Fundamental Study on End Resistance of Bored Precast Pile
Toshihide Yamashita
Maeda Co., Ltd., Chiyoda, Tokyo, Japan

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

Yutaka Kubo and Keizo Minagawa
System Keisoku Co., Ltd., Sumida, Tokyo, Japan

Katsuhiro Kanuka
Kanuka Design Co., Ltd, Yokohama, Kanagawa, JapanABSTRACT
In the bored precast pile construction method, precast concrete pile orsteel pipe pile is used, and small sized piles also apply to this method.
The objective of this study is to develop an effective pile end shape for asmall-diameter steel pipe pile, which can work even if the stiffness of thecement paste around the pile tip would be insufficient. Laboratory
experiment for static axial compressive load test of single pile in which
the tip was covered with plaster was performed. The end resistance waslarger than that of a bared pile, but when the displacement progressed,
the resistance decreased because of destruction of the root part. The
behavior of the end resistance and relationship of the strength of the rootpart were discussed, and a simplified calculation method was proposed.

KEY WORDS: Bored precast pile; end bearing capacity; laboratory
experiment; simplified analysis.

INTRODUCTION
The bored precast pile method is a major pile construction method inJapan. The application number of this method is increasing in urban areas
because of less noise and low vibration. Recently, small sized piles also
apply to this method for small building's foundations such as detachedresidences or retaining walls.
It is necessary to build a homogeneous and strong root solidifying partin the pile end zone. The root part is designed to have sufficient strength,
and is managed as strictly as possible. However, the strength of the rootpart may be insufficient due to the unevenness and dispersion of theground layer, and some errors of the control of the installation. If the pile
end is fixed to a hard layer, such as dense sand or gravel layer, thesufficient end resistance can be obtained because it is relatively easy tobuild the root hardening part with good quality. However, in the case ofa soft cohesive layer or saturated medium sandy layer, the end resistancecan be short. It is surmised that the cause of this phenomenon isdestruction of the root part, and it occurs due to weakness of the root part
and/or insufficient stiffness of the soil around the pile end.
Some destruction patterns in the root hardening part have been observedin previous research. Ueno and Kurachi (1993) reported compressive
destruction and punching destruction as typical patterns. Kiya and Kato(2007) pointed out a thin root hardening part can be the cause of thepunching destruction, while a thick case can cause cleavage destruction.
On the other hand, Ishikawa and Ito (2012) stated the destructive formof the root part is affected by the thickness and the strength of the root.
Tsuchiya and Kuwabara (2012) reported the strength of the root part isdetermined mainly by the effective cement-water ratio which considers
the water content in the original ground. Miyazaki and Sato (2015) andSato and Kanuka (2016a) indicated that the end disk plate attached to thepile tip is effective to exhibit good performance of the end bearing
capacity for a small-diameter steel pipe pile. Furthermore, Sato andKanuka (2016b) reported that un-hardened root could not transmit thestress from the pile body to the ground below by the visualization
experiments.
The objective of this study is to make clear the characters of the end
resistance for the bored precast pile with the weak root part. The authors
performed small sized model tests of static axial compressive loading forsingle piles. Air dry silica sand was used for model ground. The test pileof which tip was covered with plaster was set up simulating actual stresscondition of the prebored pile construction method in the pile end area.
We discuss the end resistance and destruction pattern of the root part.
Then, we try to propose a simplified calculation method that can estimate
the end resistance of bored precast pile in consideration of the destructionof the root hardening part.

VERTICAL LOADING TEST
Test Piles and Test Series
A steel pipe, 48.6 mm in diameter (D) and 3.2 mm in thickness (t), wasused as test pile. The length of the pile is 600 mm and the pile ends werecovered with plaster in order to imitate root hardening parts withinsufficient stiffness.
Figs. 1, 2 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 aclosed end pile (SP). In this paper, the SP is not covered with plaster andis treated as the standard pile. The embedment length (H) of SP is 243
mm (5D). A test series of C, from CPT-60 to CPT-120 are performed toFig. 1 End shape of test pile

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 the modelground. The test ground is prepared by the air-pluviation technique withdouble nets for dispersion. The characteristics of the model ground areshown in table 2.
In order to produce pile tip peripheral ground conditions approximate to
the bored pile construction method, model ground preparation andsetting up of the test pile is executed as follows.

50

1. The model ground is built up to the bottom of the root part.
2. The test pile covered by plaster is placed in the center of the tank.
3. The test tank is filled to the rim.

25.7 48.6 25.7
100
Unit： mmFig. 2 Detail of root partP-series

A test series of P is carried out to discuss the effect of end disk plate. At
the pile tips of the P-series, holed disks with an opening diameter (Di) of
32.2 mm and an external diameter (De) of 58.3, 67.6 and 75.9 mm are
fixed to the pile tips. The pile ends are also covered with plaster of which
W/P is 120 %. The dimensions of the root part are the same as C-series,
and the embedment length is also 300 mm.

150
PlasterSP(Standard pile)investigate the influence of the root strength. The pile ends are covered
with plaster in which water-plaster ratios (W/P) are 60% to 120%. The
diameter of the root part is 100 mm and the thickness of the plaster is 50
mm. The embedment length (H) of C-series is 300 mm.

(C-series)
Table 1 Test pile Factors
SeriesC-series

P-series

SymbolSP(Standard)
C-60
C-80
C-100
C-120
P-58
P-68
P-75

WaterPlaster
Ratio
W/P (%)
60
80
100120120

External
diameterof pile end
De (mm)Pile end
diameter
D (mm)
48.6

48.6

58.3
67.6
75.9

100

100

Table 2 Model ground characteristics
SandSilica sand (No.5)

MethodAir-pluviation

Density (ρ)

1760 kg/m3

Void ratio (e)0.512Relative density(Dr)Internal friction
angle (φ)

87.50 %
45°

Fig. 3 Distribution of N-values

Fig. 4 Experimental apparatus

Fig. 3 shows the distribution of the converted N-values of SPT (standardpenetration test). The converted N-value is obtained by dividing thespecified penetration length of the SPT (300 mm) by the penetration
length of one drop. From fig. 3, the N-value of the pile end (Nt) isestimated.
Loading and Measurement Method
For axual loading test, a displacement controlling type loading device isused (see fig. 4), and the loading speed is 0.1 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 thepile end are almost equal to the values of the pile head, because the pilelength is relatively short and frictional resistance of the pile surface isexceedingly small. Accordingly, in this paper, the pile head load andpilehead displacement are estimated as the pile end resistance and thepile end displacement, respectively.

2C-1004Water-Plasterratio W/P (%)

Compressive
strength (N/mm2 )

6080
100
120

5.25
4.43
2.79
1.67

C-80

6C-120C-60

8SP101214Fig. 5 Relationship between displacement and pile end
resistance (C-series)

Pile end resistance R (kN)
5100

150
Water-plaster ratio = 120 %2P-58

4C-12010.0200mm

Table 3 Relationship between water-plaster ratio and compressive
strength

Stress of pile end (N/mm2)

Displacement δ (mm)Test results of C-series
The test piles with root part (from C-60 to C-120) show larger pile end
resistances (R) than that of the standard pile (SP) because of enlargeddiameter effect. And the test piles of C-60 to C-120 show almost thesame resistance values until 3.5 mm of displacement. However, after that,the increase rates of the resistance are reduced, and the resistance values
decrease suddenly. Then, it is surmised that the destruction of the root
hardening part has arisen. The magnitude order of the maximum
resistance (R) is C-60 > C-80 > C-100 > C-120, and the smaller the waterplaster ratio, the larger the maximum resistance.

200Displacement δ (mm)Figs. 5 to 6 shows the test results of the show the results of the C-series
and P-series, the relationships between the pile end resistance(R) anddisplacement of pile(δ), and the test pile without root part (SP: standardpile) is added to the figures.

Table 3 show the relationship between water-plaster ratio (W/P) andcompressive strength. Fig. 7 shows the relationship between the water-Pile end resistance R (kN)
510150RESULTS OF LABORATORY EXPERIMENT

100mm

C-60

8.0

C-80

6.0

C-100

4.0

C-120
Max (Destruction)
After Destruction
Plaster

2.00.06080

100120

Water-plaster ratio W/P(%)
Fig. 7 Relationship between water-plaster ratio anddestruction stress of pile end (C-series)

P-68

68SP

P-76

101214
Fig. 6 Relationship between displacement and pile end
resistance (P-series)

Fig. 8 Relationship between external diameter of the end
plate and destruction stress of pile end (P-series)plaster ratio and the stress of the pile end before and after destruction of
the root part. The stress is the quotient of the end resistance divided by
the pile tip area, and plaster compressive strength is added to the figure.
The pile end stresses at the destruction (maximum values) areapproximately 1.5 times large as the compressive strengths of the plaster.
And even after the destruction, the pile end stresses do not show thesmaller values than the plaster's strengths. This phenomenon is presumed
to be due to side reaction stress around the pile tip.
Test results of P-series
In the P-series, the R value of P-58 (the diameter of the end disk is De=58.3 mm) is as the almost same as the C-120's. However, the R valuesof P-68 and P-76 (De = 67.6 mm and 75.9 mm) are larger than that of theC-120's, and the larger the diameter, the larger the pile end resistance.
Furthermore, similar to the C-series, the tip resistance has decreased after
the maximum load, but the rate of decrease is relatively small.
Fig. 8 shows relationship between the external diameter (De) of the end
disk plate and the stress of the pile end before and after destruction of theroot part. The pile end stresses of C-series at the root breakage are almost
the same value of 2.5 N/mm2, and is also same as that of C-120's. The
end resistance of a pile with the end disk plate does not decrease greatly
even after the root breakage because the pile end has a large effective
area. And the pile with the end disk plate will exhibit sufficient end
resistance even if the stiffness of the root hardening part would beinsufficient.
Destruction shape of root part
After the loading test, the test pile is excavated and the destruction
condition of the root part is observed. Fig. 9 shows the destruction shapeof C-60 (W/P = 60 %, closed end pile) and P-68 (W/P = 120 %, pile withend disk plate). Both specimens exhibit a similar destructive shape. The
both roots part. Cracks are observed in a wedge shape just below the piletip, and the occurrence of the split failure is presumed in surroundingarea of the wedge.

resistance of surrounding the body (Pr). And the pile end resistance (R)is given as the sum of these as follows;
R = Pp + Pr

(1)

The load-displacement relationship (q-y curve) of the each spring is set
as follows.
Relationship between pile end stress and displacement
The stress of pile end (q) and displacement (y) are defined by thereference displacement (0.1D) and the stress of qmax, which is the stressat the reference displacement (0.1D).
qmax = α Nt

(2)where, α is the bearing capacity factor and Nt is the N-value at pile end.
Then, a trilinear skeleton curve is assumed as shown in fig. 10. Thefunction of each line is as follows;
1st line:qyq max 6 D

(2nd line:qy1

q max 12D 6(0.02 y 0.1)
D(4)

2y
30 D 3(0.1 y)D(5)3rd line:

qq maxy 0.02)
D

Pile end resistance force
The resistance force Pp and Pr are calculated as follows;
Pp = α Nt Ap , Pr = α Nt Ar

Fig. 10 Calculation Model(a) C-60, W/P=60%(b) P-68, W/P=120%(Closed end pile)(Pile with end disk plate)Fig. 9 Destruction condition of root part

SIMPLIFIED CALCULATION METHOD
Calculation model
It was understood that the end resistance of pile with root part was larger
than that of the pile without root even if the root strength was weak.
Furthermore, the end resistance of pile with root did not become smaller
than that without root even after the root part broke.
Therefore, we propose a simple calculation model as shown in fig. 10.
This model assumes two nonlinear springs, one spring handles theresistance of the pile body (Pp), and another spring responds for the(3)Fig. 11 Pile end resistance (R-y curve)(6)

where, Ap and Ar are the areas of the body portion and the surrounding
portion, respectively. Moreover, as shown in fig. 11, it is assumed thatthe surrounding spring force Pr gradually decreases until 4 times of thedisplacement of ye (ye: breakage displacement) after the root break. In
above manner, the relationship between pile end resistance force anddisplacement, i.e. Pp-y curve and Pr-y curve are determined.

can estimate the relationship between pile tip resistance anddisplacement considering the destruction of the root part. However,when applying this method to an actual pile, it is necessary to determine
the bearing capacity factor (α) and the destruction strength of the root
taking into consideration the characteristics of the supporting layer andthe scale effect.

Comparison between test and calculation results
To examine the validity of the calculation method, comparisons with thelaboratory test results are carried out. In case of the test without root (SP),of which depth of embedment is H = 250 mm, the N-value at the pile end
is set as Nt = 2, and in other cases, that is, the test with the root (H = 300mm) are set as Nt=3 (see fig. 3). The bearing capacity factor (α) which
depends on the construction method and end shape of pile is set as α =600 (kN/m2) in C-series, and is set as α =500 (kN/m2) in P-series.

CONCLUSION
The pile end resistance of the bored pile is affected by the strength of theroot part, and when the root part breaks, the resistance decreases
suddenly. The pile which has a large disk plate onto the tip, does not
suffer a large load reduction even if the root is destroyed. The simplified
calculation method was proposed which can consider the behavior of thedestruction of the root part. In this method, the load-displacement
relationship can be estimated from only the N-value and the bearing
capacity factor. The calculation method was comparatively in good
agreement with the laboratory experiment results by applying theappropriate destruction stress of the root.

The destruction stress of the root part applies as 1.5 times of compressive
strength of the plaster (see figs. 7 and 8).
Figs 12 and 13 show a comparison between laboratory test andcalculation results. The calculated value of the SP-pile is in goodagreement with the experimental result. This result shows that thecharacteristics of the pile body spring (Pp-y curve) are suitable.

Pile end resistance R (kN)51015

Displacement δ (mm)0C-120C-100
SPC-8068C-6010

Pile end resistance R (kN)
480

120

Test Rssult
Calculation

2420

Test Rssult
Calculation2
Displacement δ (mm)

04
6P-76
P-58

81012121414Fig. 12 Comparison between test and calculation results(C-series)

Fig. 13 Comparison between test and calculation results(P-series)

In the C-series, the root destruction load of the calculation and test resultsshows approximately same value. And after the destruction, the declineof the load is also roughly the same. However, at the phase of the loadincreasing, the calculation values are larger than the test results. It is
presumed that this phenomenon is caused by lack of the stiffness of theroot part, and some improvements of the Pr-y curve will be required.

P-68

ACKNOWLEDGEMENTS
Special thanks are due to Mr. S. Matsumoto and S. Miyazaki, graduate
students of Nihon University, for their cooperation in the laboratory
experiment.
REFERENCES

In the P-series, as same as the C-series, although the calculation values
are larger than the test results at the load increasing stage, the calculation
results are well simulating the destruction of the root and subsequenttransition of the resistance.
This calculation method can be applied to a safer pile design because it

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