construction control and pile body tensile stresses distribution pattern during driving

Dow

nloa

ded

from

asc

elib

rary

.org

by

MIS

SOU

RI,

UN

IV O

F/C

OL

UM

BIA

on

03/1

5/13

. Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

Construction Control and Pile Body Tensile StressesDistribution Pattern during Driving

Liyun Zhou1; Jianbin Chen2; and Weikang Lao3

Abstract: In this paper, the pile body tensile stresses distribution pattern during driving is discussed, based on dynamic piling testingresults of 13 driven piles attained by the laboratory and in situ tests. These tests indicate that the maximum driving tensile stresses occurin the upper portion of the pile, especially at the initial stages of driving the pile tip seated on soft soils or if the pile foundation is builton a soft foundation. After serious study, it can be concluded that the maximum driving tensile stress often occurs at �1/4�l �l stands forthe length of pile� from pile top with its value accounts for 50% of the driving compressive stresses at the same cross section. This highdriving tensile stress would lead to the occurrence of transverse cracks and even the breakage of the pile, which should claim the attentionof the engineer. The tests also demonstrate that high driving tensile stresses often occur due to the usage of a conventional pile cap in thedriving of a prestressed concrete �tubular� pile. To the contrary, the tensile stresses tend to diminish within the control tensile stresses�5 MPa� due to the usage of a new type of disk spring pile cap during driving. Moreover, this new pile cap can prevent driving deviationfrom alignment, ensure an even distribution of the hammer blow on pile top, and protect the integrity of the pile body.

DOI: 10.1061/�ASCE�1090-0241�2007�133:9�1102�

CE Database subject headings: Pile driving; Tensile stress; Concrete, prestressed; Pile caps.

Introduction

Some problems often occur in the driving construction of precastreinforced concrete piles:1. The occurrence of longitudinal cracks along pile axis;2. The breakage of pile top; and3. The occurrence of transverse cracks.

In the case of the first two problems the damage is due to theoccurrence of maximum driving compressive stresses on the topof the pile. However, the maximum driving tensile stresses aloneon the axis of the pile eventually lead to the third problem occur-ring during driving. In engineering, people always pay more at-tention to the former two than to the third one. So it is no wonderthat research and reports on the driving compressive stresses areeven more than that of driving tensile stresses, much less thanin situ tests and inspections. In order to fully understand the oc-currence and distribution pattern of driving tensile stresses of pile,dynamic testing of 13 driven piles are seriously studied by labo-ratory and in situ tests. This paper mainly contains the followingthree parts: the first part is the in situ test of two driven large-

1Advanced Engineer, College of Civil Engineering, Wuhan Univ.,Wuhan 430072, People’s Republic of China. E-mail: [email protected]

2Ph.D. Wuhan Municipal Engineering Design and Research InstituteCo., Ltd., Wuhan 430015, People’s Republic of China. E-mail: [email protected]

3Postgraduate, College of Civil Engineering, Wuhan Univ. Wuhan430072, People’s Republic of China.

Note. Discussion open until February 1, 2008. Separate discussionsmust be submitted for individual papers. To extend the closing date byone month, a written request must be filed with the ASCE ManagingEditor. The manuscript for this paper was submitted for review and pos-sible publication on December 14, 2005; approved on December 22,2006. This paper is part of the Journal of Geotechnical and Geoenvi-ronmental Engineering, Vol. 133, No. 9, September 1, 2007. ©ASCE,
ISSN 1090-0241/2007/9-1102–1109/$25.00.
1102 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

J. Geotech. Geoenviron. Eng.

diameter steel tubular piles of a wharf in Wuhan China; then isthe laboratory test of six driven small-diameter steel tubular pilesin a test pit; and the last part deals with test results of five pre-stressed concrete �tubular� piles. In order to provide convincingtestimony about the crushing or splitting of concrete pile due todriving tensile stresses, the steel tubular pile is chosen to study.This can be explained since the steel tubular pile is formed bylinear elastic material and the data attained are more convictiveand representative.

In Situ Test

General Situation of Test

As the piling foundation of an approach, 164 steel tubular pileswere driven at the site of a wharf in Wuhan China. The mainstructure of this wharf is a beam-and-slab pile foundation struc-ture. In order to determine the bearing capacity of a single pile,not only static load testing but also dynamic testing are carriedout on two test piles. They are 41.49 m �S1� and 41.41 m �S2� inlength, respectively, 800 mm outside diameter with a wall thick-ness of 16 mm. There are four rows �Testing Lines A, B, C, andD� of instrumentation in each test pile. In each row, six electricalresistance strain gauges �Testing Points 1–6� are bonded securelyto the internal surface of the test pile, as shown in Fig. 1. In thedriving process, there are six records �Testing Processes 1–6� ofthe driving stresses, each record includes 5–10 blows. In order tofully understand the propagation of forces and stress waves alongthe axis of pile, the stress values at every measurement point arerecorded at the same time. Moreover, the average penetration ineach soil stratum is recorded, too.

Both test piles are driven by an MB70 hammer, which containsa pile cap with a pinewood cushion of 30 cm thickness. Eventu-
ally, the two pile toes seated on the pebble stratum after 1,061 and
EERING © ASCE / SEPTEMBER 2007

2007.133:1102-1109.

Dow

nloa

ded

from

asc

elib

rary

.org

by

MIS

SOU

RI,

UN

IV O

F/C

OL

UM

BIA

on

03/1

5/13

. Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

1,491 blows, respectively, with elevation differences of 63 cm,6 m in pile spacing, and the same geologic condition.

Driving stresses are recorded by oscilloscope and tape re-corder, and amplified by a dynamic strain gauge.

Test Results

Test results are time-varying driving stress waves of measurementpoints recorded by an oscilloscope. In this paper, only Test Pile S2

is analyzed because general conditions and test results of the twotest piles are basically the same. Fig. 2 shows the stress waves ofTesting Line A of Test Pile S2 from the Testing Processes 1 to 6,and Fig. 3 also shows the stress distributions during the driving ofthis pile. Table 1 gives the measured tensile and compressivestresses �positive sign stands for compressive stresses and minusstand for tensile stresses� along the pile shaft of Test Pile S2.

Analysis of Test Results

It can be seen in Table 1 and Figs. 2 and 3 that:�a� The maximum driving tensile stresses occur in the upper

portion of the pile, at the initial stages of driving, and with theincrease of penetration depth of the pile, this stress in the upper

Fig. 1. Layout of testing points

portion of pile diminishes gradually, as does the stress ratio of

JOURNAL OF GEOTECHNICAL AND GEOEN


tensile stress to compressive stress. For example, during TestingProcess 1, tensile stress in the upper portion of the pile reachesthe maximum value of −66.4 MPa �A3�, 54% of the compressivestress at the same cross section. In Testing Process 2, this tensilestress is −60 MPa �A2�, 33% of the compressive stress at thesame cross section. In Testing Process 3, this tensile stress is−46.2 MPa �A1�, 22% of the compressive stress at the same crosssection. In Testing Process 4, this tensile stress is −37.8 MPa�A1�, 17% of the compressive stress at the same cross section.During Testing Processes 5 and 6, no tensile stress occurs at A1

and A2.�b� At the former stages of driving �Testing Processes 1–4�, the

maximum driving tensile stress occurs in the upper portion of thepile, namely, the undriven portion of the pile above the soil sur-face level �l1�. While at the final stages of driving �Testing Pro-cesses 5 and 6�, the maximum driving tensile stress occurs in thelower portion of the pile, namely, the buried portion of the pile�l2�. It can be concluded that the magnitude and the location ofthe driving tensile stress are closely related to the buried portionof the pile �l2� and the undriven portion of the pile above the soilsurface level �l1� or the overall length of pile �l�. For example,during Testing Process 1, when the pile tip is �1/4�l deep into thesoil, the maximum driving tensile stress occurs at 0.6l1 above thesoil surface level, namely, �1/4�l from pile top. During TestingProcess 2, when the pile tip is �1/3�l deep into the soil, the maxi-mum driving tensile stress occurs at 0.9l1 above the soil surfacelevel, namely, 0.06l from the pile top. During Testing Process 3,when the pile tip is �2/5�l deep into the soil, the maximum driv-ing tensile stress occurs at the pile top. And then, during TestingProcesses 5 and 6, when the pile tip is more than �3/4�l deep intothe soil, the maximum driving tensile stress occurs at 0.33l2

below the soil surface �A4�.�c� During Testing Process 1, the pile tip is 10.46 m deep into

the soil, thus, Testing Points A1–A5 are all above soil surface,leaving A6 under the surface. The driving tensile stresses recordedin A1–A5 range from −37.8 to −66.4 MPa, and all of these dataare over 32% greater than the compressive stresses of their sametesting point. These longitudinal tensile stresses are caused by theextremely small driving resistance provided by the soft or loosesoils beneath the toe of the pile. In this stage, the measured stresswaves fluctuate dramatically, with tensile stresses and compres-sive stresses reflecting many times between pile top and pile toewith a period of 2L /C �L�length of pile; and C�velocity of thestress wave propagation in the pile�. Thus, tensile cracks easilyoccur at this initial stage of driving. However, when the pile is30 m deeper into the soil �Testing Processes 5 and 6� with its toeseated on a harder stratum of gravel, which provided muchgreater penetration resistance of pile, the measured stress wavesfluctuate the least, with tensile stresses and compressive stressesreflecting only a few times between pile top and pile toe with aperiod of 4L /C, and there is no occurrence of tensile stressthough Testing Point A1, and A2 is still above the soil surface.

�d� After the pile shaft is much deeper into the soil �TestingProcesses 5 and 6�, the measured tensile stresses are much smaller�often smaller than −10 Mpa�, only 3–6% of the compressivestresses of their same testing point, except for Testing Point A4

during Testing Process 5, whose measured tensile stress is−27.1 MPa, 16% of its compressive stresses. After careful study,this is because Testing Point A4 is exactly located at fine siltysand, which is between clayey soils. According to the rock andsoil mechanics theory, when the pile toe is seated on a soft orloose soil being overlaid with a hard stratum, it tends to cause the
occurrence of tensile stress.
VIRONMENTAL ENGINEERING © ASCE / SEPTEMBER 2007 / 1103

2007.133:1102-1109.

Dow

nloa

ded

from

asc

elib

rary

.org

by

MIS

SOU

RI,

UN

IV O

F/C

OL

UM

BIA

on

03/1

5/13

. Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

Fig. 2. Stress waves of Testing Line A of Test Pile S2 from Testing Processes 1–6

1104 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / SEPTEMBER 2007

J. Geotech. Geoenviron. Eng. 2007.133:1102-1109.

Dow

nloa

ded

from

asc

elib

rary

.org

by

MIS

SOU

RI,

UN

IV O

F/C

OL

UM

BIA

on

03/1

5/13

. Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

Analysis by Smith Wave Equation

According to the principle of the Smith wave equation, hammer,pile cap, and pile body can be divided into 10 elements. The

Fig. 3. Stress distributions of Test Pile S2: �1� mucky clayey soil; �2�6� fine silty sand; �7� clayey soil; �8� fine silty sand; and �9� gravel

Finite-difference method is used to solve the wave equation. The



input parameters for calculation are as follows: hammerweight�72 kN; pile cap weight�37 kN, drop height of hammers�2.4 m; hammer efficiency�0.9; pile element�4.149 m longfor S1 and 4.141 m long for S2; elastic modulus of pile�210 Gpa;

silty sand; �3� mucky clayey soil; �4� fine silty sand; �5� clayey soil;
� fine

2007.133:1102-1109.

Dow

nloa

ded

from

asc

elib

rary

.org

by

MIS

SOU

RI,

UN

IV O

F/C

OL

UM

BIA

on

03/1

5/13

. Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

pile element weighs 12.3 kN; cross section of pile�394 cm2;soil damping coefficient at pile tip�0.48; soil damping coeffi-cient at pile side�0.16; and maximum elastic displacement ofsoil�0.254 cm; percent of soil resistance at the pile tip to totalresistance�18% for S1 and 31% for S2. Penetration at last strikemeasured in the field test�1.68 blow/cm for S1 and 3.7 blow/cmfor S2. By calculation, the curves of the hammering stress andresponse of the pile driving are listed in Fig. 4. From the curve, it

Table 1. Measured Tensile and Compressive Stresses of Test Pile S2

Testingpoint Testing process 1

A1 Tensile stresses �t �MPa� −50.4

Compressive stresses �c �MPa� 155.4

�t /�c�100% 32



�t /�c�100% 41



�t /�c�100% 54



�t /�c�100% 40



�t /�c�100% 35



�t /�c�100% 6

Fig. 4. Pile top measurements of stress waves with differentthicknesses of sand



can be seen that the static resistance during driving is 8,600 kNfor S1 and 10,500 kN for S2.

Laboratory Test

Testing Method

The laboratory test is carried out in a test pit, which is

2 3 4 5 6

54.6 −46.2 −37.8 0 0

00.0 205.8 222.6 218.4 208.0

27 22 17 0 0

60.0 −42.0 −35.9 0 0

80 177.0 182.9 182.9 171.2

33 24 2 0 0

42.2 −18.1 −21.2 −6.1 −3.0

69.1 163.1 172.2 166.1 162.3

25 11 12 4 1.8

18.1 0 −12.0 −27.1 −10.2

73.9 168.0 173.9 164.9 157.2

10 0 7 16 6

14.7 −8.4 −8.4 −8.4 −4.2

55.4 151.2 130.2 142.8 137.3

9 6 6 6 3

0 0 0 −2.9 −2.9

56.9 77.9 63.0 105.0 86.0

0 0 0 3 3

Fig. 5. Pile top measurements of stress waves with different pile caps

−

2

−

1

−

1

−

1

−

1


2007.133:1102-1109.

Dow

nloa

ded

from

asc

elib

rary

.org

by

MIS

SOU

RI,

UN

IV O

F/C

OL

UM

BIA

on

03/1

5/13

. Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

1.5�1.7 m in section and 1.7 m in height. Below the bottom ofthis pit there is a layer of hard clay, with its water content at 15%,and liquidity index 0.25. Inside, the pit is filled with homoge-neous sand with specific gravity G=2.64; and average relativedensity Dr=0.43. Among the four sides of this pit, two are con-crete walls and the other two are hard clay, which have the sameproperties with the underlying soil.

The sand used in the test is sand commonly used in construc-tion, which is screened out and filled the test pit and then com-pacted layer by layer. At the same time, in order to model the hardgravel or rock, a 50 mm thick concrete board lays on the bottomof the test pit. In the first stage of the test, only one test pile isdriven, in which there are three cases:

�a� No sand in the pit. The test pile is driven by a hammer withits tip directly seated on the concrete board, and dynamic drivingstresses are recorded.

�b� Fill the pit with 680 mm thick sand with the pile tip stillseated on the concrete board, then drive the pile and record thedynamic driving stresses similar to Case �a�.

�c� The same as in Case �b� except that the sand is 1,500 mmin thick. The test piles are small-diameter steel tubular piles withtheir outside diameter of 60 mm, 4 mm in wall thickness, and1,600 mm in length. There two rows of instrumentation in eachpile. In each row, there are five electrical resistance strain gaugesbonded to the outside surface of the test pile �Zhou and Li 1992�.In the second stage of the test, in order to fully understand theperformance of different pile caps, five test piles with differentpile caps �pinewood, disk spring pile cap �Shi 1995; Hu 1987;Zhou and Li 1994; Zhou and Zhou 2001�, or without pile cap� aredriven, with a transducer tack welded between the pile and the

Table 2. Measured Stresses of Pile Top

Peak value�MPa�

Operating condition�c

�MPa��t

�MPa

No sand in the pit 31.4 −18.

Fill the pit with 680 mm thickness sand 38.7 −16.

Fill the pit with 1,500 mm thickness sand 45.4 −10.

Fig. 6. Pile top stress waves and pile shaft stress envelope �1� clayeclayey sand



pile cap. The pile driver is a simple minidrop hammer pile driverdesigned and made by the writers, with a 500 N free drop ham-mer and maximum drop height 2.5 m.

Test Results and Analysis

Fig. 5 shows the pile top measurements of stress waves of differ-ent thicknesses of sand in the test pit, with the stress values listedin Table 2, for the convenient of analysis of tensile stress distri-bution patterns. The pile top �five test piles with different pilecaps� measurements of stress waves for the final blow by thehammer are illustrated in Fig. 6.

From Figs. 5 and 6 and Table 2, it can be concluded that:�a� With the increase of penetration of the pile, the compres-

sive stresses in pile shaft increase gradually, while the tensilestresses diminish gradually. The driving tensile stress reaches themaximum value of −18.8 MPa, 60% of the compressive stress atthe same cross section, which is closed to the value 54% obtainedin the in situ test.

�b� The magnitude and the location of driving tensile stress areclosely related to the buried portion of the pile �l2� and the un-driven portion of the pile above the soil surface level �l1�. Whenthe pile tip is 0.94l deep into the soil, the driving tensile stress ofthe pile top is still 22% of the compressive stress of the pile top,which is greater than that value obtained in the in situ test. Thisinconsistency can be explained by the defective tightness betweenthe pile–soil interface, which leads to less than full mobilizationof skin friction.

�c� Driving tensile stress occurs when the pile is driven with

��t /�c��100%�%�

Buried portionof the pile

�l2�

Undriven portionof the pile

�l1�

60 0 l

42 0.43l 0.57l

22 0.94l 0.06l

�2� mucky clayey soil; �3� mucky clay soil; �4� clayey soil; and �5�

�

8

2

1

y soil;


2007.133:1102-1109.

Dow

nloa

ded

from

asc

elib

rary

.org

by

MIS

SOU

RI,

UN

IV O

F/C

OL

UM

BIA

on

03/1

5/13

. Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

the use of a pinewood pile cap or without a pile cap. On thecontrary, the tensile stress tends to disappear with the use of thenew type of disk spring pile cap during driving.

Example for Prestressed Concrete „Tubular… Piles

Prestressed concrete pile, because of its high strength combinedwith its advantage of withstanding severe driving, remains widelyused for piling. However, its disadvantages and limitations in-clude noise, vibration, ground heave, and even the occurrence oftensile stresses caused by driving. The control tensile stress ofprestressed concrete pile is often 5 Mpa �Zhou et al. 2001; Zhouand Chen 2006�. Based on research by Zhou �2006� and Tang�1990�, five practical examples are analyzed in the following text.

Example 1

On an engineering site in Shanghai, prestressed concrete piles,45 m in length, 450 mm in square section, are driven by a K-35diesel hammer with a ram having a mass of 35 kN. The conven-tional pile cap is used, which has a mass of 12.0 kN, pile cushionspring constant 8.66 MN/cm, elastic modulus E=3.5�104 MPa.The measured stress curves are shown in Fig. 6.

As shown in Fig. 6, the pile tip is seated on a clayey soilstratum being overlaid with a soft mucky clay soil. The maximumdriving tensile stress is −100 KPa, 48% of the compressive stressat the same cross section, which is closed to the 54% value ob-tained in the in situ test. This tensile stress occurs at 0.22l fromthe pile top �which is close to the value �1/4�l obtained in thein-situ test�, exactly located at the mucky clayey soil stratum.

Example 2

Large-diameter prestressed concrete tubular piles are driven by anMB-70 diesel hammer with a ram having a mass of 72 kN on thesite of a Honggang City wharf in Wuhan China. These piles are

Table 3. Measured Dynamic Stresses of Pile Top

Pile shaft stre�MPa�

Maximum Minimum

Pilenumber

Pilecap type

Compressivestress

Tensilestress

Compressivestress

0 Conventional 27.4 −8.22 19.4

2 Disk spring 13.33 −3.70 11.56

3 Disk spring 13.33 −3.71 12.35

4 Conventional 17.95 −5.41 12.95

Table 4. Measured Dynamic Stresses of Pile Shaft

Pile shaft stresses�MPa�

Pile cap type

Maximumcompressivestresses �c

Maximumtensile

stresses �t

��t /�c��1�%�

Conventional 15.16 −5.88 39

Disk spring 6.77 −1.38 20



1,000 mm in outside diameter, 40 m in length, and C60 of con-crete grade. The conventional pile cap and disk spring pile cap areused, respectively. And the conventional pile cap has a mass of35 kN, with a 300 mm thick pinewood cushion inside this cap.The disk spring pile cap is 50 kN in weight. Table 3 �Shi 1995�shows the in situ measured stresses.

Example 3

On the site of a Ligang wharf in Jiangyin, China, concrete piles,38 m in length, 600 mm in square section, are driven by an MH80diesel pile driver barge with a ram having a mass of 80 kN. Theconventional pile cap and disk spring pile cap are used, respec-tively. The measured pile shaft stresses are shown in Table 4�Tang 1990�.

Example 4

On the site of a wharf in Nantong, China, prestressed concretetubular piles, 40 m in length, 800 mm outside diameter, aredriven by an MH80 diesel pile driver barge with a ram having amass of 80 kN. The conventional pile cap and disk spring pile capare used respectively. The measured pile shaft stresses are shownin Table 5 �Tang 1990�.

Based on the Examples 2–4, the following conclusions can bedrawn:

�a� High driving tensile stresses �exceeding the control tensilestress of 5 MPa�, which cause the occurrence of cracking, evenbreakage of pile shaft, and pile toe’s failure to reach the requireddepth, often occur due to the usage of conventional pile cap in thepile driving. On the contrary, these tensile stresses tend to dimin-ish within the control tensile stress �5 MPa� due to the usage ofthe new type of disk spring pile cap in driving. Moreover, thisnew pile cap ensures the integrity of the pile body and driving thepile to the required depth.

�b� Compared to disk spring pile cap, the conventional onealways makes the maximum driving tensile stresses much greater

Average value

Compressivestress

Tensilestress

Soils beneathpile toe Pile integrity

24.8 −7.45 Gravel Pile top breakageand pile shaft cracking

12.35 −3.71 Gravel No damage

12.94 −3.88 Gravel No damage

15.99 −4.80 Gravel Pile top breakageand pile shaft cracking


Clayey soil near the fine silty sand Breakage of pile shaft

Clayey soil near the fine silty sand No damage

sses

Tensilestress

−5.82

−3.47

−3.70

−3.81

00%


2007.133:1102-1109.

Dow

nloa

ded

from

asc

elib

rary

.org

by

MIS

SOU

RI,

UN

IV O

F/C

OL

UM

BIA

on

03/1

5/13

. Cop

yrig

ht A

SCE

. For

per

sona

l use

onl

y; a

ll ri

ghts

res

erve

d.

than the minimum tensile stresses, while the disk spring pile capmakes the difference between the maximum and minimum tensilestresses become small, and often makes them close to the averagevalue. All of these demonstrate that this new pile cap can effec-tively prevent driving deviation from alignment.

Example 5

Prestressed concrete tubular piles are driven by a diesel hammerwith a ram having a mass of 60 KN on the site of a third exten-sion engineering of a power station in Wuhan, China. These pilesare 600 mm in outside diameter, 18 m in length, C80 of concretegrade. And the conventional pile cap is used in driving. The soilprofile consists of artificial plain soil with gravels to a depth vary-ing from 1 to 9 m below ground level overlaying medium weath-ered rock as bearing stratum. Below this depth is clay and siltyclay about 10 m in thickness. Although there is no dynamic test-ing of driven piles, the writer had observed the driving process ofa pile. When the pile is about 3 m deep into the soils, near the piledriver several transverse cracks occur in the upper portion of thepile above the soil surface level. These cracks are up to 10 mm inwidth with a length exceeding one-half of the pile circumferenceand with spacing from 2 to 3 m. A level instrument had measuredthe great deviation of the pile from the true alignment to the piledriver. Thus, not only compressive stress concentration but alsothe tensile stress concentration �the main factor that causes thecracking breakage of the pile� is due to this deviation. Finally, thispile failed to reach the required depth, remaining one-half of thepile length above the soil surface.

Conclusion

�a� In the pile driving, with the increase of penetration of the pile,the compressive stresses in pile shaft increase gradually, while thetensile stresses diminish gradually. Both laboratory and in situtests show that the magnitude and the location of the drivingtensile stress are closely related to the buried length of the pile, tothe undriven length of the pile above soil surface level, and evento the soil physical property. These tests also indicate that themaximum driving tensile stresses occur in the upper portion ofthe pile, especially at the initial stages of driving the pile seatedon a soft or loose soils. This stress often occurs at �1/4�l �l stands

Table 5. Measured Dynamic Stresses of the Pile Shaft

Pile shaft stresses�MPa�

Pile cap type

Maximumcompressivestresses �c

Maximumtensile

stresses �t

��

Conventional 15.9 −6.62

Disk spring 11.58 −4.84

for the length of pile� from the pile top with its value accounting



for 50% of the driving compressive stresses at the same crosssection. This high driving tensile stress would lead to the occur-rence of transverse cracks and even the breakage of the pile,which should draw the attention of the engineer. But it is a pitythat engineers always neglect this problem.

�b� High driving tensile stresses �exceeding the control tensilestresses� often occur due to the use of a conventional pile cap inthe driving of prestressed concrete �tubular� pile, and this stressoften causes the occurrence of cracks and even breakage of piles.On the other hand, the disk spring pile cap can effectively dimin-ish the driving tensile stresses and protect the integrity of the pilebody.

�c� In driving, deviation of the pile from the true alignmenttends to cause breakage in the upper portion of the pile above thesoil surface level, which make the driving tensile stresses evengreater. The disk spring pile cap can effectively prevent drivingdeviation from alignment because this new cap can transfer thedriving force through its piston, which ensure an even distributionof the hammer blow on the pile top. Thus, in driving practice,besides avoiding driving deviation from alignment, the writersrecommend the disk spring pile cap.

References

Hu, R. �1987�. Analysis and design of bridge pile foundation, ChinaRailway, Beijing.

Shi, P. �1995�. Pile engineering handbook, China-Building, Beijing.Tang, N. C. �1990�. “The Research Report of Dh-8000 disk spring pile

cap.” Academic Research of the Third Navigational Fairs Bureau ofMinistry of Communications, Navigational Fairs Bureau of Ministryof Communications, 560–566.

Zhou, L., and Li, P. �1992�. “Application effects of pile cushion and diskspring driving cap—indoor test and study.” Port Waterway Eng.(China), 9�1�, 37–42.

Zhou, L., and Li, P. �1994�. “Development of a new type of disc springcap and its application.” Chinese J. Geotech. Eng., 16�4�, 47–55.

Zhou, L. Y., and Chen, J. B. �2006�. “Mechanical principles of a newtwo-way composite disk spring cap for pile driving.” J. Test. Eval.,34�4�, 357–361.

Zhou, L. Y., Chen, J. B., and Zhou, X. �2001�. “On the recovery coeffi-cient and parameter selection for a new type of disk spring pile cap.”J. Test. Eval., 29�6�, 582–587.

Zhou, L. Y., and Zhou, X. �2001�. “Proper selection of the stiffness of the

100%�


Fine sand Cracking and failure to reachthe required depth

Fine sand No damage

t /�c��%

42

41

disk spring pile cap in pile driving.” J. Test. Eval., 29�2�, 208–213.


2007.133:1102-1109.

construction control and pile body tensile stresses distribution pattern during driving

Documents