Fundamental Study on End
Resistance of Bored Precast Pile
Toshihide YAMASHITA1) ,Hideto SATO2)
Yutaka KUBO3)， Keizo MINAGAWA3)
Katsuhiro KANUKA4)1) Maeda Co.,Ltd
3) System Keisoku Co.,Ltd

2) Nihon Univ.
4) Kanuka Design Co.,Ltd

E-mail : yamshitt@jcity.maeda.co.jp
sato.hideto@nihon-u.ac.jp

INTRODUCTION- In Japan, many buildings were constructedusing the driven pile construction method in
1950-1965. However, the loud noise and large
vibration generated from machines were aserious problems in the big city.
- And the Noise Regulation Act came into effectin 1968. Since then, the driven pile construction method has been prohibited in urban areaof Japan.

LowNoiseLow
Vibration- Therefore, the cast-in-place pile construction
method and the bored precast pile construction method were developed. These methods
were progressed very much, and various
methods have been developing.
Bored precast pile construction method
ISOPE 2019, Hawaii, USA

INTRODUCTION- Recently, small sized piles also apply to the bored precast pile construction
method for small buildings such as detached residences or retaining walls.
And in this method, quality control of the root part is very important.
- When the strong root can bebuilt, the pile end area contributing to the resistance increase
and large end bearing capacity
will be performed.
But if not, the pile end resistancewill be lack and the upper structure may be subject to terrible
damage.
Retaining wall with small steel pile
constructed by pre-bored method
ISOPE 2019, Hawaii, USAINTRODUCTION- It is necessary to build a homogeneous and a strong root solidifying
part in the pile end zone.
- If the pile end is fixed in a hard layer, it is relatively easy to build theroot hardening part with good quality.
- However, in the case of a soft cohesive layer, or much water is flowing,
the strength of the root part can be short.
- And small-diameter steel pipe piles are often fixed in such layers.

N>50

Sandy Soil or Gravel

N≒20

Cohesive Soil or Much Water is flowing
ISOPE 2019, Hawaii, USAINTRODUCTION
The objective of this study:
To make clear the characters of the end resistancefor the bored precast pile with the weak root part.
VERTICAL LOADING TEST (small sized model test)
・We performed static axial compressive loading test.
・The pile tip is covered with plaster.
・The model piles set up with simulating actual stress condition of thepre-bored method.

SIMPLIFIED CALCULATION METHOD
・We discuss the test results. (End resistance, Destruction pattern, etc.)
・We propose a simplified calculation method.
・It can estimate the end resistance of bored pile in consideration of theroot part destruction.
ISOPE 2019, Hawaii, USA

VERTICAL LOADING TEST

993.2

Plaster 3.2

32.2

48.6 (D)Standard pile(SP)Test Piles and Test Series

C-series(SP + Plaster)

Unit[mm]

58.3~75.9(De)P-series- A steel pipe was used as test pile (D=48.6, t=3.2 mm),(Steel pipe
and the pile ends were covered with plaster (to imitate+ Holed disk)
the weak root part).
- SP pile : Standard pile (Not covered with plaster)- C-series : Standard pile covered with plaster- P-series : Steel pipe + Holed disk (De=58.3~75.9 mm) and covered with plaster
ISOPE 2019, Hawaii, USAVERTICAL LOADING TEST

Test Piles and Test Series

Table 1 Test pile Factors

SeriesSymbolC-series

P-series

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

External
Waterdiameter
Plaster Ratioof pile end
W/P (%)De (mm)6080
100120

48.6

120

58.3
67.6
75.9

Pile enddiameter
D (mm)
48.6
100

100

- The C-series (C-60 to C-120) : To investigate influence of the root strength.
Plaster: W/P (Water-plaster ratio)=60% to 120%.
- The P-series (P-58, P-68, P-75) : To discuss the effect of end disk plate.
W/P of the P-series is 120% (very weak).

ISOPE 2019, Hawaii, USAVERTICAL LOADING TEST Model Ground Preparation

Table 2 Model ground characteristics
Density (ρ)
1760 kg/m3
Void ratio (e)0.512Relative density (Dr)87.50 %
Internal friction angle (φ) 45°

1000- A steel circular test tank is used.
Diameter is 1000 mm.
Depth is 1000 mm.
- No. 5 silica sand in dry condition is used
for the model ground.
- The test ground is prepared by the air
pluviation technique with double nets
for dispersion.
- The N-values of the pile end is N=2or N=3.

φ1000
Fig. 4 Experimental apparatus
ISOPE 2019, Hawaii, USA

RESULTS OF LABORATORY EXPERIMENT
Test results of C-series

Pile end resistance R (kN)
05101520Displacement δ (mm)

02

C-100

4C-80

68
10C-120

C-60

GroundSPSP

Plaster
GroundC-60 ~ C-120(C-series)

1214Fig. 5 Relationship between displacement
and pile end resistance (C-series)- The test piles with root part(fromC-60 to C-120) show larger pile endresistances(R) than that of the standard pile (SP) because of enlargeddiameter effect.
- C-series show almost the same resistance values until 3.5 mm ofdisplacement.
- However, after that, the increase rates of the resistance are reduced, andthe resistance values decrease suddenly.
ISOPE 2019, Hawaii, USA

RESULTS OF LABORATORY EXPERIMENT
Test results of C-series

Pile end resistance R (kN)05101520Displacement δ (mm)

02

C-100

4C-80

68
10C-120

C-60

GroundSPSP

Plaster
GroundC-60 ~ C-120(C-series)

1214Fig. 5 Relationship between displacement
and pile end resistance (C-series)- Then, it is surmised that the destruction of the root hardening part hasarisen.
- The magnitude order of the maximum resistance(R) is the smaller thewater-plaster ratio, the larger the maximum resistance.

ISOPE 2019, Hawaii, USA

Stress of pile end (N/mm2)

RESULTS OF LABORATORY EXPERIMENT
Test results of C-series
10.0

C-60

8.0

C-80

C-100

6.0

4.0

C-120
Max (Destruction)
After Destruction
Plaster

2.0
0.0
60

80

100120

Water-plaster ratio W/P(%)
Fig. 7 Relationship between water-plaster ratio anddestruction stress of pile end (C-series)- 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) are approximately 1.5 times large as the compressive strengths of the plaster.
- Even after the destruction, the pile end stresses do not show the smaller
values than the plaster's strengths. This phenomenon is presumed to be due
to side reaction stress around the pile tip.
ISOPE 2019, Hawaii, USA

RESULTS OF LABORATORY EXPERIMENT
Pile end resistance R (kN)
051015

Test results of P-series

Displacement δ (mm)0Water-plaster ratio = 120 %2P-58 C-120

4P-68

Plaster
Ground68SPP-76

1032.21258.3~75.9

14(De)

P-series
Fig. 6 Relationship between displacement and pile end resistance (P-series)- In the P-series, the R value of P-58 (the diameter of the end disk is De =58.3mm) is as the almost same as the C-120's.
- However, the R values of P-68 and P-76 (De = 67.6 mm and 75.9 mm) arelarger than that of the C-120's, and the larger the diameter, the larger thepile end resistance.
ISOPE 2019, Hawaii, USARESULTS OF LABORATORY EXPERIMENT
Pile end resistance R (kN)
051015

Test results of P-series

Displacement δ (mm)0Water-plaster ratio = 120 %2P-58 C-120

4P-68

Plaster
Ground68SP

P-76

1032.21258.3~75.9

14(De)

P-series
Fig. 6 Relationship between displacement and pile end resistance (P-series)- Furthermore, similar to the C-series, the tip resistance has decreased after
the maximum load, but the rate of decrease is relatively small.

ISOPE 2019, Hawaii, USA

RESULTS OF LABORATORY EXPERIMENTStress of pile end (N/mm2)

Test results of P-series
3.5C-120

3.0

P-58

P-68

P-76

2.5

2.0
1.5
1.0

Max (Destruction)
After Destruction
Plaster

0.5
0.0405060

7080

External Diameter (mm)Fig. 8 Relationship between external diameter of the end plateand destruction stress of pile end (P-series)- The pile end stresses of P-series at the root breakage are almost the same valueof 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
Testgreatlyendeven after the root breakage because the pile end has a large effective area.
- The pile with the end disk plate will exhibit sufficient end resistance even if thestiffness of the root hardening part would be insufficient.
ISOPE 2019, Hawaii, USA

SIMPLIFIED CALCULATIONMETHOD
杭先端荷重～基準化変位関係(R－δ/D曲線）
Calculation model

R = Pp + Pr

Fig. 10 Calculation Model- We propose a simple calculation model as shown in fig.10.
- This model assumes two nonlinear springs, one spring handles the resistanceof the pile body (Pp), and another spring responds for the resistance ofsurrounding the body (Pr).
- The pile end resistance (R) is given as the sum of these.
ISOPE 2019, Hawaii, USA

SIMPLIFIED杭先端荷重～基準化変位関係(RCALCULATION METHOD－δ/D曲線）
Relationship between pile end stress and displacement- The stress of pile end (q) and displacement (y) are defined by the reference
displacement (0.1D) and the stress of qmax, which is the stress at the reference
displacement (0.1D).

qmax=αNt- 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. 11.

1st line

2nd line

3rd line𝑞𝑦=𝑞𝑚𝑎𝑥 6𝐷𝑞𝑦1=+𝑞𝑚𝑎𝑥 12𝐷 6𝑞𝑦2=+𝑞𝑚𝑎𝑥 30𝐷 3𝑦𝐷( < 0.02)(0.02 ≤𝑦< 0.1)
𝐷𝑦𝐷(0.1 ≤ )Fig. 11 Pile end resistance (R-y curve)
ISOPE 2019, Hawaii, USA

SIMPLIFIED杭先端荷重～基準化変位関係(RCALCULATION METHOD－δ/D曲線）
Pile end resistans force- The resistance force Pp and Pr are
calculated as follows;

Pp=α Nt Ap

Pr=α Nt Ar

Fig. 11 Pile end resistance (R-y curve)- Ap and Ar are the areas of the body portion and the surrounding portion,respectively.
- Moreover, as shown in fig. 11, it is assumed that the surrounding spring force
Pr gradually decreases until 4 times of the displacement of ye (ye: breakagedisplacement) 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.
ISOPE 2019, Hawaii, USA

SIMPLIFIED杭先端荷重～基準化変位関係(RCALCULATION METHOD－δ/D曲線）
Comparison between laboratory test and calculation results(C-series)
Pile end resistance R (kN)
051015200

Displacement δ (mm)C-120

2Test Result
Calculation
C-100

4SP

C-80

6C-60

8101214Fig. 12 Comparison between test andcalculation results (C-series)- The calculated value of the SP-pile isin good agreement with the experimental result.
- This result shows that the characteristics of the Pile body spring(Pp-y curve) are suitable.
- In the C-series, the root destruction
load of the calculation and testresults shows approximately samevalue.
- However, at the phase of the loadincreasing, the calculation values arelarger than the test results.
- It is presumed that this phenomenon
is caused by lack of the stiffness ofthe root part, and some improvements of the Pr-y curve will berequired.
ISOPE 2019, Hawaii, USA

SIMPLIFIED杭先端荷重～基準化変位関係(RCALCULATION METHOD－δ/D曲線）
Comparison between laboratory test and calculation results(P-series)
Pile end resistance R (kN)04

8120Test Result
Calculation

Displacement δ (mm)24P-76

6P-58- In the P-series although the calculation values are larger than the test
results at the load increasing stage.
- The calculation results are wellsimulating the destruction of the rootand subsequent transition of theresistance.

81012P-68

14Fig. 13 Comparison between test andcalculation results (P-series)- This calculation method can beapplied to a safer pile design because
it can estimate the relationshipbetween pile tip resistance anddisplacement considering thedestruction of the root part.
ISOPE 2019, Hawaii, USA

SIMPLIFIED杭先端荷重～基準化変位関係(RCALCULATION METHOD－δ/D曲線）
Comparison between laboratory test and calculation results(P-series)- However, when applying this method
to an actual pile, it is necessary to812determine the bearing capacity factor
Test Result(α) and the destruction strength of theCalculation
root taking into consideration thecharacteristics of the supporting layer
and the scale effect.
P-76

Pile end resistance R (kN)
040Displacement δ (mm)

246
P-58

81012P-68

14Fig. 13 Comparison between test andcalculation results (P-series)
ISOPE 2019, Hawaii, USA

CONCLUSION- The pile end resistance of the bored pile is affected by thestrength of the root part, and when the root part breaks, theresistance decreases suddenly.
- The pile which has a large disk plate into the tip, does not suffera large load reduction even if the root is destroyed.
- The simplified calculation method was proposed which canconsider the behavior of the destruction of the root part.
- In this method, the load-displacement relationship can beestimated from only the N-value and the bearing capacity
factor.
- The calculation method was comparatively in good agreementwith the laboratory experiment results by applying theappropriate destruction stress of the root.
ISOPE 2015, Hawaii, USAThank you very much for your kind attention

E-mail : yamasitt@jcity.maeda.co.jp
sato.hideto@nihon-u.ac.jp