the pcc piling method - tu delft

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The PCC piling method

Analysis, design and applications

Master Thesis Geotechnical Engineering

Faculty of Civil Engineering Delft University of Technology

Wouter Karreman

1004956

August 2006


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Analysis, design and applications

Master Thesis as a final work on the study of Civil Engineering

at the Delft University of Technology

Wouter Karreman

Graduation committee

Prof. dr. ir. A.F. van Tol (Delft University of Technology, Geotechnical Engineering)

ing. H.J. Everts (Delft University of Technology Geotechnical Engineering)

Dr. ir. C. van der Veen (Delft University of Technology, Structural and Building Engineering)

Front page photo: PCC pile installation [Michiel van der Ruyt]


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Preface

This is report titled “the PCC piling method” was written as a finalization of my Master

study in Delft at the Geotechnical Engineering section of the Faculty of Civil engineering of

the Delft University of Technology.

The research on the PCC piling method was done in the framework of the

cooperation between the geotechnical institutes of GeoDelft, Delft and GeoHohai, Nanjing,

China. I was allowed to go to Nanjing to study the PCC piles and bring back information on

the possibilities of this system.

I would like to thank the members of my graduation committee, prof. dr. ir. Van Tol,

ing. Everts and dr. ir. Van der Veen for their guidance and support during the months of my

study in Delft.

At GeoDelft I would like to thank mr. Martin van Staveren for creating the possibility

for me to visit GeoHohai and making all sorts of arrangements make my stay there as

enjoyable as it was. Thanks also go out to all my colleagues at the departments of foundation

engineering and construction and soil constructions for their support during my study. I

would especially like to thank Ad Verweij for his useful comments and discussions which were

sometimes even about the PCC pile.

Last but certainly not least I would like to thank all the members of GeoHohai for

their warm welcome and great support in my study. Special thanks go out to prof. Liu

Hanlong, Dr. Zhou Yundong, Dr. Zhang Ting and Dr. Tan Hiuming. All the students of prof.

Liu with whom I shared an office are thanked for their support and tireless efforts to show

me as much of Nanjing and China as possible in the short time of my stay.

Delft, August 31, 2006

Wouter Karreman


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Summary

At the GeoHohai research institute at Hohai University in Nanjing, China a new

ground improving piling system is developed. The function of this system is to minimize the

settlements and differential settlements after the construction of an embankment on soft soil.

The PCC pile is an open, cast-in-place, hollow, concrete tube pile constructed using a

casing consisting of two steel pipes with different diameter placed inside of each other. The

space between the two pipes is closed off at the bottom and the pile is vibrated into the soil.

At the design depth concrete is cast in the hollow area created between the two pipe piles

and the casing is retracted. This process opens the closure between the two casings allowing

the concrete tube to remain in the soil while the casing is being retracted. The final pile has a

diameter of 1.0 to 1.5 m, a wall thickness of 100 to 150 mm, a length of up to 25 m and a

centre-to-centre distance of approximately 3 to 3.5 m. On the complete pile field a geotextile

mattress consisting of 3 layers of geotextile with gravel in between is laid to redistribute the

load of the fill to the piles. It is found that the installation speed is quite slow but that by

streamlining the concreting a lot of time can be gained.

Several tests can be done to verify the quality of the single pile and the entire soil

improvement. These include low strain test, load tests on single piles and on the composite

foundation, partly excavations and long term monitoring. The results of these tests point to

the PCC pile having a high and consistent quality. Some possible problems noted to occur in

the Netherlands during construction of cast-in-place piles, like demixing of the concrete at

higher depths of the pile and necking are not yet sufficiently addressed.

A comparison is made between the PCC piling method and two new ground

improving piling systems developed in the Netherlands, the HSP and the AuGeo piling

methods. Both the HSP and AuGeo systems apply small cast in place solid concrete pile with

diameters ranging from 150 to 180 mm and lengths up to 17 m. They can be installed much

faster than the PCC pile and with reinforcement but have lower bearing capacity and smaller

spacing. A rough comparison shows that a field of PCC piles would it be applied in the

Netherlands can be made faster than an AuGeo pile field with slightly more concrete. In thick

layers of soft soil the PCC pile’s advantage of high shaft bearing over HSP and AuGeo

because more prevalent.

The currently used design methods in China for the bearing capacity and settlement

of the PCC pile and the pile improved area are very simple and are not according to the

Dutch norms. A detailed calculation method for the single pile bearing capacity and

settlement has been obtained from the Dutch norms and the advice for plugging calculations.

The method includes:

• Shaft bearing capacity,

• Negative skin friction,


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• Plugging according to the spring method and

• Tip bearing capacity.

The calculation method was verified on the Yan-Tong case where load tests were

executed on a single PCC pile. The results of the calculation, based on the soil investigation in

the area, closely approximated the measured bearing capacity and settlement.

The high quality and high single pile bearing capacity open the door for the

application of the PCC pile in other cases then soil improvement. Two cases are considered:

• PCC pile in building foundation in the Rotterdam area including tension forces due to pull-

out and head moments and comparison to the Vibro-pile and

• PCC pile in bridge abutment where the pile is subjected to horizontal forces and the

resulting bending moments in the pile.

When applied in a building foundation it was found that the bearing capacity of the pile when

placed 2 m into a stiff sand layer is a little higher than that of the Vibro pile per unit of

concrete. The unique shape of the pile also reduces the tension forces due to the bending

moment when the pile is loaded eccentrically. This same reduction compared to solid piles is

found in the application of the PCC pile in a bridge abutment. Some tension forces are

however occurring in the pile and reinforcement is found to be required.

Three reinforcement methods are discussed:

• Traditional reinforcement which is cheap but due to the required reinforcement netting

difficult to apply,

• High strength reinforcement which requires less bars and netting but is susceptible to

corrosion and expensive and

• Reinforcing with steel fibres which has no problems with netting and cover but add only a

limited tension strength.

Installation of the reinforcement into the thin walled casing is a difficult problem with the

attachment of the bars to the casing as a possible solution.

Import conclusions of the report are:

• Application of the PCC pile in the Netherlands as a ground improving method is viable in

many cases and a robust design and installation method is available and

• The PCC pile is applicable in foundations and bridge abutments, if necessary

reinforcement is possible.


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1 Introduction ............................................................................................................9 2 Research question .................................................................................................11

2.1 Problem Definition.........................................................................................11 2.2 Aspects ........................................................................................................11

2.2.1 Execution..................................................................................................11 2.2.2 Pile quality ................................................................................................11 2.2.3 Comparison pile based ground improvement................................................11 2.2.4 Cost .........................................................................................................11 2.2.5 Bearing capacity........................................................................................11 2.2.6 Improvements for foundation application.....................................................12 2.2.7 Comparison pile foundation ........................................................................12

3 The PCC pile..........................................................................................................13 3.1 Introduction..................................................................................................13 3.2 Installation method .......................................................................................13

3.2.1 Introduction..............................................................................................13 3.2.2 Installation Method....................................................................................13 3.2.3 Equipment and personnel...........................................................................16 3.2.4 Attention points.........................................................................................19

3.3 Quality assurance..........................................................................................24 3.3.1 Introduction..............................................................................................24 3.3.2 Low strain test ..........................................................................................24 3.3.3 Excavation ................................................................................................26 3.3.4 Load test ..................................................................................................28 3.3.5 Long term settlement monitoring ................................................................31

4 Comparison other methods.....................................................................................36 4.1 Introduction..................................................................................................36 4.2 HSP..............................................................................................................36

4.2.1 Introduction..............................................................................................36 4.2.2 Pile properties ...........................................................................................37 4.2.3 Installation process....................................................................................37

4.3 AuGeo ..........................................................................................................38 4.3.1 Introduction..............................................................................................38 4.3.2 Pile properties ...........................................................................................38 4.3.3 Installation process....................................................................................39 4.3.4 Quality control...........................................................................................40

4.4 Comparison ..................................................................................................41 4.4.1 Introduction..............................................................................................41


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4.4.2 General ....................................................................................................41 4.4.3 Construction .............................................................................................41 4.4.4 Economic comparison ................................................................................41 4.4.5 Conclusion ................................................................................................42

4.5 Conclusion ....................................................................................................43 5 Calculation method ................................................................................................44

5.1 Introduction..................................................................................................44 5.2 Bearing capacity............................................................................................44

5.2.1 Introduction..............................................................................................44 5.2.2 Chinese practice ........................................................................................44 5.2.3 Dutch practice...........................................................................................46 5.2.4 Negative shaft friction................................................................................51 5.2.5 Plugging ...................................................................................................54 5.2.6 Soil bearing capacity..................................................................................66

5.3 Settlement....................................................................................................68 5.3.1 Chinese practice ........................................................................................68 5.3.2 Dutch practice...........................................................................................72 5.3.3 Piled raft and pile group responses .............................................................74

5.4 Yan-Tong example case .................................................................................82 5.4.1 Introduction..............................................................................................82 5.4.2 Site description .........................................................................................82 5.4.3 Chosen calculation methods .......................................................................84 5.4.4 Bearing capacity single pile ........................................................................85 5.4.5 Soil bearing capacity..................................................................................92 5.4.6 Bearing capacity composite foundation........................................................92 5.4.7 Settlement single pile.................................................................................93 5.4.8 Settlement of the composite foundation ......................................................95

5.5 Conclusion ....................................................................................................96 6 Further applications ...............................................................................................98

6.1 Introduction..................................................................................................98 6.2 Requirements................................................................................................98

6.2.1 Introduction..............................................................................................98 6.2.2 Vertical loading .........................................................................................98 6.2.3 Horizontal load ........................................................................................ 101 6.2.4 Settlement .............................................................................................. 101

6.3 Case studies ............................................................................................... 102 6.3.1 Introduction............................................................................................ 102 6.3.2 Case 1: Foundation on tip bearing ............................................................ 102


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6.3.3 Case 2: Piled bridge abutment .................................................................. 112 6.3.4 Conclusion .............................................................................................. 114

6.4 Reinforcement ............................................................................................ 115 6.4.1 Introduction............................................................................................ 115 6.4.2 Traditional reinforcement ......................................................................... 115 6.4.3 Dywidag bars .......................................................................................... 116 6.4.4 Fibre reinforcement ................................................................................. 117

6.5 Design changes........................................................................................... 121 6.5.1 Introduction............................................................................................ 121 6.5.2 Reinforcement......................................................................................... 122 6.5.3 Pile properties ......................................................................................... 124 6.5.4 Equipment .............................................................................................. 124

6.6 Conclusion .................................................................................................. 125 7 Conclusions and recommendations ........................................................................ 127

7.1 Conclusions................................................................................................. 127 7.2 Recommendations ....................................................................................... 128


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1 Introduction

During and after construction of embankments on soft subsoil large settlements often

occur due to the consolidation and creep of the soft material. Because of the load on the soil

caused by the embankment, excess pore pressures occur. The outflow of water leads to a

volume loss of the soft soil.

The consolidation time and settlement are mainly dependent on the thickness and

permeability of the soft soil layers but can be years and meters. This can cause problems

during construction of, for example, a highway embankment for which after a certain period

of time only small settlements are allowed. Differential settlements, where some parts of the

construction settle more than others, can also occur in inhomogeneous subsoil.

To reduce the settlement several soil improving methods are available. One of the

methods is placing the embankment on piles. Several methods for this kind of ground

improving piles are available all over the world.

In recent years the Chinese Geotechnical Institute of HoHai University in Nanjing

(GeoHohai) has developed a new ground improving pile system, the PCC piling system. The

PCC pile is essentially a hollow, cast in place, concrete pile. This pile has an advantage over

solid concrete piles in that it can obtain a higher shaft resistance with the same amount of

concrete. The main advantage over pre-casting is that no reinforcement is needed to prevent

damage during transportation.

During construction a hollow double steel casing is vibrated into the subsoil. The

space between the two casings is sealed at the pile tip while vibrating the pile into the soil.

The seal, see figure 1-1, consists of several spring plates attached to the outer casing, which

will be pushed against the inner casing due to the vertical downward movement.

Figure 1-1: Seal that connects the two tubes [33];


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After vibrating the double casing to its appropriate depth the space between the

casings is filled with a grout mix. The grout mix is poured through an opening at the top part

of the double casing, see figure 1-2.

Figure 1-2: Casing with pour mouth at the top [33];

The total amount of grout mix depends on the length of the pile and the width of the

hollow space between the two casings. After filling the space between the casings, the pile is

vibrated out of the soil while approxrimately 30% additional grout mix is added to

compensate for expansion and volume loss due to the vibration. The top 0.5 m of soil above

the pile is excavated and replaced by concrete to provide a solid concrete head at the top of

the pile.

This type of soil improvement has been used effectively for several projects in thick

soft soil layers in China. It might therefore be interesting to apply this method in the soft soils

of the Netherlands, as a ground improving method or as a foundation pile. To determine if

the PCC method is viable in the Netherlands an analysis of the method and a comparison with

existing soil improving piles and foundation piles will have to be made. The purpose of this

Master Thesis is to provide this analysis and comparison.


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2 Research question

2.1 Problem Definition

To reduce settlement and provide stability to embankments, GeoHoHai has

developed the PCC pile. To determine if this type of pile is viable in the Netherlands as a

ground improving method and/or as a foundation pile, the technical and economical feasibility

needs to be determined. This includes an analysis of the execution method, the calculation

method and a comparison with existing ground improving methods and foundation piles.

2.2 Aspects

2.2.1 Execution

The execution method of the PCC pile will be analysed. During execution of cast-in-

place piles certain problems can be expected. Demixing of the grout can, for example, occur

which can result in the forming of gravel arches that prevent the grout from filling the entire

volume of the hollow space.

2.2.2 Pile quality

The quality of the pile determines its ability to function. A pile that is not continous

over its length or has other failures can have a lower bearing capacity than designed. The

quality of the pile can be determined by a number of tests.

2.2.3 Comparison pile based ground improvement

At the moment several types of piles are used in the Netherlands for ground

improvement purposes. Two of the newest methods, HSP and Augeo piles, will be reviewed

and compared to the PCC piling method.

2.2.4 Cost

The cost of the pile is dependent on the material, the equipment, the personnel

required and the installation time. Differences between the Netherlands and China in cost

ratios between these categories can be expected.

2.2.5 Bearing capacity

The lateral and vertical bearing capacity of the PCC piles, and the method in which

these are calculated, is very important to determine the effectiveness and applicability of the

PCC pile in the Netherlands.


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2.2.6 Improvements for foundation application

For application of the PCC method in foundations of buildings some adjustments will

have to be made. Reinforcement of the head or of the entire pile can, for example, be

considered.

2.2.7 Comparison pile foundation

At the moment several types of piles are used in the Netherlands for foundation

construction. A comparable foundation piling system, the Vibro pile, will be considered and

compared to the PCC pile.


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3 The PCC pile

3.1 Introduction

A general introduction on the PCC pile is given in chapter 0. In this chapter the

installation method and quality assurance of the PCC pile will be described in detail.

In paragraph 3.2 the installation method is discussed based on observations in the

field and available literature. Required equipment and personnel are described and attention

points during installation including possible solutions are discussed.

In paragraph 3.3 the quality assurance during and after construction is described

from the results of different tests executed at the Yan-Tong project.

3.2 Installation method

3.2.1 Introduction

This chapter gives a description of the installation method based on the observation

of the installation of two PCC piles and available literature on PCC pile installation. This

chapter also contains an analysis of the attention points during PCC pile installation is

obtained based on past experiences from literature.

The observed PCC pile installation was part of the construction of an embankment for

a secondary road just outside Nanjing, the Jin Yang Road in ZhenJiang. The piles where

installed to a depth of 15.4 m below ground level and have a 1000 mm outer diameter, a 120

mm wall thickness and a centre-to-centre distance of 3 m. The subsoil consists of

approximately 2 m thick stiff clay on a thick base of softer clay.

The PCC piles are applied in this case to limit the settlement due to compression of

the soft soil by redistributing the load of the embankment to the stiffer lower clay layers via

the shaft and tip bearing of the piles. The load of the embankment is brought to the piles by

application of a geotextile mattress which is laid on top of the piles [18]. This application of

the PCC pile is the most common in China at the moment of writing.

3.2.2 Installation Method

The installation method consists of roughly four steps, which are extensively

explained in the following paragraphs:

• Vibrating the steel casing into the soil,

• Pouring the concrete,

• Vibrating the steel casing out of the soil and

• Installation of the pile head.


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Vibrating the casing into the soil

A double walled steel casing is vibrated into the soil using a double headed vibratory

hammer. The casing is closed with flaps at the pile tip, see figure 3-1.

Figure 3-1: The flaps at the tip of the pile are closed using a temporary steel wire;

The flaps are closed during installation with a steel wire to prevent penetration of water and

soil into the open area between the two steel tubes. The casing is then pushed into the soil,

firstly using only the dead weight of the hammer and the installation platform to minimize

vibration, later also with help from the vibratory hammer. The process is continuous with a

speed of 3-4 m/min (dependent on soil resistance) until the design depth is reached. An

alternative for the use of the flaps is using a concrete expendable driving shoe to close off

the area between the two piles [18].

The driving, vibration and squeezing effects of the penetration, and to a certain

extent the extraction, of the casing compacts the soil. The degree of this compaction depends

on the wall thickness, the vibratory hammer and the soil properties.

In this project there is an existing landfill of 2 m on the top of the soft soil layers. If

the building manager suspects that this landfill is heavily compacted, for example if the pile is

installed on a route used by the concrete truck, the pile is installed slightly differently. First

the pile is pushed through the fill and then pulled out and cleaned before pushing it in again.

This is to prevent the stiff fill material from blocking the opening of the flaps in a later stage.

Pouring the concrete

When the design depth is reached, concrete is added through shunt at the top of the

pile, see figure 3-2.


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Figure 3-2: Pouring of the concrete;

The annulus area between the two steel pipes is filled with concrete until above

ground level. By hitting the outer pile with a hammer and by experience the level of the

concrete can be estimated. The pressure of the concrete column breaks the wire and opens

the flaps at the tip of the pile. If for any reason the flaps do not open, the pile is retracted,

cleaned and installed again. The concrete is produced in a small concrete plant on site, see

figure 3-3. Alternatively the concrete can be delivered by truck from a regular concrete plant,

but this is less economical.

Figure 3-3: Concrete plant on site;

Vibrating the steel casing out of the soil

When a sufficient level of concrete is reached the casing is vibrated out of the soil at

a speed of 3-4 m/min. At certain times the extraction is stopped to add more concrete to

compensate for the loss of volume due to outflow and compaction of the concrete and the

filling of the volume of the walls of the tubes. The added quantity of concrete is 10-40% for

most projects but depends on the soil properties; in this case 30% was found to be sufficient.

After a few piles the value is known by the crew and no further monitoring of the concrete

level is required. A sufficiently large volume is added in order to ascertain that when the


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casing leaves the ground there is still concrete left in the annulus area, thus guaranteeing a

continuous pile.

Figure 3-4: Sufficient concrete is added during extraction of the casing;

Installation of the pile head

After the concrete has had sufficient time to harden, the top 0.5 m of the inside of

the pile is excavated by the construction crew using shovels. Concrete with the same

properties as the pile is cast into this top 0.5 m to assure a good connection between the

geotextile mattresses placed over the piles, see figure 3-5.

Figure 3-5: Excavated and concreted pile head [20];

3.2.3 Equipment and personnel

The required equipment for the installation of the PCC pile is the following:

• Jacket,

• Vibratory Hammer,

• Casing and

• Concrete Plant.

Personnel are required to man all the installation equipment.


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Jacket

The components of the installation jacket are shown in figure 3-7.

Figure 3-6: Jacket overview;

Figure 3-7: Components of installation jacket [18];

The jacket is able to move itself on two rails connected to the base, see figure 3-8. During

installation, the tube at the side of the pile can be moved while the jacket rests on the pile.

Figure 3-8: The jacket can move itself, to the right the control station can be seen;

The movement of the jacket is controlled from the base; the vibratory hammer is also

controlled from here. The winch used for lifting the concrete container to the shunt is

remotely controlled the concreting crew.

Vibratory Hammer

For the installation of the casing a double vibratory hammer is mounted in such a

way that the two hammers reduce the horizontal movement and amplify the vertical


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movement of the casing. In this case he double vibratory hammer can develop a pressure of

48 t on the casing and the dead weight of the jacket and hammer of 30 t, see figure 3-9. If

the force delivered by the dead weight of the jacket is insufficient for installation the vibratory

hammer is used. The high capacity of the hammer and the deadweight are to ensure a high

penetration speed of 3-4 m/min. The type of hammer used in the installation depends on the

properties of the subsoil.

Figure 3-9: Vibratory Hammer;

Casing

The casing consists of two steel pipe piles with different diameters. The spacing

between the two pipes is 100 mm in this case. So the typical wall thickness is also at least

100 mm. The piles are connected at the tip with flaps, see figure 3-1. The flaps are closed

during installation with a steel wire to prevent penetration of water and soil into the area

between the piles. Each 4-5 m steel blocks are welded between the steel pipe piles to assure

a continuous wall thickness. A shunt is connected to the top of the casing to allow for entry

of concrete in the annulus area, see figure 3-7.

Concrete plant

The concrete plant for this project is located on site. The concrete quality is typically

C15-C20. This strength is considered enough, higher quality is more expensive to make and is

less fluid thus decreasing the workability. Additives are typically only used to guarantee

workability at low temperatures.

Personnel

The vibratory hammer and movement of the jacket are controlled by 1-2 men

personnel. The casting of the concrete is done by 2-3 men personnel. The concrete plant is

operated by 3 people and 1 person transports the concrete to the pile location. The total

personnel required for this project is 8 people, excluding building managers.


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Time Planning

The installation time of the PCC pile in the Jin Yang Road project is approximately 50-

60 minutes per pile. This time is divided as follows:

• 2-3 minutes for moving the jacket to the pile location,

• 10 minutes for casing preparation and installation (highly dependent on geology) and

• 40-50 minutes for concreting and retracting.

As can be seen the concreting takes the most time; this is because the distance between the

concrete plant and installation site is quite large and approximately twenty batches (of 0.27

m3) of concrete are required to fill the pile.

3.2.4 Attention points

A few points of attention for the installation of the PCC piles are obtained from

literature. Most of the attention points are based on experiences with the Vibro pile which is

very similar in method of construction [43] and some general notes on pile installation [1].

Vibrating the steel casing into the soil

Attention points during the penetration of the steel casing into the soil are:

• Negative effects of vibration on adjacent piles

The vibration of the casing can have a negative effect on adjacent piles. The level of

the concrete can rise due to the installation of a pile close by due to the horizontal

displacement of the soil. If the level of the fresh concrete observed to be lowering during

installation of an adjacent pile an increase of the pile diameter is taking place due to

compaction of the soil.

• Negative effects of vibration on adjacent buildings

The installation can cause settlement of foundations of nearby buildings due to the

dynamic load on the foundation or the densification of the soil. This can lead to unwanted

settlements and/or damage to the buildings.

• Encountering rocks and debris

The soil can contain loose rocks and debris not detected by the soil investigation. If

encountered the installation of the pile can be blocked or seriously hampered. Damage to

the installation equipment can also occur.

• Water and soil entering the pile at the tip

Water and soil entering the tip of the pile during installation can have a negative

effect on the concrete quality at the pile tip. Mixing the concrete with soil and/or water

will reduce its quality and thus the bearing capacity of the entire pile.


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To avoid the occurrence of problems a number of steps that can be taken is given below; first

the recommendations from [43] are given. After that the current practice as observed and

described in PCC literature is given.

• Negative effects of vibration on adjacent piles

Monitoring of the concrete level in the piles adjacent to the pile currently being

installed. If the concrete level in the pile increases a change in installation order has to be

considered. If the level in the pile lowers the distance between the piles should be

increased to three times the tip diameter. The visual inspection of the pile to 0.5 m below

ground level or the execution of low strain tests is advised [43].

In this project no steps are taken to monitor the adjacent piles, although the distance

between the piles is less than three times the tip diameter. Although vibration of the soil

can clearly be felt at the location of the adjacent piles the concrete level can not be

visually monitored because of the extra concrete cast on the pile head, see figure 3-4, so

no decrease or increase of concrete level could be observed.

• Negative effects of vibration on adjacent buildings

Monitoring of vibration and settlement in adjacent buildings. If the vibration and load

become larger than allowed, stop the installation. The damage can be minimized by

increasing the distance between the building and the PCC piles.

In this project no buildings are located adjacent to the construction. In general, if

buildings are located close to the pile installation, a vibration reduction trench is

sometimes used.

• Encountering rocks and debris

Sufficient soil investigation can minimize the chance of encountering rocks and

debris. If rocks are encountered that prevent further installation, the casing will have to

be retracted and installed somewhere else.

In this project rock and debris are not often encountered. In general, if soil

investigation determines rock to be located at planned installation locations the design is

changed. If rock is encountered during installation the decision to either construct a new

pile or leave the pile somewhat shorter than designed, falls to the designer.

• Water and soil entering the pile at the tip

To prevent water and soil from entering the pile tip, the tip has to be sufficiently

closed. After installation the presence of water in the shaft can be verified by use of

measuring tube on a line. The decision if the water level in the shaft is unacceptable lies

with the contractor, in general not more than 50 mm water and no soil can be accepted.

The PCC piles are closed of with steel wire over the flaps. No tests are done to

determine if water or soil has entered the pipe pile. In literature [18] it is mentioned that

concrete can be cast in the shaft just above the ground water table. Installation is then

continued. This was not observed in practice.


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Pouring of the concrete

• Demixing of concrete

Demixing of the concrete has taken place when the gravel in the concrete is

separated from the cement. This can result in the formation of gravel ‘nests’ in the pile which

have a very low bearing strength. The demixing is dependent on the collisions of the concrete

with the steel. This effect will only have influence on the first few batches of concrete, this

will be explained below.

• Concrete penetration

The quality of the pile is heavily determined by the ability of the concrete to reach

every part of the pile. This depends on the quality of the concrete and the arching effect. The

arching effect is described in [43] as the creation of a concrete ‘arch’ over the diameter of the

wall, mostly on reinforcement rings. This arch prevents the penetration of more concrete in

the tube thus reducing the strength. An example of arching occurring in insitu concrete piles

(Vibro piles) is given in figure 3-10.

Figure 3-10: Arching over reinforcement ring in Vibro pile [43];

To avoid the occurrence of problems a number of steps that can be taken is given

below, first the recommendations from [43] are given. After that the current practice as

observed and described in PCC literature is given.

• Demixing of concrete

The demixing of the concrete is mainly caused by the interaction with (clean) steel.

The use of ‘clean’ steel equipment, for example at the start of the day will result in

cement forming a coating on the steel and thus demixing the concrete. The first batch of

concrete should therefore contain more cement or consist of only cement. Furthermore

the presence of obstacles during casting, for example reinforcement netting should be

minimized. If used large windows and thick reinforcement rods are preferred to small


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diameter rods and small windows. The presence of broken gravel instead of rounded

gravel also increases the chances of demixing.

To avoid demixing of the concrete due to ‘clean’ steel pipe piles the contact areas are

treated with oil before the tubes are connected. This treatment is done at the start of the

project and is assumed to guarantee no demixing. The piles are cleaned at the end of

each day but the coating will stay on. No reinforcement is currently used in the PCC pile

but the contact area is relatively large compared to non-pipe piles. Except low strain

tests, minimal measurement is done to determine the concrete quality at the bottom of

the pile so no data on the demixing is available.

• Concrete penetration

As with demixing the concrete penetration is best when as little obstructions as

possible are present. The presence of horizontal reinforcement rings enlarges the chance

of arching and should be minimized. The pipe pile casing should have a constant

diameter of the entire length. The fluidity of the concrete has to be high enough to fill the

pile from the tip to ground level; the concrete can also be cast under pressure.

No reinforcement is present in the PCC pile casings and the fluidity of the concrete of

quality C15-C20 is considered high enough. Excavation of the top 5-10 m and low strain

tests are done to guarantee the continuity of the pile, see paragraph 3.3 but the pile tip

is not checked.

Vibrating the steel casing out of the soil

Attention points during the extraction of the steel casing are:

• Different permeability of soil layers

In permeable sand layers the concrete can drain very fast, while in less permeable

layers, the concrete will drain/harden much slower. After installation the diameter in the

lower part of the pile expands a little, this triggers some flow of wet concrete. The

concrete in the permeable layer will be less able to flow due to the fast drying. This can

lead to a loss of concrete in the transition area between permeable and non-permeable

layers, which results in the formation of a bottleneck. This effect is called necking, see

figure 3-11.


23

Figure 3-11: “Necking” in Vibro pile due to permeability differences in soil layers;

• Support pressure provided by soil

In very weak soils, like peat, the lateral support pressure provided by the soil may

not be sufficient to prevent outflow of the concrete..

• Compaction of the concrete

To assure sufficient bearing capacity the pile the concrete has to be compacted.

• Adding of the additional concrete

If the additional concrete is added while the tip of the casing is in a weak soil layer,

the diameter of the pile just under the casing will become oversized due to the pressure

of the concrete.

To avoid the occurrence of problems a number of steps that can be taken is given

below, first the recommendations from [43] are given. After that the current practice as

observed and described in PCC literature is given.

• Different permeability of soil layers

If different soil layers with large differences in permeability are encountered it is

advised to change the installation order of the piles to avoid increase of pile diameter due

to soil compaction following from adjacent pile installation. This to avoid the occurance of

“necking”.

The distance between the piles is considered sufficient to avoid this problem with the

PCC piles.

• Support pressure provided by soil

If not enough support pressure can be delivered by the soil it is better to use another

piling technique.

The soils where PCC piles are applied provide enough support pressure.

• Compaction of the concrete

The retraction speed has no influence on the quality if not higher then 3-4 m/min. It

is considered important [43] to jack the pile forty times in place after casting the concrete


24

(for vibro-piles) to let the trapped air escape. Compaction will not take place because

water is insufficiently able to escape from the casing.

When retracting the pile is vibrated in place for a short time and then retracted at a

speed of 3-4 m/min.

• Adding of the additional concrete

The additional concrete should be added while the head of the casing is in a relatively

stiff soil layer. After adding additional concrete some extra jacking has to be done (10

blows for Vibro piles). The concrete level should never fall below ground level.

During execution of the PCC piles it is monitored if the level of concrete is above

ground level. Extra vibration is done after adding concrete but no special care is taken to

add the concrete in stiff soil layers.

3.3 Quality assurance

3.3.1 Introduction

Next to the possibilities to assure the quality of the PCC pile during construction,

several methods are also available to verify the consistency, bearing capacity and predicted

settlement. This chapter gives an overview of the commonly used methods in China. These

are:

• Low strain tests,

• Excavation of the pile head,

• Load tests on single pile and pile treated area and

• Long term monitoring.

Results are shown from measurements of the Yan-Tong highway project [17].

3.3.2 Low strain test

The low strain test (or short wave test) is used to verify the length and integrity of

pile by sending a short wave through the pile and measuring the reflection. A picture of the

equipment can be seen in figure 3-12.


25

Figure 3-12: Pile integrity testing system, low strain [23];

Because of the special nature of the PCC pile it is unclear how the integrity can be

correctly verified with the low strain test. A recent study by GeoHohai on prefab hollow

concrete piles states that the full pile can be checked by testing two points on the pile, a

quarter of the perimeter apart, see figure 3-13.

1

2

Figure 3-13: locations for low strain test;

Low strain tests were executed on the piles of the Yan-Tong highway embankment.

Sixty piles were checked, some typical results are shown in figure 3-14.


26

Figure 3-14: Low strain test result for Yan-Tong highway [17];

Although some of the peaks in the test result are not so clear which can be attributed to the

shape of the PCC pile it is possible to determine the pile length and no obvious discontinuities

are observed.

3.3.3 Excavation

The pile heads of some piles are excavated to 5 or 10 m below ground level. This is

an excellent way to visually inspect the quality of the pile. Examples of excavated piles are

shown in figure 3-15.

Figure 3-15: Excavated pile heads [17];


27

From the excavation it can be seen that the inner surface of the PCC pile is smooth,

pile rupture, segregation of concrete and contraction of the diameter are not observed.

However at several piles a slight deflection of the pile head was observed and the wall

thickness was found to not be uniform. This phenomenon is likely to be caused by the extra

concrete that extrudes from the casing when the pile is finished. It is therefore advised to

remove the extra concrete after driving and lift the casing higher [17].

Measurements were also done to determine the wall thickness, see table 3-1.

Table 3-1: Measured wall thickness of PCC pile [17];

Section Num. length [m]

Design value of diamete

r [m]

Design value of wall

thickness [cm]

Average value of wall thickness in

measure [cm]

Minimum value of wall thickness in

measure [cm]

A8-21 15.0 1.0 12 13.9 13.6 K30+740~ K30+778 A4-10 15.0 1.0 12 14.1 13.6

A2-19 15.0 1.24 12 14.1 13.8 K30+778~ K30+808 A1-8 15.0 1.24 12 14.2 13.9

A6-22 15.5 1.0 12 14.0 13.6 K30+808~ K30+838 A10-20 15.5 1.0 12 14.3 13.8

A3-21 15.5 1.0 12 14.0 13.5 K30+838~ K30+868 A2-19 15.5 1.0 12 13.9 13.6

A4-3 15.5 1.0 12 14.2 13.9 K30+868~ K30+898 A6-4 15.5 1.0 12 13.9 13.6

A5-5 15.5 1.0 10 11.8 12.1 K31+509~ K31+559 A6-4 15.5 1.0 10 12.1 11.8

A8-3 15.5 1.0 12 14.1 13.8 K31+559~ K31+600 A3-2 15.5 1.0 12 14.2 13.9

As can be expected the wall thickness is larger than the design value (the distance

between the casings). The extra diameter is approximately 1.5 - 2 cm. For the head

excavation it was found that the minimum wall thickness was always found near the pile

head. It is advised to keep the pulling speed of the casing at 0.8 to 1.2 m/min at the pile

head to keep the wall thickness guaranteed [17].

Samples were also taken for uni-axial compression tests. The results are shown in

table 3-2.

Table 3-2: Compression test results [17];

Section Num. Sample depth [m]

Compressive Strength [MPa]

Age [days]

A8-21 2.0 21 38 K30+740~ K30+778 A4-10 10 24 45

A2-19 2.0 19 63 K30+778~ K30+808 A1-8 10.0 20 61

A6-22 2.0 21 70 K30+808~ K30+838 A10-20 10.0 23 77


28

A3-21 2.0 20 89 K30+838~ K30+868 A2-19 10.0 23 88

A4-3 10.0 19 67 K30+868~ K30+898 A6-4 2.0 22 65

A5-5 2.0 27 106 K31+509~ K31+559 A6-4 10.0 22 107

A8-3 2.0 21 66 K31+559~ K31+600 A3-2 10.0 24 72

It can be seen from the compression tests results that the compressive strength of

the concrete is always higher than the design value of C12/15 concrete of 15 N/mm2. It can

also be concluded that in general the compressive strength of the concrete becomes larger

with larger depth.

3.3.4 Load test

Load tests were executed on a number of piles. The principle of load testing is to find

the maximum load that can be sustained by a part of the construction. A pile can fail in two

ways: constructional (load is higher than strength of the pile shaft) or geotechnical

(deformation of the soil is higher than the deformation defined as failure). The test load can

be applied dynamically or statically. To divide the load over the pile cross section a cap plate

is placed over the pile head. The loading can be executed in 2 ways: the load (ballast) is

placed directly on the pile or the load is placed on the pile using a jack. The jack is placed

between the pile head and the loading frame [17].

In this case a ballast load is used, see figure 3-16. Four trial piles were chosen for the

static loading test of a single pile. The relationship curves of load Q and displacement s on

the top of piles are shown in figure 3-17.

Figure 3-16: Load test on single pile [17];


29

Figure 3-17: Load - settlement diagram static loading [17];

The length of all tested piles is 15.5 m, the diameter of piles A14 and A15 are 1000 mm and

the diameter of piles A5 and A6 are 1240 mm.

Based on the load settlement diagram, the limit bearing capacity of a single pile with

a length of 15 meter and a diameter of 1240 mm is found to be 1650 kN, 9% higher than the

theoretical design values [17]. The failure is geotechnical. The diameter of the piles A6-20

and A5-18 is 1240 mm and the other piles 1000 mm; it can clearly be seen that a larger

diameter gives a higher bearing capacity.

Load tests were also executed on two composite foundations. The composite

foundation is defined as a single PCC pile and its influence area of surrounding soil. This test

is done to better simulate a highway embankment. The equipment setup can be seen in

figure 3-18.

Figure 3-18: Test set-up for composite foundation load test [17];


30

The test is executed on an area of 3.3 by 3.3 m consisting of a single PCC pile and its

influence area. The results of the test are shown in figure 3-19.

Figure 3-19: Results of load test on composite foundation [17];

Pile A7-12 has a diameter of 1240 mm and a wall thickness of 120 mm while pile A7-18 has a

diameter of 1000 mm and a wall thickness of 100 mm. The slope of A7-12 is smaller than A7-

18, while the settlement is higher, which indicates that the settlement of PCC pile composite

foundation increases with the decrease of displacement ratio that is defined as the ratio

between the pile and soil cross-sections.

To study the working mechanism of PCC pile composite foundation under loads, the

inner soil, pile body and soil between piles are equipped with soil pressure boxes. These

boxes can monitor the soil pressure during every stage of the test loading, the test results

are shown in Figure 3-20.

Figure 3-20: Static load test on composite foundation, stress measurements [17];


31

It can be seen in figure 3-20 that the stress in both pile head and soil between piles

increases as the load increases, in the beginning there is only a small difference. When the

load is increased the load transfers to the pile head and a steep increase in the pile stress can

be seen while the soil pressure stays approximately constant. It can also be seen that the

stress in the inside soil stays very low during the test. Literature [19] indicates that the inner

soil of the PCC pile has little contribution to the bearing capacity. As a result, in Chinese

practice the effect of inner soil is neglected when calculating the bearing capacity of the PCC

pile in complicated soft soil foundations.

An overview of the load test results is shown in table 3-3.

Table 3-3: Results load tests on single piles and composite foundation [17];

Area

Pile Total load [kN]

Loaded unto

failure?

Ultimate capacity

pile [kN]

Ultimate capacity

composite foundation

[kPa]

Settlement at the ultimate

capacity [mm]

Maximum elastic

deformation [mm]

A5-18 1650 × 1650 13.63 7.53 A6-20 1800 √ 1650 42.74 21.64 K30+778

~K30+808 A7-12 2995 × 137.5 13.61 7.40 A7-18 2700 × 124.0 12.11 6.18 K30+868

~K30+898 A8-16 1500 √ 1350 50.98 12.81 A14-10 1000 × 1000 13.18 6.22 K31+509

~K31+559 A15-8 900 × 900 11.70 5.33

The piles loaded unto failure have an ultimate capacity of respectively 1650 and 1350 kN.

This corresponds to the outer diameters and wall diameter of 1240 mm and 120 mm and

1000 mm and 100 mm respectively. As can be expected the bigger piles have a higher

bearing capacity due to larger shaft friction and larger tip bearing. Measurements of the

ultimate capacity of the composite foundations are also influenced by the pile type 137.5 kPa

for the 1240/120mm piles and 124.0 for the 1000/100mm piles. Since the composite

foundation was not loaded unto failure no definite ultimate capacity can however be given.

3.3.5 Long term settlement monitoring

After pile installation the highway embankment is constructed over a period of seven

months. During this construction monitoring of settlement, soil pressure, differential

settlement, horizontal displacement and pore pressure is done. The different types of

monitoring for each area are shown in table 3-4.

Table 3-4: Long term monitoring per section [17];

Section Num. Monitoring content 1 K30+756.5 Surface settlement, soil pressure 2 K30+794.5 Surface settlement, differential settlement, horizontal displacement,

soil pressure, pore pressure


32

3 K30+822 Surface settlement, differential settlement, horizontal displacement, soil pressure, pore pressure

4 K30+853 Surface settlement, differential settlement, horizontal displacement, soil pressure, pore pressure

5 K30+884.5 Surface settlement, differential settlement, horizontal displacement, soil pressure, pore pressure

6 K31+535.4 Surface settlement, differential settlement, horizontal displacement, soil pressure, pore pressure

7 K31+578.8 Surface settlement, soil pressure

The surface settlement for section 1 is shown in figure 3-21. It can be seen that the

settlement of the pile is the lowest and the settlement of the centre of the embankment the

highest. The final settlement is almost reached for the pile settlement at 252 mm and 351

mm for the centre of the road. These values are much higher than that of the composite load

test; the difference can be attributed to consolidation and creep effects.

Figure 3-21: Settlement of section 1, K30+765.5 [17];

The rate of settlement is shown in figure 3-22. It is clearly visible that the settlement

rate increases when the last, larger, part of the fill is applied. As can be seen in Figure 3-21,

at first the soil settles more than the pile while the pile rate of settlement (shown by the

triangles in the graph) picks up late which indicates that a settlement of the soil is required

before the load is distributed to the piles. After a while the settlement of the pile and soil has

the same rate indicating that there is neither positive nor negative skin friction. The equal

settlement rate of soil and pile indicates that there is only compression of the soil layers

below the pile tip.


33

Figure 3-22: Settlement rate of section 1, K30+765.5 [17];

The difference in settlement rate can also be observed from figure 3-23, where the

differential settlement between the pile and the soil is shown. It is clear that the differential

settlement does not increase after December 25.

Figure 3-23: Different settlement between soil and pile of K30+756.5 [17];

Figure 3-24 gives an overview of the measured settlement of the soil next to the pile

with the depth. It can be seen that the largest settlements take place in the top layers since

this is the cumulative settlement of all layers but also that the soil below the pile tip settles

approximately 250 mm.


34

Figure 3-24: Settlement in depth of K30+794.5 [17];

From this and the previous graphs it can be concluded that the soil layers next to the

pile settle approximately 100 mm more than the pile while the group of piles and soil settle

another 250 mm due to compression of the layers below the pile tip.

Figure 3-25 gives an overview of the measured soil pressures. It is clear that the soil

pressure on the pile head reaches a much higher value than the soil pressure. It can also be

seen that the pressure on the top of the soil inside the pile is almost zero.

Figure 3-25: Soil pressure of K30+794.5 [17];

In figure 3-26, the pore pressure measured is presented. It can be seen that the

influence of the small load steps on the pore pressure is almost unnoticeable. For the small

load steps the reaction of the soil can be considered undrained. The last, and largest, load


35

step gives a large increase in pore pressure on all levels which indicates undrained behaviour.

After about three months the pore pressure is almost fully dissipated.

Figure 3-26: Pore pressure for K30+794.5 [17];


36

4 Comparison other methods

4.1 Introduction

The PCC pile is used in Chinese practice as a ground improving pile, part of the

research question is to determine the viability of the application of the PCC piling method in

Dutch practice, see paragraph 2.2.3.

To this end the PCC piling method is compared to two relatively new ground

improving piling systems developed by Dutch companies and applied in the Netherlands.

In paragraph 4.2 the HSP system developed by Voorbij Foundation Technics is

described and in paragraph 4.3 the AuGeo system developed by Cofra.

In paragraph 4.4 a comparison between the PCC piling method and the two Dutch

systems is made.

4.2 HSP

4.2.1 Introduction

The Voton-HSP system is developed by Voorbij Foundation technics as a cast-in-situ

ground displacing pile system [40]. The pile is installed using a vibrated steel casing. During

construction the concrete pressure, speed, resistance and depth are measured continuously.

The pile can be reinforced with traditional steel reinforcement or steel fibre reinforcement

and the length is, depending on local conditions, maximal 17 m. The production speed is

about 200 piles of 15 m per day.

Different types of HSP piles are:

• Standard, various reinforcement and diameter, used as standard pressure pile

• HSP VV, higher concrete pressure, used when higher bearing capacity is required

• HSP Renovation pile, steel tube, used for jacking inside of buildings with limited space.

• HSP tension element, production process is halted for reinforcement, used in under water

concrete floors.


37

Figure 4-1: Typical HSP field [40];

4.2.2 Pile properties

The pile is made with concrete of C20/25 or C30/35 grading. The standard pile

diameter is 170 or 180 mm but can be made larger if required for a project. The pile length is

2-17 m and can be varied per pile and the bearing capacity value for the standard pile is 250

kN. The reinforcement consists of one steel bar of 16 mm diameter and a length of 1.5 m,

but also 4 bars of 10 mm diameter can be used in a frame up to a length of 6.0 m.

Alternatively steel fibre concrete can be used. The practical minimum for the centre-to-centre

distance of the piles is 1.0 m.

4.2.3 Installation process

An overview of the installation process is shown in figure 4-2. First a test pile is made

without concrete to verify the computer program. The steel tube is vibrated into the soil using

a high frequency vibratory hammer. If necessary the process is supported by fluidization at

the tip of the pile. In the steel casing a tube is located for transporting the concrete. The tube

is closed of by a valve that is controlled by a central computer. When the required depth is

reached the valve is opened and the injection of concrete starts. The concrete pumping is

continuous and after the required pressure is reached the casing is pulled. If the available

winch force is not enough to pull the casing, the vibratory hammer is automatically used.

During the pulling of the casing concrete is added continuously so the concrete pressure

remains constant. At ground level the valve is closed automatically. The whole process takes

about two minutes and the installation machine can move on to the next pile location.

Heavy loads between the new piles have to be avoided to minimize horizontal

pressures. If reinforcement or the pile head is required it can be added at this stage. All the

processes are monitored and controlled by computer. Bottlenecks are avoided and a good

connection between the surrounding soil and the pile is obtained.


38

Vibratory hammer

Concrete mixture

Valve

Reinforcement

Figure 4-2: Construction method HSP [40];

Figure 4-3: Concrete mixer and pile installation equipment for HSP [40];

4.3 AuGeo

4.3.1 Introduction

The AuGeo system consists of lightweight piles with an enlarged pile cap and foot

founded in a stable sand or gravel layer. To limit the settlements of the soil the load of the

embankment is transferred to the pile caps by a geogrid mattress.


The pile properties are shown in table 4-1.


39

Table 4-1: AuGeo pile specifications [6]

AuGeo specifications

Casing material PE Diameter [mm] 150

Wall thickness [mm] 14 Pile length [m] 2-15

Bearing capacity pile [kN] 150 Height fill [m] 1 - 7

Tension strength geotextile [kN/m] 50-350 Strain geogrid [%] <4

4.3.3 Installation process

Firstly borings are done to determine the expected pile tip depth, the foundation

layer thickness, the presence of stiff intermediate layers and the presence of soft layers under

the foundation layer. This last point is important to determine the chance of punching

through the foundation layer. If necessary a working layer of 1 m sand is placed to support

the installation equipment. The HDPE tube is cut to the required length and the cap is

attached to the tube, see figure 4-4.

Figure 4-4: AuGeo Tubes [6];

The pile is inserted using a drain “stitcher” commonly used for installing vertical

drains. The HDPE tube is loaded into a steel casing (mandrel) with dimensions 200x200x10

mm [34] which is pushed into the soil until a certain resistance is reached, see figure 4-5.


40

Figure 4-5: Loading the casing on the stitcher [6];

This resistance will mostly be found in layers with CPT values of 6 MPa and SPT

values of 12-15 blows per 300 mm or higher. At this depth the mandrel is retracted and the

casing is cut of at the required level.

Alternatively the mandrel can be pressed into the soil without the HDPE tube. In this

case the bottom plate is placed below the mandrel and pressed into the soil. At the design

depth the HDPE tube is placed in the casing.

Six steel reinforcement bars are placed into the casing and the casing is filled with

self-compacting foam concrete with a compressive strength of 30 N/mm2 after 28 days. A

concrete cap of 300x300x16 mm is placed on top [34]. The final bearing capacity of the 150

mm diameter pile will have a maximum value of 350 kN. The shaft bearing capacity is not

considered to add to the total bearing capacity.

The installation speed is about 20 piles per hour when a stable working floor is

present with the advantages of no vibration, no noise and no problems with heavy prefab

piles.

4.3.4 Quality control

A test field was executed in ‘s Gravendeel, The Netherlands [34]. Before installation

the verticality of the rig was assured. During installation the following data was logged:

position of the pile, soil resistance, maximum installation depth and resistance, amount of

piles installed and date and time. Low-strain tests (sonic wave tests) were executed to verify

the diameter of the installed piles. This is especially important at close pile spacing, when

newly installed piles can influence the quality of the piles already in place. Settlements of the

embankment were all well within the maximum allowance. Dynamic loading tests were also


41

executed which showed that the constructed embankment was suitable for trains with a

maximum speed of 350 km/h.

4.4 Comparison

4.4.1 Introduction

A comparison is made between the PCC piling method for soil improvement and the

HSP and AuGeo piling methods. The comparison is qualitative not quantitative and an

indication and a cost comparison will not be given but the comparison is based on the

construction method and bearing capacity.

4.4.2 General

In table 4-2 an overview of the properties of the different piles is given. It can be

seen that the PCC pile can be applied to a larger depth and has a larger diameter.

Table 4-2: Overview general properties HSP, AuGeo and PCC piles;

HSP AuGeo PCC General Solid circular pile Solid circular pile Pipe pile Maximum pile length 17 m 15 m 25 m Embankment force redistribution

Geotextile mattress (pile cap optional)

Geotextile mattress and pile cap

Geotextile mattress

Diameter 170-180 mm 150 mm 1000-1500 mm Wall thickness n.a. 14 mm 100-150 mm Casing material n.a. HDPE n.a. Type of fill material Concrete C20/25 or

C30/35 Foam concrete

C27/30 Concrete C12/15

Reinforcement Steel bars Steel bars or fibres none

4.4.3 Construction

In table 4-3 an overview of installation time and equipment per piling method is

given. The installation time of the PCC pile is much longer than the HSP and AuGeo piles and

requires more personnel to be constructed.

Table 4-3: Overview construction properties HSP, AuGeo and PCC piles;

HSP AuGeo PCC Equipment Vibratory hammer

on rig Drain stitcher Double Vibratory

hammer on rig Force capacity Ca 55 tons 25 tons 78 tons Pressing and retracting casing 2-3 minutes 2-3 minutes 10 minutes Total time including filling 2-3 minutes* 2-3 minutes 45-60 minutes Personnel required 3-4 persons** 6-7 persons** 8 persons *installation device can move on before filling

**estimation

4.4.4 Economic comparison

A rough comparison between the PCC piling system and the AuGeo piling system

based on available data is made. For an embankment of approximately 5 m (equal to the


42

Yan-Tong case [17]) a typical PCC application is compared to a typical AuGeo application. Pile

data is taken from [17] and [34] and shown in table 4-4.

Table 4-4: Pile specifications for 5 m fill;

PCC AuGeo Centre-to-centre distance 3.3 m 0.8 m Number of pile per day 20 200 Pile cross section 0.41 m2 0.018 m2

Based on the measurements done at the Jin Yang Road project, see paragraph 3.2.3, the

PCC pile production per day would be assumed lower at approximately 10 piles per day.

However as already described there most time is spend by transporting the concrete from the

plant to the pile, it is roughly estimated that using concrete trucks and/or pumps about a

double pile production per day can be reached in the Netherlands.

If we assume a 26 m wide pile area with a length of 260 meter and the required pile

length would be 15 m for both types of pile, this would result in the requirements given in

table 4-5.

Table 4-5: Example case PCC and AuGeo, 5 m fill;

PCC AuGeo Number of piles required 640 10890 Total construction time 32 days 54 days Load on single pile 950 kN 55 kN Volume of concrete 3940 m3 2940 m3

It can be seen that the construction time of the PCC piled embankment is about 60% that of

the AuGeo piled embankment, this because a much smaller number of piles is required. The

load on a single PCC pile, based on the assumption that the total load of the embankment is

placed on the piles, is much larger than that on a single AuGeo pile. However the PCC pile

has been proven to be able to sustain even larger loads, see paragraph 3.3.4. The volume of

concrete required is larger for the PCC pile.

It should be noted that in this rough comparison the required equipment and

personnel is not taken into account. Another very important not is that the centre-to-centre

distance of the PCC piles is based on the experiences in China where the soil properties are

different from the Netherlands. It is therefore required to determine if a distance of 3.3 m is

allowable in Dutch soil too. As can be seen the number of piles and thus the installation time

depends heavily on the centre-to-centre distance.

4.4.5 Conclusion

An overview of the advantages and disadvantages of the PCC pile compared to the

HSP and AuGeo piles is given in this paragraph based on the comparison given in the

previous paragraphs.


43

Advantages

• High shaft bearing capacity due to large circumference,

• High tip bearing capacity especially when plugged,

• Large application depth,

• Larger pile spacing reduces installation effects,

• Smaller construction time for entire project,

• Economical ratio between concrete and bearing capacity and

• Large pile head ensures good connection with geotextile.

Disadvantages

• Heavy machinery required during construction,

• Much personnel required during construction,

• More concrete required,

• Larger pile spacing requires more load redistribution so a thicker mattress,

• No reinforcement possible and

• Large circumference leads to larger negative skin friction (when applicable).

Analysis

From the economical analysis the PCC pile seems to be comparable to the AuGeo

piling system. The larger application depth of the PCC pile makes it an even more viable

solution in subsoil with deep stiff layers where its larger wall friction becomes a great

advantage. When a stiff layer at a depth of less than 15 meters below surface exists the PCC

is possibly more expansive to build than the AuGeo and HSP systems. Reinforcement is also

not yet possible for the PCC pile.

The high shaft bearing capacity and efficient use of concrete makes the PCC pile

applicable for the foundations of buildings on soft soil. This would however require some

reinforcement of the pile or pile head to sustain horizontal forces without damage.

4.5 Conclusion

The PCC pile is mainly applied in very thick (more than 20 m) packets of soft layers.

Its high shaft bearing capacity can effectively reduce settlements in this kind of subsoil. In

less thick soft layer packets when the pile tip can be placed in a stiff layer it is possibly a

competing method for the HSP and AuGeo.

With adjustments it might be possible to apply the PCC pile in building foundations.

Aspects that will have to be considered are:

• Bending moments,

• Bearing capacity,

• Connection to the foundation,

• Equipment adjustment.


44

5 Calculation method

5.1 Introduction

The possibilities for application of the PCC pile in the Netherlands are highly

dependent on the existence of a robust system for bearing capacity and settlement

calculations.

In paragraph 5.2 calculation methods for the bearing capacity of the PCC pile are

evaluated.

In paragraph 5.3 calculation methods for the settlement of the single PCC pile and

the composite foundation are evaluated.

In paragraph 5.4 a choice is made based on the evaluations and a number of

methods are applied to an existing case in the China.

5.2 Bearing capacity

5.2.1 Introduction

In this paragraph the calculation methods for the bearing capacity of the PCC pile

and the surrounding soil is given. First the Chinese practice is described followed by several

methods obtained from the Dutch practice adapted to the PCC pile. Two special aspects of

the PCC pile, negative shaft friction and plugging are described separately.

5.2.2 Chinese practice

The calculation method for the bearing capacity used by GeoHohai is fairly basic since

the maximum bearing capacity is mostly determined with pile load tests. Because of the

ground improving nature of the pile the bearing capacity is considered less important than

the ability of the pile to reduce settlement and increase the bearing capacity of the soil.

Nevertheless two methods are available for the calculation of the bearing capacity of the pile

improved soil:

• The most commonly used method is the separate calculation of bearing capacity of the

pile and the soil and addition of those bearing capacities.

• The second method is to consider the pile and soil to be one body and calculate the

bearing capacity of this body. This method is seldom used and no description is available.

Method: Separate calculation pile and soil

The bearing capacity is determined by separately calculating the bearing capacity of

the soil and the pile and adding the results under certain conditions. The allowable load on

the composite foundation consisting of soil and PCC pile is defined as:


45

Kpp cf

cc = (5.1)

where:

pcc bearing capacity of the composite foundation

pcf ultimate bearing capacity of the composite foundation

K factor of safety

The ultimate bearing capacity of the composite foundation is defined by:

sfpfcf pmKpmKp )1(2211 −+⋅= λλ (5.2)

where:

ppf ultimate bearing capacity of the PCC pile, from calculation or load test on a

single pile

psf ultimate bearing capacity of the soil, based on CPT and/or vane tests

K1 correction coefficient for the PCC pile in a composite foundation (generally

larger than 1)

K2 correction coefficient for the soil in a composite foundation (may be larger or

smaller than 1)

λ1 load factor PCC pile (smaller or equal to 1)

λ2 load factor soil (smaller or equal to 1)

m exchange ratio composite foundation

total

p

AA

=

Ap cross section of the pile in the horizontal plane

Atotal total influence area of the pile in the horizontal plane, for square pile layout

the square of the centre-to-centre distance

The factor K1 is used for the conversion of the bearing capacity of a single pile to the bearing

capacity of a pile group, including the strengthening effect on the soil caused by the pile

installation. The factor K2 describes the effect of the pile installation on the bearing capacity

of the soil. The factors K1 and K2 are based on the properties of the soil and experience. The

factors λ1 and λ2 are reduction factors for the determined bearing capacity depending on

whether or not the pile or soil is loaded to ultimate capacity. If the pile is loaded to bearing

capacity (λ1 = 1) and the soil is not (λ2 < 1) the formula for the bearing capacity becomes:

sfsfcf pmpmp )1(2 −+⋅= βλ (5.3)


46

where:

β coefficient of soil strength

ppf ultimate bearing capacity of the PCC pile, from load test on a single pile,

including K1

psf ultimate bearing capacity of the soil, based on CPT and/or vane tests,

including K2

The ultimate limit state bearing capacity of the PCC pile is determined using the

following formula:

p

isiksf A

lfup ∑= (5.4)

where:

u pile perimeter

li layer thickness

fsik layer friction, obtained from CPT

Limitations of this method are:

• Many of the design factors are based on experience,

• It is unclear how the bearing capacity of the soil is calculated and

• The ultimate bearing capacity of the pile is based on only the shaft friction; plugging, tip-

bearing and negative skin friction are not taken into account.

5.2.3 Dutch practice

Tip bearing capacity

The bearing capacity of the tip of the PCC pile can be calculated by two well known

methods:

• As a pile, according to Koppejan or

• As a deep foundation wall, according to Prandtl.

Method 1: Koppejan

The 4D-8D method as described in [12] is used for pile tip bearing capacity of solid

piles. The name refers to the logarithmic-spiral failure zone around the pile tip, which extends

to a distance of 0.7 D0 to 4 D0 beneath the pile tip and 8 D0 above the pile tip, where D0 is

the outer diameter of the pile. The ultimate bearing capacity of the pile tip is determined by:

tipmax;r;tiptipmax;r; pAF = (5.5)


47

where:

Fr;max;tip ultimate tip resistance force

Atip surface of the pile tip

pr;max;tip ultimate tip resistance

limavgIIIcavgIIcavgIc

p qqqq

s ≤⎟⎟⎠

⎞⎜⎜⎝

⎛+

+⋅⋅⋅= ;;

;;;;

25.0βα

αp pile class factor, 1.0 for cast-in-place piles soil displacing

β pile tip shape factor, 1.0 for PCC pile.

s pile tip shape factor, 0.6 for PCC pile.

qc;I;avg mean value of cone resistance in trajectory I that runs from the pile tip to a

level that is at least 0.7 times and at most 4 times the equivalent pile

diameter deeper. It must be selected in such a way that qb is minimal.

qc;II;avg mean value of the cone resistance in trajectory II that runs from the bottom

of trajectory I to the pile point level, where the value used for the cone

resistance may never be higher than the previous value in the trajectory.

qc;III;avg value of the cone resistance in trajectory III that runs from the pile point to a

level that is 8 times the equivalent pile diameter higher, where the value

used for the cone resistance may never be higher than the previous value in

the trajectory.


• The method is designed for solid piles so the influence of plugging is not taken into

account and

• The method is empirical.

Method 2: Prandtl

The bearing capacity of a strip foundation has been analyzed by Prandtl and others.

The schematization is given in figure 5-1 and the commonly used formula is [38]:

Figure 5-1: Prandtl method, left shallow foundation, right deep foundation [38];


48

yqcr BNdNNcp '5.0''max; γγ ++= (5.6)

where:

pr;max maximum tip bearing resistance

c’ effective cohesion

B width of the foundation

d depth of the foundation level

γ’ effective volumetric weight of the soil

Nc bearing capacity factor for the influence of the cohesion

Nq bearing capacity factor for the influence of the soil cover

Nγ bearing capacity factor for the influence of the effective volumetric weight of

the soil under the foundation surface

The factors N can be calculated by [13]:

( )( )( )

( ) deq

deq

deqc

NN

eN

NNde

;

2;

'tan

;

'tan12

'5.045tan

'cot1,

ϕ

ϕ

ϕ

γ

ϕπ

−=

+°=

−=

(5.7)

where:

φ’e,d design value of the effective angle of internal friction

The friction angle, cohesion and unit weight are determined as an average parameter for an

influence depth of 1.5 B.

Equation (5.6) can also be used for different cross-sections like piles with the

following shape factors [13]:

⎟⎠

⎞⎜⎝

⎛−=

⎟⎠

⎞⎜⎝

⎛+=

−

−=

LBs

LBs

NNs

s

r

q

q

qqc

3.01

sin1

1

1

ϕ (5.8)

where:

L length of the foundation

If the soil inside the PCC pile forms a stiff soil plug and the friction along this plug

exceeds the tip resistance of the inner soil then the PCC pile can be seen as solid pile. The


49

bearing capacity can then be calculated with equation (5.6) and (5.8). However, according to

the GeoHohai literature [19] full plugging does not occur and in that case it is uncertain how

much bearing capacity can be attributed to the soil in the pile. It is therefore considered safe

to assume the PCC pile tube as a circular wall with no inner bearing capacity. The length of

the equivalent wall is the average perimeter of the wall and the width of the equivalent wall

is equal to the wall thickness. While for a solid circular plate the ratio B/L would be 1, for a

PCC pile of 1000 mm outer diameter and 100 mm wall thickness the ratio B/L would become:

035.0

21001000

2

100=

−⋅⋅

=πL

B

The small B/L ratio leads to shape factors sq and sγ to be close to 1 so the PCC pile

schematized like this is closely corresponding to an infinite wall.

Limitations of the method are:

• The method is designed for shallow foundations and

• The method is designed for infinitely long strip foundations and although shape factors

are available for piles the PCC pile’s shape is difficult to model.

Shaft bearing capacity

The friction along the shaft of the PCC pile is commonly calculated with two different

methods:

• Based on the cone resistance

• Based on the shear strength

Method 1: Cone resistance (according to NEN 6743)

The equation for the ultimate shaft friction is given as [12]:

azcszshaftr qp ;;;max;; α= (5.9)

where:

pr;max;shaft ultimate unit shaft friction

αs pile class factor for compression, see table 5-1

qc;z;a cone resistance at depth z, with a maximum value of 12 MPa, for layers < 1

m thickness and 15 MPa for layers > 1 m thickness


50

Table 5-1: Values for αs [12];

soil type relative depth αs

clay/silt qc ≤ 1 MPa 5 < z/d < 20 0.025 clay/silt qc ≤ 1 MPa z/d ≥ 20 0.055 clay/silt qc > 1 MPa - 0.035

Limitations of this model are:

• The method is empirical.

Method 2: Shear strength

This method is also called the slip method and is based on the analysis of the stress

situation and the pile properties on the boundary between the pile shaft and the soil. The

friction force follows from [38]:

δσδσ tan'tan' ;;;;max;; zvzszhzshaftr Kp == (5.10)

where:

Ks horizontal ground pressure coefficient after pile installation

σ'v average value of the effective vertical stress for the considered layer

δ friction angle between soil and shaft

Typical values for Ks and δ are shown in table 5-2.

Table 5-2: values for the wall friction angle and the horizontal earth pressure coefficient [38];

Earth pressure coefficient for sand Pile type Friction angleLow density

Rd < 0.6 High density

Rd > 0.6 Soil displacing - Prefab concrete 3/4 φ 1.0 2.0 - Cast-in-place φ 1.0 2.0 - Jacked wooden 2/3 φ 1.5 4.0 - Jacked closed steel tube 20º 1.0 2.0 Little soil displacement - Steel profiles 20º 0.5 1.0 - Open steel tubes 20º 0.5 1.0 Ground removing 3/4 φ Horizontal earth pressure based

On 80% of grout/water pressure


• The average value of the effective stress is difficult to determine in situ and

• The soil-wall friction angle and earth pressure coefficient are empirical.


51

5.2.4 Negative shaft friction

If piles are installed through soft soil layers and the pile tips are placed in soil layers

with a high bearing capacity, the piles will behave almost settlement free. The soft soil layers

will then, if they settle, load the piles with a downward friction force. This friction force on the

pile shaft is called the negative shaft friction. The friction force is already maximized by

displacements of 20 mm or less. In that case it is not expected that negative skin friction will

occur over the full height of the soft layers. It is assumed that for ground level settlements of

more than 100 mm, there will be negative skin friction in all soft layers. In a soil profile with

fully settled soft layers it can be safely assumed that the upper boundary of the negative skin

friction is equal to the added load on the soil due to a fill or construction. The negative skin

friction can be calculated with two methods [38]:

• The slip method

• The method Zeevaert - De Beer

Method 1: Slip method

The slip method [38] is the simplest method for negative skin friction calculation and

gives an upper boundary for the negative skin friction. The schematization of the slip method

is shown in figure 5-2.

Figure 5-2: Schematisation of the slip method [38];

The method is based on the stress situation and friction properties of the boundary surface

between the pile shaft and the soil. The downward friction force can be determined by:

δσ tan'0; vsnks hKOF = (5.11)

where:

Fs;nk force due to negative skin friction

Os perimeter of the pile shaft

h thickness of the considered layer

K0 neutral earth pressure coefficient


52

σ’v mean value of the vertical effective stress in the considered layer

δ friction angle between pile and soil

When the friction angle between pile and soil is taken equal to that of the soil and

the earth pressure coefficient K0 = 1-sinφ, it follows for clay and peat layers that K0 tan δ

varies from 0.2 to 0.3. In practice a value of 0.25 is used. For displacement piles a value of

0.50 is more realistic [38].


• Only an upper boundary of the shaft friction is determined and

• The earth pressure coefficient and friction angle between soil and pile are determined

using empirical formulae.

Method 2: Zeevaert - De Beer

This method is based on the vertical equilibrium of a small layer of soil around a pile

and is used for pile groups instead of the slip method. The difference with the slip method is

that in this case the vertical effective stress is reduced by the already conveyed negative skin

friction. This is done by taking into account the mean vertical stress on the top and bottom

side of small area (A) with a height of Δz and to distribute the shear stress along the pile over

the area A, see figure 5-3.

Figure 5-3: Method Zeevaert-De Beer [38];


53

The equation for the friction force is given by [12]:

( )∑=

=

−=ni

irepihmrepihnfreps AF

1;;;;;;0;; '' σσ (5.12)

with:

repirephlimrepih h ;1;1;;;;0 ''' γσσ += −

reprepihm p ;0;1;;' =−σ for the first layer

( ) ( )iiii hmrepihm

hm

i

repirepihm ee

m−

−− +−= ;1;;

';;; '1

'' σ

γσ

where:

Fs;nf;rep representative value of the friction force due to negative skin friction

A influence area of the pile

σm;l;i;rep representative value of the effective vertical stress in the soil for layer i

σ0;l;i;d representative value of the effective stress due to the top load for layer i

p0;d representative value of the top load

mi factor for layer i

A

Ku repirepi ;;;0 tanδ⋅=

u pile perimeter

For a single pile (A becomes infinite) the method Zeevaert gives the same value for

the negative skin friction as the slip method because m = 0. This value is an overestimation

because, also in case of a single pile, shaft friction occurs, which results in a decrease of the

effective stress. De Beer proposed to limit the size of area A dependent on the layer thickness

h. For a top load A is limited to 1/4πh2 and for only characteristic weight of the soil to

1/16πh2. If more than one soft layer is present the negative skin friction is calculated with h1

for the top layer and then the weight of the top layer is reduced by the negative skin friction

that works as a load on the second layer. The second layer is calculated with h2. The

disadvantage of this method is that the negative skin friction is also determined by the order

of the layers. This can not be proven theoretically and may lead to reduction of the negative

skin friction. Especially for large pile distances it is advised to take the relation between layer

thickness and pile distance in the order of 2.


• The shaft friction for a single pile cannot be correctly calculated and

• The earth pressure coefficient and friction angle between soil and pile are determined



54

5.2.5 Plugging

General

Plugging is the phenomenon where an open-ended pipe pile develops a rigid soil

“plug” at the bottom which prevents soil from entering the pile and essentially makes the

pipe pile behave as a closed-ended pile. The phenomenon can occur at two moments:

• During driving, at some point during driving the friction builds up sufficiently between the

soil plug and inner pile wall so that the pile becomes plugged, preventing additional soil

from entering [27]. When a plug is formed the installation effort increases.

• During loading, the pile is considered to fail plugged when the shear capacity along the

length of the soil plug exceeds the end-bearing capacity at the base of the plug [31] [12].

According to [31] plugging is less likely to occur during driving because the inertia of the soil

plug encourages slip relative to the pile preventing plugging.

A pile can be driven in either fully coring mode, with no plug formation at all (the soil

enters the pile at the same rate it advances) fully plugging, where no soil enters the pile, or

partially plugging, with some plug formation, see figure 5-4. After plug formation a fully

plugged pile behaves in the same way as a closed-ended pile [26]. The plug formation

depends on several variables, mainly the shape of the pile tip, the pile geometry, the

installation method and the soil type [27].

Several methods are available to describe the plugging effect in pipe piles. The spring

model and the inner friction model as described in [2] and the one-dimensional analysis of

soil plugs according to [31] will be discussed here.

Figure 5-4: Plugging [31];

Method 1: Spring model

In the spring model [2] for steel pipe piles the soil inside and outside of the pile is

modelled as a spring. The end-bearing response is modelled as a combination of three spring


55

systems: a spring underneath the pile wall with a spring constant Kw, a spring underneath the

soil plug with spring constant Ki and a spring inside the pile representing the compression of

the soil plug due to end bearing loading with a spring constant Kpl, see figure 5-5.

Figure 5-5: spring analogy open ended pipe pile [22];

A load on the pile wall only will result in wall displacement and plug displacement:

w

www K

Qu = (5.13)

wwplw uu ⋅= 1α (5.14)

In the same way a load on the plug only will result in plug displacement and wall

displacement:

i

plplpl K

Qu = (5.15)

plplwpl uu ⋅= 2α (5.16)

where:


56

uww pile wall displacement due to load on pile wall

uplpl displacement soil below the plug due to load on the plug

Qpl load on the plug

Qw load on the pile wall

Kw spring constant soil below pile wall

Ki spring constant soil below plug

uplw displacement soil below the plug due to load on pile wall

uwpl pile wall displacement due to load on plug

α1 spring interaction factor soil below pile wall to soil below plug

α2 spring interaction factor soil below plug to soil below pile wall

The interaction factors α1 and α2 represents the influence of the wall displacement on the

plug displacement and visa versa. Combining the equations gives (if no failure occurs):

i

pl

w

wwplwww K

QKQ

uuu 2α+=+= (5.17)

w

w

i

plplwplplpl K

QKQ

uuu 1α+=+= (5.18)

The compression of the soil plug inside the pile is given as the relative displacement of the

pile wall to the displacement of the soil below the plug:

pl

plplw K

Quu =− (5.19)

where:

Kpl spring constant soil in plug

and the total end bearing load:

plw QQQ += (5.20)

where:

Q total end bearing load

The spring constants can be obtained from literature [32] [28]:


57

212

ν−= isoiltip

i

REK (5.21)

( ) ( )n

REK osoiltip

w ων 212

−= (5.22)

Maipl DRK βπ2= (5.23)

with:

( ) plM ED)21(1

1νν

ν−+

−=

where:

Esoiltip stiffness of the soil at the pile tip

ω(n) factor dependent on inner and outer radius of the pile, see table 5-3

Ri inner diameter pile

Ro outer diameter pile

βa active friction ratio

paK δtan=

Ka ratio between the radial effective stress and vertical effective stress

δp passive angle of friction between pile wall and soil plug

DM stiffness factor [32]

Epl stiffness soil in plug

ν Poisson’s ratio soil

Table 5-3: ω(n) values [28];

o

i

RR

n =

0 0.2 0.4 0.6 0.8 0.9 0.95

ω(n) 0.50 0.50 0.51 0.52 0.57 0.60 0.65

Combining these equations for the equations of a closed ended pile in:

2

2

211

)(121

n

nnn

−−

⎟⎠⎞⎜

⎝⎛ −−

=ω

α (5.24)


58

)(12

112

2

1nn

nn

ωνα

−

−+−= (5.25)

w

pl

w

i

wQ

KK

KK

Q

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

++−

−= 1

)1(

1

2

1

α

α (5.26)

i

w

w

ww K

QQKQ

u−

+= 2α (5.27)

Assuming that the soil plug is sufficiently long that failure can only occur due to failure of the

wall, while the soil inside and below the soil plug still behaves elastically, this results in an

ultimate wall resistance (Qw;u) and total end bearing load (Q1):

uw

pl

w

i

wQ

KK

KK

Q ;

2

11 1

)1(

1

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

++−

−=

α

α (5.28)

i

uw

w

uww K

QQK

Qu ;

2;

1;

−+= α (5.29)

Additional load is only carried by the plug:

plQQ Δ=Δ (5.30)

QKK

uipl

Δ⎟⎟⎠

⎞⎜⎜⎝

⎛+=Δ

11 (5.31)

This will continue until failure of the plug occurs.

It is also suggested to add another spring to the model because not all the soil in the

pile will form a plug. Furthermore it is noted that this model only models the pile tip reaction,

for good measure there should also be a spring modelling of the effect of the skin friction.

The spring constant for the shaft resistance (Kshaft) is modelled based on the deformations

required to obtain fully mobilized shaft friction. According to [12] for soil displacing piles the

tip displacement required for full mobilization of the shaft friction is about 10 mm. If the


59

mobilized shaft friction is considered to be linear (which it is not, but this is an approximation)

then the spring constant becomes:

0.01Q

K maxshaft = for 01.0≤u m

w

maxshaft u

QK = for 01.0>u m

where:

Kshaft spring constant for shaft

Qmax maximum bearing capacity of the pile shaft (calculated with [12])

In words: The bearing capacity of the shaft is linearly increasing between 0 and 10 mm wall

displacement and constant after 10 mm wall displacement. In this model there is no

interaction between the friction spring and the springs at the tip. It is assumed the load taken

by the soil dissipates in horizontal direction and does not influence the soil layers below the

pile tip. This is a simplification. The spring model now becomes:

Kshaft

Figure 5-6: Suggested adjustments to spring theory [2] (edidted);

The length of the plug spring is advised to have a length of 2 to 4 times the inner

diameter of the pile. The value of Kinner should be based on the average cone resistance


60

inside the pile. It should also be noted that a plug spring should only be applied by sufficient

L/D ratio when a soil plug is expected.


• Model has been created for sand plugs,

• Soil stiffness is hard to determine in situ,

• The increase of the soil stiffness with higher loads is not taken into account,

• For settlement calculations the group effect is not taken into account and

• The earth pressure coefficient and friction angle between pile and soil are determined


Method 2: Inner friction model

The equilibrium of forces requires that the added bearing capacity due to plugging is

the lesser of the physical maximum of the total inner shaft resistance and the end bearing

capacity of the soil below the plug [31] [12][24].

In [2] an analytical derivation of the silo formula concerning plugging is given, see

figure 5-7.

R⋅2

z'σ

dzz

zz ∂

∂+

σ σ

τ τ

Figure 5-7: forces on a disc shaped layer of soil inside a pile [2]:

The equilibrium of downward and upward forces leads to the following equation:

⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

+=++ dzz

RdzRdzRR zzz

σσπγπτπσπ 222 '2 (5.32)

If the friction is fully mobilized:

δστ tan' zKc += (5.33)


61

Combining these equations and solving the differential equation and then rewriting for the

vertical stress gives:

( ) RK

z

z

eKcRKK

cR δ

δσγδδ

γσ

tan2

0 tan2'2'tan21

tan2'2'

++++

−= (5.34)

where:

γ’ effective volume weight inner soil

R radius of the pile

K lateral earth pressure coefficient

δ friction angle pile-soil

σ0 surface load

c’ effective cohesion

As can be seen the influence of the lateral earth pressure coefficient K is very large in

equation (5.34), to use this formula in practice a method has to exist to determine K tan δ

exactly.

The inner soil can be divided in an “active” plug and a “passive” plug, see figure 5-8.

La

Lp

Figure 5-8: Division of soil in the pile in active and passive soil plug;

An approach is given to calculate the minimal active plug length (La), for which the calculated

end bearing equals the wall-end bearing plus plug-end bearing. The unit plug-end bearing is

calculated as follows:

a

L

aaebplug

aaepqθγ

θγ θ ''

−⎟⎟⎠

⎞⎜⎜⎝

⎛+= (5.35)


62

with:

Da

aβ

θ4

= (5.36)

and a formula for the active friction ratio, (βa) at active failure:

30.075.0

25.005.1 DrDL

D aa

−−

⎟⎟⎠

⎞⎜⎜⎝

⎛=β (5.37)

where:

γ' effective unit weight (≈ 10 kN/m3 in sand)

pa vertical effective stress acting on the top of the soil plug

'γpL=

La active plug length

Lp passive plug length

D plug diameter

Dr relative density, calculated with the Jamiolkowsky formula [2], with K0 taken

0.4

If upward direct shear stresses were to occur between the “passive” soil plug and the pile

wall (silo type arching behaviour), the value of pa would be considerably less than Lp γ':

( )ppL

pp ep θ

θγ −−= 1' (5.38)

with:

Dp

p

βθ

4= (5.39)

and the “passive” friction ratio, (βp) is given by:

v

zrppp K

'tan ,

στ

δβ == (5.40)

where:

pp vertical effective pressure at the bottom of the “passive plug” with length Lp


63

γ' effective unit weight in the passive plug (assumed to be the same as in the

active plug)

Kp ratio between the radial effective stress and vertical effective stress in the

passive plug (assumed to be constant along the entire length Lp)

δp angle of friction between pile wall and passive soil plug (assumed to be

constant along the entire length Lp)

In [2] it is recommended to assume a vertical effective stress acting on the top of the

soil plug (pa) equal to the vertical effective pressure at the bottom of the “passive” plug. For

infinite values of Lp/D this becomes:

( ) DD

KD

eD

pppp

p '25.04

'tan4'

14

'γ

γδ

γβ

γ≈

⋅≈=−= ∞− (5.41)

This leads to an overestimation of the active plug length if the vertical stress acting on top of

the soil plug is larger than this (low) value. For small piles (L/D<5) it is recommended to take

pa equal to zero. The plug end bearing is related to the inner pile area and the wall-end

bearing to the rim area. The equation, which has to supply the active plug length and

therefore the minimal penetration depth, becomes:

( ) ebplugebwalleb qnqnq 221 +−= (5.42)

with

0DD

n i=

The active plug length will have to be found iteratively. The report presents some

calculations which find La/Do to be between 4 and 8 although the cut-off point for the friction

formula La/Do = 2 is still valid because almost no friction is developed between 2 and 4.


• Model has been created for sand plugs,

• Determination of the active and passive plug length is difficult and

• Determination of active and passive earth pressure coefficient is difficult.

Method 3: One-dimensional soil plug analysis

In [31] it is stated that most open-ended piles will fill up with soil during driving and

then fail as closed-ended piles during static loading. The end bearing capacity of the pile is

taken to be the lesser of:


64

⎪⎭

⎪⎬

⎫

=

=

bbc

ibo

qdQ

dhQ

4

2π

τπ (5.43)

with:

Qbo bearing capacity in open-ended failure mode

Qbc bearing capacity in closed-ended failure mode

i pile inner diameter

h soil plug length (height of soil in pile)

qb end bearing stress

τi inner shaft friction per layer

v'βσ=

The value for β depends on the ratio of horizontal to vertical effective stress in the plug. A

minimum value of β is obtained assuming that the soil near the plug is at active failure, from

the Mohr’s circle in figure 5-9 it can be seen that:

( )( )δϕδϕ

στ

β−Δ+

−Δ==

cossin1sinsin

'vi

ϕδ

sinsin

sin =Δ

where

φ internal friction angle of the soil in the plug

δ friction angle between the soil and the pipe pile


65

Figure 5-9: Mohr’s circle of soil plug at active failure [31];

If the plug is considered to be loaded undrained there will be no compression of the

soil plug and the response will be rigid-plastic with the following capacity:

dh

pd

hq bu

2'45.0

4 βγβ+= (5.44)

If the plug is considered to be drained the limiting end-bearing capacity in excess of

the initial effective stress is:

hh

hdh

peq dh

bd '4'

'1

4

γβ

γγ

β

−⎭⎬⎫

⎩⎨⎧

+⎟⎟⎠

⎞⎜⎜⎝

⎛−= (5.45)

If the end-bearing stress is raised incrementally in the drained case, the internal skin friction

will be mobilized over an active length of the plug l, as shown in figure 5-10. The vertical

stress increment over the active plug length is then given directly by:

zd

he dz

v '4

')1('1'

4

γβγ

γσβ

−⎭⎬⎫

⎩⎨⎧

+−⎟⎟⎠

⎞⎜⎜⎝

⎛−=Δ (5.46)


66

Figure 5-10: Active and passive plug length [31];

With an elastic soil plug with a one-dimensional modulus E0 the active plug compression, or

tip displacement, may be approximated by:

04 Eqdw Δ

≈Δβ

(5.47)

The main limitations of this model are:

• The value of β is dependent on the stress, via the change of the friction angles due to

grain crushing,

• The influence of installation on the in situ stress state is not taken into account and

• The analysis has concentrated on carbonate soils, which are known to undergo volume

reduction during shearing, which in turn will affect the response of the plug.

5.2.6 Soil bearing capacity

The bearing capacity of the soil can be calculated with two methods:

• The Brinch-Hansen method for shallow foundations and

• The settlement based bearing capacity.


67

Method 1: Brinch-Hansen

The undrained bearing capacity of the soil can be calculated with the Brinch-Hansen

method [13]. This method is applicable for stiff shallow foundations and describes the

formation of a failure surface, see figure 5-11.

Figure 5-11: Failure surface for shallow foundation [13];

According to [13] the bearing capacity of the soil on ground level (undrained state) is defined

by:

( ) dzvccdimdrd isf ;0;;;max; '2' σπσ +⋅⋅+= q (5.48)

with:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛

⋅−+=

5.0;115.0

us

hsc cA

Fi

ef

efc L

Bs 2.01 +=

( )∑=

=

−⋅=ni

icharigfdzv ud

1;;0;;' γγσ

where:

σ'max;d design value of the maximum foundation pressure on the effective

foundation surface

fundr;d design value of the undrained shear strength according to [11]

σ'v;z;o;d design value of the original vertical soil stress at a depth z

di thickness of layer i

γchar characteristic volumetric weight of the soil

u water pressure according to [11]

γf;g load factor according to [14]


68

n number of horizontal soil layers

Lef length of the effective foundation area

Bef width of the effective foundation area

Fs;h;d design value of the load component on the foundation area

ic reduction factor for the slope of the load

sc shape factor for the influence of the cohesion


• Stiff foundations are assumed while for the PCC pile flexible geotextile is used and,

• The influence of the piles and the pile installation is not taken into account in the bearing

capacity.

Method 2: Settlement based bearing capacity

It is important to remember that the reason for installing the PCC piles is a reduction

of settlement of the soil. The two aforementioned methods describe two failure mechanisms

that can occur caused by the load of the fill. However, it is possible, even probable, for

unacceptable large settlements to occur before failure via one of the above methods. The

bearing capacity of the soil can also be based on the acceptable settlement. The method for

calculating the settlement belonging to a load on the soil will be given in paragraph 5.3.


• A maximum bearing capacity will still have to be calculated and

• Acceptable settlement has to be defined correctly and verified.

5.3 Settlement

5.3.1 Chinese practice

The total settlement of the composite foundation is composed of two parts: the

settlement of the pile-improved area and the settlement of the underlying stratum.

Settlement of the pile improved area

For the settlement of the composite foundation two methods are available:

• The composite modulus of elasticity method

• The stress correction method.


69

Figure 5-12: Settlement of the composite foundation consists of two parts [17

Method 1: Composite modulus of elasticity

The settlement of the pile-improved area is based on the modus of elasticity of the

composite foundation. The stiffness of the soil will be improved due to installation of the pile.

In this method the increase in stiffness is based on the exchange ratio of the composite

foundation, m, as given in equation (5.2) . The settlement of the pile improved area becomes

[19]:

i

n

i csi

i HE

ps ∑

=

Δ=

11 (5.49)

where:

s1 settlement of pile improved area

n number of soil layers in pile improved area

Hi thickness of the i-th soil layer

Δpi additional stress due to fill

Ecsi modulus of elasticity of the composite foundation

ssips EmmE )1( −+=

Eps modulus of elasticity of the pile

Essi modulus of elasticity of the i-th soil layer

m exchange ratio of the composite foundation


• The soil and pile are assumed to behave as one block while in paragraph 3.3.5 it can be

seen that the soil settles more than the pile,

• The modulus of elasticity of the soil is difficult to determine and


70

• The effect of the geotextile is not taken into account for calculation of the composite

modulus of elasticity; the exchange ratio is only based on the surface ratio.

Method 2: Stress correction method

The stress correction method is based on the stress ratio between pile and soil. Only

the settlement of the soil is calculated but the load on the soil is reduced by the load taken by

the pile. The load on the soil becomes [19]:

pnm

pp ss μ=−+

=)1(1

(5.50)

with:

s

pnσσ

= (5.51)

where:

ps load on the soil

n stress ratio

σp stress from the fill on the pile head

σp stress from the fill on the pile head

μs reduction factor

m exchange ratio composite foundation

The settlement of the pile treated area now becomes:

ssi

n

i csi

si sHEp

s 11

1 μ=Δ

= ∑=

(5.52)

where:

s1s settlement of the soil without improvement

A limitation of this method is:

• The stress ratio, n is hard to determine when using a geotextile mattress on top of the

piles without load testing on the composite foundation.

Settlement of the underlying stratum

For the underlying stratum the layer summation system is used [19]:


71

i

n

ni si

b HEp

s ∑+=

Δ=

max

12 (5.53)

where:

s2 settlement of the underlying soil stratum

The load on the underlying stratum can be defined with two methods [17]:

• The pressure diffusion method

• The equivalent entity method

Method 1: Pressure diffusion

Figure 5-13: Pressure diffusion method [17];

The load of the embankment is assumed to spread over de pile improved area, see

figure 5-13. The equation for the 2D situation becomes:

))tan(2( βhBBppb +

= (5.54)

where:

pb load on the lower strata

B width of the fill

p load due to the fill

β spreading angle

This is a safe approximation because the load also spreads in the underlying stratum,

thus reducing the settlement.


72


• The soil and pile are assumed to settle as a composite block and

• The spreading angle can only be determined based on experience.

Method 2: Equivalent entity

The improved area is considered to settle as one equivalent solid soil body. The

friction between the loaded and the not loaded soil is taken into account, as can be seen in

figure 5-14.

Underlying stratum

Pile improved area

z

Top load

hpb

B

Figure 5-14: Equivalent entity method [17];

The load on the underlying stratums now becomes:

hfGppb 2−Δ+= (5.55)

where:

ΔG weight of the pile

h length of the pile

f friction with the soil that is not loaded

The value of the friction with the soil that is not loaded, f, is determined from the CPT tests.


• The soil and pile are assumed to settle as a composite block and

• The friction with the soil that is not loaded has to be determined with special CPT tests.

5.3.2 Dutch practice

According to [12] the calculation value of the settlement of a foundation is calculated

as follows:


73

ddd www ;2;1 += (5.56)

where:

wd design value of the settlement of the foundation

w1;d design value of the settlement of the head of the pile

w2;d settlement due to the compression of the soil layers under the pile tip (group

effect)

The settlement of the head of the pile consists of two parts:

ddtipd www ;2;;1 += (5.57)

where:

wtip;d design value of the settlement of the pile head due to the load on the pile

wel;d design value of the settlement of the pile head due to elasticity of the pile

To calculate the settlement of the pile tip for pile groups the values of the maximal shaft and

tip forces have to reduced by a factor dependant on the number of piles and the number of

CPT’s given in table 1 of [12]. The value of the settlement of the pile head due to the load on

the pile is then constructed from the graphs given in Appendix A, based on the resulted load

on one pile. The settlement due to pile elasticity is given by:

dmatpdshaft

davgdel EA

FLw

;;;

;; ⋅

⋅= (5.58)

where:

Fgem;d design value for the average force in the pile, determined according to [11]

L

FFILFI dtiprdtotsdtots ))((5.0 ;;;;;; +−+⋅=

Fs;tot;d design value of the total load on the pile head

Fr;tip;d design value of force in the pile tip

L distance between pile tip and pile head

I distance between the highest point in the pile for which shaft friction is

presumed and the head of the pile

Ashafd;d design value of the surface of the pile shaft

Ep;mat;d design value of the modulus of elasticity of the pile shaft material


74

For a distance of more than 10 times the smallest diameter of the pile, w2;d is

assumed to be zero, else:

davgg

DDvd E

Amw

;;

44;;2

9.0'* ⋅⋅=

σ (5.59)

where:

σ’v;4D effective vertical stress on the surface b1 times b2 on a depth of 4D under the

pile points

D smallest cross section of the pile tip

b1,b2 dimensions of the loaded surface at a depth of 4D under the pile points

A4D loaded surface at a depth of 4D under the pile points

Eg;avg;d design value of the average modulus of elasticity at the 4D level,

E = 5qc;z;avg

m* factor dependent on the shape of the loaded surface, see table 5 of [12]


• Only the settlement of a single pile is modelled and

• Many empirical factors are used that are not (yet) determined for the PCC pile

5.3.3 Piled raft and pile group responses

In literature different methods are proposed for the settlement calculation of piled

raft and pile group foundations:

• Settlement ratio method

• Equivalent raft method

• Equivalent pier method

• Piled raft

Method 1: Settlement ratio method

In this method the stiffness of a pile group can be expressed as a fraction of the sum

of the individual pile stiffness [30]:

nkK η= (5.60)

where:

K pile group stiffness

η group efficiency

en −≈

k pile head stiffness of a single pile


75

n number of piles

While the exponent e will lie between 0.3 and 0.5 for primarily friction piles it actually

depends on the pile slenderness ratio, L/d, the pile stiffness ratio, λ=Ep/Gl, the pile spacing

ratio, s/d, the homogeneity of the soil, ρ and the Poisson’s ratio, υ. Design charts for the

group efficiency are shown in Appendix B

An elastic solution for the axial response of a single pile is presented in [30], based

on the separate treatment of the pile shaft, using the linear load transfer function and the

pile base, using the Boussinesq solution for a rigid punch acting on an elastic half space:

( )νη

πλ

μμ

ζπρ

ξνη

−+

+−

=

141

1

tanh2)1(

4

0

0

rl

ll

wrGP

ll

t (5.61)

with:

⎟⎟⎠

⎞⎜⎜⎝

⎛=

0

lnrrmζ

( )( )( )25.015.225.0 −−+⋅= νρξlrm

⎟⎟⎠

⎞⎜⎜⎝

⎛=

0

2rll

ζλμ

where:

Gl shear modulus at z = l

Pt load at the top of the pile

wt displacement at the top of the pile

η under-reamed piles (rb/r0)

ξ end-bearing piles (Gl/Gb)

ρ heterogeneity of soil modulus (Gavg/Gl)

λ pile-soil stiffness ratio (Ep/Gl)

rm maximum radius of influence

μl pile compressibility

NB: the subscript b refers tot conditions at or below the pile base.

To simulate a group of piles the load transfer parameter should be replaced by:


76

∑=

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

n

i

i

rs

n2 0

* lnζζ (5.62)

The base stiffness is adjusted:

⎥⎥⎦

⎤

⎢⎢⎣

⎡+= ∑

=

n

i

b

sr

21

* 21

πξξ (5.63)

where:

si spacing of the ith pile from pile 1

Finally the settlement of a group pile can be calculated by:

ssinlgegroup

group RK

Pw δ==

with:

knK η=

If a soft soil layer is present below the pile tip this will cause extra settlement due to

the load of the pile group. The rate of stress spreading below the pile area is taken the rate

of 1:4 [30], see figure 5-15:

( )( )[ ]Lrm ξυρ 25.015.225.0 −−+=


77

Figure 5-15: Use of equivalent raft for calculating the effect of a soft layer underlying the pile group

[30];

The theoretical values for the settlement ratio for floating pile groups are shown in

Appendix C. The settlement ratio for floating pile groups is reduced by the presence of a stiff

layer, the Poisson’s ratio and the distribution of the soil modulus. Graphs for reduction factors

are given in Appendix C


• Only pile groups and no single piles are discussed,

• The method is based on stiff rafts not geotextile and

• The pile improved area is assumed to settle as a composite body.

Method 2: Equivalent raft method

The foundation is considered as a whole. Traditionally an ‘equivalent’ raft is

considered, located two-thirds of the way down the part of the piles which penetrate the

main founding stratum, or at the level of the pile bases for end-bearing piles, see figure 5-16.


78

Figure 5-16: Equivalent raft approach for pile groups [30];

The average settlement at the ground level is then calculated by:

www raftavg Δ+= (5.64)

where:

wavg average settlement at ground level

wraft raft settlement

Δw elastic compression of the piles above the level of the equivalent raft

(treated as free-standing columns)

The main advantage of this method is that it enables the variations in soil stiffness below the

level of the raft to be taken into account. The raft settlement is evaluated by integrating the

vertical strains, allowing for variations in soil modulus and correcting for the raft embedment

below the ground surface:


79

ii

n

i sDraft h

EI

qFw ∑=

⎟⎟⎠

⎞⎜⎜⎝

⎛=

1

ε (5.65)

where:

q average pressure applied to the raft

Iε influence factor from which the vertical strain may be calculated, see

figure 5-17

hi thickness of the ith soil layer

Esi Young’s Modulus of the ith soil layer

FD correction factor

•

Figure 5-17: Influence factor for the vertical stain [30] ;





Method 3: Equivalent pier method

An alternative for the equivalent raft method is to consider the region of soil in which

the piles are embedded as an equivalent continuum. For a pile group the equivalent pier

diameter may be taken as:

ggeq AAd 13.14

==π

(5.66)

where:


80

deq equivalent pier diameter

Ag pile group area

The Young’s modulus of the pier is:

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛−+=

g

pspseq A

AEEEE (5.67)

where:

Eeq Equivalent Young’s modulus

Ep Young’s modulus of the piles

Es Average Young’s modulus of the soil penetrated by the piles

The advantages of the equivalent raft and pier approaches may be assessed by considering

the overall aspect ratio of the pile group, as shown in figure 5-18.

Figure 5-18: Replacement of pile group by equivalent pier [30];

The overall aspect ratio also depends on the degree of interaction between the piles (l/s) and

becomes:


81

lnsR = (5.68)

where

R aspect ratio

n number of piles

s spacing piles

l embedded length

For values of R greater than 4, it is shown that the pattern of differential settlement is very

similar to that of a raft foundation. An equivalent raft would then be a logical solution. For

smaller values the outset of an equivalent pier is more logical.





Method 4: Piled raft

The settlement of the soil and PCC pile can be compared to the settlement of a piled

raft foundation where the raft foundation where the stiffness of the raft is equivalent to that

of the soil. With subscripts p for the pile group and r for the raft the settlement may be

expressed as [30]:

⎭⎬⎫

⎩⎨⎧

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

=⎭⎬⎫

⎩⎨⎧

r

p

rp

rp

r

rp

p

r

p

PP

kk

kkww

1

1

α

α

(5.69)

where:

w settlement

αpr interaction factor

αrp interaction factor

P load

k stiffness

For large pile groups αrp becomes a constant value of 0.8 and the piled raft stiffness:


82

p

p

r

p

r

pr k

kk

kk

k

⎟⎟⎠

⎞⎜⎜⎝

⎛−

⎟⎟⎠

⎞⎜⎜⎝

⎛−

=

64.01

6.01

(5.70)

Similar the ratio of loads carried by the pile cap (or raft) and the pile group is:

p

r

p

rp

r

kk

kkP

P

⎟⎟⎠

⎞⎜⎜⎝

⎛−

=

8.01

2.0 (5.71)





5.4 Yan-Tong example case

5.4.1 Introduction

As an illustration of the calculation method the case of the Yan-Tong highway

embankment is considered. PCC piles were applied here to limit the settlement. Before,

during and after construction measurements were done to determine the settlement and the

bearing capacity of the piles and the pile treated area.

5.4.2 Site description

The Yan-Tong highway embankment is placed on the soft soil layers of the shore

plains of the Yellow Sea. The soil is made up of silty soil to a depth of about 30 meters. table

5-4 shows the soil properties. The distribution of the different types PCC piles over the area is

shown in table 5-5.


83

Table 5-4: Soil properties at Yan-Tong site [17];

Table 5-5: Pile properties [17];

Pile area Application

length [m]

Design length [m]

Diameter [mm]

Wall thickness

[mm]

Centre-to-centre [m]

Number of piles

Final length [m]

K30+740∼ K30+778 38 16 1000 120 3.3 142 15.0

K30+778∼ K30+808 30 18 1240 120 3.3 113 15.0

K30+808∼ K30+838 30 18 1000 120 2.8 158 15.5

K30+838∼ K30+868 30 18 1000 120 3.0 139 15.5

K30+868∼ K30+898 30 18 1000 120 3.3 116 15.5

K31+509∼ K31+559 50 18 1000 100 3.3 193 15.5

K31+559∼ K31+600 41 16 1000 120 3.3 155 15.5

Measurements have been done to determine the physical and mechanical properties

of the soil. The results are shown in Appendix D.

The cross section K30+793 is chosen for the example calculations, it should be noted

however that not all soil properties are determined on this exact location, some differences

with the measured results are therefore to be expected. Based on the CPT test and borings

the soil profile is shown in . According to table 5-6, the pile at K30+793 has a length of 15.0

m, a wall thickness of 120 mm, a diameter of 1240 mm and a centre-to-centre distance of

3.3 m.


84

Table 5-6: Layer properties for K30+793 [17];

Layer number 1 2 3 4 5Soil type Clay Silty Clay Clay Silt Sandy SiltTop [m] 0.00 1.70 13.10 18.70 28.00Bottom [m] 1.70 13.10 18.70 28.00 28.50Thickness [m] 1.70 11.40 5.60 9.30 -Water Table [m] -2.90 -2.90 -2.90 -2.90 -q_c [MPa] 1.82 0.67 1.65 7.58 10.94f_s [kPa] 46.31 11.68 51.49 81.41 95.06R_f [%] 2.54 1.75 3.22 1.09 0.87E_s [MPa] 6.70 3.34 6.22 18.42 22.02c_u [kPa] - 30.48 - -sigma_0 [kPa] 130 80 140 160 180tau [kPa] 35 17 44 43 45

Water content [%] 25 37 29 29 -Natural density [g/cm3] 1.85 1.45 1.44 1.76 -Dry density [g/cm3] 1.48 1.34 1.44 1.47 -Saturated density [g/cm3] 81.00 98.00 98.00 93.67 -Void ratio [-] 0.82 1.02 0.885 0.84 -Specific density [-] 2.70 2.70 2.72 2.69 -Liquid limit [-] 28 31 46 30 -Plastic limit [-] 20 23 26 24 -Plasticity index [-] 8 9 20 6 -Liquidity index [-] 0.59 1.73 0.13 0.90 -Coefficient of compressibility [-] 0.33 0.61 0.27 0.18 -Modulus of compressibility [MPa] 5.34 4.23 6.77 11.57 -Preconsolidation pressure [kPa] 240 158 270 292 -Compression index [-] 0.17 0.27 0.257 0.07 -Relaxation index [-] 0.01 0.23 0.257 0.04 -Coefficient of consolidation (100kPa) [10E-03 cm2/s] 3.72 4.43 4.65 5.10 -Coefficient of consolidation (200kPa) [10E-03 cm2/s] 4.03 4.01 2.97 0.62 -Permeability coefficient vertical [10E-06 cm/s] 5.73 2.41 2.75 29.68 -Permeability coefficient horizontal [cm/s] 1.43 0.60 0.69 3.50 -Cohesion [kPa] 41 43 46 9 -Angle of internal friction [degrees] 19.5 16.3 15.7 41.7 -Effective cohesion [kPa] 31 27 29 4 -Effective angle of internal friction [degrees] 26.9 29.5 25.3 31.7 -

5.4.3 Chosen calculation methods

In the previous chapters a number of calculation methods for bearing capacity and

settlement are given. As decided in paragraph 4.5 the main focus will lie on the bearing

capacity and settlement of a single pile. Some verification of the Chinese and Dutch practice

methods will also be executed. Based on the limitations of the models described in the

previous chapters the following methods will be used:

• Bearing capacity single pile:

o PCC pile bearing capacity from the Chinese practice

o Tip bearing capacity with Koppejan

o Negative shaft friction with the method Zeevaert - De Beer

o Shaft bearing capacity according to NEN6740

o Plugging with the spring model

• Soil bearing capacity

o Brinch-Hansen

• Bearing capacity composite foundation


85

o Chinese practice

• Settlement single pile

o NEN6740 method

o Spring model

• Settlement composite foundation

o Chinese practice

o Equivalent raft / Equivalent pier method

5.4.4 Bearing capacity single pile

Bearing capacity calculations of single piles for the Yan-Tong case are made with the

above mentioned calculation methods, based on the descriptions in the previous chapters and

the pile and soil data of paragraph 5.4.2.

Chinese practice

A calculation of the bearing capacity is made with the most commonly used GeoHohai

method.

The bearing capacity of a single PCC pile is defined by equation (5.4) :

)90.149.5140.1168.1170.113.46(9.3 ⋅+⋅+⋅== ∑ isiksf lfuF = 1200 kN

Pile tip bearing capacity with Koppejan

The pile tip bearing capacity is calculated with equation (5.5), the cone resistances

are determined from the CPT, see Appendix E:

⎟⎟⎠

⎞⎜⎜⎝

⎛+

+⋅⋅⋅⋅= avgIIIc

avgIIcavgIcptiptipmax;r; q

qqsAF ;;

;;;;

25.0βα

⎟⎠

⎞⎜⎝

⎛ ++

⋅⋅⋅⋅= 6.12

7.18.15.06.01142.0 = 450 kN

Negative shaft friction

The negative shaft friction is based on the Method Zeevaert – De Beer. The

maximum negative shaft friction follows from equation (5.12). The value of the load on the

soil used in the Zeevaert-De Beer method depends on the fill height, the fill material, the pile

layout and the geotextile mattress. Since the load redistribution by the geotextile mattress is

beyond the scope of this report the load on the soil is taken from the long term monitoring

executed on the composite foundation, see paragraph 3.3.5. For the Yan-Tong case the

stress at the top of the soil is found to be approximately 50 kPa. The soil around the pile is

schematized as layer 2 for the entire pile length because of the calculation errors occurring


86

when using thin soil layers in the Zeevaert – De Beer model [12]. Layer 2 is chosen because

this is the thickest layer. The shaft friction then becomes:

( ) ( )4011868.9''1

;;;;;;0;; −⋅=−= ∑=

=

ni

idihmdihdnfs AF σσ =755 kN

with:

==− ddihm p ;0;1;;'σ 50 kPa

=⋅+=+= − 53.41550''' ;1;1;;;;;0 didihmdih h γσσ 118 kPa

( ) ( ) ( ) ( ) =+−=+−= ⋅−⋅−−−

− 1512.01512.0;1;;

1

;;;; 501

12.053.4

'1'

' eeeem

iiii hmdihm

hmdidihm σ

γσ 40 kPa

There are only soft soil layers present in this profile and the calculated negative shaft

friction is larger than the total load on the soil of 480 kN, calculated based on the measured

soil stress. This can be explained by the measurements in paragraph 3.3.5 where the

settlement of the soil is approximately 100 mm more than that of the pile, while in [38] a

minimum differential settlement of 200 mm is required for full mobilisation of the shaft

friction in all the soft layers. To try to make a realistic approach of the negative shaft friction

it is assumed that there is negative shaft friction until the level where the entire load on the

soil is brought to the pile. So h for Fs;ns = 480 kN. A value of h of 9.4 m is then found

iteratively.

Only from this level on, the shaft bearing capacity can be calculated, since when

negative skin friction occurs no positive skin friction can be mobilized for the bearing

capacity. The positive shaft friction, or pile shaft bearing capacity is only active over the

length of the pile between 9.4 and 15 m below pile head level. The pile can be divided into

two parts, one where negative shaft friction occurs and one where positive shaft friction

occurs, see figure 5-19.


87

Positive skin friction, pile settles more than soil

Negative skin friction , soil settles more than pile

Tip bearing capacity

Inner shaft bearing

capacity or plug bearing

Figure 5-19: All interaction forces with the soil for a schematised single PCC pile;

Pile shaft bearing capacity according to NEN6743

The pile shaft bearing capacity is calculated according to the Koppejan method [12].

Two values are calculated: with and without the negative skin friction, corresponding to

respectively the pile load test and the long term monitoring. The shaft bearing resistance

follows from equation (5.9):

With negative skin friction:

( )9.185.17.367.0025.090.3;;;max;; ⋅+⋅⋅== ∑ iiscszshaftr HquF α = 550 kN

Without negative skin friction (shaft bearing over full pile length):

( )9.165.14.1167.07.182.1025.090.3;;;max;; ⋅+⋅+⋅⋅== ∑ iiscszshaftr HquF α = 1350 kN

Plugging with the spring model

The bearing capacity of the plug and the resulting settlement are determined with

the spring model, see paragraph 5.2.5. Since the spring model is not often applied in practice

and there is little experience with the model the effect of variation of parameters on the plug

bearing capacity is determined. This is done based on the parameters of the Yan-Tong

project.

There are large differences between the load test on the single pile and the

measurements during embankment construction:

• Load test: quick loading until failure with small settlements and large load on the pile,

undrained.


88

• Embankment: slow loading with stresses in the pile far below bearing capacity but with

large settlements due to consolidation and group-effect, drained.

Since the main focus of this analysis is to determine the bearing capacity of a single pile only

the load test on a single pile is discussed here. An overview of the relevant parameters is

shown in table 5-7.

Table 5-7: Relevant soil parameters plugging;

Parameter Load test Calculation Undrained Soil stiffness at tip 8.14 MPa Soil stiffness active plug 6.20 MPa Soil stiffness passive plug 3.84 MPa Load on the pile 1650 kN Pile head displacement 15 mm Poisson’s ratio soil 0.45 Friction angle soil-pile infinite

The spring model consists of five different springs with the following spring constants

(based on table 5-7):

=−

⋅⋅=

−=

2

3

2

_

45.015.01014.8

21

2υ

itipsoili

REK 10.2 MN/m

=−

⋅⋅=

−=

2

3

2

_

45.015.01084.3

21

2υ

ipilesoilinner

REK 4.82 MN/m

=⋅−⋅⋅

=−

=57.0)33.01(62.01014.8

)()1( 2

3

2

0_

n

REK tipsoil

w ωυ 11.1 MN/m

=⋅⋅⋅⋅== 31019.917.05.022 πβπ Maipl DRK 19.1 MN/m

where:

=⋅⋅⋅−+

−=

−+−

= 31020.6)45.021)(45.01(

45.01)21)(1(

1plM ED

υυυ

9.19E+03 kN/m2

=°−

== 5.17tan45.01

45.0tan0 δβ Ka 0.26

The interaction factors between the springs are given by:

57.045.0181.02

81.01181.0

)(12

112

2

2

2

1⋅−⋅

−+−=

−

−+−=

nn

nn

ωυα = 0.48

2

2

2

2

281.011

57.081.0121(81.0

11

)(121

−−

⋅−−=

−−

⎟⎠⎞⎜

⎝⎛ −−

=n

nnn ωα = 0.64.


89

The contribution of the wall bearing to the total end bearing load follows from these formulas

and the relationship between the wall and plug settlement:

www

totalpl

w

i

wQQQ

KK

KK

Q ⋅=⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜

⎝

⎛

++−

−=

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

++−

−= 16.11

9.31.11

2.101.11

)64.01(

48.011

)1(

1

.2

1

α

α

where:

totalplplinnertotalpl

KKKK .

.

111→+= = 3.9 MN/m

So the total end bearing load is 1.16 times the wall end bearing load, which means the plug

only has about 16% of the bearing capacity of the wall in the load test according to the

spring theory.

The wall displacement, which equals the pile head displacement, can be calculated

with:

i

w

w

ww K

QQKQu −

+= 2α

The shaft bearing is fully mobilized if the displacement is more than 10 mm, it is assumed

this will be the case and this will be verified later. The ultimate wall bearing load can now be

calculated from the total pile bearing (3.3.5) and the calculated maximum shaft resistance,

calculated above. This gives: Qu = 1650-1350 = 300 kN, which results in a settlement at

failure of:

2.1016.1/300300

64.01.1116.1/300;

2;

;−

+=−

+=i

uw

w

uwuw uK

QQK

Qu α = 34 mm

This is about a factor 2 larger than the measured settlement of 15 mm. Note that only the

compression of the layers 4D below the pile tip is considered. The assumption of the full shaft

mobilization is correct.

The parameters in the spring model that are determined from soil tests and are of

influence on the plug bearing capacity and overall settlement are the following:

• Soil stiffness

• Poisson’s ratio: ν


90

The soil stiffness in table 5-4 is taken from the measurements of the Yan-Tong project

geotechnical survey and is based on borings, the resulting stiffness is assumed to be

determined undrained. This undrained stiffness is as a rule of thumb reduced to a quarter of

the original value in the drained case. The Poisson’s ratio is not known but assumed to be

around 0.45 for the undrained case and around 0.33 for the drained case. In figure 5-20 the

soil stiffness of the soil below the pile, the soil in the passive plug and the soil in the active

plug is varied from 10% to 1000% of the values given in table 5-7.

0.000

0.020

0.040

0.060

0.080

0.100

0.120

0.140

0.160

0.180

0.200

0.10 1.00 10.00

E/E measured

u w [m

]

Stiffness soil below tip

Stiffness soil in passive plug

Stiffness soil in active plug

Measured settlement

Figure 5-20: Sensitivity of spring model to soil stiffness variations, undrained case;

From the figure it can be seen that the influence of the soil stiffness in the plug, both active

and passive, is very small. For the variation of 10-1000% of the soil stiffness the final

settlement only lies between 25 and 27 mm. The soil stiffness below the plug is much more

influential, for the given variation the calculated settlement lies between 3 and 235 mm.

The relationship between the active plug and passive plug soil stiffness and the

spring constants is linear, see equations (5.21), (5.22) and (5.23), so the spring properties of

the soil plug in the considered range have almost no effect on the end settlement of the pile.

This can be explained by (5.26) and (5.27) where it is seen that an increase in the overall

plug stiffness will lead to a higher part of the load taken by the soil under the pile plug,

expressed in the relationship between Q and Qw. Since the spring constants of the soil below


91

the pile wall (Kw) and the plug (Ki) are comparable and the factor α2 is not very small (~0.7 in

this case), the calculated settlement only varies slightly.

Although the calculated settlement with different stiffness of the soil in the pile hardly

changes, the ultimate bearing capacity does. A strong spring in the plug will lead to a

distribution of forces on the soil below the plug and the soil below the pile wall. A weak plug

however will result in a large force on the soil below the pile wall and a smaller force on the

soil below the plug. In the last case failure of the soil will be reached earlier than in the first

case.

In figure 5-21 the Poisson’s ratio of the soil is varied between the 0.20 and 0.50.

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.20 0.25 0.30 0.35 0.40 0.45 0.50

Poisson's Ratio [-]

u w [

m]

Poisson's ratio

Measured settlement

Figure 5-21: Sensitivity of spring model to Poisson’s ratio variations, undrained case;

It can be seen that the variation of the Poisson’s ratio between 0.20-0.50 leads to a variation

in the calculated settlement of 24-32 mm. The influence of the Poisson’s ratio is significant

but, as can be seen from figure 5-21, not large enough to explain the difference between the

measured value for the settlement and the calculated value.

In conclusion it can be said that for the settlement of the loaded pile both the soil

stiffness below the pile and the Poisson’s ratio are of large influence of the settlement. These

parameters are hard to determine correctly in situ thus limiting the viability of the spring

method. For a correct settlement result the soil stiffness below the pile should be about two

times larger than measured in the Yan-Tong case.

Total bearing capacity single pile

The total bearing capacity of a single PCC pile in the Yan-Tong highway embankment

according to the different methods is given in table 5-8.


92

Table 5-8: Overview calculated bearing capacity single pile according to Dutch practice;

Method Bearing

capacity

Chinese practice (only friction) 1200 kN

Dutch practice 1750 kN

• Tip capacity (Koppejan) 340 kN

• Shaft capacity (Koppejan) 1350 kN

• Plug capacity (16% of tip capacity) 55 kN

Measured capacity single pile 1650 kN

5.4.5 Soil bearing capacity

Brinch-Hansen

As discussed in paragraph 5.2.6 the bearing capacity from the soil can best be

determined by the acceptable settlement, with the Brinch-Hansen failure mechanism for the

undrained case as an upper limit. The maximum bearing capacity according to the Brinch-

Hansen criterion is given in equation (5.48). The factors sc and ic both become unity since the

length of the foundation is very large compared to the width. For the limit of the bearing

capacity at a depth z = 0, follows:

( ) ( ) 01148.302'2' ;;;; +⋅⋅⋅+=+⋅⋅+= πσπσ dozvccdundrdmax; isf = 157 kPa

Note that the undrained shear strength is taken for layer 2 since most of the slip circle will be

in this layer. The ratio te/Bef, where te is the influence depth of the slip circle, is around unity

for a internal friction angle of 25-30 degrees. The undrained shear strength of layer 1 and 3

is higher so layer 2 is governing.

5.4.6 Bearing capacity composite foundation

Chinese practice

The bearing capacity of the soil is taken from the CPT test result and is 100 kPa

according to [17]. The correction and loading factors are based on experience and are also

given in [17], where λ2 is chosen 0.9 and β as 1.0. With a value of m of Ahead/Atotal = 0.11,

the bearing capacity of the composite foundation is given by equation:

100)11.01(19.0100011.0)1(2 ⋅−⋅+⋅=−+⋅= sfpfcf pmpmp βλ = 190 kPa


93

Note that the bearing capacity of the single pile is taken from the GeoHohai calculation

method in paragraph 5.2.2. If the pile bearing capacity according to the Dutch practice is

taken combined with the soil bearing capacity according to the Brinch-Hansen method it

follows that:

157)11.01(19.0145011.0)1(2 ⋅−⋅+⋅=−+⋅= sfpfcf pmpmp βλ = 290 kPa

It has to be evaluated if a load of this magnitude will not lead to unacceptable high

settlements.

5.4.7 Settlement single pile

NEN 6743

A single pile without group effect is considered. The maximum load of 1650 kN is

taken from the measurements of the load test on the single pile, see paragraph 3.3.5.

Because the group effect is not taken into account w2;d in equation(5.56) is zero and the total

settlement becomes equal to the calculation value of the settlement of the top of the pile as

in equation (5.57). This settlement consists of two parts, the compression of the pile and the

settlement of the pile tip due to the load on the pile.

The settlement of the pile tip is calculated in figure 5-22, where the maximum shaft

resistance and the maximum tip resistance are shown. The equivalent pile diameter is defined

as the smallest cross section of the pile surface, taken in this case as the wall thickness. The

tip bearing capacity and shaft bearing capacity are taken from table 5-8 where the plug

bearing capacity is added to the tip bearing capacity.

600 200 4008001000 200 0400

wtip;d

Fr;tip;d [kN]Fr;shaft;d [kN]

5

10

15

1650 kN

12001400

Figure 5-22: Calculation pile head settlement [12];

As the total bearing capacity of the pile determined by the Dutch practice for displacement

piles is higher than the measured settlement, the full pile head displacement is not yet


94

reached in figure 5-22. The calculated pile head settlement for 1650 kN is approximately 9

mm while the maximum settlement is 12 mm and will be reached at a load of 1750 kN.

The elastic compression of the pile head is calculated with equation (5.58) where:

4;;

;; 102041.0

100015⋅⋅

⋅=

⋅

⋅=

dmatptip

davgdel EA

FLw = 1.1 mm

with

( )( )15

35016500155.016500;

+−+⋅=davgF = 1000 kN

Since there is positive shaft friction over the entire length of the pile, I is zero.

In figure 5-23 the calculated settlement with the NEN method and the settlement

measured during the Yan-Tong project are plotted. It can be seen that the NEN method

underestimates the settlements. This could be caused by simplification or insufficient exact

soil data.

0

2

4

6

8

10

12

14

0 250 500 750 1000 1250 1500 1750

Load [kN]

Dis

plac

emen

t [m

m]

NEN 6743 Calculation

Yan-Tong measurements

Figure 5-23: Comparison calculated settlements and measurements for Yan-Tong project;


95

Spring model

The settlement calculated with the spring model is shown in paragraph 5.4.4 and is

about 34 mm. A sensitivity analysis of the input parameters is also given there.

5.4.8 Settlement of the composite foundation

Composite modulus of elasticity of Chinese practice

The settlement of the pile improved area of the Yan-Tong project is calculated with

the modulus of elasticity of the composite foundation, equation (5.49):

=++=Δ

= ∑=

9.1588105

4.11586105

7.1587105

1

n

ii

csi

ii H

Ep

s = 3 mm

where:

=⋅−+⋅⋅=−+= 34.5)04.01(104.104.0)1( 411 sspscs EmmEE 587 MPa

=⋅−+⋅⋅=−+= 23.4)04.01(104.104.0)1( 422 sspscs EmmEE 586 MPa

=⋅−+⋅⋅=−+= 77.6)04.01(104.104.0)1( 433 sspscs EmmEE 588 MPa

The load on the lower strata is commonly calculated with the pressure diffusion

method of equation (5.54), with a spreading angle of 1:2 gives:

( ) ( ))45tan(1526210562

)tan(2 ⋅+⋅

=+

=βhB

Bppb = 71 kPa

The settlement of the underlying stratum is calculated with equation (5.53) where

the location of the stiff layer is assumed at 50 meter below surface (based on table 5-4):

220.22

711.1

4.1871

7.38.6

71max

12 ++=

Δ= ∑

+=

n

nii

si

bi HEps = 145 mm (drained situation)

This is assuming that layer 5 continues to a depth of 50 m below surface and that the

stiffness remains constant, which is not the case. In reality the stiffness will increase with

depth.

The total calculated settlement now become 148 mm with very rough and

conservative estimate for the lower strata. The measured settlement of the piles was

approximately 250 mm and that of the soil between the piles was approximately 350 mm. It

can be seen that using the composite modulus of elasticity underestimates the settlement of


96

the composite foundation. This method also does not take into account consolidation and

creep effects.

Equivalent raft/pier method

The first step is to determine which of the two schematizations can be best applied.

In paragraph 5.3.3 it is stated that an equivalent raft can best be applied when the value of R

is greater than 4 in equation (5.68) and an equivalent pier if the value is smaller than 4.

Assuming a square foundation layout (20x20 piles, where 20 is the number of piles in the

cross section of the Yan-Tong embankment) this gives:

153.3400 ⋅

==l

nsR = 9.4 > 4

which gives the equivalent raft method as best approximation.

The equivalent raft is located at 2/3 of the pile length in the bearing stratum for

primarily friction piles. For calculation purposes it is proposed that the “soft soil” layers in

figure 5-16 are defined as the layers where negative skin friction occurs because these layers

do not add to the bearing capacity of the piles. The bearing stratum therefore starts from z =

9.4 (see paragraph 5.4.4) below ground level. The elastic compression of the pile is small and

can be neglected. Equation (5.65) then gives the settlement of the equivalent raft, calculated

for all the layers below the equivalent raft (modulus of elasticity for the undrained case):

∑=

⎟⎟⎠

⎞⎜⎜⎝

⎛=

n

ii

isDraft h

EI

qFw1

ε

=⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛+⎟⎠

⎞⎜⎝

⎛+⎟⎠

⎞⎜⎝

⎛+⎟⎠

⎞⎜⎝

⎛⋅= 0.2202.2250.0

30.942.1875.0

6.577.670.0

7.323.450.0

1051 198 mm

Note that the factor FD is kept at 1.0 and that a stiff raft is assumed on top of the piles. Again

layer 5 is assumed to continue until a depth of 50 meters below surface and again no

consolidation and creep is taken into account.

5.5 Conclusion

The results can be compared with the data obtained from the long term monitoring

and load tests. The comparison is shown in table 5-9.

Table 5-9: Comparsion calculation methods with measurements for single pile;

Method Bearing capacity Offset Final settlement Offset


97

Load test 1650 kN - 15 mm -

Chinese practice 1200 kN -27% n.a. -

Dutch practice* 1750 kN +6% 11 mm** -27%

Spring model n.a - 34 mm +127%

*including plugging

**for a load of 1650 kN, for 1750 kN a settlement of 13 mm is calculated

Table 5-10: Comparsion calculation methods with measurements for embankment;

Method Bearing capacity Final settlement Offset

Embankment 150 kPa* 250-350 mm -

Chinese practice 190 kPa 148 mm -50%

Equivalent raft - 198 mm -34%

*not loaded until failure

The NEN method gives a rather good approach for both the bearing capacity of the

single pile and the settlement. The somewhat simplistic GeoHohai method for bearing

capacity calculations gives a margin of error of less than 30%. The spring model

overestimates the settlement noting that the influence of the, difficult to determine, soil

stiffness is very large on this result.

Although conservative assumptions are made for the soil properties of the layers

below -28 m both the methods underestimate the settlement. For the Chinese practice this

overestimation can be explained due to the use the composite stiffness modulus. The

negative offset of the equivalent raft can partly be explained by the assumption of a stiff raft.

Both methods also only include direct compression and when using the drained modulus of

elasticity not the consolidation and creep which takes place in the period of time of the Yan-

Tong embankment construction.

For the settlement of a single PCC pile the NEN method is advised with inclusion of

negative shaft friction and plugging.


98

6 Further applications

6.1 Introduction

In chapter 4 the application of the PCC pile as a soil improving pile is evaluated

compared to the HSP and AuGeo piles. Part of the conclusion was the notion that the quality

of the PCC pile was such that it would be feasible to apply the system apart from soil

improvements. This part of the report will discuss the possibilities.

In paragraph 6.3, two applications, building foundations and bridge abutments, are

discussed. The bearing capacity for both compression and tension loading is calculated.

Following these calculations different reinforcement solutions for resisting a bending

moment are described in paragraph 6.4.

In paragraph 6.5 the application of these solutions to the PCC pile and the required

changes to the design are described.

Based on the previous chapters a conclusion drawn in chapter 5 to determine the

viability of the PCC pile in the discussed cases.

6.2 Requirements

6.2.1 Introduction

To determine if and how the PCC piling method can best be applied outside the

traditional soil improvement field an overview of two possible applications is given. The

applications are:

• Foundation of a large building and

• Foundation of bridge abutments.

Both applications have their own problems and demands on the piling system used. The

focus will be on application in soft soil since the PCC pile is considered to be especially suited

for that case.

6.2.2 Vertical loading

One of the most important functions of the foundation pile is the ability to carry the

load applied on it. An extensive description of the vertical bearing capacity of the single PCC

pile is given in paragraph 5.2 of this report. Below some special comments for the application

in a foundation are made.

Pressure loading

The main purpose of the foundation of a large building is to limit the settlement and

differential settlement of the building when constructed on soft soils. This can be done by

transferring the load of the building to a stiffer layer which will settle less than the soft layers.


99

The commonly used method for this load transfer is a pile foundation connected to the

building, the pile tip placed in a bearing sand layer.

Sufficient piles must be applied so that the load on a single pile is not higher than the

bearing capacity of this pile. The soil mechanical bearing capacity consists of pile end

bearing, plug bearing, shaft bearing (including negative shaft friction), see paragraph 5.2.

Tension loading

Apart from pressure loading a foundation pile can also be subject to tension loading

when the pile is used as a tension pile or when high wind loads cause a number of piles in

the foundation to be subjected to tension, see figure 6-1.

Wind load

Tension or reduced compression

Compression

Figure 6-1: Tension and pressure forces in piles due to wind loading;

Since un-reinforced concrete can only sustain small tension forces [42],

reinforcement over the length of the pile might be required to sustain the tension forces in

the pile. The amount of reinforcement required is dependent on the tensile load on the pile.

The upper value of the tension force in a pile as a result of the tensile load is the force

required to pull the pile out of the soil, the pull-out capacity.

The pull out capacity of a single pile is given in [4] as:

∫ ⋅=L

dzravgpdtenstionr dzpuF0

;;;;; (6.1)

where:

Fr;tension;d design value of the bearing capacity of a pile loaded on tension

up;avg mean circumference of the pile


100

L pile length over which shaft friction is calculated

pr;z;d design value of the shaft friction at depth z

m

zct

qγ

ξα ;⋅=

z depth

αt factor from table 6-2 depending on the installation process

qc;z cone resistance at depth z

ξ safety factor dependent on number of piles (0.8 in this case)

γm material safety factor (1.4 in this case)

Table 6-1: Maximum values for αt for sand and sand with gravel;

Pile class/type αt ground displacing methods • Driven prefabricated concrete piles and steel tube piles with closed tip • Cast-in-place where the concrete column is pressed against the soil and tube

back by driving • Idem, tube back by vibration • Driven MV piles • Continuous flight auger piles:

o Grout injection or mixing

0.007 0.0012 0.010 0.012 0.009

profiles with little soil displacements • Steel profiles, driven (including open steel tubes and sheet pile walls) 0.004 piles with soil removal • Bored piles 0.0045

Table 6-2: Values for αt for clay and silt [4];

Soil type Relative depth αt* Clay/silt qc ≤ 1 MPa 0 < z/Deq < 20 0.02 Clay/silt qc ≤ 1 MPa z/Deq > 20 0.025 Clay/silt qc > 1 MPa - 0.025 *Values are based on literature

The occurrence of a soil plug influences the tension capacity of the PCC pile. In [12]

it is stated that the plug compression bearing capacity is equal to the tip bearing capacity of

the plug or the inner shaft bearing capacity, whichever is the lowest. In the same way it can

be derived that the pull out resistance of the plug in the PCC pile is the lowest of the inner

shaft capacity calculated with equation (6.1) and the dead weight of the soil inside the pile.

Either the wall friction is large enough to let the whole plug cling to the pile or the soil weight

is large enough to break the wall friction. In the first case the soil plug is pulled out with the

pile and in the second case the pile is pulled out of the soil leaving the plug behind.


101

6.2.3 Horizontal load

When applied as a soil improvement the PCC pile will only have to bear very small

horizontal loads. When applied in foundations however, the pile head is connected rigidly to

the construction and significant horizontal loads can occur. In designing these connections it

is common practice to assume a certain eccentricity of the load on the pile. This eccentricity

results in a moment on the head of the pile resulting in a tension load on the concrete. A

concrete pile has only a very limited capacity for tension loading, so reinforcement of the

head can be required.

The eccentricity of the load is dependent on the installation of the pile and the

position of the connection between pile and building. In design the eccentricity is commonly

assumed to be 50 mm. Because of its large radius a larger eccentricity is assumed for the

PCC compared to commonly used foundation piles. The eccentricity is taken at an arbitrary

and conservative value 100 mm for the PCC pile. The occurring moment is defined as force

multiplied by arm. In this case the load on a single pile multiplied by the 100 mm:

eccdheaddhead uFM ⋅= ;; (6.2)

where:

Mhead;d design value of the maximum moment in pile head

Fhead;d design value of the maximum load on pile head

uecc eccentricity of load (100 mm)

Bending moments in the pile can also be caused by horizontal displacements of the

soil. Using analytical programs like MSheet, the resulting tension force in the pile can be

calculated.

6.2.4 Settlement

The main reason for applying piles in a foundation is to limit the settlement and the

differential settlement thus preventing damage to the building. The settlement calculation of

a single PCC pile and a pile group is extensively described in paragraph 5.3. The allowable

settlement for buildings is generally much smaller than that of embankments. This should be

considered in the design.

An important item is the connection between the foundation and the building.

Difference can be made between piled rafts, where there are piles connected to stiff plates,

and pile groups, where a summation of each single pile is made [30]. Both methods are

described in chapter 5


102

6.3 Case studies

6.3.1 Introduction

The case studies serve as a means for identifying possible applications of the PCC

pile. The application as ground improvement is already discussed in previous parts. In the

western part of The Netherlands where the big cities and thus big building projects are

located the soil profile mainly consists of a number of soft soil layers underlain by a stiff sand

layer at depth of 15-20 meter below ground level. Case 1 considers a foundation in this kind

of soil.

To test the limits of the PCC pile a case is considered where the pile is used in a

foundation for a bridge abutment, where piles are generally subjected to high bending

moments. Case 2 considers the application of the PCC pile here.

The PCC pile might also be usable as a building foundation pile on friction only. Since

the field of application for this kind of pile in The Netherlands is rather small due to the

presence of stiff layers at medium depths, this case is left out.

6.3.2 Case 1: Foundation on tip bearing

Description

The construction of a fictional high rise building in the Rotterdam area is considered

where foundation piles have to be applied. A representative CPT test is shown in Appendix F.

The calculation of the piles will be made based on this. The purpose of this case is to

compare the bearing capacity of the PCC pile and the Vibro pile.

The considered piles are the PCC pile as described in the Yan-Tong case in chapter 5.

The Vibro pile is basically a cast-in-place circular concrete pile, comparable to the PCC pile

but solid instead of hollow, the design is based on the description in [43] without an

increased footplate as is normally applied. The pile properties are given in table 6-3.

Table 6-3: Pile properties after installation;

Pile Value PCC pile • Outer diameter • Inner diameter • Cross-section surface • Equivalent diameter • Centre-to-centre distance• Concrete

1240 mm1000 mm0.42 m2 120 mm 3.3 m C12/15

Vibro pile • Outer diameter • Cross-section surface • Equivalent diameter • Centre-to-centre distance• Concrete

400 mm 0.13 m2 400 mm 1.8 m C30/35


103

The soil properties are given in table 6-4.

Table 6-4: Soil properties;

Layer 1 Layer 2 Layer 3 Layer 4Soil type [-] Sand Peat Clay SandTop level (NAP) [m] -0.5 -5.0 -11.0 -17.0Bottom level [m] -5.0 -11.0 -17.0 -35.0Layer thickness [m] 4.5 6.0 6.0 18.0Saturated unit weight [kN/m3] 21.0 12.0 20.0 20.0Dry unit weight [kN/m3] 19.0 12.0 20.0 19.0Angle of internal friction [degrees] 32.5 15.0 25.0 32.5

The ground water table is assumed at ground level (-0.5 m NAP). In this case the piles will be

installed with the pile tip 2 m in the stiff sand layer at 17 m below NAP. The efficiency of the

piles will be determined by the volume of concrete used per meter length compared to the

bearing capacity.

Bearing capacity

The bearing capacity of the PCC and Vibro piles is calculated with the 4D-8D method

[12] as described in paragraph 5.2.

Tip bearing

The bearing capacity of a single PCC pile has been calculated based on the CPT

values, see Appendix G:

⎟⎟⎠

⎞⎜⎜⎝

⎛+

+⋅⋅⋅= avgIIIc

avgIIcavgIcptiptipr q

qqsAF ;;

;;;;max;; 2

5.0βα

=⎟⎠

⎞⎜⎝

⎛+

+⋅⋅⋅⋅= 0.13

25.145.16

5.06.01142.0 3.6 MN

The same calculation for the Vibro pile, see Appendix H, results in:

⎟⎟⎠

⎞⎜⎜⎝

⎛+

+⋅⋅⋅= avgIIIc

avgIIcavgIcptiptipr q

qqsAF ;;

;;;;max;; 2

5.0βα

=⎟⎠

⎞⎜⎝

⎛ ++

⋅⋅⋅⋅= 52

2.10125.011113.0 1.0 MN

The equivalent diameter of the PCC pile is smaller than that of the Vibro pile which

results in smaller values of 4D and 8D because the failure planes are also smaller. Therefore

higher values for the qc can be taken. This is a positive aspect of the PCC pile however the tip


104

shape also results in a reduction via the shape factor s because of the non-square/round

shape of the pile tip.

Positive shaft friction

Since the ground level in the Rotterdam area is settling at a rate of approximately 1 cm/year,

negative shaft friction is assumed over the height of the soft soil layers and positive shaft

friction over the foundation layer. The design value of the positive shaft friction is calculated

by:

=⋅⋅⋅=⋅⋅⋅= 10000012.029.3;zcsposzshaft;max;r; qLuF α 935 kN

For the Vibro pile this value is calculated in the same way:

=⋅⋅⋅=⋅⋅⋅= 10000012.0238.0;zcsposzshaft;max;r; qLuF α 90 kN

The circumference of the Vibro pile is much smaller, resulting in a lower shaft bearing

capacity.

Negative shaft friction

The negative shaft friction is calculated with the Zeevaert-De Beer method according

to [12] for pile groups with multiple soil layers, see paragraph 5.2.4

( )∑=

=

−=ni

irepihmrepihnfreps AF

1;;;;;;0;; '' σσ

The values of K0;i;reptanδi;rep for calculation of m as described in paragraph 5.2.4 are assumed

to be 0.5 for sand and 0.25 for clay and peat [38]. Now follows:

reprepihm p ;0;1;;' =−σ 0 kPa (no top load on the soil)

repirephlimrepih h ;1;1;;;;0 ''' γσσ += −

( ) ( ) ( ) ( ) =⋅+−=+−= ⋅−⋅−−− 5.418.05.418.0;0;;

1

'1;1;; 01

18.011

'1'

' 1111 eeeem

hmrephm

hmreprephm σ

γσ 34 kPa

( ) ( ) ( ) ( ) =⋅+−=+−= ⋅−⋅−−− 609.0609.0;1;;

;2;2;; 341

09.02

'1'

' 2222 eeeem

hmrephm

hm

i

reprephm σ

γσ 29 kPa

( ) ( ) ( ) ( ) =⋅+−=+−= ⋅−⋅−−− 609.0609.0;2;;

3

;3;3;; 341

09.010

'1'

' 3333 eeeem

hmrephm

hmreprephm σ

γσ 63 kPa

and:


105

=⋅+=+= 115.40''' ;11;0;;1;;0 reprephlmreph h γσσ 50 kPa

=⋅+=+= 2634''' ;22;1;;2;;0 reprephlmreph h γσσ 46 kPa

=⋅+=+= 10629''' ;33;2;;3;;0 reprephlmreph h γσσ 89 kPa

This results in:

( ) ( ) ( ) ( )( ) =−+−+−=−= ∑=

=

6389294634509.10''1

;;;;;;0;;

ni

irepihmrepihnfreps AF σσ

630 kN

The same method is used for the calculation of the negative skin friction for the Vibro

pile. The area of effect of the Vibro pile is dependent on the pile spacing. For, a centre-to-

centre distance of 1.8 m the negative skin friction becomes:

( ) ( ) ( ) ( )( ) =−+−+−=−= ∑=

=

951084856445024.3''1

;;;;;;0;;

ni

irepihmrepihnfreps AF σσ

85 kN

As expected, the negative shaft friction is also much smaller due to the smaller size of the

Vibro pile’s shaft.

Plugging

Since the piles are placed 2 m in the stiff bearing layer some plugging might occur in

the PCC pile. In [31] the active plug length in a steel pipe pile in sand is considered to be 2-4

times the inner diameter of the pile. It is therefore not guaranteed that installing the PCC pile

2 m into the sand will result into full plugging. In [2] the inner friction model (as described

paragraph 5.2.5) is used to calculate the plug bearing of a pile in sand:

a

L

aaebplug

aaepqθγ

θγ θ ''

−⎟⎟⎠

⎞⎜⎜⎝

⎛+=

with:

Da

aβ

θ4

=

( )( )δϕδϕ

στ

β−Δ+

−Δ==

cossin1sinsin

'vi active friction ratio for the pile at failure

ϕδ

sinsin

sin =Δ


106

( )ppL

pp ep θ

θγ −−= 1'

passive plug bearing

with:

Dp

p

βθ

4=

( )( )δϕδϕ

β−Δ−

−Δ=

cossin1sinsin

p passive friction ratio for the pile at failure

The subscript a and p stand for “active” and “passive” plug. This results in:

32sin32sin

sin =Δ = 1

( )( )

( )( ) =−−−

=−Δ−

−Δ=

3290cos32sin13290sin32sin

cossin1sinsin

δϕδϕ

β p 0.43

( )( )

( )( ) =−+−

=−Δ+

−Δ=

3290cos32sin13290sin32sin

cossin1sinsin

δϕδϕ

β a 0.35

=⋅

==1

35.044D

aa

βθ 1.41 and =

⋅==

143.044

Dp

p

βθ 1.73

( ) ( ) =−=−= ⋅−− 1573.1173.14.7

1'

eep ppL

pp

θ

θγ

4.25 kPa

=−⎟⎠

⎞⎜⎝

⎛ +=−⎟⎟⎠

⎞⎜⎜⎝

⎛+= ⋅

41.110

41.110

25.4'' 2.141.1eepq

a

L

aaebplug

aa

θγ

θγ θ 182 kPa

The results in a value for the plug bearing capacity of:

innerebplugplug uqF ⋅=max; 140 kN

Overview

Table 6-5 gives an overview of the results of the pile bearing capacity calculation. All

results are maximum values..

Table 6-5: Maximum values of bearing capacity calculation case 1;

PCC pile Vibro pile Tip bearing capacity +3800 kN +1200 kN Shaft bearing capacity +935 kN +90 kN Negative skin friction (representative) -630 kN -85 kN Plug bearing capacity +140 kN - Total bearing capacity single pile 4245 kN 1205 kN Concrete volume per m length 0.422m3 0.126m3 Bearing capacity per unit concrete volume 610 kN/m3 580 kN/m3


107

The design value of the bearing capacity is calculated by [12]:

m

maxr;dmax;fund;r;

FF

γξ ⋅=

where

ξ safety factor dependent on number of piles (0.8 in this case)

γm material safety factor (1.25 in this case)

This gives for the design value of the bearing capacity in case 1:

=⋅=⋅=25.1

42458.0

m

maxr;dmax;fund;r;

FF

γξ 2710 kN for the PCC pile and

=⋅=⋅=25.1

12058.0

m

maxr;dmax;fund;r;

FF

γξ 770 kN for the Vibro pile.

As was to be expected based on the size of the piles, the bearing capacity of the single PCC

pile is larger than that of the single Vibro pile. The bearing capacity per unit of concrete

volume of the PCC pile is about 5% higher than that of the Vibro pile. If only concrete volume

is considered in price calculation, the PCC pile is 5% cheaper in this case.

For the PCC pile the representative value of the compression stress in a single

concrete pile also includes twice the negative skin friction:

=⋅+

=⋅+

=42.0

630242452max;max;

pile

nfrep;r;rc A

FFf 13.1 N/mm2

and for the Vibro pile:

=⋅+

=⋅+

=13.0

85212052max;max;

pile

nfrep;r;rc A

FFf 10.6 N/mm2

The design value of the uni-axial compression strength of C12/15 and C30/35

concrete is given by:

ckb ff '6.0' =

where:


108

f’ck characteristic cube compression strength, 15 N/mm2 for C12/15 and 35 N/mm2 for

C30/35.

This gives for the PCC pile:

=⋅== 156.0'6.0' ckb ff 9.0 N/mm2

and for the Vibro pile:

=⋅== 356.0'6.0' ckb ff 21.0 N/mm2

The design value of the bearing capacity for the PCC pile based on the compression strength

of the concrete becomes:

=⋅=⋅= 42.00.9' pilebdmax;fund;r; AfF 3780 kN

This is larger than the design value based on the bearing capacity so the concrete is

sufficiently strong.

For the Vibro pile the design value of the maximum bearing capacity based on the

concrete strength, is calculated in the same way at 1890 kN. This is significantly larger than

the calculated design bearing capacity of 1205 kN.

Tension capacity

Tension due to pull-out

The absolute maximum of the tension force in the PCC pile is equal to the force

required to pull the pile out of the soil.

Since the tension capacity of the pile will differ per project only the order of

magnitude is calculated here. Case 1, as described in paragraph 6.3.2, is used as example

case for the pull-out capacity calculation. The pull out capacity is calculated with equation

(6.1) and the smallest of the dead weight of the soil and the inner shaft bearing capacity

(also calculated with equation (6.1)). The dead weight of the soil is taken as the effective soil

weight since the usability limit state is considered. The calculation value of the pull-out force

for case 1 is given by:

dzpuFL

maxzroutavgpmaxout;tension;r; ∫ ⋅=0

;;;;

( ) ( )( ) =⋅⋅+⋅+⋅+⋅⋅⋅= 210010.065.065.05.40.4020.090.3 2650 kN


109

plus:

dzpuFL

maxzrinavgpmaxin;tension;r; ∫ ⋅=0

;;;;

( ) ( )( ) =⋅⋅+⋅+⋅+⋅⋅⋅= 210010.065.065.05.40.4020.014.3 2140 kN

or:

( )∫ =⋅+⋅+⋅⋅=⋅=L

zsoilinnerweighttensionr dzAF0

;;; 210625.41179.0γ 65 kN

Since Fr;tension;weight < Fr;tension;in the total pull out capacity is 2715 kN.

The maximum tension force in the PCC pile is therefore also taken 2910 kN to

determine the reinforcement required in the pile. Divided over the cross-section of 0.42 m2

this results in a tension of 6.9 N/mm2 in the concrete.

Tension due to bending moment

The single pile bearing capacity in Case 1 is approximately 4245 kN according to the

calculations in paragraph 6.3.2. Assuming and eccentricity of the load of 100 mm (paragraph

6.2.3) and loading of the pile until bearing capacity the maximum moment at the pile head is

given by equation (6.2):

=⋅=⋅= 1004245max; eccmax;headhead uFM 425 kNm

Tension resistance: General

The representative value of the long term uni-axial tension strength of concrete

without reinforcement is related to the characteristic cube pressure strength [42]:

( )ckdb ff '05.0150.0; += (6.3)

where:

fb;d design value of the long term uni-axial tension strength

f’ck characteristic cube pressure strength

The characteristic cube compression strength of the C12/15 concrete used in Case 1 is: 15

N/mm2. This results in a tension strength of:

=⋅+= )1505.01(50.0;repbf 0.9 N/mm2


110

Tension resistance: Pull-out

The tension strength in the concrete when pull-out capacity is reached is 6.9 N/mm2 so the

concrete will fail under pull-out tension.

Using C12/15 concrete the representative value of the maximum tension force

allowable on the unreinforced PCC pile is be given by:

=⋅=⋅= 42.09.0;; pilerepbrepmax;tension AfF 380 kN

Depending on the pile application the tension forces might not exceed this value; in that case

no reinforcement would be necessary. For higher concrete qualities the tension strength of

the PCC pile increases. This might be a viable alternative for reinforcement but attention will

have to be paid to the workability.

Tension resistance: Bending moment

The resistance against tension force without reinforcement in concrete is rather

small. For concrete without any reinforcement the cracking moment when bending occurs is

given in [42] as:

WfM brr ⋅= (6.4)

where:

Mr cracking moment

fbr bending tension strength

W section modulus

The long term bending tension strength is related to the long term uni-axial tension strength

according to [42] with the relation:

( ) 16.1;

>−= hff

db

br (6.5)

The bending strength is larger than the uni-axial tension strength since when rupture occurs

the concrete still keeps some of its strength, only after a certain rupture width (more than

0.15 mm) is reached the stress will drop to zero. Therefore the stress distribution over the

cross-section is not linear and a certain reserve is available. This is proven to be dependent

on the element width [42]. For hollow piles the relationship between fbr and fb;rep is taken

1:1.


111

The section modulus of a concrete tube is defined as:

( )o

iotube R

RRW

44

41 −

= π (6.6)

where:

Wtube section modulus of a tube

Ro outside radius of the tube

Ri inside radius of the tube

Using the values for the bending moment applied to the pile head and the inertia, the

representative value of the occurring bending tension stress can be calculated:

( )=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛⎟⎠

⎞⎜⎝

⎛−⎟⎠

⎞⎜⎝

⎛

=−

=

224.1

200.1

224.1

41

41

44

44

ππo

iotube R

RRW 108E+06 mm3

gives:

=⋅⋅

== 6

6

; 1010810425

tube

maxhead;reqb W

Mf 3.9 N/mm2

The compression stress in the concrete at design load is:

=⋅+

=⋅+

=42.0

630227102;;

pile

nfrep;r;drdc A

FFf 9.4 N/mm2

It can be seen that there will be no tension stress in the pile since the stress due to the

bending moment does not exceed the normal compression stress due to the maximum top

load. However the moment does not only cause a tension stress but also a compression

stress at the opposite side of the pile cross section. This is not entirely correct since the

calculated compression stress only takes place at the edge of the pile and some redistribution

of the stress over the pile width is possible. The compression strength of the concrete can

therefore be a bit lower than the calculated compression stress including bending moment

although never lower than the compression stress due to the top load.. Considering C12/15

concrete and 100 mm eccentricity of the load and considering linear elastic behaviour the

design value of the pile bearing capacity may not exceed:


112

=

⋅+

⋅

=+

=

56 102.41

10108100

91

'

piletube

ecc

bdfund,r;

AWu

fF 2720 kN

6.3.3 Case 2: Piled bridge abutment

Description

To investigate the limits of the applications of the PCC pile a piled bridge abutment is

considered. When a single pile is applied it is subjected to a small vertical load but a large

horizontal force, see figure 6-2.

Figure 6-2: Loads on piled bridge abutment originating from the approach embankment [36];

Since in this case the possibility of applying the PCC pile as abutment foundation is

considered some assumptions are made to allow for a quick scan of the occurring bending

moments and loads on the pile.

A single row of PCC piles is assumed under the abutment. This makes it possible to

schematize the row of PCC piles as a sheet pile wall. This simplification allows for the use of

the GeoDelft program MSheet to model the PCC pile behaviour as an abutment foundation.

Simulation MSheet

Input

The row of PCC piles under the abutment is modelled as a sheet pile wall. The

centre-to-centre distance of the piles is taken as twice the outside diameter thus creating a

spacing between the piles equal to the pile diameter. It is assumed that this spacing is small

enough that no soil can pass between the piles (arching effect) allowing the modelling of the

pile row as a wall. The bending stiffness of the sheet pile wall in the model is per meter wall


113

length equal to half the bending stiffness of the PCC pile. Considering the same pile as in

Case 1 the moment of inertia of the pile can be calculated:

=⋅⋅=⋅=2

124010108 6

outrWI 6.70E+10 mm4

The bending stiffness is also dependent on the pile stiffness, for C12/15 concrete this is

respectively [42]:

=⋅+=⋅+= 1525022250'2502225015/12 ckC fE 2.6E+04 N/mm2

In this calculation the time effects like creep are not taken into account, a reduction of the

pile stiffness for these effects is normally included. The resulting bending stiffness for a

C12/15 PCC pile is:

=⋅⋅⋅= 104 1070.6106.2EI 1.74+06 kNm2

The bending stiffness for the equivalent wall then becomes 8.70E+05 kNm2.

The soil profile is taken comparable to figure 6-2, where a sand embankment is

placed on clayey subsoil. Under the 15 m thick clay layer a stiff sand layer is located. The PCC

pile is embedded 2 m into this sand layer. The abutment is simulated as the continuous

equivalent PCC sheet pile wall to a level of 4 m above ground level. The bridge cover is

modelled as a support allowing rotation of the equivalent wall but blocking any horizontal

movement. An overview of the input is given in figure 6-3.

Sand fill

Clay

Stiff sand

Clay

Stiff sand

PCC equivalent wall

Bridge cover

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30m

-25

-20

-15

-10

-5

0

m

Figure 6-3: Input soil profile MSheet;


114

The soil properties are given in table 6-6 based on typical values from [11] and [3].

Table 6-6: Soil properties MSheet simulation;

Properties Sand fill Clay Stiff sand Unsaturated unit weight [kN/m3] 18.0 17.0 19.0 Saturated unit weight [kN/m3] 20.0 17.0 21.0 Cohesion [kN/m2] 0.0 10.0 0.0 Friction angle [degrees] 32.5 17.5 35.0 Modulus of subgrade reaction 50% [kN/m3] Modulus of subgrade reaction 80% [kN/m3] Modulus of subgrade reaction 100% [kN/m3]

20000 10000 5000

40002000800

40000 20000 10000

Two construction stages are considered. In the initial stage the equivalent wall is

placed and in the final stage the embankment, including inner slope, is placed. For each side

and for each stage MSheet derives the lateral earth pressure coefficients using Culmann’s

method. This method is based on straight slip surfaces and includes the influence of soil

weight, non-horizontal ground surfaces and non-uniform surcharge.

Results

The resulting graphs for the bending moments, shear forces a displacement can be

found in Appendix I. The maximum bending occurring moment is located about 1 m below

the embankment - clay layer boundary and is approximately 165 kNm. This moment is given

per m length of the equivalent sheet pile wall. Since the pile spacing is taken as two times

the pile outer diameter of 1240 mm, each pile is equivalent to 2.48 m wall. The bending

moment on a single pile is then 2.48 times 165 equals 410 kNm. The tension force in the

concrete resulting from this bending moment is:

=⋅⋅

==6

6

; 1010810410

tube

dmax;db W

Mf 3.8 N/mm2

In the governing situation there is no load placed on the pile and so no compression force is

present in the concrete so there is no reduction of the tension force. The tension capacity of

the C12/15 concrete is calculated as 0.90 N/mm2 so the concrete cannot sustain this tension

force without additional reinforcement.

6.3.4 Conclusion

The high bending stiffness and section modulus of the PCC pile reduce the tension

forces in the pile. No reinforcement is therefore necessary when the PCC pile is applied in a

foundation with an eccentric load.


115

However when applied as the foundation of an bridge abutment or as a tension pile

the pile will sustain much larger tension forces and reinforcement will be necessary.

6.4 Reinforcement

6.4.1 Introduction

For application of the PCC in foundations the main change in the requirements is the

addition of a resistance against tension forces in the concrete. Since concrete has a low

tension resistance steel reinforcement is normally applied to bear the tension forces in

concrete constructions.

The nature of the PCC pile, with its thin walls makes application of reinforcement

more of a challenge than in solid piles. In this chapter three possible reinforcement methods

are described and in the next chapter the methods will be evaluated. The methods are:

• Traditional steel reinforcement,

• High tensile alloy steel (Dywidag) and

• Fiber reinforcement (steel and carbon).

6.4.2 Traditional reinforcement

For installation of Vibro piles which are cast in place, round concrete piles for which

the installation method is comparable to the PCC pile, reinforcement netting is used which

includes a number of steel bars connected and kept in place with steel rings and/or spirals

[43], see figure 6-4

Spiral ∅ 6 mm

6 xl ∅ 16 mm

0.15 m

Ring ∅ 10 mm

Figure 6-4: Typical reinforcement in Vibro pile [43];


116

In [42] some notes for the application of reinforcement steel are given. It is advisable to use:

• As large as possible steel diameters and

• As little different steel diameters as possible.

The minimal distance between the bars should be:

• 4/3 of the largest grain diameter of the aggregate,

• the largest bar diameter or

• 30 mm for beams and columns, 25 mm for floors and 50 mm for walls.

This to avoid the formation of gravel “nests” between the bars.

A certain concrete cover is required to prevent the steel of being damaged by the

groundwater. The concrete cover is determined by the environment, in a salt or otherwise

hostile environment more cover is required. In [15] an overview of the environmental classes

and corresponding covers is given, partly shown in table 6-7.

Table 6-7: Concrete cover for a selection of safety classes [15];

Environment Class Cover* Column, no risk of corrosion X0 - Column in contact with water, seldom dry XC2 35 mm Column in contact with water with chlorides other than from sea, seldom dry XD2 40 mm

Column in contact with water with chlorides from sea, seldom dry XS2 40 mm

*another 5 mm is added when the concrete surface is: treated after construction, not checked or has a characteristic

cube pressure strength of less then 25 N/mm2 (superposed if more than one condition occurs)

In [42] and overview is given of the commonly used reinforcement steel properties,

see table 6-8.

Table 6-8: Properties of reinforcement steel [42]

Concrete steel type fsrep N/mm2

fs N/mm2

εsu

% FeB 220 HWL FeB 400 HWL, HK FeB 500 HWL, HK FeB 500 HKN

220 400 500 500

190 350 435 435

5.004.003.252.75

FeB 500 HKN, HWN 500 435 2.75

The modulus of elasticity of the steel is Es = 2.0E105 N/mm2 and the steel is available in

diameters of 6,8,10,12,16,20,25,32 and 40 mm [42].

6.4.3 Dywidag bars

Dywidag is a high tensile alloy steel bar which features a coarse thread over its full

length [9]. The system is developed by Dywidag-systems international and has a wide range

of applications including reinforcement. The application is essentially the same as normal

reinforcement but features much higher tensile strength bars, see table 6-9.


117

Table 6-9: Technical date for Dywidag steel threadbar

Nominal diameter

[mm]

Steel grade

[N/mm2]

Ultimatestrength

[kN]

Yield strength

[kN] 15 900/1100 195 159 20 900/1100 345 283

26.5 900/1050 579 523 32 900/1050 844 764 36 900/1050 1069 967 40 900/1050 1320 1194 47 900/1050 1822 1648

The modulus of elasticity of the used alloy is 2.05E105 N/mm2 and like normal steel bars the

Dywidag bars need to be protected against corrosion.

A large disadvantage of high quality steel bars is that they are very susceptible to

corrosion in aggressive climates, like groundwater. Therefore only small ruptures in the

concrete are allowed which occur much faster than the yield strength of the bar is reached.

This makes this type of solution hard to apply in foundation piles.

6.4.4 Fibre reinforcement

General

An alternative for the use of steel bars is the use of steel fibres instead. Steel fibres

are used when installing the reinforcement netting is difficult or time expensive [8].

The first purpose of steel fibre reinforcement is the strengthening of the cement

matrix (micro scale). Fibres lying in different directions in the concrete will prevent micro

cracks from forming. The effect depends on the size and the concentration of the fibres. Non-

metal fibres function better in this way because they are less susceptible to temperature

changes. The second purpose is reinforcement (macro scale). The reinforcement is based on

the effect that the fibres will slip through the concrete and not break [37].

Composition

Steel fibres can be seen as an extra supplement to the concrete. It has certain

influences on the design of the mixture [22]:

• The fibre length should be chosen such that the L/3 or L/2 is not smaller than the

aggregate diameter to avoid the forming of clumps of fibres.

• The sand diameter influences the ability to pump the mixture. A small diameter (max 2

mm) makes it easier to pump the mixture and thus increases the tension burst strength.

A diameter of 4 mm or larger prevents the sinking of the fibres during compaction.

• 10% more cement is advised because the fibres disturb the packing of the grains so

more cement is required to fill the hollow spaces.


118

• The maximum percentage of fibres is determined by the workability of the concrete. The

maximum lies around 50-100 kg/m3.

• More water increases the workability of the concrete but decreases the strength and

durability.

The ACI commission (1993) gives recommendations for concrete mixtures with steel

fibres, see table 6-10.

Table 6-10: ACI recommendation for steel fiber concrete mixture [22]

Particle size Mixture characteristics dmax= 10 mm dmax= 20 mm dmax= 38 mm Water/cement factor 0.35-0.45 0.35-0.50 0.35-0.55 Cement (kg/m3) 360-600 300-540 280-420 Fine material/total (%) 45-60 45-55 40-55 Air content (%) 4-8 4-6 4-5 Vf straight fibers (%) 0.8-2.0 0.6-1.6 0.4-1.4 Vf deformed fibres (%) 0.4-1.0 0.3-0.8 0.2-0.7

Strength

The strength characteristics of fibres reinforced concrete are [22]:

• The fibres influence the shear strength of the concrete by working as a dowel (“bridging”

effect) and forcing the shear to develop over a number of small cracks.

• The compression strength of the fibre concrete will be the same as that of normal

concrete although the fibres will reduces the damage at failure. The “bridging” of the

fibres lets the concrete maintain some strength after failure occurs.

• The tension strength of the concrete increases, according to CUR recommendation 36, to

1.05 times the tension strength of normal concrete. The tension strength after cracking

of the concrete is much higher than that without fibres.

• The bending tension strength of fibre concrete will be approximately 35 % higher than

concrete without reinforcement. The value of the bending toughness (the surface under

the load-deformation graph after reaching the bending tension strength) is up to 40 times

that of normal concrete, due to the “bridging” effect.

• The modulus of elasticity of the fibre concrete will not change compared to normal

concrete.

• Concrete creep is not changed by adding fibres to the concrete.

Due to the fact that the fibers are distributed evenly in the concrete some damage to

the fibres at the edges of the fibre concrete element can be expected. Because the fibres are

small and unconnected, the rusting of the fibres will not lead to large damages [22].

CUR 35 provides a method for calculation of bending strength at breaking and

bending strength after breaking based on a three point bending test. The ultimate bending

strength, Fu, is determined from the F-δ graph as the highest value of F in an area left of a

line parallel (0.05 mm to the right) to the graph before failure, see figure 6-5.


119

Steel fiber concrete Steel fiber concrete

Figure 6-5: Breaking load Fu-[22];

The equivalent breaking bending strength is determined from the test-setup and the ultimate

bending strength:

2; 5.1bhF

f uequctb = (6.7)

The characteristic crack bending tension strength is determined according to [22]:

pavgequctbcharctb Skff ⋅−= ;;; (6.8)

where:

fctb,equ,avg average result from the tests

k 1.64 for 5% chance of exceeding

Sp standard deviation of the test results

The calculation value of the tension strength is determined by dividing fctb,char by a material

factor. The breaking toughness (Df) is determined by the surface under the F-δ graph, see

figure 6-2, between the zero and deflections δ2 and δ3:

22,

2,,5.1

5.1bh

Df f

equftb⋅

= (6.9)

23,

3,,0.3

5.1bh

Df f

equfctb⋅

= (6.10)


120

where:

Df,2 surface under the load-deflection graph “0” to δ2 = 1.5 mm

Df,3 surface under the load-deflection graph “0” to δ3 = 3.0 mm

Steel fiber concrete Steel fiber concrete

Figure 6-6: Definition of the breaking energy and the toughness [22];

The characteristic equivalent bending strength after breaking forming follows from:

pavgequfctbcharequfctb Skff ⋅−= 3/2,,,3/2,,, (6.11)

The design value of the tension strength is determined by dividing ffctb,char by a material

factor.

The modulus of elasticity of the fibre concrete is [37]:

mmffc VEVEE += 21ηη (6.12)

where:

Ec modulus of elasticity of composite material

η1 efficiency factor dependent on fibre orientation

η2 efficiency factor dependent on fibre length

Ef fibre modulus of elasticity

Vf volume fraction of the fibres

Em concrete modulus of elasticity

Vm volume fraction of the concrete


121

Attention points and experience

Fibre ball

During addition of the steel fibres the stiffness of the concrete mix increases and a

fibre ball can be formed. It is advisable to limit the number of fibres per unit of volume to

limit this effect. For the popular Dramix fibre the allowable fibres per unit volume, dependent

on the L/D ratio of the fibre and the concrete particle size, are given in Table 6-11.

Table 6-11: Maximum allowable fibres per unit of volume for the Dramix fibre [37];

Concrete dmax

L/D = 60 normal

L/D = 60 pumping

L/D = 75 normal

L/D = 75 pumping

L/D = 100 normal

L/D = 100 pumping

4 mm 160 kg/m3 120 kg/m3 125 kg/m3 95 kg/m3 95 kg/m3 70 kg/m3 8 mm 125 kg/m3 95 kg/m3 100 kg/m3 75 kg/m3 75 kg/m3 55 kg/m3 16 mm 85 kg/m3 65 kg/m3 70 kg/m3 55 kg/m3 55 kg/m3 40 kg/m3 32 mm 50 kg/m3 40 kg/m3 40 kg/m3 30 kg/m3 30 kg/m3 25 kg/m3

Fibre concrete in Vibro piles

For the application of Vibro piles in the soft soil layers in the west of the Netherlands,

Mebin has designed a polypropylene fibre mortar (Fibrecrete) to increase the coherence of

the concrete and prevent large diameters from forming. It is investigated of the function of

the reinforcement netting in Vibro-piles can be taken over by a type of Fibrecrete. Tests were

done with synthetic fibre concrete and steel fibre concrete. The following conclusions were

drawn [8]:

• The piles where of excellent quality, no de-mixing of the concrete has taken place for

piles of 300 mm diameter and 16 m length, although a central steel bar was used (to pull

out the piles).

• The tested cubes of synthetic fibre and steel fibre concrete had the same pressure

strength properties as normally used concrete.

• The soil pressure at the pile tip is essentially identical for the fibre concretes compared to

the reference piles (made with the usual method).

• The experiments has proven that the steel fibre reinforced cross sections (30 kg RC-

80/60-BN) have equal or higher bending moment capacity and guarantee the prismatic

dimensions of the pile.

• The bending moment capacity of a steel fibre concrete pile (30 kg RC-80/60-BN) is

comparable to a traditional Vibro pile with 4 Ø 10 mm steel reinforcement bars with a

cover of 40 mm.

6.5 Design changes

6.5.1 Introduction

An overview of the required design changes to the PCC pile for application in building

foundations and bridge abutments is given in this chapter. The required changes are based


122

on the case studies in chapter 6.3 and take into account the reinforcement, the pile

properties and the required equipment.

6.5.2 Reinforcement

For an estimation of the required reinforcement the pull-out capacity as calculated in

Case 1 and the bending moment calculated in Case 2 are taken. This gives tension in the

concrete of respectively 6.9 N/mm2 and 2.7 N/mm2.

Traditional reinforcement

The traditional reinforcement consists of a number of vertically placed steel bars kept

in place using steel rings or spirals. When traditional reinforcement steel bars of Ø16 mm FeB

500 steel are applied, corresponding to the Vibro pile reinforcement, the required number of

bars is given in table 6-12.

Table 6-12: Required traditional reinforcement;

Case Tension force

Required steel area

Number of bars

Bar spacing

Tension pull-out 6.9 N/mm2 6660 mm2 34 100 mm Bending moment 2.7 N/mm2 2600 mm2 14 240 mm

The bar spacing more than sufficient to meet the demands of [15] although a large

number of bars need to be applied. The concrete cover if the bars are placed in the middle of

the pile wall is given by:

=−−

=−

=2

16100012402

barwallcover

dwd 112 mm

which is more than the required 40 mm [16].

Advantages of this solution are:

• Common solution with lots of experience and

• Low cost.

Disadvantages of this solution are:

• Large reinforcement netting required and

• Difficulty in placing the reinforcement after concreting in long and slim PCC pile walls

or

• Possible de-mixing of concrete due to reinforcement netting when netting is present

during concreting.


123

Dywidag bars

The Dywidag bars have larger tension strength than the traditional reinforcement

bars and the required number of bars is therefore smaller. Considering 15 mm Dywidag bars

the results are given in table 6-13.

Table 6-13: Required Dywidag reinforcement;

Case Tension force

Number of bars

Bar spacing

Tension pull-out 6.9 N/mm2 19 190 mmBending moment 2.7 N/mm2 8 450 mm

As with the traditional reinforcement the pile spacing and concrete cover is more than

sufficient.

Advantages of this solution are:

• Smaller number of bars required

Disadvantages of this solution are:

• Higher single bar cost,

• Highly susceptible to corrosion,

• Large reinforcement netting and

• Difficulty in placing the reinforcement after concreting in long and slim PCC pile walls

or

• Possible de-mixing of concrete due to reinforcement netting when netting is present

during concreting.

Fibre reinforcement

The use of fibre concrete gives only a limited extra tension strength, about 5% of the

un-reinforced concrete tension strength, of the concrete [22] but allows for a larger

remaining tension strength after cracking of the concrete.

The fibre reinforced bending strength will increase by approximately 35% of the

concrete strength using fibre concrete. The exact value of the bending strength is determined

based on load tests according to the most commonly used methods [22]. The PCC pile

executed with C20/25 concrete would then have a tension capacity of:

=⋅+⋅=⋅= )2505.01(5.005.105.1 ;repbbf ff 1.18 N/mm2

and a bending strength of

=⋅+⋅=⋅= )2505.01(5.035.135.1 ;repbbf ff 1.51 N/mm2


124

These values are estimations, the real tension strength and bending moment should be

calculated based on load tests on the improved concrete.

Advantages of this method are:

• No reinforcement netting required which lessens the chance of de-mixing and arching,

• No changes to the casing for reinforcement placement and

• Less loss of tension strength after rupture of the concrete.

Disadvantages are:

• Less experience with this method,

• Not a very large increase in tension and bending capacity and

• Reinforcement of the concrete in all directions instead of only the required direction gives

economical loss since more steel is required.


Case 1 has shown that the compression strength of C12/15 concrete is insufficient to

sustain the pressure in the concrete at a load close to the maximum bearing capacity of the

pile. It is therefore advisable to make the pile out of C20/25 concrete which has sufficient

strength. However it should be noted that the lower fluidity of stronger concrete might cause

problems for the concrete to reach all areas of the casing, especially when also reinforcement

is applied, see Part 2.

Tests will have to be executed to verify the continuity of the pile cast with higher quality

concrete. Enlarging the wall thickness facilitates the flow of the concrete but increases the

concrete use.

6.5.4 Equipment

Changes in casing design are only required when reinforcement steel bars are applied. In

that case the reinforcement has to be connected to the casing or lowered into it after

vibration of the casing. This can cause problems with both the flaps that close off the casing

and with the spacers that keep the distance between the inner and outer casing wall

constant, see Part 2. If a small number of bars are applied, for example the Dywidag system,

it might be possible to combine the function of spacing the inner and outer casing and

holding the bars in place in one system, see figure 6-7.


125

Inner wall

Outer wall

Reinforcement bar

Vertical cross section Detail, top view

Figure 6-7: Possible reinforcement frame;

In the figure the reinforcement bar is placed through the holes in all the spacers and is left

behind during retraction of the casing ensuring no displacement of the bars and sufficient

concrete cover.

6.6 Conclusion

Based on the discussed cases it is concluded that the PCC piling system is a system that

can compete with the Vibro piling system as a foundation piling system based solely on

bearing capacity per unit of concrete volume, although a stronger type of concrete that the

currently used C12/15 is needed to insure sufficient concrete compression strength. C20/25

concrete is sufficient in the example cases.

The high bending stiffness of a single pile as a result of its tube shape make the system

resistant against bending moments and in some cases reinforcement of the concrete is not

required.

When reinforcement would be required difficulties in execution of the pile occur due to

the thin walls and current design of the installation casing. Reinforcement netting to ensure

the distance between the bars and the wall and between the bars themselves is difficult to

place after concreting due to the small wall thickness and potentially long pile length. Placing

the netting before concreting can cause de-mixing of the concrete and concrete quality in

lower parts of the pile. High strength bars are difficult to apply in this environment but the

application of fibre concrete might be a solution that does not have this problem. This is a

field for further research.


126

Changes to the installation equipment are expected to be required, especially when steel

bar reinforcement is used. A possibility might be the combination of wall spacers and

reinforcement bar distance holders.


127

7 Conclusions and recommendations

In this final chapter the conclusions of the report and the recommendations for

further research are given.

7.1 Conclusions

For the conclusion we look back to the beginning of the report where the research

question and the different aspects that would be looked at are stated. The purpose of the

report was to determine if the PCC pile is viable in the Netherlands as a ground improving

method and/or as a foundation pile. The technical and economical feasibility needed to be

determined including and an analysis of the execution method, the calculation method and a

comparison with existing ground improving methods and foundation piles.

It is concluded that the PCC piling method is a robust and viable method for ground

improvement. The studied tests and observations in the field show that the method is

successful in reducing settlement of a highway embankment on soft soil in the Chinese

practice.

Analysis of the execution method shows a number of attention points in the different

construction phases that can possibly cause problems with the pile quality when not sufficient

attention is paid during pile construction. Possible demixing of the concrete mixture during

casting and vibration damage caused by nearby pile installation are two of these attention

points.

To determine the viability of the PCC in the Netherlands a comparison with two new

ground improving piles developed in the Netherlands, the HSP and the AuGeo pile, is made.

The PCC pile appears to be a competing solution based on the installation time, required

amount of concrete and bearing capacity. Both the HSP and AuGeo systems however allow

for installation of steel reinforcement while this is not yet possible for the PCC pile. The PCC

pile is especially suitable when thick soft soil layers are present where its higher shaft bearing

capacity becomes a significant advantage.

To extend the basic Chinese calculation method a calculation method for the PCC pile

is derived from the Dutch norms and recommendations for plugging calculations. This

calculation method is verified based on load tests on a single pile executed in China. The

results of the calculation method according to the Dutch norms give a good approximation for

the single pile bearing capacity.

Further applications of the PCC pile as a foundation pile and as a bridge abutment

are discussed. Comparison of the PCC pile with the and a commonly used Dutch cast-in-place

pile, the Vibro pile, which is commonly used in the Netherlands gives a slightly better ratio of

concrete to bearing capacity. The high bending stiffness and section modulus of the PCC pile


128

due to its unique shape make it also suitable for application in bridge abutments with

relatively little reinforcement necessary.

Due to the shape and installation method of the PCC pile traditional steel bar

reinforcement is difficult to apply. A number of solutions for reinforcement of the PCC pile are

discussed; both the Dywidag system and fibre concrete are solutions for strengthening the

thin wall of the PCC pile against tension forces. Some changes to the design of the PCC pile

and the installation equipment are necessary.

7.2 Recommendations

The next step in the introduction of the PCC pile in The Netherlands might be the

installation of a number of test piles by a Dutch contractor. This is necessary to familiarize

with the equipment and get some experience with possible problems during installation. This

can best be done in cooperation with a Chinese contractor that already has experience with

the method.

Possible problems during construction mainly lie with the concreting where the

integrity of the wall will have to be ascertained. The thin wall combined with the large contact

area, even larger when reinforcement is applied, creates a risk of inconsistencies in the pile.

The PCC pile is competing with the Dutch soil improvements pile because of its large

single pile bearing capacity and the large centre-to-centre distance which results in a fast

construction of the project. It has to be verified if a centre-to-centre distance as applied in

China is also allowed on soils in the Netherlands. The viability of the PCC is dependent on

this.

Additional testing will have to be done for determining the PCC pile applicable for

building foundations. The connection between building and pile needs to be detailed and

detailed calculations for and design of the reinforcement is needed.

Verification of the bearing capacity calculation method by additional load tests in the

Dutch soil is also advised. Tests to verify the plug bearing capacity are also advised since little

experience with the calculation of plugging in clay and sand is available.


129

References

1. Bles, T. Risks during execution of pilefoundations: descision model for

identification, estimation and weighing of risks (Risico’s bij uitvoering

paalfunderingen; Beslissingsmodel, door identificatie, inschatting en afweging

van risico’s), Master Thesis Delft University of Technology, 2003.

2. Centre for Civil Engineering Research and Codes, Bearing capacity of steel

pipe piles, CUR 2001-8, 2001

3. Centre for Civil Engineering Research and Codes, Building with soil: Soil

construction on and in weak and compressible subsoil, CUR 166, 1992.

4. Centre for Civil Engineering Research and Codes, Design rules for tension

piles, CUR 2001-4, 2001.

5. Centre for Underground Construction (COB), Manual for underground

construction part 2: gravel and sand columns (Handboek ondergronds

bouwen deel 2 – grind en zandkolommen), Balkeman, 2000.

6. Cofra, Cofra website, www.cofra.com

7. Cortlever, N.G., Design of double track railway Bidor-Rawang on AuGeo Piling

system according to BS8006 and PLAXIS numerical analysis.

8. Dekker, L.J.G., Hoekstra, A., Nohl, W. Fibre concrete in Vibro piles

(Vezelbeton in vibropalen), Cement, 2004

9. DSI, Dywidag Prestressing steel Threadbar system

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Compression piles, NEN 6743, 1991.

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1991.

14. Dutch Norm Institute, TGB 1990: Technical principals for building codes

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http://www.cofra.com/�


130

pile the state of practice. Part II Application

20. GeoHohai, Photo database GeoHohai

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of stress wave theory to piles, Balkema Rotterdam, 2000.

24. Matsumoto, T., Sekiguchi, H., Shibata, T., Fuse, Y., Performance of steel pipe

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131

on soft clay for loading from lateral soil movements, Geotechnique 44, 1994.

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132

Appendices

The PCC piling method: Appendices

Appendix A: Settlement chart from NEN6743 133

dtipmaxr

dtipr

FF

,,.

,,

eq

dtip

Dw ,

Relationship between Fr;max;tip;d in percents of Fr;max;tip;d and wtip;d in percents of Deq (1=ground

displacing pile, 2=augercast pile and 3=drilled pile);

dshaftmaxr

dshaftr

FF

,,,

,,

dtipw ,

Relation between Fr;shaft;d in percents and Fr;max;shaft;d and wtipt in millimetres


Appendix B: Design charts for the group efficiency 134

In the first graph the value of e1 is based on typical values for the pile geometry and soil

properties: Ep/Gl = 1000, s/d = 3, ρ = 0.75, υ = 0.3. For a known slenderness ratio the

value of e1 can be determined. This value can then be corrected with the second graph.


Appendix C: Theoretical values for the settlement ratio 135

Table C-1: Theoretical values for the settlement ratio Rs for friction pile groups with rigid cap in a deep

uniform soil mass;

Rs values for other numbers of piles may be interpolated from this table. For groups of more

than 16 piles it has been found that Rs varies approximately linearly with the square root of

the number of piles in the group:

( )( ) 255.0

1625 5 RnRRRs +−−=

where:

R25 value for Rs for 25-pile group

R16 value for Rs for 16-pile group

n number of piles in the group

The pile stiffness factor is defined as:

As

p REE

K =

with:

( )4/2dAR P

A π=



Reduction factors for settlement ratio

The influence of the presence of a rigid layer on the settlement of typical pile groups

is shown in figure C-1

Figure C-1: Reduction coefficient for effect of finite layer (Poulos and Davis (1980))

The influence of the Poisson’s ratio on the settlement ratio is shown in figure C-2, it

can be seen that the effect becomes more pronounced for larger number of piles.



Figure C-2: Correction factor for effect of Poisson’s ratio (Poulos and Davis (1980));

The influence of the distribution of the soil modulus for a typical situation is shown in

figure C-3. The difference between uniform and non uniform soil become larger when the

number of piles increases.

Figure C-3: Effect of distribution of soil modulus on settlement ratio (Poulos and Davis (1980))


Appendix D: Physical and mechanical soil properties Yan-Tong 138

Physical mechanic property parameters of soil before and after pile driven (A—after driven,

B—before driven)

Plas

ticity

lim

it

Wat

er c

onte

nt

[％]

Nat

ural

dens

ity γ

soil

[g/c

m3 ]

Dry

den

sity

γdr

y

[g/c

m3 ]

Satu

rate

dens

ity

γ sat [

g/cm

3 ]

Void

rat

io e

[-]

Spec

ific

grav

ity

γ soi

l/ γ w

ater

[-]

Liqu

id li

mit

76g

[％]

Plas

tic li

mit

[％]

plas

ticity

inde

x

[-]

liqui

dity

inde

x

[-]

Coef

ficie

nt o

f

com

pres

sibi

lity

a v

[MPa

-1]

B A

B A

B A

B A

B A

B A

B A

B A

B A

B A

B A

1.1-

1.3

1.

2 6 . 5

30.0

1.

92

1.91

1.

52

1.47

92

96

0.

779

0.84

52.

70

2.71

28

.8

31.7

20

.5

19.3

8

12

0.72

0.

86

0.23

0.

39

4.1-

4.5

40.8

37

.4

1.79

1.

83

1.27

1.

33

98

98

1.12

4 1.

027

2.70

2.

70

34.3

31

.1

26.5

21

.5

8 10

1.

83

1.66

0.

92

0.51

5.3-

5.5

39.7

39

.1

1.81

1.

81

1.30

1.

30

99

98

1.08

4 1.

075

2.70

2.

70

33.1

31

.4

25.1

23

.3

8 8

1.83

1.

95

0.68

0.

51

8.3-

8.7

34.0

37

.4

1.87

1.

87

1.40

1.

36

98

100

0.93

5 0.

991

2.70

2.

71

35.2

33

.4

27.6

20

.1

8 13

0.

84

1.3

0.22

0.

30

11.3

-11.

7 41

.4

40.8

1.

82

1.79

1.

29

1.27

10

0 98

1.

098

1.13

22.

70

2.71

31

.5

33.3

23

.3

20.4

8

13

2.21

1.

58

0.73

1.

06

15.8

-16.

0 31

.4

31.8

1.

84

1.95

1.

40

1.48

90

10

0 0.

95

0.83

22.

73

2.71

51

.4

36.3

29

.4

21.6

22

15

0.

09

0.69

0.

51

0.27

18.5

-18.

7 29

.4

1.

90

1.

47

95

0.82

5

2.68

27.7

23.2

5

1.38

0.07

0.

09

21.2

-21.

4 29

.1

1.

92

1.

49

97

0.80

9

2.69

30.0

25.5

5

0.80

0.11

K31+

534

24.7

-24.

9 31

.2

1.

90

1.

45

98

0.85

8

2.69

31.5

25.1

6

1.02

0.19

0.8-

1.0

24.6

1.85

1.48

81

0.

818

2.

70

28

.0

19

.7

8

0.

59

0.

33

3.2-

3.4

29.8

1.92

1.48

97

0.

825

2.

70

29

.4

21

.5

8

1.

05

0.

25

5.2-

5.4

39.3

1.28

1.28

96

1.

105

2.

69

32

.1

26

.1

6

2.

20

0.

94

10.2

-10.

4 38

.5

1.

35

1.

35

10

0

1.00

0

2.70

31.8

21.1

11

1.

63

0.

37

12.2

-12.

4 42

.0

1.

26

1.

26

99

1.14

2

2.70

31.9

22.0

10

2.

02

0.

87

15.2

-15.

4 28

.9

1.

44

1.

44

98

0.88

5

2.72

46.3

26.4

20

0.

13

0.

27

K30+

793

18.7

-18.

9 26

.9

1.

46

1.

46

86

0.84

5

2.69

27.6

21.0

7

0.89

0.23

prop

erty

pile

de

pth

（m）


Appendix D: Physical and mechanical soil properties Yan-Tong 139

Perm

eabi

lity

coef

ficie

nt

CU t

est

Mod

ulus

of

com

pres

sibi

lity

[Mpa

]

pre-

cons

olid

atio

n

pres

sure

[kPa

]

Com

pres

sion

inde

x

[-]

rela

xatio

n in

dex

[-]

Coef

ficie

nt o

f

cons

olid

atio

n

100k

Pa

[*10

-3cm

2/s]

Coef

ficie

nt o

f

cons

olid

atio

n 20

0kPa

[*10

-3cm

2/s]

ve

rtic

al *

10-6

[cm

/s]

horiz

onta

l

[cm

/s]

Cohe

sion

[kPa

]

Inte

rnal

fric

tion

angl

e

[°]

effe

ctiv

e co

hesi

o

[kPa

]

B A

B A

B A

B A

B A

B A

B A

B A

B A

B A

1.1-

1.3

7.72

4.

54

420

157

0.07

7 0.

20

0.01

27

0.01

8 9.

43

2.49

0.

13

2.49

7.

23

1.48

4.1-

4.5

2.3

3.78

11

0 15

9 0.

307

0.26

0.

0127

0.

025

10.1

7 4.

31

0.13

4.

31

2.58

4.

37

5.3-

5.5

3.05

3.

85

165

127

0.22

7 0.

25

0.01

73

0.01

7 9.

39

4.8

0.13

4.

8 2.

6.

54

0.75

7

8.3-

8.7

8.77

6.

34

335

173

0.07

3 0.

18

0.01

09

0.01

9 4.

21

2.78

0.

13

2.78

1.

37

1.10

11.3

-11.

7 2.

87

1.94

11

0 12

0 0.

243

0.43

0.

0188

0.

035

4.55

4.

61

0.13

4.

61

9.08

15.8

-16.

0 3.

85

6.60

28

0 24

7 0.

168

0.24

0.

0457

0.

023

7.84

3.

76

0.13

3.

76

0.02

57

18.5

-18.

7 25

.64

23

5

0.02

4

0.00

58

3.

46

0.

14

46

.5

21.2

-21.

4 15

.87

27

5

0.03

8

0.00

51

7.

8

0.06

7

79.1

1.

K31+

5

34

24.7

-24.

9 10

.00

32

4

0.06

2

0.00

93

5.

63

0.

13

8.

74

0.8-

1.0

5.34

240

0.

174

0.

01

3.

72

4.

03

5.

73

40

.9

53.8

19

.5

18.0

30

.5

20.5

3.2-

3.4

7.05

290

0.

164

0.

019

4.

61

5.

07

1.

46

86

.4

37.4

15

.9

27.1

28

.2

35.5

5.2-

5.4

2.06

80

0.

292

0.

292

4.

61

3.

45

1.

68

3.

5 78

.2

25.9

34

.2

22.3

66

.8

10.2

-10.

4 5.

41

19

4

0.20

4

0.20

4

3.83

3.94

1.51

32.4

19

.9

13.2

15

.0

16.1

11

.9

12.2

-12.

4 2.

38

68

0.42

0

0.42

0

4.66

3.58

2

4.98

50.8

1.

4 10

.1

17.9

39

.4

19.7

15.2

-15.

4 6.

77

27

0

0.25

7

0.25

7

4.65

2.97

2.75

46.3

38

.9

15.7

22

.7

28.8

8.

3

K30+

793

18.7

-18.

9 8.

83

27

8

0.12

0

0.12

0

1.86

1.66

1.21

9.3

25

41.7

30

.7

0.0

4.1

prop

erty

pile

dept

h

[m

]


Appendix E: CPT Yan-Tong project 140

q_cII_a q_cI_a

q_cIII_a

pile level

8D level

4D level


Appendix F: CPT Rotterdam 141


Appendix G: CPT Rotterdam with 4D8D PCC pile 142

Pile level

8D level

4D level

qc;III = 13.0

qc;II = 14.5

qc;I = 16.5


Appendix H: CPT Rotterdam with 4D8D Vibro pile 143

Pile level

8D level

4D level

qc;III = 5.0

qc;II = 10.2 qc;I = 12.0


Appendix I: Results MSheet calculation 144

the pcc piling method - tu delft

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