in-situ testing of a carbon/epoxy isotruss reinforced
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Brigham Young University Brigham Young University
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Theses and Dissertations
2006-04-14
In-Situ Testing of a Carbon/Epoxy IsoTruss Reinforced Concrete In-Situ Testing of a Carbon/Epoxy IsoTruss Reinforced Concrete
Foundation Pile Foundation Pile
Sarah Richardson Brigham Young University - Provo
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IN-SITU TESTING OF A CARBON/EPOXY ISOTRUSS
REINFORCED CONCRETE FOUNDATION PILE
by
Sarah Richardson
A thesis submitted to the faculty of
Brigham Young University
in partial fulfillment of the requirements for the degree of
Master of Science
Department of Civil and Environmental Engineering
Brigham Young University
April 2006
BRIGHAM YOUNG UNIVERSITY
GRADUATE COMMITTEE APPROVAL
of a thesis submitted by
Sarah Richardson
This thesis has been read by each member of the following graduate committee andby majority vote has been found to be satisfactory.
Date David W. Jensen, Chair
Date Kyle M. Rollins
Date Fernando S. Fonseca
BRIGHAM YOUNG UNIVERSITY
As chair of the candidate’s graduate committee, I have read the thesis of SarahRichardson in its final form and have found that (1) its format, citations, andbibliographical style are consistent and acceptable and fulfill university anddepartment style requirements; (2) its illustrative materials including figures, tables,and charts are in place; and (3) the final manuscript is satisfactory to the graduatecommittee and is ready for submission to the university library.
Date David W. JensenChair, Graduate Committee
Accepted for the Department
E. James NelsonGraduate Coordinator
Accepted for the College
Alan R. ParkinsonDean, Ira A. Fulton College of Engineeringand Technology
ABSTRACT
IN-SITU TESTING OF A CARBON/EPOXY ISOTRUSS
REINFORCED CONCRETE FOUNDATION PILE
Sarah Richardson
Department of Civil and Environmental Engineering
Master of Science
This thesis focuses on the field performance of IsoTruss R©-reinforced concrete
beam columns for use in driven piles. Experimental investigation included one
instrumented carbon/epoxy IsoTruss R©-reinforced concrete pile (IRC pile) and one
instrumented steel-reinforced concrete pile (SRC pile) which were driven into a clay
profile at a test site. These two piles, each 30 ft (9 m) in length and 14 in (36 cm) in
diameter, were quasi-statically loaded laterally until failure. Behavior was predicted
using three different methods: 1) a commercial finite difference-based computer
program called Lpile; 2) a Winkler foundation model; and, 3) a simple analysis
based on fundamental mechanics of materials principles.
Both Lpile and Winkler foundation model predictions concluded that the
IRC pile should hold approximately twice the load of the SRC pile. Applying
mechanics of materials principles found the predicted stiffness of the piles to be
consistent with the laboratory results. Due to unresolveable errors, experimental
field test data for the SRC pile is inconclusive. However, analysis predictions in
conjunction with field test data for the IRC pile show that the IRC pile should
perform similar to predictions and laboratory test results. Therefore, IsoTruss R©
grid-structures are a suitable alternative to steel as reinforcement in driven piles.
Table of Contents
List of Tables x
List of Figures xi
1 Introduction 1
1.1 Brief History of Reinforced-Concrete . . . . . . . . . . . . . . . . . . 2
1.2 Driven Piles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Introduction to the IsoTruss R© . . . . . . . . . . . . . . . . . . . . . 4
1.3.1 IsoTruss R© Geometry . . . . . . . . . . . . . . . . . . . . . . . 5
1.3.2 Benefits of the IsoTruss R© In Deep Foundation Piles . . . . . . 6
1.4 Description of Research . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Summary of Pile Design and Fabrication 9
2.1 Design of the Pile Reinforcement . . . . . . . . . . . . . . . . . . . . 9
2.2 Fabrication of Reinforced Concrete Piles . . . . . . . . . . . . . . . . 12
2.3 Pile Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3 Summary of Pile Lab Tests 19
3.1 Lab Test Description . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2 Pile Stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.3 Pile Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.4 Pile Failure Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
vi
3.5 Pile Toughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.6 Review of Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.7 Recommendations and Conclusions . . . . . . . . . . . . . . . . . . . 26
4 Field Test Set-Up 29
4.1 Test Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.2 Pile Driving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.2.1 Accelerometer Installation . . . . . . . . . . . . . . . . . . . . 31
4.2.2 Pile Cushions . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.2.3 Pile Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . 32
4.3 Data Acquisition Equipment . . . . . . . . . . . . . . . . . . . . . . . 33
4.3.1 Strain Gages . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.3.2 String Potentiometers . . . . . . . . . . . . . . . . . . . . . . 34
4.3.3 Inclinometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.3.4 Load Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.4 Test Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.4.1 Hydraulic Jack and Extensions . . . . . . . . . . . . . . . . . 38
4.4.2 Hydraulic Jack Placement . . . . . . . . . . . . . . . . . . . . 40
4.4.3 Equipment Check . . . . . . . . . . . . . . . . . . . . . . . . . 40
5 Experimental Procedure 43
5.1 IsoTruss R© Reinforced Concrete Pile Test . . . . . . . . . . . . . . . . 43
5.2 Steel Reinforced Concrete Pile Test . . . . . . . . . . . . . . . . . . . 45
5.3 Inclinometer Data Reduction . . . . . . . . . . . . . . . . . . . . . . . 45
5.3.1 Strain and String Potentiometer Data Reduction . . . . . . . 50
vii
5.3.1.1 Data Consolidation . . . . . . . . . . . . . . . . . . . 50
5.3.2 Data Reversal Correction . . . . . . . . . . . . . . . . . . . . . 52
6 Experimental Results 55
6.1 Loading Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
6.2 Deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
6.2.1 String Potentiometer . . . . . . . . . . . . . . . . . . . . . . . 56
6.2.2 Inclinometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
6.2.3 Strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
7 Analytical Procedure 65
7.1 Lpile Program Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 65
7.1.1 Soil Properties Input . . . . . . . . . . . . . . . . . . . . . . . 65
7.1.2 Pile Properties Input . . . . . . . . . . . . . . . . . . . . . . . 66
7.2 Winkler Foundation Model Analysis . . . . . . . . . . . . . . . . . . . 73
7.3 Application of Mechanics of Materials . . . . . . . . . . . . . . . . . . 80
7.3.1 Cracked Moment of Inertia . . . . . . . . . . . . . . . . . . . . 80
7.3.2 Pile Moment Capacity . . . . . . . . . . . . . . . . . . . . . . 86
8 Analytical Results 89
8.1 Lpile Deflection Predictions . . . . . . . . . . . . . . . . . . . . . . . 89
8.2 Winkler Foundation Model Deflection Predictions . . . . . . . . . . . 92
9 Discussion of Results 97
9.1 Pile Stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
9.1.1 Comparison to Lab Stiffness Results . . . . . . . . . . . . . . 98
9.1.2 Verification of Lab Stiffness Results . . . . . . . . . . . . . . . 99
viii
9.2 Deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
9.3 Loading Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
9.4 Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
9.5 Energy-Modified Results . . . . . . . . . . . . . . . . . . . . . . . . . 104
9.6 Lpile Adjusted Soil Predictions . . . . . . . . . . . . . . . . . . . . . 107
9.7 Lpile SRC Pile Adjusted Reinforcement Predictions . . . . . . . . . . 112
9.8 Error Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
9.9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
10 Conclusions and Recommendations 117
10.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
10.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
References 121
ix
List of Tables
2.1 Pile Lengths [1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2 Pile Geometric Properties . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3 Reinforcement Weights [1] . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1 Summary of Lab Pile Strength Results for Lab Tests [2] . . . . . . . 24
3.2 Comparison of Stiffness, Moment, Curvature, Ductility and Toughnessof the Lab Piles [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1 Inclinometer Readings Taken During Field Testing . . . . . . . . . . 46
5.2 Example of Inclinometer Data . . . . . . . . . . . . . . . . . . . . . . 47
7.1 Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
7.2 Mechanics of Materials Analysis Results . . . . . . . . . . . . . . . . 87
7.3 Pile Failure Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
8.1 Lpile Prediction Notation . . . . . . . . . . . . . . . . . . . . . . . . 90
9.1 Comparison of Laboratory Test and Predicted Stiffness Values . . . . 100
9.2 Original and Adjusted Soil Properties for the Top Two Layers in theSoil Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
x
List of Figures
1.1 Applications for Deep Foundation Piles . . . . . . . . . . . . . . . . . 5
1.2 Schematic of an 8-Node IsoTruss R© Grid-Structure . . . . . . . . . . . 6
2.1 IsoTruss R© End Views: (a) Standard IsoTruss R© ; and,(b) IsoTruss R©with Rounded Nodes [1] . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Cutting of First IsoTruss R© Structure: (a) As Manufactured; and, (b)As Tested [1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3 Cutting of Second IsoTruss R© Structure: (a) As Manufactured; and,(b) As Tested [1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4 Steel Extension to IsoTruss R© Reinforcement [1] . . . . . . . . . . . . 14
2.5 Steel Reinforcement Splice [1] . . . . . . . . . . . . . . . . . . . . . . 15
2.6 Pile Cross Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.1 Lab Test Pile with Strain Gage Locations Marked [2] . . . . . . . . . 20
3.2 SRC Pile Ready to Be Tested in the Laboratory [2] . . . . . . . . . . 20
3.3 Lab Results for Average Deflections of All Piles [2] . . . . . . . . . . 21
3.4 Lab Results for Moments vs. Curvature of All Piles [2] . . . . . . . . 23
3.5 Lab Results for Average Moment vs. EI for the IRC and SRC Piles inthe Center Region (gages 4-8) [2] . . . . . . . . . . . . . . . . . . . . 24
4.1 Plan View of the Pile Testing Site [2] . . . . . . . . . . . . . . . . . . 30
4.2 Pile Cushions Attached with Pieces of the Cardboard Concrete Forms[2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
xi
4.3 Strain Gage Offset to Intended Line of Force [2] . . . . . . . . . . . . 33
4.4 Drawing Showing a Plan View of the Beam, Loads, and ResistingForces [2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.5 Inclinometer Casing . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.6 Photo of an Inclinometer Probe . . . . . . . . . . . . . . . . . . . . . 36
4.7 Hydraulic Jack, Load Cell, Swivel Head, and Pile Cradle . . . . . . . 37
4.8 Gap Between Piles and Reaction Load Points . . . . . . . . . . . . . 39
4.9 Jack Extension Layout . . . . . . . . . . . . . . . . . . . . . . . . . . 41
5.1 Taking Inclinometer Readings for the IRC Pile . . . . . . . . . . . . 44
5.2 Diagram of the Angle of Inclination and Related Lateral Deviation . 48
5.3 Slice of the Top of the Pile Showing the Angle Offset from Line of Loadto Inclinometer Readings . . . . . . . . . . . . . . . . . . . . . . . . 49
5.4 Comparison of Data: (a) Raw; (b) Consolidated Using the Consolida-tion Macro; and, (c) Both . . . . . . . . . . . . . . . . . . . . . . . . 52
5.5 Comparison of Data: (a) Raw; (b) Adjusted Using the Reversal Macro,and; (c) Both . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.1 Load vs. Time from Field Tests . . . . . . . . . . . . . . . . . . . . . 56
6.2 String Potentiometer Deflection from Field Tests . . . . . . . . . . . . 57
6.3 Deflection at point of Load Application based on Inclinometer Readingsfrom Field Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
6.4 Deflected Shape of the IRC Pile based on Inclinometer Readings fromField Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
6.5 Deflected Shape of the SRC Pile based on Inclinometer Readings fromField Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
6.6 Deflected Shape of the IRC and SRC Piles based on Inclinometer Read-ings from Field Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
6.7 Strain vs. Load of the IRC Pile from Field Tests . . . . . . . . . . . . 62
xii
6.8 Strain vs. Load of the SRC Pile from Field Tests . . . . . . . . . . . 63
6.9 Strain vs. Load of the IRC and SRC Piles from Field Tests . . . . . . 63
7.1 Soil Properties at the Test Site [3] . . . . . . . . . . . . . . . . . . . 67
7.2 Moment-Stiffness Generated by Lpile given SRC Pile Properties . . . 68
7.3 Lab Test Moment vs. Curvature Data . . . . . . . . . . . . . . . . . 69
7.4 Chauvenet’s Criterion Envelope for Lab Test SRC Pile 2 Gage 8 . . . 70
7.5 Moment vs Stiffness from Laboratory Testing . . . . . . . . . . . . . 71
7.6 Moment vs. Stiffness curve from Laboratory Testing with SimplifiedCurve for Lpile Input . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
7.7 Elastic Foundation Model: (a) As Loaded; and, (b) Statically AdjustedLoad for Winkler Foundation Model . . . . . . . . . . . . . . . . . . . 74
7.8 Three Displacement Components for Pile . . . . . . . . . . . . . . . . 76
7.9 Deflection of the Beam due to Rotation at the Ground Surface . . . . 78
7.10 Shifted Neutral Axis of Cracked Concrete Pile . . . . . . . . . . . . . 81
7.11 Area of a Circular Segment [4] . . . . . . . . . . . . . . . . . . . . . . 83
7.12 Stress Distribution in Concrete Compression Region . . . . . . . . . . 84
8.1 Lpile Prediction 1: Load vs. Deflection of the SRC Pile from the FieldTests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
8.2 Lpile Prediction 2: Load vs. Deflection of the IRC and SRC Piles fromField Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
8.3 Lpile Prediction 1 and 2: Load vs. Deflection of the IRC and SRCPiles from Field Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
8.4 Winkler Foundation Model Predicted Deflection at Point of Load Ap-plication of the IRC Pile from Field Tests . . . . . . . . . . . . . . . . 93
8.5 Winkler Foundation Model Predicted Deflection at Point of Load Ap-plication of the SRC Pile from Field Tests . . . . . . . . . . . . . . . 94
xiii
9.1 Deflections of All Piles in Lab Tests . . . . . . . . . . . . . . . . . . . 98
9.2 Load vs. Deflection based on String Potentiometer Readings from FieldTests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
9.3 String Potentiometer and Inclinometer Tip Deflection Results fromField Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
9.4 Load vs. Time from Field Tests . . . . . . . . . . . . . . . . . . . . . 102
9.5 Adjusted Load vs. Time from Field Tests . . . . . . . . . . . . . . . . 102
9.6 Soil Compaction Energy of the IRC and SRC Piles . . . . . . . . . . 105
9.7 Energy-Modified Load vs. Deflection Data from Field Tests . . . . . . 106
9.8 Lpile Deflection Prediction for the SRC Pile Compared to String Po-tentiometer Deflection Results for the IRC Pile in the Field . . . . . . 107
9.9 Lpile Deflection Prediction for the SRC Pile Compared to String Po-tentiometer Deflection Results for the SRC Pile in the Field . . . . . 108
9.10 Lpile Deflection Prediction for the SRC Pile Compared to AdjustedString Potentiometer Deflection Results for the SRC Pile in the Field 109
9.11 Actual Load vs. Deflection Behavior Compared to Lpile Predictionsbased on Adjusted Soil Properties . . . . . . . . . . . . . . . . . . . . 110
9.12 Actual Deflected Shape of the SRC Pile Compared to Lpile PredictionsBased on Original and Adjusted Soil Properties . . . . . . . . . . . . 111
9.13 SRC Pile Adjusted Reinforcement Predictions . . . . . . . . . . . . . 113
9.14 Lpile Deflection Prediction for the SRC Pile Compared to String Po-tentiometer Deflection Results for the IRC Pile in the Field . . . . . . 115
xiv
Chapter 1
Introduction
This thesis focuses on the field performance of IsoTruss R© grid-reinforced
concrete beam columns for use in driven piles. Experimental investigation included
one instrumented carbon/epoxy IsoTruss R© grid-reinforced concrete (IRC) pile and
one instrumented steel-reinforced concrete (SRC) pile which were driven into a clay
profile at a test site. These two piles were quasi-statically loaded laterally until
failure. Behavior was predicted using three different methods: 1) a commercial finite
difference-based computer program called Lpile; 2) a Winkler foundation model;
and, 3) a simple analysis based on fundamental mechanics of materials principles.
This thesis is the concluding section of a three-part investigation of the
suitability of IsoTruss R© grid-reinforced concrete columns for use as driven piles.
Part one, performed by David McCune [1], included the design and fabrication of
the test piles. Part two, performed by Monica Ferrell [2], assessed the strength and
stiffness of IsoTruss R© grid-reinforced concrete piles through laboratory testing and
preliminary field test design. Due to the significance of this research to the
1
investigation performed in this thesis, McCune’s and Ferrell’s work is summarized in
Chapters 2 and 3, respectively with some of Ferrell’s field test design in Chapter 4.
This chapter includes a brief history of reinforced concrete which introduces
the reader to previous research and the reasons for conducting further investigation
in the area of reinforced concrete. An introduction to driven piles as well as a
description of the IsoTruss R© grid-structure used as reinforcement is also provided.
A description of the research performed for this thesis concludes the chapter.
1.1 Brief History of Reinforced-Concrete
In the mid seventeen hundreds, pebbles were added to a cement paste
introducing the world to what would become a great power in structural materials,
concrete. Concrete underwent another improvement when French gardener, Joseph
Monier, added steel wire to his concrete pots. The use of steel in concrete was
expanded to rail ties, pipes, floors, arches, and bridges [5]. Today this steel and
concrete mixture, known as reinforced concrete, is used in almost every modern
structure. Reinforced concrete has allowed engineers to design with the compressive
strength of concrete combined with the tensile strength of steel thus making a
strong, economic building material. Unfortunately, the addition of steel to concrete
was not without flaws. Steel tends to corrode when exposed to water and chemical
agents. As a result of this corrosion, the steel reinforcement looses strength and
de-bonds from the concrete.
2
To increase the life of steel-reinforced concrete structures, fiber-reinforced
polymer (FRP) wraps have been researched and implemented in many situations.
Research indicates that FRP wraps increase the flexural and shear strength of
existing steel-reinforced structures [6, 7]. These FRP wraps have also been found to
increase the fatigue life of steel-reinforced concrete structures, which is important in
cases of frequent freeze-thaw [8].
Not only are FRP being used for repair, they are also entering the concrete
field as a primary reinforcement material that is lighter and more corrosion-resistant
than steel with increased stiffness and tensile capacity [9]. However, with these
advantages, FRP reinforcement generally has a lower bonding quality than steel and
tends to be brittle [10]. Different shapes of FRP-reinforcement have proven to
increase the strength and bond characteristics [10, 11].
An improvement to the one-dimensional FRP bars are FRP grids. FRP grids
have shown to be both predictable and reliable [12]. The grid allows for a good
transfer of load from the concrete to the reinforcement thus making a great
alternative to steel as reinforcement in concrete [13, 12]. The IsoTruss R©, which is
discussed in further detail later in this chapter, is a superior type of FRP grid
structure which could prove to be the most innovative improvement concrete has
undergone since its invention over a century ago.
3
1.2 Driven Piles
Pile foundations are long, slender structural elements driven into the soil
profile to develop sufficient bearing resistance to support high-rise buildings and
bridges. Piles typically consist of timber, steel pipe, or reinforced concrete columns.
Piles are becoming more advantageous as America’s infrastructure increases in size
and diversity. Soils once considered unsuitable for building can be developed with
the addition of piles. New buildings are taller and new bridges span greater
distances than before and therefore require greater strength from the subsurface
materials. Piles can play a key role in providing this strength.
Figure 1.1 shows several applications for foundation piles. One application is
to transfer loads from weak or active upper layers of soil to stronger, more stable
layers of soil and rock found deep in the earth. Piles are also used to resist
horizontal loads introduced by earthquakes or strong winds. They can reduce uplift
or provide more bearing strength in cases of erosion. Piles therefore resist primarily
high bending and compression forces [14].
1.3 Introduction to the IsoTruss R©
The IsoTruss R© is a composite structural grid built of strong fibers held
together by polymer resin. The efficient shape and innovative material of the
IsoTruss R© make it a strong structure with several benefits for deep foundations.
4
� � � � � � � � � � � � � � � � � � � �� � � �
� � � � � � � � � �� � � � � � � � � � � � �
Figure 1.1: Applications for Deep Foundation Piles
1.3.1 IsoTruss R© Geometry
The unique geometry of the IsoTruss R© gives it incredible strength at very
low weights. Loads are carried in the IsoTruss R© through two different sets of
members. Longitudinal members run parallel to the length of the IsoTruss R© and
carry most of the compression and tension forces, as well as the bending forces in
the structure. A second set of members wraps around the core of the IsoTruss R© ,
crossing the longitudinal members at regular intervals between 30 and 60 degrees
relative to the longitudinal axis of the IsoTruss R©. These members, called helicals,
resist the torsional and shear loads. When not placed in concrete, the helical
members also play a critical role by bracing the longitudinal members to decrease
their effective length and consequently reduce the onset of buckling. Load is
transferred from one member to another through interweaving of the fibers at the
intersections. Figure 1-2 shows these two types of members and how they form the
5
Figure 1.2: Schematic of an 8-Node IsoTruss R© Grid-Structure
IsoTruss R© grid-structure. The longitudinal members are represented in black and
the helical members are represented in gray.
1.3.2 Benefits of the IsoTruss R© In Deep Foundation Piles
Traditional foundation piles have been constructed of steel, concrete, and
timber. Steel and concrete piles can be very strong but are limited to land
applications due to their corrosive nature in water. Timber fares better in water but
provides significantly less strength than concrete or steel piles.
Even on land, the deterioration of steel reinforcement is a significant problem
that has plagued the reinforced concrete industry for decades. This deterioration is
becoming an even greater concern as our world’s infrastructure is getting older. In
6
Corrosion of Steel in Concrete, the author states that: The economic loss and
damage caused by the corrosion of steel in concrete makes it arguably the largest
single infrastructure problem facing industrialized countries [15].” The IsoTruss R©
provides a nice solution to the corrosion problem encountered by foundation piles
without sacrificing strength. Because of its non-metallic material, the IsoTruss R©
resists the chemical agents and water that rusts and weakens steel reinforcement.
In addition to being non-corrosive, the IsoTruss R© is significantly lighter than
other building materials. Steel rebar is heavy and therefore more labor is required
for its transport and installation.
1.4 Description of Research
Research performed for this thesis focused on the field performance of an
IsoTruss R© reinforced concrete pile. Because the IsoTruss R© is an alternative to steel
reinforcement, the strength of an IsoTruss R© reinforced concrete (IRC) pile was
compared to that of a similar steel reinforced concrete (SRC) pile. Both
experimental procedure as well as analysis were performed to understand the pile
behavior.
Experimental testing was performed on two reinforced concrete foundation
piles: one with composite reinforcement and the other with similar steel
reinforcement. Each pile was 30 ft (9.14m) long and 14 in (35.56 cm) in diameter.
After the piles had been driven at the test site, a static lateral load test was
7
performed on each pile. The results of these tests were analyzed to compare the
flexural strength and stiffness of the piles.
Three different methods were used to predict the flexural strength and
stiffness of the driven piles. The first method used a commercial software program
called Lpile and the second method applied a Winkler elastic foundation model.
These approaches were used to predict the flexural strength of the piles. The third
method was based on mechanics of materials principles. The third approach
included calculations to predict the cracked moment of inertia , stiffness, and
bending strength of the pile. Both laboratory test data and material properties were
used as input for these analyses.
8
Chapter 2
Summary of Pile Design and Fabrication
This chapter provides an overview of the design and fabrication process
McCune followed to construct the piles studied in this thesis. A more detailed
description of the design and manufacturing process is provided in Reference 1.
2.1 Design of the Pile Reinforcement
The process followed to design the IRC and SRC piles focused on creating
two separate types of piles which would be comparable in application. Each pile was
designed to have the same pile diameter, length, and stiffness.
The IRC pile was designed such that it: (1) efficiently held the desired pile
loading; (2) met typical pile form dimensions; and, (3) could be easily compared to
the steel reinforcing cage. In order to meet these requirements, slight changes were
made to the usual IsoTruss R© geometry and corresponding equations that describe
the modified geometry were developed.
9
The overall diameter of the IRC pile was determined by the size of a typical
concrete form, 14 in (37 cm). Because composite materials are very corrosion
resistant, a 1.0 in (2.5 cm) cover was used and therefore a 13 in (33 cm) outer
diameter was chosen for the IsoTruss R© reinforcement.
The longitudinal members were designed to match the bending stiffness of
the #4 grade 60 steel rebar used in the steel reinforced pile. The number of fibers in
the longitudinal members determines the size and stiffness of the longitudinal
members. Therefore the fiber number was adjusted until the longitudinal stiffness
matched the rebar stiffness. The size of the longitudinal IsoTruss R© members is
expressed in tows, or bundles of 12,000 fibers. The final design was determined to
be 8 longitudinal members consisting of 133 tows each, for a total member
cross-sectional area of 0.15 in2 (0.97 cm2). The helical IsoTruss R© members were
designed with respect to the longitudinal IsoTruss R© members. Typically, a ratio of
the longitudinal members to the helical members for an IsoTruss R© of 12
to 23
has
been used. A ratio of 23
was chosen for the piles resulting in a helical design of 89
tows with a cross-sectional area of 0.10 in2 (0.65 cm2).
The most novel change made to the IsoTruss R© geometry was the rounding of
the usually pointed nodes of the helical members. The change in the IsoTruss R©
nodes was motivated by a desire to maximize the bending strength of the IsoTruss R©
reinforcement in the confined geometry. Bending strength is a function of the
material properties and moment of inertia. To maximize the moment of inertia, the
10
longitudinal members were positioned as far away as possible from the center of the
IsoTruss R© within the constraints of the pile and IsoTruss R©. This was achieved in a
volume-constrained application by rounding the nodes of the helical members.
Figure 2.1 shows a cross-section of a typical IsoTruss R© and the comparative
position of the longitudinal members with the new rounded nodes. By moving the
longitudinal members further out, the moment of inertia was increased 70%,
resulting in a corresponding increase in the bending strength of the IsoTruss R© .
( a ) ( b )Figure 2.1: IsoTruss R© End Views: (a) Standard IsoTruss R© ; and,(b)IsoTruss R© with Rounded Nodes [1]
Careful design of the steel reinforcement was important to ensure the SRC
piles were comparable to the IRC piles. Eight #4 grade 60 steel bars were chosen
for the longitudinal steel reinforcement for two reasons. First, eight bars is
consistent with the 8-node design of the IsoTruss R© structure. Second, #4 bars
11
permit testing with reasonable loads. The final step was to design the transverse
reinforcement in the steel pile to be equivalent to the helical members of the
IsoTruss R© grid-reinforcement. The helical members spiral around the IsoTruss R© .
Therefore, comparing the composite helicals to the transverse steel reinforcement
required estimation of the strength of the helical members in the direction of the
transverse steel reinforcement based on the angles that the helical members form
with a cross-section of the pile.
2.2 Fabrication of Reinforced Concrete Piles
The two piles were fabricated using different processes. The IsoTruss R©
reinforcement was manufactured from T300C 200NT 12K tow carbon fiber
pre-impregnated with TCR UF3325-95 epoxy resin. Fabrication of the IsoTruss R©
reinforcement required three main steps. First, the pre-impregnated carbon fiber
tows were wrapped around a collapsible form, called a mandrel. Layer upon layer of
carbon fiber was wound onto the mandrel in bundles of 4 to 6 tows alternating
between helical and longitudinal members in a predetermined pattern. This process
formed interwoven joints and continued until the required amount of fiber was
placed in each member. Second, the members were consolidated by wrapping
Dunston Hi-shrink tape tightly around each member. Finally, the IsoTruss R© was
cured in a rudimentary plywood oven according to the curing instructions for
Thiokol UF 3325-95 resin.
12
Figure 2.2: Cutting of First IsoTruss R© Structure: (a) As Manufactured;and, (b) As Tested [1]
Figure 2.3: Cutting of Second IsoTruss R© Structure: (a) As Manufac-tured; and, (b) As Tested [1]
Two 30 ft (9 m) long IsoTruss R© structures were manufactured for testing
purposes. The first IsoTruss R© is shown in Figure 2.2. A short section measuring
32.75 in (83 cm) was cut from each pile to be used in compression testing for quality
control purposes. A longer section measuring 26.9 ft (8.2 m) was cut for the in-situ
testing. The second IsoTruss R© is shown in Figure 2.3. Two sections measuring
13.38 ft (8.2 m) were cut for lab bending tests. In addition, small pieces from the
second IsoTruss R© were tested to assess the local member strength.
13
Figure 2.4: Steel Extension to IsoTruss R© Reinforcement [1]
The IRC pile to be tested in the field was designed to be 30 ft (9 m) long;
however, Figure 2.2 shows that 32.75 in (83 cm) was removed from the end of the
pile for compression testing. To compensate for the lost length, a short section of
steel cage reinforcement was attached to the end of the pile. Figure 2.4 shows the
splice between the IsoTruss R© and the steel rebar.
The steel reinforcement was constructed according to industry methods. The
longitudinal bars were attached to the transverse hoops in an 8-bar pattern. The
#4 bars used for the longitudinal reinforcement came in lengths of 20 ft (6 m) and
therefore splices were only necessary for the 30 ft (9 m) long in-situ SRC pile
reinforcement. Figure 2.5 shows how the splices were alternated each bar so four of
the splices were at one end of the pile and the other four were at the other end.
Texas Measurements FLA-3-11-3LT strain gages were placed in several
locations on the longitudinal members of the IsoTruss R© and on the longitudinal
14
Figure 2.5: Steel Reinforcement Splice [1]
steel reinforcement. A special pipe was inserted in each of the piles in order to take
inclinometer readings. The pipe has an outer diameter of 2.75 in. (6.99 cm), and an
inner diameter of 2.32 in (5.89 cm). To complete the pile construction, the
reinforcements were placed in 14 in (36 cm) diameter Kolumn Forms forms
purchased from CaraustarTM. The concrete was placed by Eagle Precast Company.
2.3 Pile Properties
Four of the piles were for laboratory testing, two piles with IsoTruss R©
reinforcement and two with steel reinforcement. Each of the laboratory piles was 13
ft (4 m) in length. Two of the piles, 30 ft (9m) in length, were for field testing.
Table 2.1 reports the lengths of each of the piles fabricated.
A cross section of the pile is shown in Figure 2.6 and the specific
measurements for each pile is shown in Table 2.2.
15
Table 2.1: Pile Lengths [1]L e n g t hP i l e [ f t ( m ) ]S R C 1 6 . 5 8 ( 2 . 0 1 )S R C 2 6 . 5 8 ( 2 . 0 1 )I R C 1 6 . 6 7 ( 2 . 0 3 )I R C 2 6 . 6 5 ( 2 . 0 3 )Center Line
d2
d1
Rp
Figure 2.6: Pile Cross Section
Something interesting to note is the difference in weight between the
IsoTruss R© and steel reinforcements, as shown in Table 2.3. For approximately the
same length and diameter, the IsoTruss R© reinforcement is only about 37% as heavy
as the steel reinforcement.
16
Table 2.2: Pile Geometric Properties
Property IRC Pile SRC Pile
Radius of the Pile, Rp
[in (cm)] 7 (17.8) 7 (17.8)
Radius of the Reinforcement, rr
[in (cm)] 0.22 (0.56) 0.25 (0.64)
Cross Sectional Area of the Reinforcement, Ar
[in2 (cm2)] 0.15 (0.38) 0.2 (0.51)
Distance from Center to Bottom Layer of Reinforcement, d1
[in (cm)] 5.69 (14.5) 4.25 (10.8)
Distance from Center to Second Layer of Reinforcement, d2
[in (cm)] 4.020 (10.2) 3.010 (7.6)
Moment of Inertia of the Longitudinal Reinforcement, Im
[in4 (cm4)] 0.00184 (0.077) 0.00310 (0.12)
Table 2.3: Reinforcement Weights [1]
Weight Sample Type Reinforcement Pile
[lb (kg)]
1 97 (44) Steel
2 97 (44)
1 37 (17) Lab
IsoTruss®
2 37 (17)
Steel 1 232 (104)
IsoTruss® w/o steel piece 1 76 (34) Field
IsoTruss® w/ steel piece 1 110 (50)
17
18
Chapter 3
Summary of Pile Lab Tests
This chapter summarizes the basic testing procedure Ferrell followed with a
summary of results obtained from the four pile sections tested in the laboratory. A
more detailed description can be found in Reference 2.
3.1 Lab Test Description
Four-point bending tests were performed in the laboratory on two
instrumented carbon/epoxy IsoTruss R© reinforced concrete piles (IRC piles) and two
instrumented steel-reinforced concrete piles (SRC piles). The piles were were loaded
to failure while monitoring load, deflection, and strain data. As shown in Figure 3.1,
strain gages were located on opposite sides of the reinforcement at nine different
locations on the test piles. Figure 3.2 shows one of the SRC piles in the test fixture,
ready to be tested. Each of the four piles was tested to failure in the same manner.
Lab testing revealed much about the stiffness, load capacity, failure mode,
toughness, and ductility of the two piles. Each of these properties is addressed
individually in the following sections.
19
P i l e98765432 11 4 . 5 i n ( 3 6 . 8 c m ) 1 4 . 5 i n ( 3 6 . 8 c m )1 5 . 1 i n ( 3 8 . 3 c m )7 . 4 4 i n ( 1 8 . 9 c m )1 4 . 8 i n ( 3 7 . 6 c m )2 9 . 1 i n ( 7 3 . 8 c m )
7 . 4 4 i n ( 1 8 . 9 c m )1 5 . 0 i n ( 3 8 . 1 c m )
Figure 3.1: Lab Test Pile with Strain Gage Locations Marked [2]
Figure 3.2: SRC Pile Ready to Be Tested in the Laboratory [2]
20
0
10
20
30
40
50
60
70
0 1 2 3 4 5 6
Deflection [in]
To
tal T
ran
sver
se L
oad
[k
ips]
0
50
100
150
200
250
300
0 5 10 15 20 25
Deflection [cm]
Tota
l T
ran
sver
se L
oa
d [
kN
]
S1-SRC S1-IRC
L4-SRC L4-IRC
L3-SRC L3-IRC
L2-SRC L2-IRC
L1-SRC L1-IRC
C-SRC C-IRC
R1-SRC R1-IRC
R2-SRC R2-IRC
R3-SRC R3-IRC
R4-SRC R4-IRC
S2-SRC S2-IRC
R4R3R2R1CL1L2L3L4
S2S1 Load Cell 2Load Cell 1
Figure 3.3: Lab Results for Average Deflections of All Piles [2]
3.2 Pile Stiffness
The steel and IsoTruss R© reinforcement were designed to have the same
stiffness. Lab testing was useful in verifying the equality of stiffness in the two
differently-reinforced piles. The stiffness is represented by the slope of the load vs.
deflection curves, shown in Figure 3.3. Both types of piles exhibit similar
displacements for the same load level until the steel in the SRC pile begins to yield,
leading to eventual failure.
Another verification of the pile stiffness was obtained from the strain data
gathered. Stiffness can be related to moment, M , and curvature, κ, through the
21
following relationship:
M = EIκ (3.1)
where the product of E (modulus of elasticity) and I (moment of inertia) is
stiffness. The moment was easily obtained from statics by multiplying the applied
load by the distance to the strain gage locations marked in Figure 3.1. Assuming a
linear strain distribution through the thickness (diameter) of the pile, the curvature
is a function of the longitudinal strain:
κ =εl − εu
h(3.2)
where εu and εl are the strains on the upper and lower reinforcements, respectively,
and h is the distance between the two strain gages. This distance was 9.0 in (23 cm)
for the SRC piles and 12.0 in (31 cm) for the IRC piles.
Moment curvature plots were developed for each of the nine locations on
both piles. Two specimens of each pile type were tested and therefore averaged
plots were made from the two moment vs. curvature plots. These plots are shown in
Figure 3.4.
As given in Equation 3.1, stiffness is the moment divided by the curvature,
or the slope of the moment vs. curvature plot in Figure 3.4. These stiffness values
are plotted as a function of moment in Figure 3.5.
22
0
500
1000
1500
2000
0 200 400 600 800 1000
Curvature [microstrain/in]
Mo
men
t [k
ip-i
n]
0
50
100
150
200
0 50 100 150 200 250 300 350
Curvature [microstrain/cm]
Mo
men
t [k
N-m
]
1-SRC 1-IRC
2-SRC 2-IRC3-SRC 3-IRC
4-SRC 4-IRC5-SRC 5-IRC
6-SRC 6-IRC7-SRC 7-IRC
8-SRC 8-IRC9-SRC 9-IRC
S1
6
Load Cell 1 Load Cell 2 S2
5 4 3 2 1789
Figure 3.4: Lab Results for Moments vs. Curvature of All Piles [2]
Using a linear regression function in Excel, the average slope of the curves
was calculated. The region between curvatures of 100 and 140 micro strain were
chosen for these calculations because it is a region just after the initial noise and
before yielding of the piles. These slope values were 3.8 kip-in2 (109 kN-cm2) for the
SRC piles and 3.4 kip-in2 (98 kN-cm2) for the IRC piles. The closeness of the two
stiffness values verifies the design objective of similar stiffness values for the two
different reinforcement materials.
3.3 Pile Strength
Laboratory testing of the IRC and SRC piles showed that the IRC piles held
nearly twice the bending moment as the SRC piles at failure. The IRC piles failed
23
0
5
10
15
20
25
30
0 200 400 600 800 1000
Moment [kip-in]
EI
x 1
06 [
kip
-in2
]
0
10
20
30
40
0 100 200 300 400 500 600 700
Moment [kN-cm]
EI
x 1
06 [
kN
-cm
2]
SRC
IRC
S1
6
Load Cell 1 Load Cell 2 S2
5 4 3 2 1789
Figure 3.5: Lab Results for Average Moment vs. EI for the IRC andSRC Piles in the Center Region (gages 4-8) [2]
at an average moment of 1,719 kip-in (194 kN-m) while the SRC piles failed at an
average moment of 895 kip-in (101 kN-m). Table 3.3 summarizes the ultimate load
held by each of the four piles.
Table 3.1: Summary of Lab Pile Strength Results for Lab Tests [2]
Ultimate Load
[kips (kN)] Specimen
SRC Piles IRC Piles
1 36.2 (161) 63 (280)
2 37.9 (169) 65.4 (291)
Average 37.1 (165) 64.2 (286)
Standard Deviation 1.20 (5.66) 1.70 (7.78)
24
3.4 Pile Failure Mode
The failure modes for the two different types of piles were very different. The
failures of the SRC piles were ductile, as expected, while the failures of the IRC
piles lacked ductility. Figure 3.3 shows that the deflection of the IRC pile increases
linearly until failure while the SRC pile yields significantly prior to failure.
Ferrell explains the observed physical failure of the piles in the following
statement:
From the very beginning of load application the IRC piles behaved
differently than the SRC piles. At loads where the SRC piles had yielded
and were heavily cracked throughout the region between load points, the
IRC pile had much smaller deflections, and hence, much smaller hair-line
cracks. The IRC pile seemed to be able to take the load much better and
maintain its shape until loads much higher than the total capacity of the
SRC piles [2].
3.5 Pile Toughness
The energy required to fracture a material is known as the toughness.
Toughness is calculated by determining the area under the load vs. deflection
curves. As a result of the brittle fracture of the IRC piles, the SRC piles absorbed
approximately twice as much total energy as the IRC piles before failure. However,
if toughness is calculated at the maximum loads rather than the maximum
25
Table 3.2: Comparison of Stiffness, Moment, Curvature, Ductility andToughness of the Lab Piles [2]
Property SRC IRC
Flexural Stiffness
[kip-in2 (kN-cm
2)] 3.8 (109) 3.4 (98)
Maximum Moment
[kip-in (kN-m)] 895 (101) 1719 (194)
Maximum Curvature from Strain Gage
[ � E/in ( � E/cm)] 1049 (413) 505 (199)
Maximum Curvature from Deflections
[�E/in (
�E/cm)] 1200 (472) 1200 (472)
Maximum Strain in Reinforcement
[ � E] 5400 (5400) 7200 (7200)
Toughness at Maximum Displacement
[kip-in (kN-m)] 168 (1900) 83 (940)
Toughness at Maximum Loads
[kip-in (kN-m)] 74 (836) 83 (940)
deflections, the toughness of the IRC piles is 83 kip-in (940 kN-cm) while the
toughness of the SRC piles is only 74 kip-in (836 kN-cm). This comparison seems
more indicative of the pile capacity when considering that piles are designed for a
specific load rather than a specific deflection.
3.6 Review of Results
Table 3.6 displays the results for stiffness, moment, curvature, and
toughness.
3.7 Recommendations and Conclusions
Despite the lack of ductility observed in these tests, the IRC piles are
nevertheless still suitable for use as pile foundations, due to their substantially
greater strength than SRC piles. However, further investigations are recommended
26
to improve the ductility of the IRC piles, since ductility has been observed in other
IsoTruss R© grid-reinforced concrete piles.
27
28
Chapter 4
Field Test Set-Up
Field test set-up, initiated by Ferrell [2] and completed as part of this thesis,
included choosing a testing site, driving the piles, ensuring the data acquisition
instruments were functioning properly and getting the load from the jack to the
pile. Data acquisition tests were conducted before the field tests were performed
and pile cradles and jack extensions were fabricated to ensure the load was
distributed to the piles effectively.
4.1 Test Site
The site chosen for testing the piles had a predominatly clay profile and was
located near South Temple in Salt Lake City, Utah. Two freeways pass over the site
and a railroad track is located several meters away from the test piles. The site was
partially excavated in order to expose an old freeway concrete footing. This footing
provided a surface against which the actuator could push to load the IRC pile. A
pile made completely of steel was driven and provided a surface against which the
actuator could push to test the SRC pile. Careful consideration was taken to ensure
29
2 4 ' ( 7 m )2 8 ' ( 9 m )
1 7 ' ( 5 m )6 ' ( 2 m )7 ' ( 2 m )4 ' ( 1 m )
1 5 ' ( 5 m )7 ' ( 2 m ) 8 ' ( 2 . 5 m )
7 ' 1 1 " ( 2 . 4 m ) 7 ' 4 " ( 2 . 2 m )2 ' 1 1 " ( 0 . 1 m )D 1 4 " ( 3 6 c m )12 3
12 3 S t e e l P i l eL e g e n dS R C P i l eI R C P i l e
Figure 4.1: Plan View of the Pile Testing Site [2]
that the testing of one pile did not disturb the soil surrounding the other piles.
Figure 4.1 shows a plan view of the test site with the piles in place.
4.2 Pile Driving
The piles were driven using an A IHC S-70 pile hammer on July 19, 2004.
The top 2.0 ft (0.6 m) of both piles was left exposed above ground. Two concerns
needed to be considered in the pile driving. First, the tops of the piles required
protection from the force of the pile driver to avoid chipping the concrete. Second,
30
the strain gages in the piles needed to be oriented parallel to the actuator so that
proper strain measurements could be recorded.
4.2.1 Accelerometer Installation
An accelerometer and strain gage was attached to measure the acceleration
and strain in the piles during driving. The data gathered from the accelerometer
and strain gage can be used to estimate the axial capacity of the pile at the end of
driving for the piles. Personnel from the Utah Department of Public Transportation
performed the installation. The first attempt to install the accelerometer in the
steel reinforced pile began at the same location as the strain gages. When this was
discovered, the drilling was stopped, the column was rotated 90 degrees, and the
installation resumed. Because the initial drilling was not deep, the wires and gages
were not likely damaged.
4.2.2 Pile Cushions
Cushions were made out of wooden disks to protect the ends of the piles
from the driving hammer. Wedges were attached to hold the disks in place while the
piles were being driven. This method proved to be ineffective when the disks
shifted, exposing the concrete to the pile hammer. A portion of concrete was
chipped from the top of the steel reinforced pile; however, this damage was not
sufficient to influence the testing. The disks were better attached using pieces of the
concrete forms as shown in Figure 4.2. This method proved to be effective.
31
Figure 4.2: Pile Cushions Attached with Pieces of the Cardboard Con-crete Forms [2]
4.2.3 Pile Orientation
The orientation of the piles was critical in ensuring a direct line of action
from the load point on the pile to the plane of the strain gages. The driving of the
SRC pile was successful in orienting the pile parallel to the actuator’s load.
However, complications arose when the IRC pile rotated during the driving process
leaving the strain gages 16.5o out of alignment from the desired orientation. This
rotation of the pile was large enough that the concrete foundation intended for use
as a surface, on which the actuator could push, was no longer in the projected line
of the strain gages. Figure 4.3 shows this offset from the projected line of the strain
gages to the line of force intended.
In order to solve the problem presented when the IRC pile rotated, a beam
was connected to the existing concrete foundation thus providing an alternate
32
2 ' � 1 1 "( 0 . 8 9 m ) 1 3 " ( 3 3 c m )1 6 . 5 °
Figure 4.3: Strain Gage Offset to Intended Line of Force [2]
surface for the hydraulic jack to push against. The new surface needed to be
oriented at the same angle as the strain gages, 16.5o. The beam also needed to hold
the large moment that would be created by the offset. The beam and connecting
bolts were designed to hold the required loading and a small ramp was attached to
the beam to provide the necessary angle. Figure 4.4 shows the beam and ramp that
was installed at the test site.
4.3 Data Acquisition Equipment
This section describes the instrumentation used during the field tests to
acquire strain, deflection and load measurements.
33
P
F s p F bF s f P wP y P x
Figure 4.4: Drawing Showing a Plan View of the Beam, Loads, andResisting Forces [2]
4.3.1 Strain Gages
Ten TML WFLA-6-11 strain gages were installed on the tension and
compression sides of the pile reinforcement. Wires were run from the actual gages,
along the reinforcement, and up through the top of the concrete. These bundles of
wire were protected in a thick plastic wrapping after fabrication and were not
exposed until the day of testing.
4.3.2 String Potentiometers
String potentiometers were placed 6.0 in (15 cm) from the top of each pile to
record tip deflection. The potentiometers were attached to an independent reference
frame consisting of a wood beam which was supported outside of the heavily
disturbed soil region.
34
4.3.3 Inclinometer
A Slope Indicator Digitilt R© Inclinometer Probe was used to take slope
readings throughout the length of the pile. This inclinometer system is composed of
four main components:
• Inclinometer Casing
• Inclinometer Probe
• Control Cable
• Inclinometer Readout Unit
The inclinometer casing provides a shaft through which the probe may pass
to take slope measurements. An inclinometer casing, shown in Figure 4.5, was
placed in the center of both piles.
Figure 4.5: Inclinometer Casing
35
The inclinometer probe was composed of an aluminum shaft with wheel
assemblies at the top and bottom of the shaft. Figure 4.6 shows a photo of the
probe. The upper and lower wheel assemblies are tilted to facilitate passage through
the casing and to differentiate between positive and negative slope readings. Tilt is
measured in the inclinometer probe by two force-balanced servo-accelerometers.
One of the accelerometers measures tilt in the plane containing the wheels, the A
axis. The other accelerometer measures tilt in the plane perpendicular to the
wheels, the B axis.
Figure 4.6: Photo of an Inclinometer Probe
The control cable is connected to the top of the inclinometer probe to
transmit readings to the inclinometer readout unit. Readings were taken at 2 ft (0.6
m) intervals in each pile starting at 2 ft (0.6 m) down from the top of the pile and
ending 2 ft (0.6 m) up from the bottom of the pile. Seven and nine sets of readings
were taken for the SRC and IRC piles, respectively. Each set of readings includes
two slope data readouts for each 2 ft (0.6 m) interval. One of the readouts comes
36
from the first pass the inclinometer makes down the inclinometer casing. This
process was repeated with the inclinometer rotated 180 degrees. In theory, the two
passes should yield the same data, although the second data set will have the
opposite sign. This practice provides redundancy in the data and eliminates bias in
the probe.
4.3.4 Load Cell
An RST Instruments model SG300 300-kip (1300 kN) capacity load cell with
a tolerance of +/- 0.1% was used to monitor the load applied to the pile. The load
cell can be viewed in Figure 4.7. The center of the applied load was 18 in (46 cm)
above the ground surface.
Figure 4.7: Hydraulic Jack, Load Cell, Swivel Head, and Pile Cradle
37
4.4 Test Preparation
Final test preparations included installation of pile cradles and jack
extensions to effectively transfer the load to the piles. The data acquisition
equipment also underwent final checks before beginning the field tests.
In order to test the piles, a flat surface that the hydraulic jack could push
against needed to be attached to the pile faces. As shown in Figure 4.7, a cradle was
built using 34
in (1.9 cm) A36 steel to provide this flat surface for the IRC and SRC
piles. An 8 in (20 cm) channel was tack welded onto the solid steel pile to provide
its flat surface.
4.4.1 Hydraulic Jack and Extensions
A Power Team 150-ton (1300 kN) hydraulic jack, shown in Figure 4.7, was
used to apply the load. However, the jack was not capable of extending the entire
gap between the piles and their respective reaction load points, extensions were
designed to shorten these gaps. The distances between the IRC pile and the SRC
pile with their reaction load points measured 68 in (170 cm) and 82 in (210 cm),
respectively. The jack itself is 22 in (56 cm) long with an additional 5 in (13 cm)
attached load cell. Two extensions were constructed to shorten the rest of the
distance shown in Figure 4.8 and provide a reaction for the compressive load.
The material available to construct these extensions was 35 ksi (24 kN/cm2),
6 in (15 cm) diameter standard steel pipe. Because the pipe was to be used in
38
( a )
( b )Figure 4.8: Gap Between Piles and Reaction Load Points
compression, it was analyzed as a column. The compressive strength was calculated
for the pipe using a conservative K value of 1 and effective lengths of 29 in (74 cm)
and 42 in (110 cm) for the extensions. Table 4-8 of the AISC Manuel of Steel
Construction lists a factored compressive strength of 158 kips (703 kN) for the pipe
at effective lengths under 6 ft (1.5 m). This capacity was well beyond the
anticipated testing load of 50 kips (220 kN) to 60 kips (270 kN) [16].
The next step was to determine the required thickness for the end plates on
the jack extensions. The AISC Manual of Steel Construction gives the following
39
equation for the minimum base plate thickness, tmin:
tmin = l
√2Pu
0.9FyBN(4.1)
where Fy is the yield strength; B and N represent the length and the width of the
plate, respectively; Pu is the ultimate required load; and l is the length of the pipe.
A conservative value of 80 kips (9360 kN) for Pu and a 12 in (31 cm) x 12 in (31
cm) plate yielded a minimum thickness of 0.66 in (1.68 cm) for the 42 in (110 cm)
pipe and 0.45 in (1.1 cm) for the 29 in (74 in) pipe. In order to accommodate both
extensions, a 0.75 in (1.9 cm) base plate thickness was selected. Figure 4.9 shows
the finished layout for the extensions.
4.4.2 Hydraulic Jack Placement
The hydraulic jack could was carefully positioned to ensure a precise load
was directed from the jack, through the extension and pile cradle, and onto the pile.
The center line of the jack, extension, and cradle was aligned and held in place as
the jack was extended enough to wedge all pieces between the pile and the reaction
load points. This procedure was followed for each pile before testing began.
4.4.3 Equipment Check
Equipment checks were performed on the strain gages, string potentiometers,
and load cells. After the strain gages were connected to the computer input, several
of the gages were either dysfunctional (showing very large strain without loading) or
nonfunctional (no data entering the computer). To ensure the connection was not to
40
( a )
( b )Figure 4.9: Jack Extension Layout
blame for the output, each gage connection that did not function properly was
rechecked several times and channels were changed until either the gage gave a
reasonable readout or the gage was determined to be faulty.
41
42
Chapter 5
Experimental Procedure
Experimenal procedure involved testing one IRC pile and one SRC pile in
the field. This chapter includes a description of the field tests as well as the
procedure followed for reducing data recorded during these tests.
5.1 IsoTruss R© Reinforced Concrete Pile Test
Testing of the IRC pile was performed October 4, 2004, 77 days after pile
driving. Once the loading devices were properly aligned and the strain gages and
string potentiometer connected to the computer input, a lateral load was applied.
The test was perfomed by applying a load sufficiant to achieve a given deflection
target after which this load was held constant for five minutes. Target deflection
levels were 0.5 in (1.3 cm). Inclinometer readings were taken at each target
deflection. Figure 5.1 shows the inclinometer readings being taken with one
operator lowering the probe and one operator at the readout unit.
43
Figure 5.1: Taking Inclinometer Readings for the IRC Pile
The process of holding the load constant led to a gradual increase in
deflection with time. Therefore, during the time that inclinometer readings were
made the load was allowed to decrease somewhat although the pile head deflection
remained essentially the same. Because the failure of the IRC pile was abrupt in the
laboratory, inclinometer readings were discontinued after the load reached 21 kips
(93 kN). This was done to avoid injury to those people right next to the pile taking
inclinometer readings in case of sudden failure of the pile.
44
The IRC pile failed abruptly at 32 kips (140 kN) of load. A pop was heard as
the pile apparently fractured at approximately 6 ft (2 m) below the ground surface.
5.2 Steel Reinforced Concrete Pile Test
The SRC pile underwent testing the day following the IRC pile test. Loading
of the pile was performed in the same manor as that of the IRC pile and the same
adjustments were made during the inclinometer reading pauses. The failure of the
SRC pile differed from the IRC pile in that early yielding was followed by a slow
ductile failure. At a load of 28 kips (120 kN), the SRC pile continued to deflect
without any increase in load.
5.3 Inclinometer Data Reduction
An inclinometer was used to measure the slope at 2 ft (0.6 m) intervals along
the depth of the piles. Readings were intended to be taken after every 0.5 in (1.3
cm) of deflection as measured by the string potentiometer, which was placed 6.0 in
(15.2 cm) down from the top of the pile. Once the piles experienced significant pile
head deflection, however, inclinometer readings were discontinued for safety reasons.
Table 5.1 shows the number of readings taken and the load on the pile at the time
of the reading [17].
An Excel file was created to convert the inclinometer readings to deflection
values. An example of the inclinometer data is shown in Table 5.2.
45
Table 5.1: Inclinometer Readings Taken During Field Testing
Deflection
Pile
Type Load
[kips (kN)]
Before Inclinometer
Reading
[in(cm)]
After Inclinometer
Reading
[in(cm)]
Reading
0 0 0 0 0 0 Initial
7.5 (33.4) 0.5 (1.3) 0.65 (1.7) 1
10.7 (47.6) 1.0 (2.5) 1.20 (3.1) 2
12.7 (56.5) 1.5 (3.8) 1.73 (4.4) 3
14.9 (66.3) 2.0 (5.1) 2.25 (5.8) 4
17.0 (75.6) 2.5 (6.4) 2.77 (7.0) 5
19.1 (85.0) 3.0 (7.6) 3.29 (8.4) 6
21.2 (94.3) 3.5 (8.9) 3.93 (10.0) 7
IRC
24.4 (108) 4.5 (11.4) 5.03 (12.8) After Failure
0 0 0 0 0 0 Initial
14.8 (65.8) 0.5 (1.3) 0.68 (1.7) 1
19.2 (85.4) 1.0 (2.5) 1.14 (2.9) 2
22.4 (98.8) 1.5 (3.9) 1.75 (4.5) 3
24.5 (109) 2.0 (5.1) 2.31 (5.9) 4
26.2 (117) 2.5 (6.4) 2.86 (7.3) 5
28.0 (125) 3.0 (7.6) 3.78 (9.6) 6
SRC
29.0 (129) 4.0 (10.2) 5.05 (12.8) After Failure
The data has seven columns. The first column, the pointer, identifies the pile
number. The IRC pile test was recorded as pointer number 4 and the SRC pile is
marked as pointer number 5. The second column marks the ridge set or the reading
set number for that pile. The third column indicates the depth of the reading
relative to the top of the pile. The last four columns are the angle of inclination
readings on the A and B axis, respectively, for the 0o and 180o passes, respectively.
Once the data file was retrieved with the inclinometer readings, a spreadsheet
was created to convert these angle of inclination readings to slope in radians and
deflection along the pile. An average of the 0oand 180o readings was calculated for
46
Table 5.2: Example of Inclinometer Data
Pointer Rdg_Set Depth A_0 A_180 B_0 B_180
4 1 2 -177 190 -511 521
4 1 4 -135 150 -328 338
4 1 6 -117 134 -198 215
4 1 8 -145 160 -262 278
4 1 10 -158 173 -298 310
4 1 12 -135 151 -307 326
4 1 14 -143 159 -325 341
4 1 16 -143 161 -342 348
4 1 18 -165 180 -437 450
4 1 20 -183 199 -389 403
4 1 22 -140 156 -244 259
4 1 24 -148 165 -268 280
4 1 26 -186 205 -433 435
4 1 28 -244 259 -790 794
each depth on each data set. The initial data set readings were considered the zero
load point and were therefore subtracted from all of the following data set readings
at higher loads. Readings were converted to slopes using the following equation:
Reading = sin θ ∗ Instrument Constant (5.1)
The instrument constant for English units is 20,000 for our inclinometer and
therefore Equation 5-1 becomes:
sin θ =Reading (English Units)
20, 000(5.2)
Knowing the angle of inclination makes it possible to find the deflected shape of the
pile, using simple geometry. The hypotenuse is the length of the pile between
measurements and the side opposite the angle of inclination is the lateral deviation.
Figure 5.2 displays this concept.
47
Inclinometer
Casing
Angle of
Inclination
(θ)θ
Lateral Deviation
(L Sinθ)
MeasurementInterval (L)
Figure 5.2: Diagram of the Angle of Inclination and Related LateralDeviation
Because the measurement interval and angle of inclination is known, the
deviation can be calculated as:
Deviation = L ∗ sin θ (5.3)
where L is the measurement interval [24 in (61 cm)] and θ is the angle of
inclination. The deflected shape of the pile is achieved by summing the deviations
from the bottom to the top of the pile.
The calculations for slope and deflection from inclinometer angle of
inclination readings assume a two-dimensional deflection. Ideally, our inclinometer
readings were in-plane with the load. However, the load was in-line with the strain
gages which were slightly out of the plane containing the grove in the inclinometer
casing. This angle, marked φ in Figure 5.3, was measured 11o and 11.5o for the IRC
and SRC piles respectively.
48
Load
Orientation of Inclinometer Readings
φ
Figure 5.3: Slice of the Top of the Pile Showing the Angle Offset fromLine of Load to Inclinometer Readings
To account for this offset, two correction methods were employed and
compared. The first method calculated the resultant slope from the average A and
B axis readings using the following equation:
Corrected Reading =√
Reading A2 + Reading B2 (5.4)
This corrected reading represents the maximum slope, which should coincide with
the direction of load application. The deviations were found using this resultant
reading value.
The second method used the A axis reading and the measured angle offset
between the inclinometer and the loading plane shown in Figure 5.3. The deviations
49
were derived from the A axis readings and adjusted using the equation,
Corrected Displacement =DisplacementfromReading A
cos φ, (5.5)
where φ equals 11o and 11.5o for the SRC and IRC piles, repectively.
Results obtained using these processes show excellent correlation. However,
corrected angle results were used in subsequent calculations and results.
5.3.1 Strain and String Potentiometer Data Reduction
Over 100,000 data points were taken for each field test and therefore a
process of consolidation was necessary to reduce the data to workable numbers.
Each set of data was unique and two different processes were used for consolidation.
These two processes are explained in the following sections.
5.3.1.1 Data Consolidation
A program developed by CASC personnel, was the primary method applied
to reduce data. This program uses the process of least-squares to reduce a curve
with many data points to a curve with data points at regular intervals. The process
uses a straight line:
y = a + bx (5.6)
to approximate a data set with many points, (x1, y1), (x2, y2), ...,(xn, yn). The
least-square error, L.S.E., is found by squaring the difference between the data
50
points and the function evaluated at those points. Or, stated mathematically:
LSE =n∑
i=1
[yi − f(xi)]2 =
n∑i=1
[yi − (a + bxi)]2 (5.7)
This error can be minimized by taking the derivative of this function, setting it
equal to zero, and solving for the unknown variables a and b. Once a and b are
known, x can be determined at any point. The data consolidation program allows
the user to specify the step between the x values of the data points and the number
of data points used to derive the equation of the line for a specific region.
A load-time curve for the IRC pile is shown in Figure 5.4. Both the raw data
curve and the curve made with the data consolidated by the data consolidation
macro are displayed to show the accuracy of the macro.
One limitation of the data consolidation macro is that it cannot process data
sets that do not pass the vertical line test. Or, in other words, the macro can only
consolidate curves with consistently increasing x values. Reversal of the x values in
the data curves can be traced to two main causes. One cause is due to the pauses
taken during the loading process when inclinometer readings were taken. As
explained previously, due to continued deflection of the piles during inclinometer
readings, the load dropped slightly, causing reversal both in load and strain.
Another reason is that some strain gages did not experience high strains from the
tests and were highly affected by physical or electrical interference outside of the
testing process. In these instances the data seamed to fall around a general curve
51
0
5
10
15
20
25
30
35
0 20 40 60 80 100 120 140
Time [min]
Lo
ad
[k
ips]
0
1
2
3
4
5
6
7
Lo
ad
[k
N]
(a)
0
5
10
15
20
25
30
35
0 20 40 60 80 100 120 140
Time [min]
Lo
ad
[k
ips]
0
1
2
3
4
5
6
7
Lo
ad
[k
N]
(b)
0
5
10
15
20
25
30
35
0 20 40 60 80 100 120 140Time [min]
Loa
d [k
ips]
0
1
2
3
4
5
6
7
Loa
d [k
N]
Raw IRC Pile Data
Consolidated IRC Pile Data
(c)
Figure 5.4: Comparison of Data: (a) Raw; (b) Consolidated Using theConsolidation Macro; and, (c) Both
causing decreasing x-values in the data sets. Therefore, to consolidate these data
sets, a slight adjustment to the procedure was implemented.
5.3.2 Data Reversal Correction
A macro was developed to delete points with diminishing x values. This
Excel macro simply stepped through the data list, checked the x-value, compared
this value to the preceding x value, and deleted the data point if the x-value was
less than the previous x value. The new curve made from the adjusted data set is
plotted with the original raw data in Figure 5.5 to show the deleted reversal regions.
52
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7 8 9
Deflection [in]
Lo
ad
[k
ips]
0
20
40
60
80
100
120
140
0 5 10 15 20
Deflection [cm]
Lo
ad
[k
N]
(a)
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7 8 9
Deflection [in]
Lo
ad
[k
ips]
0
20
40
60
80
100
120
140
0 5 10 15 20
Deflection [cm]
Lo
ad
[k
N]
(b)
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7 8 9
Deflection [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
0 5 10 15 20Deflection [cm]
Loa
d [k
N]
Raw SRC Pile Data
Data Regression Corrected SRC Pile Data
(c)
Figure 5.5: Comparison of Data: (a) Raw; (b) Adjusted Using the Re-versal Macro, and; (c) Both
Once the reversal macro was completed for a particular data set, the data was
further consolidated using the data consolidation macro.
It is important to note that the deleted reversal points did not affect the
results of this test analysis. The relaxation of the strain gages during the
inclinometer readings is irrelevant in determining the strength and stiffness of the
piles.
53
54
Chapter 6
Experimental Results
6.1 Loading Rate
As mentioned in the procedure section, the loading was paused in order to
take inclinometer readings. During these pauses, the pile continued to deform,
resulting in a loss of pressure on the pile face. Because the hydraulic jack was not
capable of applying very small levels of pressure, the adjustments made by the jack
to compensate for the displacement resulted in high pressure variance during the
pauses. The effects of the pauses in both the SRC and IRC pile testing are evident
when the loading rate is plotted as shown in Figure 6.1.
6.2 Deflection
Both string potentiometers and the inclinometer were used to retrieve
displacement data for the two piles during the loading process. Although the
displacement was measured in each case, the results produced two primary
differences:
55
0
5
10
15
20
25
30
35
0 20 40 60 80 100 120 140
Time [min]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
Loa
d [k
N]
IRC Pile
SRC Pile
Figure 6.1: Load vs. Time from Field Tests
1. Measurements by the string potentiometers were recorded every 0.5 seconds
throughout the testing, while only five or six inclinometer measurements were
taken during the testing process.
2. One single string potentiometer for each pile took measurements at the load
point while the inclinometer recorded displacements every 2 ft (0.6 m) along
the depth of the pile.
6.2.1 String Potentiometer
A single string potentiometer was used for each pile to measure lateral
deflection at the pile head. This deflection is shown in Figure 6.2. The string
potentiometer was not placed exactly at the pile head but rather at the point of
56
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
IRC Pile from String Potentiometer DataSRC Pile from String Potentiometer Data
Figure 6.2: String Potentiometer Deflection from Field Tests
load application located 6 in (15.24 cm) from the top of the pile. However, for
simplicity, any deflection measurement taken at this point will be referred to as tip
deflection.
6.2.2 Inclinometer
Inclinometer readings were used to produce a deflected shape of the pile as
described in Chapter 5. The deflection data is plotted in two different ways. In
order to compare inclinometer deflection data to the deflection data recorded by the
string potentiometers, Figure 6.3 plots the pile head deflections. Figures 6.4 and 6.5
57
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
IRC Pile from Inclinometer DataSRC Pile from Inclinometer Data
Figure 6.3: Deflection at point of Load Application based on InclinometerReadings from Field Tests
plot the deflected shape of the piles at each inclinometer reading set for the IRC
and SRC piles, respectively. The deflections are essentially zero below depths of 8 ft
(2.4 m) and 5 ft (1.5 m)for the IRC and SRC piles, respectively. A comparison of
the deflection vs. depth curves for both the IRC and SRC piles is provided in Figure
6.6. The deflected shape is considerably more shallow for the SRC than the IRC
pile.
58
IRC Pile Load [kips(kN)]
-5
0
5
10
15
20
25
30
0 0.5 1 1.5 2 2.5 3 3.5 4Displacement Based on Inclinometer Readings [in]
Dep
th B
elow
Gro
und
Surf
ace
[ft]
-1.5
0.5
2.5
4.5
6.5
8.5
0 1 2 3 4 5 6 7 8 9 10Displacement Based on Inclinometer Readings [cm]
Dep
th B
elow
Gro
und
Surf
ace
[m]
7.5 (33.4)10.7 (47.6)12.7 (56.5)14.9 (66.3)17.0 (75.6)19.1 (85.0)21.2 (94.3)
Figure 6.4: Deflected Shape of the IRC Pile based on Inclinometer Read-ings from Field Tests
59
SRC Pile Load [kips (kN)]
-5
0
5
10
15
20
25
30
0 0.5 1 1.5 2 2.5 3 3.5 4Displacement Based on Inclinometer Readings [in]
Dep
th B
elow
Gro
und
Surf
ace
[ft]
-1.5
0.5
2.5
4.5
6.5
8.5
0 1 2 3 4 5 6 7 8 9 10Displacement Based On Inclinometer Readings [cm]
Dep
th B
elow
Gro
und
Surf
ace
[m]
14.8 (65.8)19.2 (85.4)22.2 (98.8)24.5 (109)26.2 (117)28.0 (125)
Figure 6.5: Deflected Shape of the SRC Pile based on Inclinometer Read-ings from Field Tests
60
Pile Load [kips (kN)]
-5
0
5
10
15
20
25
30
0 0.5 1 1.5 2 2.5 3 3.5 4Displacement [in]
Dep
th b
elow
Gro
und
Surf
ace
[ft]
-1.5
0.5
2.5
4.5
6.5
8.5
0 1 2 3 4 5 6 7 8 9 10
Displacement Based On Inclinometer Readings [cm]
Dep
th [m
]
7.5 (33.4)-IRC 14.8 (65.8)-SRC10.7 (47.6)-IRC 19.2 (85.4)-SRC12.7 (56.5)-IRC 22.2 (98.8)-SRC14.9 (66.3)-IRC 24.5 (109)-SRC17.0 (75.6)-IRC 26.2 (117)-SRC19.1 (85.0)-IRC 28.0 (125)-SRC21.2 (94.3)-IRC
Figure 6.6: Deflected Shape of the IRC and SRC Piles based on Incli-nometer Readings from Field Tests
61
6.2.3 Strain
Strain gage readings were taken during each test and the results are shown in
Figures 6.7, 6.8, and 6.9. Not all of the strain gages functioned properly and so the
malfunctioning strain data was not included. Strain gage depths are noted in the
legend. It is interesting to note that almost no strain was measured below the 10 ft
(3.0 m) depth which is consistent with the low deflection values observed with the
inclinometer tests.
IRC Pile Depth of Strain Gage [ft (m)]
0
5
10
15
20
25
30
35
-5000 -3000 -1000 1000 3000 5000 7000 9000microstrain
Loa
d [k
ips]
0
20
40
60
80
100
120
140
Loa
d [k
N]
IRC N-4.1 (1.2) IRC N-6.1 (1.9)IRC N-10.0 (3.0) IRC N-14.05 (4.3)IRC N-21.8 (6.6) IRC S-2.1 (0.6)IRC S-4.1 (1.2) IRC S-7.85 (2.4)IRC S-10.0 (3.0) IRC S-14.05 (4.3)IRC S-18.1 (5.5) IRC S-21.8 (6.6)IRC S-25.8 (7.9)
Figure 6.7: Strain vs. Load of the IRC Pile from Field Tests
62
SRC Pile Depth of Strain Gage [ft (m)]
0
5
10
15
20
25
30
35
-5000 -3000 -1000 1000 3000 5000 7000 9000microstrain
Loa
d [k
ips]
0
20
40
60
80
100
120
140
Loa
d [k
N]
SRC N-2.1 (0.6) SRC N-4.1 (1.2)SRC N-6.1 (1.9) SRC N-7.85 (2.4)SRC N-10.0 (3.0) SRC N-14.05 (4.3)SRC S-2.1 (0.6) SRC S-4.1 (1.2)SRC S-6.1 (1.9) SRC S-10.0 (3.0)SRC S-14.05 (4.3) SRC S-21.8 (6.6)
Figure 6.8: Strain vs. Load of the SRC Pile from Field Tests
Depth of Strain Gage [ft (m)]
0
5
10
15
2 0
25
3 0
35
-50 00 -3 00 0 -10 00 10 00 3 00 0 500 0 700 0 9 00 0
microstrain
Load
[kip
s]
0
2 0
4 0
6 0
8 0
100
120
140
Load
[kN
]IRC N-4 .1 (1.2 ) SRC N-2 .1 (0 .6 )IRC N-6 .1 (1.9 ) SRC N-4 .1 (1.2 )IRC N-10 .0 (3 .0 ) SRC N-6 .1 (1.9 )IRC N-14 .0 5 (4 .3 ) SRC N-7.8 5 (2 .4 )IRC N-2 1.8 (6 .6 ) SRC N-10 .0 (3 .0 )IRC S-2 .1 (0 .6 ) SRC N-14 .0 5 (4 .3 )IRC S-4 .1 (1.2 ) SRC S-2 .1 (0 .6 )IRC S-7.85 (2 .4 ) SRC S-4 .1 (1.2 )IRC S-10 .0 (3 .0 ) SRC S-6 .1 (1.9 )IRC S-14 .05 (4 .3 ) SRC S-10 .0 (3 .0 )IRC S-18 .1 (5.5) SRC S-14 .05 (4 .3 )IRC S-21.8 (6 .6 ) SRC S-21.8 (6 .6 )IRC S-25.8 (7.9 )
Figure 6.9: Strain vs. Load of the IRC and SRC Piles from Field Tests
63
64
Chapter 7
Analytical Procedure
Three different analyses were performed: 1) a commercial finite
difference-based computer program called Lpile; 2) a Winkler foundation model;
and, 3) a simple analysis based on fundamental mechanics of materials principles.
Procedures followed for these analyses comprise this chapter.
7.1 Lpile Program Analysis
Computer analysis of the pile testing was performed using Lpile version 4M.
This program models the behavior of a pile driven into specific soil strata using
finite difference equations. Therefore, by inputting our test pile properties, soil
properties, and boundary conditions, several predictions could be developed [18].
7.1.1 Soil Properties Input
The site used for these pile tests had been analyzed previously to determine
the soil properties of the area. Lpile allows the user to choose from nine different
types of soil from which Lpile automatically generates a soil-resistance (p-y) curve,
65
based on basic soil properties. A p-y curve can also be manually input if this
information is available. Because we did not have a p-y curve for the test site, soil
properties from the site were input into the Lpile program. Figure 7.1 shows the soil
properties used for the field test analysis in the Lpile program. Included are several
properties such as unit weight, stiffness, and undrained shear strength.
7.1.2 Pile Properties Input
Lpile offers two options for pile stiffness input. The first option requires the
user to input the properties of the pile including diameter, size and placement of
reinforcement, rebar strength, and concrete strength. From this information Lpile
generates a moment-stiffness curve for the pile. This approach works for the SRC
pile but not the IRC pile because Lpile only offers steel as a reinforcement option.
The moment-stiffness curve generated by Lpile given the SRC reinforcement
properties is shown in Figure 7.2.
The second option for pile stiffness is to input a moment-stiffness curve for
the pile. The moment-curvature graphs from the laboratory test data were used to
create moment-stiffness curves for both the SRC and IRC piles. Stiffness data was
taken from the five strain gages (gages 4 - 8) located between the two center point
loads in the lab bending test. The creation of the moment-stiffness curves used in
Lpile predictions included three steps:
66
0
109.22
136.7
167.1
304.3
350
411
517.7
Depth [cm]
Soil Type 1
γ = .054976852 pci
k = 100 pcic = 5e50 = .01
γ = .18865741 pci
k = 225 pci
φ = 38°
γ = .18865741 pci
k = 225 pci
φ = 36°
φ = 36° Soil Type 2
Soil Type 1
γ = Effective Unit Weight
k = p-y Modulus
φ = Friction Angle
Sand (Reese)
Stiff Clay without Free Water
γ = Effective Unit Weightk = p-y Modulusc = Cohesive Strengthe50 = Soil Strain
Depth [in]
γ = .018865741 pci
k = 225 pci
c = 10e50 = .007
203.8
161.8
137.8
119.8
65.8
53.8
43
0
Soil Type 2
γ = .018865741 pci
k = 1000 pci
c = 15.5e50 = .005
Soil Type 1
γ = .018865741 pci
k = 1000 pci
c = 15.5e50 = .005
Soil Type 1
γ = .018865741 pci
k = 500 pciSoil Type 1
Soil Type 2
Soil Type 2
Soil Type 1
γ = .054976852 pci
k = 500 pci
c = 10
e50 = .007
Figure 7.1: Soil Properties at the Test Site [3]
67
0
5
10
15
20
25
30
0 200 400 600 800 1000 1200 1400 1600 1800 2000Moment [in*lb x 10
3]
EI
[lb*
in 2 x
10
9 ]
0
2
4
6
8
10
12
14
16
18
0 50 100 150 200Moment [N*m x 10
6]
EI
[N*m
2 x
10 6 ]
SRC Pile Input for Lpile Prediction 1
Figure 7.2: Moment-Stiffness Generated by Lpile given SRC Pile Prop-erties
1. Check moment vs. curvature graphs from the lab tests for consistency using
Chauvenet’s Criterion [19];
2. Change the moment vs. curvature graphs from the lab tests to one average
moment-stiffness curve for each pile; and,
3. Develop a simplified moment vs curvature curve that can be used in Lpile for
each pile.
Test data for the lab tests included two bending tests each for both the SRC
and IRC piles. Five strain gages were located between the two load cells and
68
0
500
1000
1500
2000
0 200 400 600 800 1000
Curvature [microstrain/in]
Mo
men
t [k
ip-i
n]
0
50
100
150
200
0 50 100 150 200 250 300 350
Curvature [microstrain/cm]
Mo
men
t [k
N-m
]
1-SRC 1-IRC
2-SRC 2-IRC3-SRC 3-IRC
4-SRC 4-IRC5-SRC 5-IRC
6-SRC 6-IRC7-SRC 7-IRC
8-SRC 8-IRC9-SRC 9-IRC
S1
6
Load Cell 1 Load Cell 2 S2
5 4 3 2 1789
Figure 7.3: Lab Test Moment vs. Curvature Data
therefore a total of 10 gage readings for each pile were used to develop stiffness
data. Figure 7.3 shows moment vs curvature data from one of the SRC pile lab
tests. From this figure, one can see that the data from gage 8 of the SRC pile
deviated from the rest of the test data [19].
Chauvenet’s Criterion was used to determine if the slightly outlying data
could be deleted from the data set. Chauvenet proposed that a data point could be
deleted from a data set with n number of readings if the probability of that point
deviating from the mean was less than 12n
. Using this criteria, an acceptable
envelope was plotted for gage 8 and is shown in Figure 7.4.
69
0
200
400
600
800
1000
1200
0 100 200 300 400 500 600 700 800 900 1000
Curvature [microstrain/in]
Mom
ent [
kip-
in]
0
20
40
60
80
100
120
0 50 100 150 200 250 300 350
Curvature [microstrain/cm]
Mom
ent [
kN-m
]
SRC Pile Gage 8 DataChauvenet Envelope
Figure 7.4: Chauvenet’s Criterion Envelope for Lab Test SRC Pile 2Gage 8
The envelope showed that 22% of the curve did not pass Chauvenet’s
Criterion. However, the failing region is barely outside of the envelope. Because
more than three-fourths of the curve fit Chauvenet’s Criterion and the failing
portion did not fall far from the envelope, the data was determined to be legitimate
and therefore kept in the data set.
Because the moment-curvature data had been consolidated at equal
intervals, creating an average curve for each pile was straight forward. The data
progressed in curvature increments of 0.5 microstrain/in (0.197 microstrain/cm);
70
0
5
10
15
20
25
30
0 200 400 600 800 1000 1200 1400 1600 1800 2000Moment [in*lb x 10
3]
EI
[lb*
in 2 x
10
9 ]
0
2
4
6
8
10
12
14
16
18
0 50 100 150 200Moment [N*m x 10
6]
EI
[N*m
2 x
10 6 ]
IRC Pile Lab Test
SRC Pile Lab Test
Figure 7.5: Moment vs Stiffness from Laboratory Testing
therefore an average moment was taken at these curvature increments using the 10
moment values.
With the moment plotted on the y-axis and the curvature on the x-axis, the
stiffness is the slope of the moment vs. curvature lab tests. A slope calculating
function in Excel was used to find the slope using 3-point data sets. For example,
the average slope of data points one through three yields the corresponding slope
value for the moment at data point two, etc. The average moment-stiffness curves
for the IRC and SRC piles are shown in Figure 7.5.
71
To allow lpile to apply the moment-stiffness curve to the analysis, the curve
needed to be further simplified. This is a fairly new application in the Lpile
program and the processes required to simplifiy the data to a form that Lpile could
use was not explained in the user or technical manuals. Therefore, the stiffness was
averaged over moment sections of 100 in*lb x 103 (17 N*m x 106) to form the
simplified curve shown in Figure 7.6.
0
5
10
15
20
25
30
0 200 400 600 800 1000 1200 1400 1600 1800 2000Moment [in*lb x 10
3]
EI
[lb*
in 2 x
10
9 ]
0
2
4
6
8
10
12
14
16
18
0 50 100 150 200Moment [N*m x 10
6]
EI
[N*m
2 x
10 6 ]
IRC Pile Lab TestSRC Pile Lab TestIRC Pile Input for Lpile Prediction 2SRC Pile Input for Lpile Prediction 2
Figure 7.6: Moment vs. Stiffness curve from Laboratory Testing withSimplified Curve for Lpile Input
72
7.2 Winkler Foundation Model Analysis
A Winkler foundation model was applied to the test piles in order to predict
the slope, deflection, and moment along the length of the pile. This analysis is
similar to the Lpile ananlysis in that it requires input of soil properties and pile
properties. The difference is that while the Lpile program uses non-linear soil and
pile stiffness values, the Winkler foundation mondel used linear soil and pile stiffness
values. The three values of soil stiffness applied were the maximum, minimum, and
weighted average soil stiffness in the soil profile. The average soil siffness was
weighted by the depth of soil in the profile it extended. Pile stiffness was taken as
the average stiffness of the piles from laboratory tests.
Using the Winkler foundation model required two assumptions that were not
exactly indicative of our test situation. The first assumption was that the soil
behaves elastically. The soil, in fact, has some plastic behavior. The second
assumption was that the beam is semi-infinite and therefore fixed on one end. The
pile is not semi-infinite, although it is long enough to exhibit cantilever behavior.
One simplification was applied to utilize a simpler form of the Winkler
equations. The point load was shifted down to the ground surface by adding a
corresponding moment. As a result of this adjustment, the equations derived from
the Winkler analysis are only valid below the ground surface. These concepts are
shown in Figure 7.7.
73
(a) (b)
x
Z,w P
M
Ground Surface
x
Z,w
P
Ground Surface
h
Figure 7.7: Elastic Foundation Model: (a) As Loaded; and, (b) StaticallyAdjusted Load for Winkler Foundation Model
The moment, M, in the pile at the ground surface is given by:
M = −Ph (7.1)
where h is the distance from the ground surface to the load application point.
The governing equation for a uniform beam on a Winkler foundation is
[Elastic Foundations]:
EId4w
dx4+ kw = q (7.2)
where x is the depth below ground surface [L], w is the deflection of the beam [L], k
is the soil stiffness [F/L2], E is the modulus of elasticity of the beam [F/l2], I is the
moment of inertia of the beam [L4], and q is the distributed load on the beam [F/L].
In the case of the pile, there is no distributed load and therefore q is zero [20].
74
When the differential equation is solved, the following Equations 7.3, 7.4,
and 7.5 are obtained for the deflection, w[L]; slope, θ[rad]; and, moment, M [F ∗ L]
respectively:
w(x) =2βP
kDβx −
2β2M
kCβx (7.3)
θ(x) =2β2P
kAβx +
4β3M
kDβx (7.4)
M(x) = −P
βBβx + MAβx (7.5)
where
β =
(k
4EI
) 14
(7.6)
and
Aβx = e−βx(cos βx + sin βx) = Dβx + Bβx (7.7)
Bβx = e−βx sin βx (7.8)
Cβx = e−βx(cos βx− sin βx) = Dβx −Bβx (7.9)
Dβx = e−βx cos βx (7.10)
Equations 7.3, 7.4, and 7.5 can be further simplified with the substitution of
Equation 7.1. This substitution yields the following Equations 7.11, 7.12, and 7.13
for w, θ, and M respectively:
w(x) =2βP
k(Dβx + βhCβx) (7.11)
θ(x) =2β2P
k(Aβx − 2βhDβx) (7.12)
M(x) = −P
(Bβx
β+ hAβx
)(7.13)
75
h
wgswθwc
Ground Surface
wPH
Figure 7.8: Three Displacement Components for Pile
Equations 7.11, 7.12, and 7.13 were used to describe the behavior of the IRC
and SRC piles below ground surface, as a function of P , x, EI, and k.
Equation 7.11 provides a relationship between displacement, w, and load, P .
However, in order to compare these results to those obtained by the string
potentiometer, these deflections must continue above the ground surface. Additional
displacement occurs at the load point which is 18 in (0.46 m) above the ground.
Therefore, there are three components to the pile displacement above the ground
surface, as shown in Figure 7.8.
76
The total deflection of the pile at the point of load application, wPH is equal
to the sum of these three components:
wPH = wgs + wθ + wc (7.14)
where wgs, wθ, and wc are as shown in Figure 7.8. The first displacement is the
deflection at the ground surface, wgs. Equation 7.11 defined this displacement for a
Winkler foundation. At the ground surface, x is zero and consequently both Dx and
Cx equal 1, simplifying the deflection equation to:
wgs =2P
k(β + β2h) (7.15)
The second displacement, wθ, component is from the rotation of the pile at
the ground surface. This concept can be seen in Figure 7.9.
Using geometry, the relationship among deflection (w), distance above
ground surface (h), and the angle of the pile at the ground surface (θ) is as follows:
tan θ0 =wθ
h(7.16)
Because θ is small, the small angle assumption (tan θ0 ≈ θ0) can be used, therefore:
wθ = θ0h (7.17)
where θ0 = θ(x = 0) as defined in Equation 7.12. The slope is desired at the ground
surface and so, similar to displacement, x is zero and the two variables Ax and Bx
equal 1, simplifying the slope equation to:
θ0 =2P
k(β2 − 2β3h) (7.18)
77
h
Ground Surface
θ0
wθ
Figure 7.9: Deflection of the Beam due to Rotation at the Ground Sur-face
Substituting Equation 7.18 into Equation 7.17 yields the equation:
wθ =2Ph
k(β2 − 2β3h) (7.19)
The third component is from cantilever bending of the pile, which basic beam
theory defines as:
wc =Ph3
3EI(7.20)
where h is the distance from the ground surface to the point of load application, 18
in (0.46 m).
Once all components are defined, the total deflection can be obtained by
substituting Equations 7.15, 7.19, and 7.20 into Equation 7.14 to yeild:
wPH =(
2P
k(β + β2h)
)+ h
(2P
k(β2 − 2β3h)
)+
Ph3
3EI(7.21)
78
Pile Head deflection can be plotted for load values up to theoretical failure. Beam
theory in combination with the Winkler equations can be used to find the
theoretical failure point. Beam theory states that:
σf =Mc
I(7.22)
where M is the maximum moment, σf is the failure stress of the reinforcement
material, I is the moment of inertia of the pile and c is the distance from the
centroid of the reinforcement to the outermost fiber of reinforcement. By
substituting Equation 7.13 into Equation 7.22, the stress at failure is:
σf = −Pc
I
(Bβx
β+ hAβx
)(7.23)
Substituting the necessary equations from Equations 7.8 and 7.7 into Equation 7.23
gives:
σf (x) = −Pc
I
[e−βx sin βx
β+ he−βx(cos βx + sin βx)
](7.24)
When Equation 7.24 is differentiated with respect to the position (x), the following
equation results:
σ′f =
dσf
dx= −Pc
I
[e−βx(− sin βx + cos βx− 2hβ sin βx)
](7.25)
Equation 7.25 can be equated to zero in order to solve for the position on the pile
where the greatest stress occurs in the reinforcement:
0 = −Pc
I
[e−βx(− sin βx + cos βx− 2hβ sin βx)
](7.26)
79
7.3 Application of Mechanics of Materials
Mechanics of materials equations were used to determine the location of the
neutral axis and moment of inertia of the cracked pile. Using these properties, the
moment capacity of the piles was calculated.
7.3.1 Cracked Moment of Inertia
After the load is applied to the pile, the portion of the concrete in tension
begins to crack and therefore changes the effective moment of inertia of the pile. The
cracked moment of inertia was determined by finding the neutral axis of the cracked
cross-section and applying the parallel axis theorem to adjust the moment of inertia.
The neutral axis is located a distance y from the center of the pile where the
compressive strength of the concrete above this axis is equal to the tensile strength
of the reinforcement below the axis. A linear stress distribution was assumed for the
concrete and the reinforcement. This concept is portrayed in Figure 7.10 where C
represents the compression strength of the concrete and Ti represents the tensile
stress on the reinforcement.
The location of the neutral axis, y, is unknown but can be determined. For
equilibrium to occur in the pile cross section, the following relationship must exist:
C =∑
i
Ti (7.27)
where C and T are both functions of y.
80
Center Line
Neutral Axis
C
T1
y
T2
T3 y
d2 d1
fc’
fy
Figure 7.10: Shifted Neutral Axis of Cracked Concrete Pile
The magnitude of the tensile strength T of the reinforcement is the product
of the reinforcement area and the tensile stress. As shown in Figure 7.10, the
bottom reinforcing bar is fully stressed and therefore the magnitude of T1 is:
T1 = Arfy (7.28)
where Ar is the area of the reinforcement and fy is the tensile (yield) strength of the
reinforcement. Using similar triangles, T2 and T3 are, respectively:
T2 =T1L2
L1
(7.29)
T3 =T1L3
L1
(7.30)
where:
L1 = y + d1 (7.31)
L2 = y + d2 (7.32)
L3 = y (7.33)
81
Using the stress distribution shown in Figure 7.10, the total tension force is
calculated:
T = T1 + 2T2 + 2T3 (7.34)
It is important to note that if the value of y was greater than d1 or d2, additional
reinforcing bars would be added to the total tension force using similar triangles as
was used to determine T2 and T3.
Like the reinforcement, the strength of the concrete section, C, is the
product of the stress on the concrete compression section and the area over which it
is applied. However, unlike the reinforcement which could be approximated as a
localized force, the area and stress of the concrete section is distributed and requires
integration. Therefore, an integral was applied to determine the strength of the
concrete section. For practical purposes a numerical integration was used instead of
an analytical integration. As shown in Figure 7.11, the area of the concrete
compression section was divided into slices defined by angle α.
The relationship between the area of the circle above the point of interest,
Ac, and the angle representing the point of interest, α, is given by:
Ac(α) = R2p(α− sin α cos α) (7.35)
where:
α = cos−1
(h
Rp
)(7.36)
82
Ac(α) Ac(α+dα)
Rp
Neutral Axis
Center Line
y α dα h
90− αΝΑ
Figure 7.11: Area of a Circular Segment [4]
Therefore the area of the ith slice is:
Ai = Ac(αi + dα)− Ac(αi) (7.37)
As shown in Figure 7.12, the top most fiber of the compression section experiences a
stress of f ′c at failure.
Using similar triangles, the stress in the slice at a distance h from the center
is:
f ′ch =
(h− y)
(Rp − y)f ′
c (7.38)
With the stress and the area of each slice defined, C is:
C =∑−i = 1N (h− y)
Rp − yf ′
cAi (7.39)
83
Center Line
Neutral Axis y
fc’
hRp
fc’h
Figure 7.12: Stress Distribution in Concrete Compression Region
An Excel spreadsheet was created to determine the neutral axis distance y
that resulted in an equal value for the compression, C, and the tension, T .
Using this value for y, the cracked moment of inertia is the sum of the
moment of inertia of the section of concrete that is not cracked and the effective
moment of inertia of the reinforcing bars outside of the uncracked section of
concrete. The moment of inertia of the concrete section about the neutral axis is [4]:
Ic =R4
p
4(αNA − sin αNA cos αNA + 2 sin3 αNA cos αNA) (7.40)
where αNA is as shown in Figure 7.11.
The moment of inertia of the ith steel bar is given by:
Ii = Ir + Ard2i (7.41)
84
Table 7.1: Material Properties
IRC Pile SRC Pile
Property From Lab Tension Tests[Toray]
From Lab Tension Tests[McCune]
Compressive Strength of the Concrete, f'c
[ksi (N/m2]7.22 (20.7) 7.22 (20.7)
Yield Strength of the Reinforcement, fy
[ksi (N/m2]262 (750) 67.8 (195)
Ultimate Strength of the Reinforcement, fu
[ksi (N/m2]262 (750) 106 (304)
Modulus of Elasticity of the Concrete, Ec
[psi (N/m2)] x 1034.8 (13.8) 4.8 (13.8)
Modulus of Elasticity of the Reinforcement, Er
[psi (N/m2)] x 10317.1 (49.2) 29.0 (83.2)
Moment of Inertia of the Reinforcement, Ir
[in4 (cm4)]27.1 (1130) 46.4 (1930)
Moment of Inertia of the Concrete, Ic
[in4 (cm4)]599 (24900) 203 (8450)
Moment of Inertia of the Pile, Ip
[in4 (cm4)]695 (28900) 481 (20000)
where Ar is the area of the reinforcement member, di is the distance to the neutral
axis, and:
Ir =πd4
r
64(7.42)
where dr is the diameter of the reinforcement. The total cracked moment of inertia
of the pile section can be stated:
Ipile = Ic =5∑
i=1
Ii (7.43)
The pile properties used to calculate the tension and compression forces and
the neutral axis are shown in Table 7.1.
85
7.3.2 Pile Moment Capacity
The moment capacity of the pile is found by summing the moments resisted
by the concrete and the steel about any axis in the pile. Using the cracked neutral
axis found previously, the magnitude of the tension forces of the steel, T , and
compression force of the concrete, C, the moment capacity can be determined. For
simplicity, the moment arms were measured from the central line of the pile. These
values are d1 and d2 for the steel and values between y and Rp for the concrete
slices. As in the moment of inertia calculations, the concrete is represented as a
summation of slices. The moment capacity of each slice is determined by
multiplying the compressive force found previously by its distance to the center line
of the pile. The total moment capacity is determined by summing the slices. The
resulting moment equation is:
M =∑
i
Cihi +∑
i
Tidi (7.44)
The results obtained for the location of the neutral axis, cracked moment of
inertia and moment capacity are shown in Table 7.2.
The moment capacity can be used to determine the ultimate load by using
Equation 7.13 and the position of maximum stress derivation, expressed in
Equation 7.26. The ultimate load is shown for different soil stiffness values in
Table 7.3. These soil stiffness values correspond to the minimum, average, and
maximum soil stiffness, respectively, at the test site.
86
Table 7.2: Mechanics of Materials Analysis Results
IRC Pile SRC Pile
Property Based on Properties From
Lab Tension Tests
[Toray]
Based on Properties From
Lab Tension Tests
[McCune]
Distance from Center to Cracked Neutral Axis, x
[in (cm]
2.53 (6.4) 4.47 (11.4)
Cracked Moment of Inertia, I
[in4 (cm4] 695 (28000) 481 (20000)
Moment Capacity, Mmax
[kip-in (kN-m)] 1131 (127) 439 (49.6)
Table 7.3: Pile Failure Loads
Soil Stiffness, k IRC Pile Failure Load SRC Pile Failure Load
[pci (kN/cm3)] [kip (kN)] [kip (kN)]
100 (7.3) 36 (160) 13.9 (61)
500 (36.4) 43.6 (194) 16.9 (75)
1000 (72.9) 46.6 (207) 18.1 (81)
87
88
Chapter 8
Analytical Results
Results for the the Lpile analysis and Winkler foundation model are
presented in this chapter.
8.1 Lpile Deflection Predictions
Using the computer program, Lpile, pile head deflections were computed for
various load levels. Predictions were made with two different types of pile input.
The first prediction, Lpile prediction 1, uses the Lpile generated moment-stiffness
for the pile given the SRC pile properties. This prediciton is only made for the SRC
pile. The second prediction, Lpile prediction 2, uses the laboratory moment-stiffness
data for the pile stiffness input. This prediction is made for the both the IRC and
SRC piles. Table 8.1 summarizes the Lpile prediction notation.
Pile head deflection from the two Lpile predictions are shown in Figures 8.1
and 8.2.
89
Table 8.1: Lpile Prediction Notation
Lpile Prediction 1 Lpile Prediction 2 Soil
Input As Measured at the Test Site As Measured at the Test Site
Pile Input
Lpile-Generated Pile Stiffness Given Pile
Properties
Laboratory Test Pile Stiffness
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
160
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
SRC Pile from Lpile Prediction 1
Figure 8.1: Lpile Prediction 1: Load vs. Deflection of the SRC Pile fromthe Field Tests
When the two predictions for the SRC pile are shown together as in
Figure 8.3, the difference in the predictions is apparent. Lpile prediction 1 assumes
field conditions. The second Lpile prediction was made with lab pile stiffness
properties. As was mentioned in the previous chapter, adequate information about
90
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
160
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
IRC Pile from Lpile Prediction 2 SRC Pile from Lpile Prediction 2
Figure 8.2: Lpile Prediction 2: Load vs. Deflection of the IRC and SRCPiles from Field Tests
Lpile prediction 2 using nonlinear pile stiffness is not available in the Lpile manuals.
Also, significant error could have been induced through the manipulation of the
stiffness data required to apply it as input for Lpile. Therefore, because the SRC
pile Lpile prediction 2 is significantly different than Lpile prediction 1, Lpile
prediction 2 may not be the best representation of the field test and will not be used
to compare to the field test results. However, because the same process and data
was used for the the IRC and SRC pile input in Lpile prediction 2, they may be
compared to one another. This comparison reveals that the IRC pile should have a
much higher load capacity than the SRC pile.
91
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
160
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
SRC Pile from Lpile Prediction 1IRC Pile from Lpile Prediction 2 SRC Pile from Lpile Prediction 2
Figure 8.3: Lpile Prediction 1 and 2: Load vs. Deflection of the IRC andSRC Piles from Field Tests
8.2 Winkler Foundation Model Deflection Predictions
Using the Winkler foundation model equations from Chapter 7, pile head
deflection was predicted and plotted at different load levels. Figures 8.4 and 8.5 plot
the deflection at the load point, located 6 in (15.2 cm) below the pile head, for the
IRC and SRC piles, respectively. Three curves are plotted for each pile. The three
curves correspond to three different soil stiffness values used for the calculations.
The three stiffness values are the minimum, weighted average, and maximum soil
stiffness for all of the soil layers found at the test site. The average was weighted by
summing the soil stiffness multiplied by the depth over which it covered and then
92
IRC Pile Winkler Model Deflection Predictions with
Soil Stiffness k [pci (kN/cm3)]
0
5
10
15
20
25
30
35
40
45
50
0 0.5 1 1.5 2 2.5 3 3.5Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
50
100
150
200
0 1 2 3 4 5 6 7 8Deflection at Point of Load Application [cm]
Loa
d [k
N]
k = 100 (7.3)k = 500 (36.4)k = 1000 (72.9)
Figure 8.4: Winkler Foundation Model Predicted Deflection at Point ofLoad Application of the IRC Pile from Field Tests
deviding by the total depth. The curves end at the predicted failure value found
using the Winkler foundation model in conjuction with the moment capacity of the
piles. For these values, please refer to Table 7.3.
The Winkler foundation predictions differ from the Lpile predictions and
actual field results because the pile stiffness is assumed to be linear. Because the
pile stiffness is similar for the IRC and SRC piles, the slope of the corresponding soil
stiffness load-deflection curves are nearly the same. The only difference in the two
figures is the predicted failure. Winkler equations predict the failure of the SRC pile
to be between 13.9 kips (61 kN) and 18.1 kips (81 kN) and the IRC pile to be
93
SRC Pile Winkler Model Deflection Predictions with
Soil Stiffness k [pci (kN/cm3)]
0
5
10
15
20
25
30
35
40
45
50
0 0.5 1 1.5 2 2.5 3 3.5
Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
50
100
150
200
0 1 2 3 4 5 6 7 8
Deflection at Point of Load Application [cm]
Loa
d [k
N]
k = 100 (7.3)k = 500 (36.4)k = 1000 (72.9)
Figure 8.5: Winkler Foundation Model Predicted Deflection at Point ofLoad Application of the SRC Pile from Field Tests
between 36 kips (160kN) and 46.6 kips (207 kN). The deflection at failure is
significantly altered by the linear pile stiffness assumption. In reality the piles would
decrease in stiffness as they neared failure. The Winker foundation prediction shows
that the SRC pile only deflects between 1 in (2.5 cm) and 1.2 in (3.0 cm) before
failure when the actual results show almost 5 in (12.7 cm) of deflection before
failure. Similarly, the Winker foundation model predicts between 2.5 in (6.35 cm)
and 3.2 in (8.1 cm) of deflection before failure of the IRC pile while field results
show nearly 8 in (20.3 cm) of deflection. In general, what the Winkler foundation
94
model shows is that the IRC pile is expected have a higher load capacity and deflect
more at failure than the SRC pile.
95
96
Chapter 9
Discussion of Results
The data gathered during the field testing of the SRC and IRC piles is
suspicious and contradictory and therefore likely erroneous. In an attempt to
extract as much information as possible, the data has been examined in both
conventional and non-conventional ways. Through study of the results and
comparisons of the results to laboratory tests, Winkler foundation model
predictions, and Lpile predictions, four potential sources of error were determined:
1. SRC pile load data;
2. Soil properties;
3. Steel reinforcement splice location; or
4. Damage to the IRC pile before testing.
The following sections describe the reasons for doubting the field test data and
evaluate the possible sources of error.
97
0
10
20
30
40
50
60
70
0 1 2 3 4 5 6
Deflection [in]
Tot
al T
rans
vers
e L
oad
[kip
s]
0
50
100
150
200
250
300
0 5 10 15 20 25
Deflection [cm]
Tot
al T
rans
vers
e L
oad
[kN
]
S1-SRC S1-IRCL4-SRC L4-IRCL3-SRC L3-IRCL2-SRC L2-IRCL1-SRC L1-IRCC-SRC C-IRCR1-SRC R1-IRCR2-SRC R2-IRCR3-SRC R3-IRCR4-SRC R4-IRCS2-SRC S2-IRC
R4R3R2R1CL1L2L3L4
S2S1 Load Cell 2Load Cell 1
Figure 9.1: Deflections of All Piles in Lab Tests
9.1 Pile Stiffness
9.1.1 Comparison to Lab Stiffness Results
The piles tested in the laboratory were constructed not only with similar
construction and materials but at the same time as the piles tested in the field.
Consequently, similar results in strength and stiffness should be expected.
Surprisingly, data retrieved from field testing did not concur with that from the lab
testing. The most obvious difference is the stiffness. Figure 9.1 (repeated from
Figure 3.3) and Figure 9.2 plot the load vs. deflection curves for the lab and field
tests respectively. The laboratory tests show that the two piles have similar stiffness
98
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
IRC Pile from String Potentiometer DataSRC Pile from String Potentiometer Data
Figure 9.2: Load vs. Deflection based on String Potentiometer Readingsfrom Field Tests
until the SRC pile begins to yield. However, the field test shows that the SRC pile
has a significantly higher initial stiffness than the IRC pile.
9.1.2 Verification of Lab Stiffness Results
Three test specimens of each pile type were tested in the laboratory, one in
axial compression and two in four-point bending tests. All three tests gave
consistent stiffness values for each pile type. In addition, stiffness values can be
calculated from the material properties. The composite stiffness is simply the sum
of the stiffness of the reinforcement and the concrete. Results for the pile stiffness
99
Table 9.1: Comparison of Laboratory Test and Predicted Stiffness Values
Stiffness
[lb-in2 x 109 (N-cm2 x 109)] Source SRC Pile IRC Pile
Lab Compression Tests 4.3 (123) 3.8 (109) Lab Bending Tests 3.8 (109) 3.4 (98) Predicted 3.7 (106) 3.8 (110) Average 3.9 (113) 3.7 (106) Standard Deviation 0.33 (9.5) 0.24 (6.9)
calculations are shown in Table 9.1.2 with the laboratory stiffness results to show
the consistency among the stiffness values.
9.2 Deflection
Two independent sources were used to gather deflection information during
the field testing. One source was the string potentiometers which gathered tip
deflection readings. The second source was the inclinometer which took slope
measurements along the length of the pile from which tip deflection was derived.
Figure 9.3 plots the tip deflection from each source on the same plot. The
agreement between the two sources is very strong; especially considering the
inclinometer deflection was derived from slope readings. The closeness of the two
independent results verifies the accuracy of the deflection data. Although the
deflection data appears accurate, the load-deflection curves are suspicious, as
indicated earlier, thus inferring that the load data is probably not correct.
100
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
IRC Pile from Inclinometer DataSRC Pile from Inclinometer DataIRC Pile from String Potentiometer DataSRC Pile from String Potentiometer Data
Figure 9.3: String Potentiometer and Inclinometer Tip Deflection Re-sults from Field Tests
9.3 Loading Rate
The oddity of the load data is also apparent when loading rate is considered.
According to the data shown in Figure 9.4, the SRC pile took over twice as much
load as the IRC pile within the first three minutes of testing. However, after the
first few minutes of testing, the rate of load change with time is very similar for
both piles. This is shown in Figure 9.5 where the SRC and IRC curves have been
collapsed so that the origin is located at the point just after the first inclinometer
reading break (as marked in Figure 9.4). The first inclinometer reading was taken
for both piles when the pile reached 0.5 in (1.3 cm) of displacement. Because the
101
0
5
10
15
20
25
30
35
0 20 40 60 80 100 120 140
Time [min]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
Loa
d [k
N]
IRC PileSRC PileIRC Pile Adjustment PointSRC Pile Adjustment Point
Figure 9.4: Load vs. Time from Field Tests
0
5
10
15
20
25
30
35
0 20 40 60 80 100 120 140Time [min]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
Loa
d [k
N]
IRC PileSRC Pile
Figure 9.5: Adjusted Load vs. Time from Field Tests
102
stiffness of the IRC and SRC piles is so similar, at such a small deflection this load
would be expected to be very similar. The load difference between the first
inclinometer readings is 6.5 kips (28.9 kN).
9.4 Energy
Further suspicion of the load data can be validated by energy considerations.
Conservation of energy states that the energy put into the pile through the
hydraulic jack should be equal to the energy absorbed by the pile and surrounding
soil. Because the piles have the same stiffness, the energy absorbed by the piles
should be equal until failure or at least until yielding begins. Therefore, at
equivalent load levels, the SRC and IRC piles should apply equal amounts of energy
on the surrounding soil.
To determine the energy absorbed by the surrounding soil, the soil was
modeled as a spring with stiffness equal to the soil stiffness. This required an
assumption that the soil behaved perfectly elastic when in reality the soil
experienced some plastic deformation. Although the elastic assumption may not be
numerically accurate, the plastic differences will be the same for both piles making
this a viable way to compare the soil compaction energies.
The equation for the energy of a spring is given by:
Us =1
2kx2 (9.1)
103
where Us is the energy of the spring, k is the spring stiffness, and x is the spring
displacement. To apply this theory to the soil, x was defined as the displacement of
the pile and k was defined as the stiffness of the soil. To convert the soil stiffness to
spring stiffness, the soil stiffness was multiplied by the pile diameter, D. To
determine the deflection, x, of the pile, a sixth-order polynomial equation was fit to
the deflected shape of the pile as recorded by the inclinometer data. Deflection, w,
is a function of the position, x, along the pile. The total soil energy was obtained by
integrating the soil energy over the length of the pile. The modified soil compaction
energy is given by:
Us =∫ L
0
D
2kw(x)2dx (9.2)
Figure 9.6 shows the results for the SRC and IRC pile soil compaction energy as
calculated using Equation 9.2. The figure makes it clear that for a particular load
level, the soil compaction energies are not the same. In fact, the calculated average
difference in load at a particular load level is 7.1 kips (31.6 kN). This value is very
similar to the 6.5 kip (28.9 kN) difference between the first inclinometer reading
loads on the load vs. time charts.
9.5 Energy-Modified Results
To account for the energy difference shown in Figure 9.6, the SRC pile load
was decreased the average difference in energy, 7.1 kips (31.6 kN). Modifying the
SRC pile test data revealed an accord with the laboratory findings, material
104
0
100
200
300
400
500
600
700
800
0 5 10 15 20 25 30 35
Load [kips]
Soil
Dis
plac
emen
t Ene
rgy
[kip
*ft])
0
200
400
600
800
1000
0 20 40 60 80 100 120 140Load [kN]
Soil
Dis
plac
emen
t Ene
rgy
[kJ]
IRC PileSRC PileDifferenceAverage Difference
Figure 9.6: Soil Compaction Energy of the IRC and SRC Piles
properties, and predictions. The modified load vs. deflection results are shown in
Figure 9.7.
The modified load vs. deflection curve shows similar behavior to lab test
results. The initial slope of the SRC pile is slightly steeper than the IRC pile,
indicating a slightly higher stiffness in the SRC pile. This difference in stiffness is
also apparent in the lab test results where the stiffness of the SRC pile was 12%
greater than the stiffness of the IRC pile. The calculated stiffness of the piles also
suggested a slightly greater stiffness for the the SRC pile.
105
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d (S
RC
Pile
Loa
d A
djus
ted)
[kip
s]
0
20
40
60
80
100
120
140
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d (S
RC
Pile
Loa
d A
djus
ted)
[kN
]
IRC Pile from String Potentiometer DataSRC Pile from String Potentiometer Data
Figure 9.7: Energy-Modified Load vs. Deflection Data from Field Tests
When Lpile prediction 1 is plotted with the IRC pile deflectin as recorded by
the string potentiometers in the field, the prediction matches the initial slope and
therefore stiffness of the IRC pile. This comparison, shown in Figure 9.14, validates
the load-deflection data for the IRC pile.
In addition, when SRC pile Lpile prediction 1 is plotted with the SRC pile
results from the field tests, as shown in Figure 9.9, the results are not equivalent.
However, if the load of the SRC piles in the field is decreased by 7.1 kips as
suggested by the energy calculations, Figure 9.10 shows that the adjusted field
results are similar to the Lpile prediction.
106
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
160
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
IRC Pile from String Potentiometer DataSRC Pile from Lpile Prediction 1
Figure 9.8: Lpile Deflection Prediction for the SRC Pile Compared toString Potentiometer Deflection Results for the IRC Pile in the Field
9.6 Lpile Adjusted Soil Predictions
Variability of soil stiffness surrounding the test piles could explain the
difference in energy. If the soil surrounding the SRC pile was stiffer than the soil
data gathered at the test site, this would produce a higher load capacity for the
SRC pile. To see the difference soil properties can make, alterations were made to
the top two layers of the soil in Lpile prediction 1. Only the top two layers were
altered because inclinometer data did not show significant deflection in deeper soil
layers. These two layers were increased in strength until the resulting load-deflection
107
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
160
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
SRC Pile from String Potentiometer Data
SRC Pile from Lpile Prediction 1
Figure 9.9: Lpile Deflection Prediction for the SRC Pile Compared toString Potentiometer Deflection Results for the SRC Pile in the Field
Table 9.2: Original and Adjusted Soil Properties for the Top Two Layersin the Soil Profile
Soil Property As Measured
at the Test Site
Adjusted to Match SRC Pile Field
Results
Soil Strain 0.007 0.5 Cohesive Strength 10 120
data matched the load-deflection data gathered in the field. The altered soil
properties are shown in Figure 9.6.
108
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d (S
RC
Pile
Loa
d A
djus
ted)
[kip
s]
0
20
40
60
80
100
120
140
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
SRC Pile from String Potentiometer DataSRC Pile Lpile Prediction 1
Figure 9.10: Lpile Deflection Prediction for the SRC Pile Compared toAdjusted String Potentiometer Deflection Results for the SRC Pile inthe Field
If the soil surrounding the SRC pile was different than the field-tested soil
properties, the resulting load-deflection curve, shown in Figure 9.11, could match
the actual field test behavior. However, the soil properties required to produce the
actual field data are unlikely if not impossible. A different subsurface material such
as an existing foundation is a more probable source of the increased load capacity of
the SRC pile. This would also be a more reasonable cause because it would produce
a very localized increase in subsurface strength where a change in soil properties in
a distance of about 15 ft (4.6 m) would be unlikely.
109
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
160
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
SRC Pile from String Potentiometer Data
SRC Pile from Lpile Prediction 1 with Soil Adjustment
Figure 9.11: Actual Load vs. Deflection Behavior Compared to LpilePredictions based on Adjusted Soil Properties
The oddity of the SRC pile results is also apparent when the Lpile-predicted
deflected shape is plotted with the actual test results. Figure 9.12 shows actual
deflected shapes of the SRC Pile from inclinometer readings with Lpile prediction 1
based on original and adjusted soil properties. All Lpile predictions represent
deflected shapes at the same load levels as the inclinometer readings. Only the
deflected shapes before failure are shown. For the original soil, Lpile prediction 1,
only one deflected shape is shown because Lpile predicted failure before the second
inclinometer reading. Two deflected shapes are shown for the adjusted soil
properties. According to Lpile, at the field test recorded loads, the SRC pile should
110
SRC Pile Load [kips (kN)]
-5
0
5
10
15
20
25
30
0 0.5 1 1.5 2 2.5 3 3.5 4Displacement [in]
Dep
th B
elow
Gro
und
Surf
ace
[ft]
-1.5
0.5
2.5
4.5
6.5
8.5
0 1 2 3 4 5 6 7 8 9 10Displacement [cm]
Dep
th [m
]14.8 (65.8) Inclinometer Data19.2 (85.4) Inclinometer Data19.2 (98.8) Inclinometer Data24.5 (109) Inclinometer Data26.2 (117) Inclinometer Data28.0 (125) Inclinometer Data14.8 (65.8) Lpile Prediction 114.8 (65.8) Lpile Prediction 1 with Soil Adjustment19.2 (98.8) Lpile Prediction 1 with Soil Adjustment
Figure 9.12: Actual Deflected Shape of the SRC Pile Compared to LpilePredictions Based on Original and Adjusted Soil Properties
111
have already yielded and failed much sooner. Even increaseing the soil strength
could not reach the high failure load of the SRC pile in the field.
Notice the shapes of the three data types. A 14.8 kip (66 kN) curve is
provided for the inclinometer data, Lpile prediction 1, and Lpile prediction 1 with
adjusted soil. The shape of Lpile prediction 1 infers a much softer soil profile with
deflection reaching greater depths. The inclinometer data and the stiffer soil Lpile
prediction have similar shapes showing that the pile does not deflect much lower
than 4 ft (1.2 m) and not at all below 6 ft (1.8 m). This comparision suggests
something stiff reacted against the SRC pile below the ground surface.
9.7 Lpile SRC Pile Adjusted Reinforcement Predictions
The SRC pile was 30 ft (9 m) long, however the steel reinforcement came in
20 ft (6 m) lengths. Splices were required to construct the pile. These splices were
alternated, every other bar, between the top and the bottom of the pile. Because
the SRC pile field data recorded that the pile withstood a significantly greater load
than was predicted, these splices became suspect. To determine the significance of
these splices, an analysis was performed using Lpile. The analysis increased the
reinforcement to see what the effect would be on the load-deflection data. The pile
tested in the field had 8 # 4 bars. One test analysis increased the reinforcement to
8 #5 bars, or, 50% greater area in each bar. Another test analysis increased the
reinforcement to 8 #6 bars, or 120% greater area in each bar. These analyses were
performed using Lpile prediction 1 input (with the exception of the adjusted
112
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
160
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
SRC Pile from String Potentiometer DataSRC Pile from Lpile Prediction 1; 8 #4 barsSRC Pile from Lpile Prediction 1; 8 #5 barsSRC Pile from Lpile Prediction 1; 8 #6 bars
(0.20 in2)(0.31 in2)(0.44 in2)
Figure 9.13: SRC Pile Adjusted Reinforcement Predictions
reinforcement). The results for these analyses, shown in Figure 9.13, make it
apparent that while the increased reinforcement does alter the load capacity of the
pile, even a gross overestimation of the splice does not produce a load capacity as
high as the SRC pile field results.
9.8 Error Evaluation
An error in the load data for the SRC pile seems likely when considering the
loading rate and the energy balance. Each method of analysis suggests an
approximate 7 kip (31 kN) adjustment in load. When this adjustment is made, the
SRC results concur with both the Lpile predictions and laboratory test results.
113
The difference in energy transfered to the soil could also be explained by a
difference in the soil surrounding the piles. Lpile predictions show that a change in
the top layers of soil can significantly alter the load-deflection data. However, the
changes in the soil necessary to match the IRC pile field data are not likely for the
test site. However, an unknown subsurface material such as an existing foundation
could cause the subsurface material to absorb energy resulting in an artificial
increase in the pile load capacity.
The splice in the SRC Pile reinforcement increased the area of the reinforcing
members; however, Lpile predictions show that even with over double the area in
each bar (a huge overestimate for the splice) the load-deflection curves are still
significantly lower than the SRC field test results. Therefore, the splice may have
altered the results, but it could not have been the single reason for the error in the
data.
The last possible error considered is damage to the IRC pile prior to the field
tests. The IRC pile stiffness is consistent with the Lpile predictions for the SRC
pile. Both laboratory testing and material property analysis showed similar stiffness
between the SRC and IRC piles. Taking the agreement of the IRC pile with analysis
in conjunction with the considerable doubt in the SRC pile field data, the IRC pile
data is most likely not the source of error.
114
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8 9Deflection at Point of Load Application [in]
Loa
d [k
ips]
0
20
40
60
80
100
120
140
160
0 5 10 15 20Deflection at Point of Load Application [cm]
Loa
d [k
N]
IRC Pile from String Potentiometer DataSRC Pile from Lpile Prediction 1
Figure 9.14: Lpile Deflection Prediction for the SRC Pile Compared toString Potentiometer Deflection Results for the IRC Pile in the Field
9.9 Summary
No evidence suggests that the IRC pile data is in error. However, substantial
evidence concludes that the SRC pile field data is flawed and therefore not useful for
comparison to the IRC pile field results. Lpile prediction 1 is a viable alternative to
experimental data for the SRC pile. To understand the comparative behavior of the
IRC pile to a similar SRC pile, the IRC pile field deflection data is shown with Lpile
prediction 1 for the SRC pile in Figure 9.14.
115
Figure 9.14 reveals that for two piles of similar stiffness, one reinforced with
an IsoTruss R© grid-structure and one with steel re-bar, the IRC pile is
approximately twice as strong. This result concurs with the laboratory tests which
also showed that the IRC pile was approximately twice as strong as the SRC pile [2].
116
Chapter 10
Conclusions and Recommendations
This thesis focused on the field performance of IsoTruss R© grid-reinforced
concrete beam columns for use in driven piles. Experimental investigation included
one instrumented carbon/epoxy IsoTruss R© grid-reinforced concrete pile (IRC pile)
and one instrumented steel-reinforced concrete pile (SRC pile) which were driven at
a clay profile test site. These two piles, each 30 ft (9 m) in length and 14 in (36 cm)
in diameter, were quasi-statically loaded laterally until failure. Behavior was
predicted using three different methods: 1) a commercial finite difference-based
computer program called Lpile; 2) a Winkler foundation model; and, 3) a simple
analysis based on fundamental mechanics of materials principles.
Due to unresolveable errors, experimental field test data for the SRC pile is
inconclusive. However, analysis predictions in conjunction with field test data for
the IRC pile show that the IRC pile should perform similar to laboratory test
results. Therefore, IsoTruss R© grid-structures are a suitable alternative to steel as
117
reinforcement in driven piles. This chapter includes the conclusions drawn from the
field research and recommendations to improve further research.
10.1 Conclusions
1. Both Lpile and Winkler foundation model predictions agree with the
laboratory results that the IRC pile is almost twice as strong as the SRC pile.
2. Experimental results were not consistent with those obtained in the laboratory
and are inconclusive due to unresolveable errors. Conservation of energy
principles also suggest that the SRC pile data was in error. Modifying the
SRC pile field test data to account for a more realistic energy balance revealed
an accord with laboratory findings and Lpile predictions.
3. Soil stiffness contributes significantly to the field performance of driven piles.
4. Applying mechanics of materials principles found the predicted stiffness of the
piles to be consistent with laboratory results.
10.2 Recommendations
1. At least two of each pile type should be tested to increase result dependability.
2. The test site should be carefully chosen and studied to ensure the soil is
undisturbed and consistent among test piles.
3. The piles need not be greater than 20 ft (6m) for field bending test.
4. Carefully protect strain gages to avoid corrupt data.
118
5. Additional field tests are required to ensure field performance of IsoTruss R©
grid-reinforced concrete piles.
119
120
References
[1] D. T. McCune, “Manufacturing quality of carbon/epoxy isotruss reinforcedconcrete structures,” Master’s thesis, Brigham Young University, 2005.
[2] M. J. Ferrell, “Flexural behavior of carbon-epoxy isotruss-reinforced concretebeam-columns,” Master’s thesis, Brigham Young University, 2005.
[3] K. Rollins, R. Olsen, J. Egbert, K. Olsen, D. Jensen, and B. Garrett,“Response, analysis, and design of pile groups subject to static and dynamiclateral loads,” Tech. Rep. UT-03.03, Research Div., Utah Departement ofTransportation, Salt Lake City, Utah, 2003.
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