclusions were obtained.
1) Using the PHRI method of type S model, the groundreaction force distribution was obtained by changing

Refferences
Moriyasu, S., Tanaka, R., Oikawa, S., Miyamoto, T.,

ks at the depth of maximum bending moment

Yoshiwara, K., Kikuchi, Y., Kawabe, S. and Mizuno,

generation depth lm, max, and as a result of estimating

R. 2013. Efforts to achieve a tough structure of

the external force distribution, in the 12 cases, thetsunami measures breakwater that utilizes steel wall.

external force estimated as the load The difference of

7th Nankai earthquake Shikoku region Academic

the resultant force was about 4~13%, and it was

Symposium. (in Japanese)

estimated with high accuracy. In the case of 3 or 6

Arikawa T., Oikawa S., Moriyasu S., Okada K., Tanaka R.,

piles, the resultant force of the external force is much

Mizutani T., Kikuchi Y., Yahiro A., and Shimosako

lower than the loading force, not all the external

K. 2015. Stability of the breakwater with steel pipe

forces from the caisson are transmitted to the pile, but

piles under tsunami overflow. Technical note of Port

the external force acting between the piles is insideand Airport Research Institute, No.1298, 44. (inthe port. It is thought that it has become resistance inJapanese)
Kikuchi Y., Kawabe S., Taenaka S., and Moriyasu S. 2015.
Horizontal loading experiments on reinforced
gravity type breakwater with steel walls. Japanese
Geotechnical Society Special Publication, 2 (35), pp.
1267-1272.
Iai, S. 1988. Similitude for Shaking Table Tests on Soil-the ground by slipping through.
2) When the number of piles is changed to 3, 6, and 12,
the distribution of external force at the same load
becomes larger as the number of piles becomes
smaller near the back-fitting part, and there is not

Structure-Fluid Model in 1g Gravitational Field.

much difference in the underground part.

Report of the port and harbour research institute, Vol.

3) The external force at the load level acts on the pile

27, No.3, pp. 3-24.

only in the ground at the initial stage of loading. It was
found that the external forces acting on the piles

Kubo, K. 1964. A New Method for the Estimation of

dominate in the vicinity of the backfilling part byLateral Resistance of Piles. Report of the port andgradually compressing the backfilling part by

harbour research institute, Vol. 2, No. 3, pp. 13-15.

displacement of the caisson.
4) The maximum bending moment generated in the pile
was not affected by the external force occurring at the
deeper point, but it was found to be affected by theexternal force acting in the vicinity of the backfilling
part.
5) When inverse analysis was carried out using the
calculation method of the PHRI method of type S
model, the measured value and the bending moment
obtained from the calculation program are in good
agreement, and by using the PHRI method of type S- 288 -