Consideration for End Shape of a Pile for Prebored Pile Construction Method

Sena MIYAZAKI
Graduate Student, College of Science & Technology, Nihon University, Funabashi, Chiba, Japan

Hideto SATO
Department of Architecture & Life Design, Nihon University Junior College, Funabashi, Chiba, Japan

Katsuhiro KANUKA
Kanuka Design Co., Ltd, Nishi-ward, Yokohama, Kanagawa, Japan

Yuuki HARIGAYA
Noix Co., Ltd, Aoi-ward, Shizuoka, Shizuoka, JapanABSTRACT
In the cement milk pile construction method, precast concrete pile orsteel pipe pile is used, and the pile ends have a circular opening. Inthis paper, we described laboratory experiments for a static axial
compressive load test of single pile. The objective of the tests is tomake clear the characteristics of the vertical end bearing capacity ofthe pile with an opening in the pile tip. We discuss the relationship ofthe bearing capacity with the tip shape, and propose a more suitable
shape of the pile end for the cement milk method. Subsequently, wetry to confirm the validity of the improved pile tip by actual size load
experiments.
KEY WORDS: Pile; prebored construction method; end bearing
capacity; laboratory load test; actual size load test.
INTRODUCTION
The cement milk pile construction method is classified as part of theprebored pile construction method. Recently, the application numberof this method has increased in urban areas of Japan because of thelow construction noise and vibration. In this method, precast concrete
hollow piles or steel pipe piles are used. Therefore, it is necessary tofill the piles with cement milk. If those pile ends are fixed in hard
sandy soil or a gravel layer with good quality cement milk, they willshow enough vertical bearing capacities. However, in soft or medium
cohesive soil, vertical resistance might decrease because of
insufficient stiffness of the cement milk. Accordingly, the cement
milk strength is often ignored in the design of the end bearing
capacity.
Goto and Katsumi (1967) pointed out that the blockade effect at thepile tip might not be demonstrated in a large diameter bored pile. In
addition, Katsumi and Kitani (1982) indicated that the end bearing
capacity of an open end pile cannot evaluate the same capacity as aclosed end pile since the capacity depends on a sectional area of thepile end.
On the other hand, in the driving pile construction method, the end
bearing capacity improves when the pile is driven into a deep

embedment since the blockade effect is promoted (Komada et al.,1973 and Shimazaki et al., 1977). Furthermore, Yamahara (1973)
reported that to make the diameter smaller or to attach divider plates
inside the pile tip are effective in order to obtain large end bearing
capacities.
The objective of this study is to develop the effective end shape of apile for vertical bearing capacity, which can work even if the stiffnessof the cement milk would be insufficient. The authors performed
small sized model tests of static axial compressive loading for single
piles installed in dry sand. We propose a design method of an end
shape for small-diameter steel-pipe piles used in foundations of smallbuildings. Then, actual size loading experiments are performed toconfirm validity of the improved pile tip.
LABORATORY EXPERIMENT
Model piles and series of test
A steel pipe, 48.6 mm in diameter (D) and 3.2 mm in thickness (t),was used as a test pile. The length of the pile was 600 mm and thelength of embedment (H) was 243 mm (5D). To eliminate thefrictional resistance, the pile surface was coated with Teflon. Fig. 1and table 1 show the test piles' shapes and details. The steel pipe isused as the open end pile (OP), and the steel pile attached a steel disk
(48.6 mm in diameter) at the tip is used as the closed end pile (CP). A
test series of S is carried out in order to discuss the influence of thepile end's opening. At the pile tips of the S-series, holed disks with anexternal diameter (De) of 48.6 mm and inner opening diameters (Do)of 15, 25 and 30 mm are attached. Furthermore, test series of P are
executed to investigate the effect of the pile end's area. Holed disk
plates with an opening diameter (Do) of 32.2 mm and external
diameters (De) of 58.3, 67.6 and 75.9 mm are fixed to the pile tips of
the P-series. Table 1 shows various factors of the test piles. An area
ratio (rA) is the ratio of an end area to the area of the closed end pile(CP).
Method of preparing model groundA steel circular test tank with a diameter of 50mm and depth of
700mm is used. No. 5 silica sand in dry condition is used for themodel ground, and the test ground is prepared by the air pluviation
technique with double nets for dispersion. The characteristics of themodel ground are shown in table 2.
In order to produce the approximate peripheral ground condition of
the pile tip of the prebored pile construction method, preparation of
the model ground and installing of the test pile is executed as follows.
1. The model ground is built up until the pile-end position + 5 mm.
2. The test pile is set and is driven 5 mm by light tapping.
3. The model ground is filled up to the surface.
Converted N-values, calculated by the amount of one fall's
displacement of SPT, are shown in Fig. 2. From Fig. 2, the average of
the N-value ( N ) of the pile end (H = 243 mm) is estimated N = 2.
Loading and measurement method
For axual loading, a displacement controlling type loading device isused (see Fig. 3), and the loading speed is 1 mm/min. The pile headload and displacement are measured in intervals of 3 seconds by loadcell and displacement gages at 2 points. The vertical earth pressure atthree points of the bed of the test tank is measured to examine thesituation of the pile tip load transmitted within the tank.
In this laboratory experiment, the resistance and the displacement of
the pile end are almost equal to the values of the pile head, because
the pile length is relatively short and the frictional resistance of thepile surface is exceedingly small. Accordingly, in this paper , the pilehead load and pile head displacement are estimated as the pile end
resistance and the pile end displacement, respectively.
RESULTS OF LABORATORY EXPERIMENTFigs. 4 to 7 show the test results of the S-series and P-series, therelationships between the pile end resistance (R) and dimensionlessdisplacement (δ/D, δ: displacement of pile end, D: diameter of pilebody), and between the average vertical earth pressure at the bed (σv)
and dimensionless displacement (δ/D). The results of the open endpile (OP) and the closed end pile (CP) are added to the figures.
In Japan, the end bearing capacity of a single pile is calculated by Eq.
1 which is generally used in pile foundation design.
(1)
R p = α ⋅ N ⋅ Ap
Where, Rp is the end bearing capacity, α is the bearing capacity factor,
N is the average of N-value at pile end and Ap is the sectional area of

上下用シャフト
御盤

0.25
1.00
0.91
0.74
0.62
1.00
1.50
2.00

Table 2. Various factors of model ground
Silica sand (No.5)

MethodAir pluviation

Density (ρ)

1760 kg/m3

Void ratio (e)0.512Relative density(Dr)

87.50 %

Internal friction
angle (φ)

45°

杭頭軸力計

Converted N-value
2 4 6 8 10

先端軸力計

05D200Depth (mm)

Sand0

回転装置

700

P-series

OPCP
S-15
S-25
S-30
P-58
P-68
P-76

243(H)

S-series

Area ratioof pileend (rA)

480mm（10D）Open end pile
Closed end pile

SymbolExternal
diameterof pile end
De (mm)
48.6
48.6
48.6
48.6
48.6
58.3
67.6
75.9

576mm（12D）

Series

圧入装置
Opening
diameterof pile end
Do (mm)
42.2
15.0
25.0
30.0
32.2
32.2
32.2

400600テフロンシート

1,400800

Fig. 2 Distribution of N-values

Fig. 3 Experimental apparatus

1000
1,100

Table 1. Various factors of test piles

2300
2,300

Fig. 1 Shapes of pile ends

the pile tip. Usually, the end bearing capacity, which is equal to thesecond-limit-resistance (regarded as an ultimate bearing capacity), isgiven when the pile-end displacement reaches 10 % of the diameter.
Table 3 shows the key values of the results. The standard value of thebearing capacity factor α for the prebored construction method is
α=200. In addition, the standard value of α for the driving
construction method is α=300.
Closed end pile and open end pile
As for the pile tip resistance (R) of the closed end pile (CP), the Rvalue is increasing from the loading initial stage. The end bearingcapacity (δ/D = 0.1) is Rp = 1.84 kN, and the end bearing capacity
factor is α = 495. These are sufficient values for end resistance of theprebored construction method.
In the case of the open end pile (OP), the pile end resistance (R) does
not large even if the dimensionless displacement (δ/D) reaches 0.1.
The end bearing capacity is Rp = 0.15 kN, and the bearing capacity
factor is α = 11.5. These values are only 8 % of the closed end pile's.
The vertical earth pressure (σv) of CP is increasing from the loading
initial stage, as well as the R-δ/D relation curve. The overburdenpressure at the bottom is presumed 12.1 kN/m3. When thedimensionless displacement (δ/D) is 0.1, the σv value indicates 2.7
kN/m3, and this value is about 1/3 of the overburden pressure at thebottom. In the CP pile, the change of the earth pressure (σv) is small,but the stress produced by deformation of a pile end is transmitting tothe bottom from the early stages of loading. On the other hand, in thecase of the OP, only a small stress is transmitted even when δ/Dreaches to 0.2.

S-series and P-series
In the S-series, the larger the opening diameter of Do at the pile end,
the smaller the pile end resistance value. The resistance of S-15 shows
almost same value of CP's, and the end pile capacity Rp and thebearing capacity factor α of S-15 are 1.71 kN and 510, respectively.
This value is 93 % of CP's.
In case of P-58, the external diameter of the disk plate is larger thanthe diameter of the upper part of the pile body. However, the opening
diameter is almost same as the S-30's. Accordingly, the area ratio of
the pile end is 1.0 in P-58. The Rp value of P-58 is indicated 40% ofCP's. This phenomenon shows that the end bearing capacity isaffected by the opening even if the end area is the same. Meanwhile,
the Rp value of P-58 is about 1.6 times larger than that of S-30. This
result is signifies that the end disk plate contributes to provide for thelarge end bearing capacity.
Table 3. Results of laboratory experiments

OPCP
S-15
S-25
S-30
P-58
P-68
P-76

120
S-15

8S-15
CP
S-25
S-30

Do

δ/D

0.1

60.05

S-25

S-30

20

48.6

0.05

δ/D

0

3α(kN/m2)
39495
461
188
138
229
546
893

Rp (kN)
0.15
1.84
1.71
0.70
0.51
0.85
2.03
3.31

σv(kN/m2)
4R(kN)0

δ/D=0.1

Aperture ratioof pile end (r)
0.25
1.00
0.91
0.74
0.62
1.00
1.50
2.00

Symbol

OP0.1Earth pressure
gage

CP
0.15

0.15
OP

0.2

0.2

Fig. 4 Relationship between displacement
and pile end resistance (S-series)

Fig. 6 Relationship between displacement
and vertical earth pressure (S-series)

R(kN)
012

30

4

50

0.15

OP

P-68

De

CP

0.2

P-68
P-58
OP

δ/D

δ/D

0.05

32.2
P-58

8P-76
CP

P-76

0.1

σv(kN/m2)

048.6

0.05

40.1

Earthpressure
gage

0.15

0.2

Fig. 5 Relationship between displacement
and pile end resistance (P-series)

Fig. 7 Relationship between displacement
and vertical earth pressure (P-series)

12

Bearing capacity factor α (kN/m2)

In the vertical earth pressure (σv) of the S-series, when the opening
diameter (Do) become large, the σv value become small. Meanwhile in
the P-series, when the external diameter (De) become large, the σvvalue also become large.
Fig. 8 shows the relationship between the aperture ratio of the pile tip(r) and the bearing capacity factor (α). The α value is calculated by eq.
1, and AP is the sectional area of the pile body. The aperture ratio r isthe ratio of the opening area to the whole area of the pile tip. In the Sseries, when aperture ratios r are larger than 0.27, α values become
smaller than the standard value of the prebored contruction method.
However in the P-series, all test results show larger values than thoseof S-series, and are also larger than the standard values.

1000900
800
700
600
500
400300
200
1000

P-76

Table 4. Various factors of test piles for field test
Average of N-value at Pile end ( N )S-pileC-pile

15.8

8.8

Diameter of pile body D (mm)165.2

Diameter of opening Do (mm)85.2External diameter of disk plate De (mm)

225.2

SP

P-68

CPS-15

S-25
00.2

P-58

α=200

S-30

OP

0.4
0.6
Aperture ratio (r)0.8

Fig. 8 Relationship between a bearing capacity
factor α and aperture ratio r

Fig. 9 Pile tip shape of field test

FIELD EXPERIMENT
Test pile and field
According to the results of the laboratory experiment, the end disk
plate is effective for improvement in the end bearing capacity of theprebored pile construction method. However, the larger disk platemight scrape the soil off a bored hole, and an enough opening isneeded to fill the inner space of the pile with the cement milk. We
designed the end shape of pile for field experiments as follows.
1. The area ratio (rA; rA=A/A0, A is actual area of pile and A0 is
sectional area of the pile body) should be more than 1.0 (rA>1.0).

2. The digging diameter ratio (rD; rD=De/DD, De is external
diameter of the end disk plate, DD is digging diameter) shouldbe less than 0.8 (rD<0.8).
3. The opening diameter (Do) of the end disk plate should be
more than 80 mm (Do >80 mm)
4. The aperture ratio (r; r =Do2/De2) should be less than 0.3(r<0.3).
Above manner is possible to apply the general design of the end shapeof small-diameter steel-pipe piles for the cement milk pileconstruction method.
The diameter of the test pile is 165.2 mm at the pile body, and theexternal diameter of the end disk plate is 225.2 mm and the opening
diameter is 85.2 mm (see table 4 and Fig. 9).
Two field loading tests are carried out. One of the test piles, named Spile is embedded into clayey sand and another pile, named C-pile isfixed on sandy clay. The test field is located in Morokawa City, in thenorth area of the Kanto plains. The boring logs and distribution of Nvalues are shown in Fig. 10. The averages of N-value at theembedment positions ( N ) are 15.8 in the S-pile and 8.8 in the C-pile,respectively.

Fig. 10 Boring logs of field test

Loading and measurement
The pile installation is performed by the cement milk pile construction
method. After filling up cement milk into the prebored hole, the pile isinstalled carefully till the embedment length. Static axial load tests areexecuted according to the test manual produced by the Japanese
Geotechnical Society using the anchor-pile-system. The strain anddisplacement of the pile tip are measured, and the pile end load isconverted by the strain.
Results of field experiment
The relationships between the pile end loads (R) and dimensionlessdisplacements (δ/D) are shown in Fig. 11.
As for the S-pile, the end bearing capacity (RP), which is the pile tip
resistance when δ/D=0.1, indicates 104 kN and the bearing capacity
factor (α) is 307. And as for the C-pile, RP = 128 kN and α = 680.
From these results, we can confirm the validity of the disk plate added
to the pile tip.
Fig. 12 shows the pile tip situation which is dug out after the test. It
seemed that the pile tip has solidified united with cement milk, andalso has enough stiffness. The pile tip resistance must be owing tobearing pressure of the pile tip area, and the friction with soil around

R(kN)
050

0100

150

C-pile(N=8.8)0.01
0.02
0.03δ/D

0.04
0.05
0.06
0.07
0.08
0.09

S-pile(N=15.8)
S-PileRp=104kN(α=307)

10.3m

12.1m

C-PileS-PileC-PileRp=128kN(α=680)0.1
Fig. 11 Relationship between displacement andpile end resistance (S-pile and C-pile)Fig. 12 Pile tip situation (S-Pile)

the pile tip zone may contribute also. Namely, It is considered that thebig block made by the end disk plate and cement milk has influenced
exertion of the end bearing capacity of pile.

CONCLUSION
In the cement milk pile construction method, there is some possibilityof decreasing the end bearing capacity due to the opening of the pileend. Adding a large disk plate to the pile end is one of the solution ofthe problem. In this paper, we examined the influence of the openings
and proposed an improvement method by the laboratory load tests.
Furthermore, we confirmed the validity of the pile tip disk by actual
sized field experiments. The piles of which the tips were improved by
the disk plates exhibited large and sufficient end bearing capacities.

ACKNOWLEDGEMENTS
Special thanks are due to Mr. K. Minagawa and Mr. Y. Kanahira,System-Keisoku Co., Ltd., for their technical support in the field test.
And we would like to acknowledge Mr. Ishikawa, students of Nihon
University, for his cooperation in the laboratory experiment.

REFERENCES
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Construction Method-”, Proc 11th Kanto Branch of The Japanese
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The Japanese Geotechnical Society (2006). "Vertical Load Test
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Yamahara, H (1973a). "Plugging Effect and Bearing Mechanism of
Steel Pipe Pile (Part-1)”, Journal of Structural and Construction
Engineering, Architectual Institute of Japan (in Japanese), 96, 2835.
Yamahara, H (1973b). "Plugging Effect and Bearing Mechanism of
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