Laboratory Experiment for End Bearing Capacity of Pile with Fragile Root Solidifying Part

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

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

Sena Miyazaki
Kanuka Design Co., Ltd, Nishi-ku, Yokohama, Kanagawa, Japan

Toru Osakabe
Division of Architecture & Civil Engineering, Ashikaga Institute of Technology, Ashikaga, Tochigi, Japan

Shozo Wada
Division of Architecture & Civil Engineering, Ashikaga Institute of Technology, Ashikaga, Tochigi, Japan

ABSTRACTThe cement milk pile construction method is classified into theprebored pile construction method, and the pile tip is solidified withthe cement paste. In this method, precast concrete pile or steel pipe
pile is used, and its pile end has a circular opening. The objective of
this study is to develop an effective pile end shape for a smalldiameter steel pipe pile, which can work even if the stiffness of thecement milk around the pile tip would be insufficient. Laboratory
experiments for static axial compressive load tests of single piles
covered with un-hardened clay around the pile end were performed.
The end bearing capacity, that is, the resistance of the pile tip when
the pile tip displacement reaches to 10% of the pile diameter, of theopen end pile was less than 20% of the standard pile. Also, the end
disk plate appended to the pile tip worked effectively. An observation
test to investigate the deformation process of the root part was alsoperformed. When the vertical displacement progressed, a swelling
was observed at the bottom and the sides of the root part, and finally,punching destruction happened.
KEY WORDS: Cement milk pile construction method; end bearing
capacity; laboratory experiment.

INTRODUCTION
Recently, the application number of the cement milk pile construction
method is increasing in urban areas of Japan because of lessconstruction noise and vibration. Furthermore, this method using
small-diameter steel pipe piles can be applied to small buildings andretaining walls. With this method, it is necessary to build ahomogeneous and a strong root solidifying part in the pile tip zone. If
the pile end is fixed in a hard sandy soil or a gravel layer with goodquality cement milk, it will show enough vertical bearing capacity.
However, in a soft or a medium cohesive soil layer, the verticalresistances can be reduced by deformation or destruction of the rootpart. Since small-diameter steel pipe piles are often fixed to suchcohesive layers, the pile end resistance may be insufficient.
Some destruction patterns in the root solidifying part have been
observed in previous research. Ueno and Kurachi (1993) reported
compressive destruction and punching destruction as typical patterns.
Kiya and Kato (2007) pointed out a thin root solidifying part can be
the cause of the punching destruction, while a thick case can cause
cracking and breakage.
On the other hand, Goto and Katsumi (1967) stated that the soil
blockade of the pile tip does not take place in the case of a large
diameter open end pile. In addition, Katsumi and Kitani (1982)
indicated that the end bearing capacity of an open end pile and aclosed end pile cannot be evaluated with same way, because thecapacities depend on the sectional areas of the pile ends. Miyazakiand Sato (2014 and 2015) reported that the end disk plate attached tothe pile tip is effective to exhibit good performance of the end bearing
capacity for a small-diameter steel pipe pile.
The objective of this study is to make clear the characteristics of theend bearing capacity of small-diameter steel pipe pile of which rootsolidifying part is weak. The authors performed small sized model
tests of static axial compressive loading for single piles. Air dry silica
sand was used and a model pile was set up simulating actual stresscondition of the prebored pile construction method in the pile end area.
We discuss the end shape of the pile and the root solidifying part toobtain sufficient resistance of the pile end.

VERTICAL LOADING TEST
Test Piles and Test Series
A steel pipe, 48.6 mm in diameter (D) and 3.2 mm in thickness (t),was used as test pile. The length of the pile is 600 mm and the lengthof embedment (H) is 243 mm (5D). To eliminate frictional resistance,
the pile surface was coated with Teflon. The pile ends were covered
with un-hardened clay in order to imitate root solidifying parts withinsufficient stiffness.
Fig. 1 and table 1 show the test pile shapes and details. A steel pipewith a steel disk (48.6 mm in diameter) attached at the tip is used as aclosed end pile (SP). In this paper, the SP is not covered with clay andis treated as the standard pile. CPT-20 and CPT-50 are performed toinvestigate the influence of the root thickness (Rt) below the pile tip. A
test pile without modified tip and covered with clay is used as an open
end pile (OPT). A test series of S is carried out to discuss the pile-endopening. At the pile tips of the S-series, holed disks with an external
diameter (De) of 48.6 mm and an inner opening diameters (Di) of 15,25 and 30 mm are attached. Furthermore, a series of P is executed toinvestigate the effect of the pile-end area. Holed disks with an
opening diameter (Di)) of 32.2 mm and external diameters (De)) of58.3, 67.6 and 75.9 mm are fixed to the pile tips. A series of T iscarried out to evaluate the blockage effect. Disks with different
shaped openings are installed to the inner part of the pile tips. TheTM-25 has a circular opening with a diameter of 25 mm, and TR-5
has 4 oval openings with an area equal to TM-25.

Closed end pile Open end pile S-series
P-series
Fig. 1 Pile end shapes

Model Ground Preparation
A steel circular test tank with a diameter of 1,000 mm and a depth of1,000mm is used. No. 5 silica sand in dry condition is used for themodel ground. 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 pile tip peripheral ground conditions approximate
to the prebored pile construction method, model ground preparationand setting up of the test pile is executed as follows.
1. The model ground is built up to the bottom of the test pile(including the root part).
2. The test pile covered by clay is placed in the center of the tank.
3. The test tank is filled to the rim.
Converted N-values, calculated by each displacement of one drive inTM pile

TR pileFig. 2 Root part dimensions

Photo 1 Pile tip

Table 1 Test pile factors
SeriesSymbolOpeningarea ratioof pile end(r)Closedend pile

SP(Standard)
CPT-20
CPT-50

0.00-

48.6

2050Openend pile

OPT

0.75

42.2

48.6

20

S-15
S-30
P-58
P-75
TM-25
TR-5

0.10
0.38
0.31
0.18
0.26
0.27

15.0
30.0
32.2
32.2
25.0
-

48.6
48.6
58.3
75.9
48.6
48.6

20
20
20
20
20
20

S-series
P-series
T-series

Opening
diameterof pile end
Do (mm)External
diameterof pile end
De (mm)Thicknessof bottom
Rt (mm)Table 2 Model ground characteristics
Silica sand (No.5)

MethodAir-pluviation

Density (ρ)

1760 kg/m3

Void ratio (e)0.512

Relative density(Dr)
Internal friction
angle (φ)

87.50 %
45°

換算N値
Converted
N-value
2 4 6 8 1005D200200(mm)Depth
深さ[mm]Sand0400400
600
600

800
800

Fig. 2 Distributionof N-values
図3 換算N値

Fig. 3 Experimental apparatus

the SPT (Standard Penetration Test), are shown in fig. 2. From fig. 2,
the average N-value ( N ) of the pile end (H = 243 mm) is estimated N= 2.
Loading and Measurement Method
For axual loading test, a displacement controlling type loading device
is used (see fig. 3), and the loading speed is 1 mm/min. The pile headload and displacement are measuared in intervals of 3 seconds by loadcell and two displacement gages.
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 frictional resistance of the pilesurface is very small. Accordingly, in this paper, the pile head loadand pile head displacement are estimated as the pile end resistanceand the pile end displacement, respectively.

RESULTS OF LABORATORY EXPERIMENT
Fig. 4 shows the test results of the close end piles and the open endpile. Figs. 5 to 6 show the results of the S-series, P-series and T-series,
the relationships between the pile end resistance (R) anddimensionless displacement (δ/D, δ: displacement of pile end, D:
diameter of pile body). Furthermore, the results of the closed end pile
without fragile root (SP: standard pile) is added to the figures.
In Japan, the end bearing capacity of a single pile is calculated by Eq.
1 which is generally used in the design of pile foundation.

R p    N  ApWhere, Rp is the end bearing capacity, α is the bearing capacity factor,
N is the average of the N-value at pile end and Ap is the area of thepile tip. The end bearing capacity is given when the pile-end
displacement reaches 10 % of the diameter. The standard value of thebearing capacity factor α for the prebored construction method isα=200. In addition, the standard value of α for the driving
construction method is α=300.
Closed End Pile and Open End Pile
Although the model piles with fragile root part (CPT-20,CPT-50 andOPT) show smaller pile end resistances (R) than that of the standard
pile (SP) in the early stage, the R values increase where δ/D exceededaround 0.3. However, R values of CPT-20, CPT-50 and OPT aresmaller than the values of SP even when the δ/D reaches 0.8. The
cause of the above phenomenon is because the energy of the verticalload was consumed only for deformation and destruction of the clay at
the root part, but the energy is not transmitted to the support layer. Asfor this phenomenon, the open end pile (OPT) gives the impression
stronger than the closed end pile. Moreover, this tendency of CPT-50
which has a thick root is stronger than CPT-20. The magnitude orderof the end resistance (Rp), when the displacement reaches 0.1D, is
CPT-20 > CPT-50 > OPT, and the percentages of those resistances
against the values of the standard pile are 38%, 27% and 18%,
respectively.

Pile end resistance R(kN)
01

2

3

4

Pile end resistance R(kN)
56

0

00.1D

0.3

CPT-20(Rt=20)

0.4

OPT

0.5

Displacement δ/DDisplacement δ/D23

456

0.2

SP（Standard pile）

S-15(Di=15)

0.3

0.4
0.5
S-30
(Di=30)0.6

CPT-50(Rt=50)

0.7

0.1D

0.1

SP（Standard pile）

0.2

0.6

0.7

0.8

0.8

Fig. 4 Relationship between displacement and pile end
resistance (open end pile, closed end pile)Fig. 5 Relationship between displacement and pile end
resistance (S-series)
Pile end resistance R(kN)

Pile end resistance R(kN)
0123

45

06

P-75(De=75.9)

0.3
0.4
0.5
P-58
(De=58.3)

0.7
0.8
Fig. 6 Relationship between displacement and pile end
resistance (P-series)

Displacement δ/DSP（Standard pile）

0.2

23

456

0.1D

0.1

0.1D

0.1

100

Displacement δ/D1

00.1

0.6(1)

SP（Standard pile）

0.2
0.3

TM-25(Circle)

0.4
0.5

0.6
0.7

TR-5(Oval)

0.8

Fig. 7 Relationship between displacement and pile end
resistance (T-series)

S-series, P-series and T-series
In the S-series, the larger the opening diameter of Di at the pile end,
the smaller the pile end resistance value. However, the end bearingcapacities (Rp) of S-15 and S-30 are almost the same. Those values are
36% of SP's in S-15, and 31% in S-30.
In the P-series, the larger the diameter of the end disk (De), the larger
the pile end resistance. The Rp value of P-75 is 1.51 kN, and thebearing capacity factor (α) is 406 kN/m2, which is 80% of thestandard. In case of P-58, the end bearing capacity is Rp=0.72 kN(α=194 kN/m2). It is about 1/2 of the P-75's and is almost the same as
Rp of the closed end pile (CPT-20). The open end piles are used in thecement milk pile construction method because the cement milk should
be filled up to the inner part of the piles. The piles with the end disk
plate will exhibit sufficient end bearing capacities even if the stiffnessof the root solidifying parts would be insufficient.
In the T-series, where the pile has an inner plate, TR-5 and TM-25
show almost the same results. The pile end resistances (R) of TR-5
and TM-25 are slightly larger than the result of the open end pile(OPT) due to the exiguous blockage effect. The inner plates have
openings, and those areas are the same but the positions and shapes
are different. However these differences have not affected the results
because the clay on the root part moves with fluidity. The Rp values of
R-5 and M-25 are 0.46 kN/m2 and 0.45kN/m2 and about 25% of thestandard pile (SP).
Bearing Capacity Factor and Aparture Ratio
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. All test
Bearing capacity factor α (kN/m2)

600
SP(Standard pile)

500

S●S■PPM ▲TRR▲TM300
CPT-20

α=200

P-58

200

S-30
S-15

100

TR-5

CPT-50 TM-25

OPT

00

0.2

0.4

0.6

OBSERVATION TEST FOR BEHAVIOR OF PILE TIPS
WITH FRAGILE ROOT PARTIn order to observe the deformation process of the root part, anobservation test was performed using a thin test tank which has
transparent glass on one side. The actual stress and strain condition
around the pile tip is a 3-dimensional axial symmetry state. Although
the condition of this test is a 2-dimensional plane state, approximate
behavior may be observed.
Test Object Preparation and Observation Method
The dimensions of the test tank are 800 mm in width, 650 mm inheight, and 35 mm in inner thickness. A rectangular box made ofaluminum is used as the test pile, and the dimensions are 50 mm inwidth x 75 mm in height x 30 mm in thickness. A steel rod, 12 mm indiameter, is attached to the test pile for vertical loading. In the samemanner as the loading test, the exterior of the pile is covered by unhardened clay with a thickness is 20 mm.
After pushing the test tank over on its side, the test pile is placed. No.
5 silica sand in dry condition is used for the model ground, and thetest ground is built up by the air-pluviation technique (see fig. 9).
After fitting the glass on the top, the test tank is raised up. The test
pile is pushed vertically with a speed of 1 mm/min, and photographs
are taken every 1 mm of displacement.
Results of the Observation Test
Fig. 10 shows the deformation process of the root part of the pile.
In the initial stage (0 - 10 mm), a small gap between the pile surface
and the root part is generated, and a little displacement can beobserved at the bottom of the root.
In the middle stage (10 - 30 mm), some cracks appeared from thecorner of the bottom. The displacement of the bottom is growing,
while a lateral displacement can be also observed.
In the final stage (30 - 40 mm), the cracks and displacements havegrown, and the bottom of the root becomes thinner. Finally, breaking
cracks reach the edge of the bottom, that is, the punching destruction
has occurred.

P-75

400results have a decreasing tendency proportional to the aperture ratio,that is, the larger aperture ratio (r), the smaller bearing capacity factor(α).
The α values of the S-series (including closed and open end piles) andT-series do not pass α>200 kN/m2, the standard of the prebored pile.
However, only P-58 and P-75, both with an end disk plate, show
larger values than the standard.

0.8

Aparture Ratio rFig. 8 Relationship between bearing capacity
factor (α) and aperture ratio (r)Table 3 Loading test results
δ/D =0.1

δ/D=0.5D

SymbolApatureratio (r)R p (kN)

α (kN/m2)

R p (kN)

α (kN/m2)

SP
CPT-20
CPT-50
OPT
S-15
S-30
P-58
P-75
TM-25
TR-5

0.00
0.00
0.00
0.75
0.10
0.38
0.31
0.18
0.26
0.27

1.84
0.68
0.48
0.33
0.65
0.56
0.75
1.47
0.46
0.45

495
184
130
89
174
151
203
396
124
122

4.03
2.75
0.73
1.21
3.20
2.77
3.23
6.68
1.59
1.46

1087
741
197
326
863747870
1801
428394

Fig. 9 Test object preparation

Initial stagea. δ=0 mm

Middle stageb. δ=10 mm

c. δ=20 mm

Final staged. δ=30 mm

e. δ=40 mm

Fig. 10 Observation test result
The punching destruction will occur easily if the bottom of the rootpart is thin. However, if the root part is thicker, the end bearing
resistance may not increase. Therefore, if the root part has not enoughstiffness, and if the pile tip shape has a sufficient performance, athinner bottom of the root might be better.

CONCLUSION
In the cement milk pile construction method, the end bearing capacity
may be reduced due to the pile end openings and the fragile root part.
Adding a large disk plate onto the pile end is one of the solutions ofthe problem. In this paper, we could examine the influence of theopenings and of the insufficient stiffness of the root solidifying part.
We could confirm the validity of the pile tip disk and of the thinbottom of the root.

ACKNOWLEDGEMENTS
Special thanks are due to Mr. K. Minagawa and Dr. Y. Kubo, SystemKeisoku Co., Ltd., for their technical support. We would like toacknowledge Mr. Matsumoto and Mr. Harashima, students of Nihon
University, for their cooperation in the laboratory experiment.

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