OBSERVED TIP RESISTANCE AT EOD & BOR USING...
Transcript of OBSERVED TIP RESISTANCE AT EOD & BOR USING...
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OBSERVED TIP RESISTANCE AT EOD & BOR USING BOTTOM TIP GAGES FOR DRIVEN PILES
By
YIPENG XIE
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2011
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ACKNOWLEDGMENTS
I am indebted to my many of my colleagues to support me with this thesis. Most
importantly, I sincerely thank Dr. Michael McVay for serving as my advisor. If I have
ever learned how to do research, or come closer to a geotechnical engineer, it is
because of his great teach by word and deed. There is no doubt that his guidance will
accompany me for the rest of my life. I am impressed by his vast and versatile erudition,
rigorous attitude towards research, and great personality throughout. To be his student
is definitely one of my whole life’s landmarks. His valuable support and encouragement
were what made this possible. Special thanks go to Dr. Reynaldo Roque for sitting on
my supervisory committee. I would also like to thank other fellow graduate students for
making the graduate study an enjoyable experience. A particular word of thanks goes to
Khiem Tran and Jiangpeng Xiang, for unselfishly sharing with me much of their
knowledge.
I would like to thank my parents also, for their self-giving love. Without them, I can
never become who I am today. I hope they will feel proud for me in the future.
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TABLE OF CONTENTS
page
ACKNOWLEDGMENTS .................................................................................................. 4
LIST OF TABLES ............................................................................................................ 7
LIST OF FIGURES .......................................................................................................... 8
ABSTRACT ................................................................................................................... 11
CHAPTER
1 FOUNDATION INTRODUCTION ............................................................................ 13
1.1 Background of Pile Foundations ....................................................................... 13
1.2 Pile Capacities .................................................................................................. 14 1.2.1 Introduction of Pile Forces ....................................................................... 14 1.2.2 Side Shear Force and Tip Resistance ..................................................... 15
2 PILE SET-UP ( FREEZE ) ...................................................................................... 18
2.1 Introduction to Pile Set-up ( Freeze ) ................................................................ 18
2.2 Principles of Pile Set-up .................................................................................... 19 2.2.1 Observation of Pile Set-up ....................................................................... 19
2.2.2 Findings and Conclusions ....................................................................... 20 2.2.3 Mechanisms of Pile Set-up ...................................................................... 22
2.3 Relationship between Pile Set-up and Logarithm of Time ................................ 24
3 PILE LOAD TESTS ................................................................................................. 28
3.1 Introduction to Pile Load Tests ......................................................................... 28
3.2 Slow and Quick Tests ....................................................................................... 30 3.3 Four Load Test Methods .................................................................................. 30 3.4 Dynamic Forces vs. Static Forces..................................................................... 34
3.4.1 Dynamic Forces Recorded by PDA ......................................................... 34 3.4.2 Match Calculated Forces to Measured Forces With CAPWAP ............... 38
3.4.3 Dynamic Tests Recorded by SmartPile Review ...................................... 40 3.4.4 Wave Theory ........................................................................................... 41
3.4.5 Unloading Point Method ......................................................................... 44
4 ENERGY METHOD ................................................................................................ 47
4.1 Theories of Energy Method ............................................................................... 47 4.1.1 Newton's Three Laws of Motion .............................................................. 47 4.1.2 Force and Energy Equilibrium ................................................................. 48
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4.1.3 Process of Energy Method ...................................................................... 51 4.1.4 Examples of Energy Method ................................................................... 59
4.1.4.1 Site – Dixie Highway .................................................................... 59
4.1.4.1.1 Compression load test – end bent no.1 ....................................... 59 4.1.4.1.2 Compression load test – Pier no.8 .............................................. 64 4.1.4.2 Site – Caminada Bay ................................................................... 68
5 OBSERVATIONS OF ENERGY APPROACH ........................................................ 72
5.1 EOD vs. BOR Predicted Static Response of Dixie Highway ............................. 72
5.2 EOD vs. BOR Predicted Static Response of Caminida Bay ............................. 74
6 CONCLUSION ........................................................................................................ 88
6.1 Summary .......................................................................................................... 88 6.2 Recommendations ............................................................................................ 89
APPENDIX: EXAMPLES OF ENERGY METHOD ........................................................ 90
LIST OF REFERENCES ............................................................................................. 102
BIOGRAPHICAL SKETCH .......................................................................................... 105
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LIST OF TABLES
Table page 5-1 EOD and BOR tip forces of dixie highway .......................................................... 83
5-2 EOD and BOR tip forces of caminida bay .......................................................... 84
5-3 EOD and BOR tip forces of I-95 DEsign Build US 192 bent3 pile5 & I-95 Eau Gallie bent1 pile1 ................................................................................................ 85
5-4 EOD and BOR tip forces comparison ................................................................. 86
5-5 EOD and BOR skin friction comparison .............................................................. 87
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LIST OF FIGURES
Figure page 1-1 Prestressed concrete pile ................................................................................... 14
1-2 Pile total capacity composed of side skin friction and tip resistance ................... 15
1-3 Forces along the pile .......................................................................................... 16
1-4 Forces acting on pile segment during driving ..................................................... 17
2-1 Seidel et at. (1988) relationship between pile capacity and log time ................. 26
3-1 Pile load test frame 1 .......................................................................................... 29
3-2 Pile load test frame 2 .......................................................................................... 29
3-3 Pile instralled with PDA strain gages and accelerometer ................................... 35
3-4 Instrumentation at 18 in from pile tip: PDI (strain and accelerometers). ............. 36
3-5 PDA Data acquisition systems............................................................................ 36
3-6 PDA analyzer ...................................................................................................... 37
3-7 CAPWAP analyze procedure.............................................................................. 40
3-8 EDC software window ........................................................................................ 41
3-9 WaveUp and WaveDown Forces passing along the pile .................................... 41
3-10 Wave traveling in the pile ................................................................................... 41
3-11 Steps of Statnamic test ....................................................................................... 45
3-12 Real field Statnamic test ..................................................................................... 45
3-13 Statnamic measured load and calculated static force ......................................... 46
4-1 Mass-Spring-Damper model ............................................................................... 50
4-2 Excel configuration sheet from SmartPile Review .............................................. 52
4-3 Excel data sheet from SmartPile Review of one blow ........................................ 51
4-4 Excel sheet from Energy Method1 ...................................................................... 53
4-5 Excel sheet from Energy Method2 ...................................................................... 53
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4-6 EDC Blow 777 Forces vs. Time at Pile Tip of Pier 8 ........................................... 55
4-7 EDC Blow 777 Forces vs. Displacement at Pile Tip of Pier 8 ............................. 56
4-8 EDC Blow 777 Energy vs. Time at Pile Tip of Pier 8 .......................................... 57
4-9 EDC Blow 779 Forces vs. Time at Pile Tip of Pier 8 ........................................... 57
4-10 EDC Blow 779 Forces vs. Displacement at Pile Tip of Pier 8 ............................. 58
4-11 EDC Blow 779 Energy vs. Time at Pile Tip of End Bent1 ................................... 58
4-12 EDC Blow 740 Energy vs. Time at Pile Tip of End Bent1 ................................... 61
4-13 EDC Blow 740 Forces vs. Displacement at Pile Tip of End Bent1...................... 61
4-14 EDC Blow 740 Energy vs. Time at Pile Tip of End Bent1 ................................... 62
4-15 EDC Blow 765 Energy vs. Time at Pile Tip of End Bent1 ................................... 62
4-16 EDC Blow 765 Forces vs. Displacement at Pile Tip of End Bent1...................... 63
4-17 EDC Blow 765 Energy vs. Time at Pile Tip of End Bent1 ................................... 63
4-18 EDC Blow 751 Forces vs. Time at Pile Tip of Pier 8 Pile .................................... 65
4-19 EDC Blow 751 Forces vs. Displacement at Pile Tip of Pier 8 Pile ...................... 66
4-20 EDC Blow 751 Energy vs. Time at Pile Tip of End Bent1 ................................... 66
4-21 EDC Blow 779 Forces vs. Time at Pile Tip of Pier 8 Pile .................................... 67
4-22 EDC Blow 779 Forces vs. Displacement at Pile Tip of Pier 8 ............................. 67
4-23 EDC Blow 779 Energy vs. Time at Pile Tip of Pier 8 .......................................... 68
4-24 EDC Blow 630 Forces vs. Time at Pile Tip of Caminida Bay Bent1 Pile1 ......... 69
4-25 EDC Blow 630 Forces vs. Disp. at Pile Tip of Caminida Bay Bent1 Pile1 ......... 69
4-26 EDC Blow 630 Energy vs. Time at Pile Tip of Caminida Bay Bent1 Pile1 .......... 70
4-27 EDC Blow 660 Forces vs. Disp. at Pile Tip of Caminida Bay Bent1 Pile1 .......... 70
4-28 EDC Blow 660 Energy vs. Time at Pile Tip of Caminida Bay Bent1 Pile1 .......... 71
4-29 EDC Blow 660 Forces vs. Time at Pile Tip of Caminida Bay Bent1 Pile1 .......... 71
5-1 Estimated tip resistance of Dixie Highway Bent 1 Pile1 blows before the load test ...................................................................................................................... 72
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5-2 Estimated tip resistance of Dixie Highway Bent 1 Pile1 blows after the load test ...................................................................................................................... 72
5-3 Estimated tip resistance of Dixie Highway Pile8 blows before the load test ....... 73
5-4 Estimated tip resistance of Dixie Highway Pile8 blows after the load test .......... 73
5-5 Estimated tip resistance of Caminida Bay1 blows before the load test ............... 74
5-6 Estimated tip resistance of Caminida Bay1 blows after the load test .................. 74
5-7 Estimated tip resistance of Caminida Bay7 blows before the load test ............... 75
5-8 Estimated tip resistance of Caminida Bay7 blows after the load test .................. 75
5-9 Davisson’s Capacity of Dixie Highway Bent1 Pile1 ............................................ 76
5-10 Davisson’s Capacity of Dixie Highway Pier8 ...................................................... 77
5-11 Davisson’s Capacity of Caminida Bay Bent1 ...................................................... 77
5-12 Davisson’s Capacity of Caminida Bay Bent7 ...................................................... 78
5-13 Predicted Tip Resistance at EOD for pile 5 at US 192 ....................................... 80
5-14 Predicted Tip Resistance at BOR for pile 5 at US 192 ....................................... 80
5-15 Predicted Tip Resistance at EOD and BOR for pile 5 at US 192 ........................ 81
5-16 Predicted Tip Resistance at EOD of I95 Eau Gallie bent1 pile1 ......................... 82
5-17 Predicted Tip Resistance at BOR of I95 Eau Gallie bent1 pile1 ......................... 82
5-18 Predicted Tip Resistance at EOD and BOR I95 Eau Gallie bent1 pile1 ............. 83
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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science
OBSERVED TIP RESISTANCE AT EOD & BOR USING BOTTOM TIP GAGES FOR
DRIVEN PILES
By
Yipeng Xie
May 2011
Chair: Michael McVay Major: Civil Engineering
Pile foundations are the important part of a structure used to carry and transfer the
load of the structure to the bearing ground. To compute the total load that can be
applied to a pile, it is necessary to compute both tip resistance and skin friction acting
on sides of the pile. Researchers develop many different methods of measuring pile
shear forces and tip force. Static testing and dynamic testing are common used today to
get pile capacities during and after driving process. During restrikes following the initial
installation, or changing cushions template removal and so on, piles may experience an
increase of total capacity. This phenomenon nowadays is well known as soil/pile set-up
(freeze). With more and more test sites observe such phenomenon, researchers believe
that pile set-up occures mostly because of pile shear force increase, where pile tip
resistance seem not to change as much as shear force. In order to have a better
evaluation of pile static tip resistance, a new method called energy method is developed
and used to analyse different pile sites, providing with consistant and gauranteed results.
This thesis focused on the introduction of this new method, also from the static tip
resistance forces measured from this approach, it shows pile set-up has less
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relationship with pile tip force compared to the increase of pile shear force. The current
application of energy method performed on different piles is considered successful.
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CHAPTER 1 FOUNDATION INTRODUCTION
1.1 Background of Pile Foundations
Pile foundations are important part of a structure used to carry and transfer the
load of the structure to the bearing ground located at some depth below ground surface.
Foundations are generally broken into two categories: shallow foundations and deep
foundations. Shallow foundation is usually, embedded a meter or so into soil. A shallow
foundation is a type of foundation which transfers building loads to the earth very near
the surface, including spread footing foundations, mat-slab foundations, slab-on-grade
foundations, rubble trench foundations, and earthbag foundations. A deep foundation is
a type of foundation distinguished from shallow foundations by the depth they are
embedded into the ground. Structural members made of steel, concrete, and/or timber.
They are expensive due to cost materials, placement (driving, drilling, etc.) vs. shallow
foundations. They are used for following reasons: 1) If the upper soil layers are
compressible or too weak to support the structural loads. 2) Structures subject to large
horizontal forces – In Florida, a common design consideration is hurricane winds, ship
impact on bridge piers, etc. 3) Foundations subject to adverse future influences: soil
erosion or scour from streams or waterways during storms (Acosta 15’ in 25yrs).
Pile foundations have been used as load carrying and load transferring systems
for many years. Two types of forces act on piles, tip resistance acts on the bottom of the
pile and skin friction acts on the sides of the pile. Piles are heavy beams of timber,
concrete, or steel, driven into the earth as a foundation or support for a structure.
Selection of a pile type is based on Cost – in south Florida commercial construction,
auger cast concrete are prevalent – develop more side friction than driven steel or
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concrete pile due to drilling into limestone (oolites) – resulting in cheaper cost.
Nowadays concrete piles are the most common piles used, which are driven into the
ground to ensure that the foundation is deep. For concrete piles, they are divided into
two categories based on construction and installation: 1) Precast prestressed concrete
pile (18” – 66”) constructed in a casting yard (Standard, Gates, etc.) and installed with
crane, leads and a hammer; 2) Cast insitu pile: Franki Pile, auger cast pile, continuous
flight auger – constructed by drilling or other hole creation, filling with concrete and steel
reinforcement.
Figure 1-1. Prestressed concrete pile
1.2 Pile Capacities
1.2.1 Introduction of Pile Forces
No matter which type the pile is, usually two kinds of forces act on it: 1) Tip
resistance acts on the bottom of the pile. 2) Skin friction acts on the sides of the pile. To
compute the total load applied to a pile, it is necessary to compute both the tip force and
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the skin friction and add them together. A modified Terzaghi bearing capacity equation
is used to find the pile capacity:
Pt = Fside + Ftip (1-1)
Figure below shows that side friction and tip resistance both composing the total
capacity.
Figure 1-2. Pile total capacity composed of side skin friction and tip resistance
1.2.2 Side Shear Force and Tip Resistance
It is common to see a block is placed on a clay surface. Now if a force is applied to
move the block, the adhesion between the block and the clay will resist the movement.
The adhesion coefficient between the clay and the block is c, and the weight of the
block is W, the force F due to adhesion will be
F = W * c (1-2)
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Figure 1-3. Forces along the pile
When load acts on the pile, it will in the same way generate side shear force along
the pile surrounded by soil. The load transferred from pile skin to the surrounding clay
soil is a function of the diameter and the surface roughness of the pile, clay properties
(cohesion, type of consolidation and level of disturbance). Generally, skin friction (FS,
force) is characterized as unit skin friction (fs, stress), times the surface area it acts over.
The unit skin friction (fs) is usually characterized as a function of the pile displacement
[u(x,t)], e.g., T-Z curve in FB-MultiPier, FB-DEEP, etc. A secant stiffness (K) is defined
as the unit skin friction per unit of displacement [u(x,t)]. Using the secant stiffness (K),
the skin friction (FS) force acting on segment dx (Figure 1-4) may be found. Next,
assuming a general damping form, i.e., viscous with coefficient (Cr), the damping force
(Fd) is obtained from particle velocity times density and surface area (Figure 1-4).
Summing the forces on the segment, results in:
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(1-3)
Figure 1-4. Forces acting on pile segment during driving
dx
B
dx FI FS = Skin Friction
Fd = Side Damping
FI = Inertia Force
2BAwhereAF
StressAxial
T
Adxx
F
dxx
B
K’
fs, u
nit s
kin
frictio
n
u(x,t), pile displacement
FS = Skin Friction = fs Asurf = K’ u(x,t) P dx
FS = Skin Friction:
Fd = Side Damping:
Fd = Cr P dx s u(x,t)/t
Particle velocity, v
Asu
rf
Radiation Damping Coeff.
Soil Density
Pile Perimeter = 4B
Summing Forces on Segment
dSITBv FFFFFF 0
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CHAPTER 2 PILE SET-UP ( FREEZE )
2.1 Introduction to Pile Set-up ( Freeze )
For geotechnical engineers, they observed nearly 100 years ago that bearing
capacity of a driven pile usually increases after its installation. During restrikes following
the initial installation, or changing cushions template removal and so on, piles may
experience a driving from easy to hard. This phenomenon nowadays is well known as
soil/pile set-up (freeze). With more installation of driven piles, it is recognized as
occurring in most parts of all the world, all driven pile types, and in all sorts of soils,
which ranges from organic and inorganic saturated clay, and loose to medium dense
silt, sandy silt, silty sand, and fine sand, and is related to both soil and pile properties.
And the timeline is from less than half an hour to several years or even longer. Over
forty years ago, geotechnical engineers observed an averaged 70% pile capacity
increase between 0.5 to 20 days. (Tavenas and Audy (1972)). They wrote the first well-
documented summarization of set-up phenomenon about performing tension load test
on steel pipe piles at sand site in France, claiming the long term set-up was in the
region of 50 to 150% of initial pile capacity. In fact, pile set-up occurs much more quickly
in sand than in clay. Usually, set-up (freeze) takes a few hours for the side-shear friction
to restore in sand, where in clay it may takes months or even years for the piles to
restore total capacity.
Although the phenomenon today is observed more and more and got recognised
in nearly all sorts of soils, the increase capacity of pile with time is not mainly
considered in pile foundation design. It can be imagined that by including the pile set-
up (freeze) into the pile foundation design, the total cost of foundation can be reduced
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as the number, length, size of the pile could be reduced. It may also generated big
saving in the total cost as foundation cost ranging from 5% to 30% in the total cost.
Although geotechnical engineers realize such advantages as if they can fully use this
phonomenon, but due to lack of understanding about the detailed mechanism, it is hard
to tell how principal factors such as soil type, pile size and material as well as its
installation method affect soil/pile set-up.
2.2 Principles of Pile Set-up
2.2.1 Observation of Pile Set-up
As mentioned before, pile set-up occurs in all kinds of soil. Many papers also
present relating findings. For example:
Bullock et al. (2005) - final research report lasting from February 1993 to April
1999 investigated five fully instrumented piles driven into a variety of soils (sand, clay,
mixed soils) at four different FDOT bridge sites. These five eighteen-inch square piles
were instrumented with strain gages, lateral total stress cells, pore pressure sensors
along their length and at their tips were cast Osterberg load cells. In the field SPT
Torque, piezo CPT and DMT stage testing were performed. Also in laboratory, they
drove a model pile in the centrifuge in flight at 50gs and then perform a static pull out
(tension) test to determine if they could duplicate freeze effects. After 16 to 1727 days
elapsed time using Osterberg cell tests separating side shear and end bearing, 12 to
32% side shear increase per log cycle of time were recorded.
C.S. Chen, S.S. Liew & Y.C. Tan (1998) - two case histories where the changes in
pile capacity were observed with time are presented. One case shows the increase in
pile capacity especially the shaft friction for piles driven into clayey deposit. The average
unit shaft friction, determined from the high strain dynamic pile test, has increased from
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33 kPa to 57 kPa from 3 days to 33 days after the installation of piles. The second case
shows a tendency of increase in pile capacity for pile driven into sandy deposit over a
period longer than needed for complete dissipation of excess pore water pressure
induced by the driving process.
Gary Axelsson (2000) - two series of full scale field tests were performed on
instrumented concrete piles, driven in loose to medium dense sand. In addition,
laboratory rod shear chamber tests were performed on driven model piles and finally
revealed that set-up is a major feature of driven piles in non-cohesive soil.
Kehoe (1989) - investigated two Florida mixed cohesive soils sites with driven
square prestressed concrete piles. Static and dynamic tests showed capacities
increases average from 58% to 200% within 11 days after driving.
W. K. Ng & M. R. Selamat, K. K. Choong (2010) - based on the assumption that
the capacity increase of pile depends on various factors, the duration of full set-up is
assumed to be dependent solely on soil type, a total 6 case studies (CS) and 11
numbers of test piles (P1 – P11) are presented to investigate from the aspect of soil/pile
set-up. All the projects located in peninsular Malaysia. Two types of pile used in the
cases such as RC square pile (with size 200 – 400mm) and spun pile (with diameter
250 – 500mm), results showing set-up effect is playing a role on time-dependent
capacity of driven pile in Malaysian soil.
2.2.2 Findings and Conclusions
Van E. Komurka and Alan B. Wagner (2003) concluded in their final report
“Estimating Soil/pile Set-up” that through a thorough review of the literature and the
state of the practice, set-up is predominately associated with an increase in soil
resistance acting on the sides (shaft) of a pile. Unit set-up has units of force divided by
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pile side area. The complete mechanisms contributing to set-up are not well
understood, but the majority of set-up is likely related primarily to dissipation of excess
porewater pressures within, and subsequent remolding and reconsolidation of soil which
is displaced and disturbed as the pile is driven. Set-up is recognized as occurring in
most parts of the world, for virtually all driven pile types, in organic and inorganic
saturated clay, and loose to medium dense silt, sandy silt, silty sand, and fine sand, and
is related to both soil and pile properties. In cohesive soils, the shear strength of the
disturbed and reconsolidated soil has been found to be higher than the soil’s
undisturbed shear strength. In fine-grained granular soils, the majority of set-up is
related to creep-induced breakdown of driving-induced arching mechanisms, and to
aging. The more permeable the soil, the faster set-up develops. Set-up rate decreases
as pile size increases. As soil around and beneath the pile is displaced and disturbed,
excess porewater pressures are generated, decreasing the effective stress of the
affected soil. The increase in porewater pressure is constant with depth (Soderberg,
1961), and can exceed the existing overburden stress within 1 pile diameter of the pile
(Pestana et al., 2002; Randolph, et al., 1979). Decrease in excess porewater pressure
is inversely proportional to the square of the distance from the pile (Pestana et al.,
2002). The time to dissipate excess porewater pressure is proportional to the square of
the horizontal pile dimension (Holloway and Beddard, 1995; Soderberg, 1961), and
inversely proportional to the soil’s horizontal coefficient of consolidation (Soderberg,
1961). Accordingly, larger-diameter piles take longer to set-up than smaller-diameter
piles (Long et al., 1999; Wang and Reese, 1989). Excess porewater pressures dissipate
slower for a pile group than for a single pile (Camp et al., 1993; Camp and Parmar,
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1999). As excess porewater pressures dissipate, the effective stress of the affected soil
increases, and set-up predominately occurs as a result of increased shear strength and
increased lateral stress against the pile.
Bullock et al. (2005), Axelsson (1998a), and Chow et al. (1998) concluded that
setup occurs primarily as a result of side shear increase, not end bearing. Penetration of
the pile pushes soil outward and away from the pile, destructuring and shearing it to a
greater extent adjacent to the side of the pile than at the pile tip, and thus reducing the
side resistance during installation (and increased aging effects).
2.2.3 Mechanisms of Pile Set-up
Time-dependent pile capacity increase depends on many factors such as soil
grain characteristics, insitu stress level, pile geometry, chemical processes and pile
installation procedure. In cohesionless soil, the excess pore water pressure dissipated
quickly. Excess pore water pressures induced by pile driving seldom exceed 20% of the
effective overburden stress. Soil/pile set-up taking place in pile in cohesionless soil is
thought to be due to the following reasons:
(a) chemical effects which may cause the sand particles to bond to the pile surface,
(b) soil ageing effects resulting in increase in shear strength and stiffness with time,
(c) gain in radial effective stress due to creep effects or relaxation on the established circumferential arching around the pile shaft during installation.
Pile installation in clay is different from pile driving in sand. Komurka et al. divided
the soil/pile set-up mechanisms into the following three phases:
Phase I: logarithmically nonlinear rate of excess pore water pressure
dissipationBecause of the highly disturbed state of the soil, the rate of dissipation of
excess porewater pressures is not constant. During this first phase of set-up, set-up rate
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corresponds to the rate of dissipation, and so is also not uniform (not linear) with
respect to the log of time for some period after driving. During this phase of non-
constant rate of dissipation of excess pore pressures, the affected soil experiences an
increase in effective and horizontal stress, consolidates, and gains strength in a manner
which is not well-understood and is difficult to model and/or predict. This first phase of
set-up has been demonstrated to account for capacity increases in a matter of minutes
after installation.
Phase II: logarithmically linear rate of excess porewater pressure dissipation
At some time after driving, the rate of excess porewater pressure dissipation
becomes constant (linear) with respect to the log of time. During this second phase of
set-up, set-up rate corresponds to the rate of excess porewater pressure dissipation,
and so for most soils is also constant (linear) with respect to the log of time for some
period after driving. During the logarithmically constant rate of dissipation, the affected
soil experiences an increase in effective vertical and horizontal stress, consolidates, and
gains shear strength according to conventional consolidation theory.
Phase III: Independent of effective stress
Infinite time is required for dissipation of excess porewater pressure to be
complete. Practically speaking, there is a time after which the rate of dissipation is so
slow as to be of no further consequence, at which time it is accepted that primary
consolidation is complete. However, secondary compression continues after primary
consolidation is complete, and is independent of effective stress. Similarly, since the
rate of set-up corresponds to the rate of excess porewater pressure dissipation, it
follows that in some cases infinite time would be required for set-up to be complete.
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Again practically speaking, and as with primary consolidation, there is likely a time after
which the rate of set-up is so slow as to be of no further consequence, and effective-
stress-related set-up is effectively complete. However, as with secondary compression,
it has been demonstrated that set-up continues after dissipation of excess porewater
pressures. During this third phase of set-up, set-up rate is independent of effective
stress. This is related to the phenomenon of aging.
For a given soil type at a given elevation along the pile shaft, there is likely some
overlap between successive phases, so, more than 1 phase may be contributing to set-
up at a time (e.g., aging may begin before essentially complete dissipation of excess
porewater pressure). In addition, unless soil conditions are uniform along the entire
length of the shaft and beneath the toe, different soils at different elevations will be in
different phases of set-up at a given time.
2.3 Relationship between Pile Set-up and Logarithm of Time
Civil engineers generally assume a log-linear relationship between pile capacity
and elapsed time. Following is Terzaghi’s one dimensional (radial) consolidation theory:
Th = 4* Ch * t / rp2 (2-1)
Where Th = Time Factor Ch = Coefficient of Radial Consolidation t = Elapsed Time since End of Driving rp = pile radius
Based on Terzaghi’s theory, Vesic (1977) found that in clays, pile capacity showed
a linear trend against the logarithm of time except for short and long setup times, which
is similar to a strain vs. log time oedometer consolidation curve. It was raised by them
that due to the dissipation of excess pore pressure as the result of pile installation, set-
up was developed because of the radial consolidation. Researchers further illustrate
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that the consolidation set-up varies as the square of pile radius, suggesting reduce
some of the variation of Set-up Factors such as pile size would warrant additional
research.
Mohr-Coulomb Equation (2-2) describes side shear increment in the following form:
τ = σ' tan(υ') + c' (2-2)
Where σ' = (σ - u), known as the principle of effective stress. σ is the total stress
applied normal to the shear plane, and u is the pore water pressure acting on the same
plane. υ' = the effective angle of shearing resistance. c' = apparent cohesion, allowing
the soil to possess some shear strength at no confining stress, or even under tensile
stress. It may also be due to diagenetic affects caused by soil aging such as chemical
bonding, cementation of grains and the effects of creep; indeed futher identified that soil
possessed no cohesion when newly remoulded. When shear tests are conducted on an
overconsolidated or dense soil, and peak strengths are plotted on a τ/σ plot, it appears
that cohesion exists as the y-intercept is non-zero. Some feel that this is not due to true
cohesion, but is the effect of interlocking of particles.
From this Mohr-Coulomb equation, some researchers have shown the possibility
which a portion of the set-up is due to the increases in the effective horizontal stress
during consolidation. Experiments demonstrated that at first near the pile the horizontal
effective stresses near the pile are low and increase as time goes by, when the excess
pore water pressure disappears. Also from the experiments done in sand, effective
stress changed from low to high the same with in clay. In conclusion, piles driven both in
clay and sand will have a change in effective stresses, and the increase of horizontal
effective stresses will be a reason for pile set-up effect.
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So far, although researchers have not come to a final conclusion of how to predict
the pile set-up exactly and uniformly, it is demonstrated a strong relationship between
pile capacity changes and the logarithm of time.
Figure 2-1. Seidel (1988) ploted relationship between pile capacity and log time
Skov and Denver (1988) reached a relationship between pile capacity and time
from four case histories illustrating set-up, as following:
Q / Q0 – 1 = A * log10 ( T / T0 ) (2-3)
Where Q = pile capacity at time T; Q0 = pile capacity at initial time T0; T = time elapsed since end of driving; T0 = initial time elapsed since end of driving, a reference time before which there is no
predictable Q0 increase as a function of elapsed time.
Skov and Denver (1988) use mostly dynamic tests to reach the A value falling
between 0.2 and 5.0. But for this equation, it is hard to tell the portion of pile side friction
and pile end bearing of the total capacity. As some researches results indicate, pile set-
up occurs primarily due to side shear forces increase instead of pile end bearing. Skov
and Denver, Kehoe and many other researchers found little change with time for pile
27
end bearing from dynamic tests. Also static tests showed such situation from Axelsson’s
research. In Equation 2-3, Q is defined as the total pile capacity which includes the pile
end bearing capacity. Since end bearing may not change a lot, it will cause the set-up
factor lower than the true values. Also further researchers need to concern is the T0.
Because there are no specific criteria to define what time T0 will ends, A will be affected
by it significantly, too. Bullock suggests using T0 = 1 day, so it will give an general
standard criteria for future set-up calculation. Bullock summarized the side shear forces
increase linearly with the log of elapsed time at five test site with different soil situation
in Florida
Bullock et al. (2005) then reviewed all the assumption of set-up factor, further
raised another Set-up factor Ashear as the side shear Semilog-Linear Setup factor to
modify Equation 2-3. He suggests that use T0=1 day to remove the difficulty of finding
the actual start of the semilog-linear set-up process, providing a global reference, also
use Ashear to describe the side shear component only, because end bearing capacity
doesn’t change a lot after end of driving. Finally, plot the EOD capacity at 1 min elapsed
time. It may give a more reliable capacity measurements at fixed times.
Bullock modified Skov and Denver’s equation into set-up factor based on side
shear, providing a more uniformly using equation:
Qs/Qs0=fs/fs0=Ashear * log ( t / t0 )+1 = ( ms / Qs0 ) * log ( t / t0) + 1 (2-4)
Where Ashear = Dimensionless set-up factor; Qs = Side shear capacity at time t; Qs0 = Side shear capacity at initial reference time t0;
fs = Unit side shear capacity at time t; t = Time elapsed since EOD, days; t0 = reference time, recommended to use 1 day; ms = Semilog-linear slope of Qs vs log t
28
CHAPTER 3 PILE LOAD TESTS
3.1 Introduction to Pile Load Tests
Geotechnical engineers find this increase of pile capacity after static load test
(SLT) or dynamic load test (DLT) taken after initial driving. So for determing when pile
set-up (freeze) actually happens or how much influence it affects the total pile bearing
capacity, it is very important that after initial driving, a load test would be conducted
later. Pile foundations are constructed depending on the stiffness of subsurface soil and
ground water conditions with a variety of construction techniques. Due to the extensive
nature of the subsurface mass that it influences, the degree of uncertainty regarding the
actual working capacity of a pile foundation is generally very higher. Load testing is
playing an important role in value engineering and the geotechnical and structural
optimization of foundation solutions. It should be recognized not only in financial terms,
but also with regard to sustainability. Load testing of piles is factored into the project
cost plan and program at an early stage. To perform load tests successfully, it should
allow sufficient time for an objective evaluation of the test results, and subsequent
design revisions engineering to be carried out. A lack of clear objectives and
understanding combined with poorly specified requirements can lead to problems that
could have been avoided such as: 1) insufficient time to carry out tests and to evaluate
the test results; 2) lack of flexibility in the testing regime; 3) no provision for value
engineering; 4) unrealistic performance criteria specified; 5) inappropriate test method
specified; 6) load test conditions are not representative of the working piles; 7) piles
infrequently loaded to failure. It is obvious that continuous improvement in foundation
design and construction practices, while at the same time fulfilling its traditional role of
29
design validation and routine quality control of the piling works can be assured. Data
from pile tests has to be collected and analyzed to enable the piling industry run
smoothly. The test pile, installation equipment and installation procedure should be
identical to that intended to be used for production piles to the extent load. The piles
should be loaded to at least two times the design load, and preferably to failure. Pile
foundations, including helical screw foundations, that have been tested to their ultimate
capacity should not be used as production piles.
Figure 3-1. Pile load test frame 1
Figure 3-2. Pile load test frame 2
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3.2 Slow and Quick Tests
Regarding to pile load test, two most common tests are slow and quick maintained
tests (see American Society for Testing and Materials 1143-81).
1) Slow Maintained:
– Load the pile in 8 equal increments (25%, 50%, 75%, 100%, 125%, 175% and 200%) of the design service load;
– Maintain each increment until the rate of settlement has decreased to 0.01 in/hour, but not longer than 2 hours;
– Maintain the 200% load for 24 hours;
– After the required holding time, remove the load in decrements of 25% with 1 hour between steps;
– After loading as above, reload pile to test load in 50% increments of design load, allowing 20 minutes between load increments;
– Then increase the load in increments of 10% of design load until failure, allowing 20 minutes between load steps.
2) Quick Maintained – Recommended by Federal Highway Administration (3 – 5 hours):
– Load the pile in 20 increments to 300% of the design load (i.e. each increment is 15% of design load);
– Maintain each load for 5 minutes with readings taken every 2.5 minutes;
– After reaching 300% - hold load for 5 minutes and then remove the load in 4 equal decrements (each 75% of design) with 5 minutes between decrements;
– Because of the quickness of the test, it is not generally recommended for settlement estimations – considered an undrained loading scenario.
3.3 Four Load Test Methods
Dynamic Pile Testing: Dynamic pile testing constitutes a comprehensive and
economical means to quantitatively evaluate the hammer-pile-soil system based on the
measurement of pile force and velocity records under hammer impacts. Measurements,
data processing and analysis are performed in real time in the field by Pile Driving
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Analyzer® (PDA) equipment from PDITM, or Smartpile ReviewTM which supplies both
top and tip gages. Testing results include estimation of pile load capacity, dynamic pile
stresses and structural integrity as well as driving system performance. The Pile Driving
Analyzer® is applicable on bored cast-in-situ, drilled shafts, continuous flight auger &
driven piles, this applies for either test pile or working pile. Dynamic pile monitoring for
construction quality control and verification testing are performed on hundreds of project
sites in America. Main objectives of dynamic pile testing include obtaining information
on the following: 1) Hammer and driving system performance for productivity and
construction control; 2) Dynamic pile stresses during and after installation. To reduce
the possibility of pile damage, stress must be kept within certain bounds; 3) Pile integrity
during and after installation; 4) Static pile bearing capacity, at the time of testing. For the
evaluation of long term capacity, piles are generally tested during re-strike some time
after installation.
To enhance analysis, CAPWAP® is used combined with PDA, which enables
people to correlate the measured data with the known pile / soil model elements. The
end result of CAPWAP®, via a rigorous and repeated signal matching solution, produces
a pile driving summary that contains pile capacity, percent end bearing / skin friction,
measured pile compression and tension stresses. Using this type of empirical and
analytical data assistance, it can validate a project's design requirements with superior
accuracy and speed. With dynamic load test, researchers want to know:
(a) Estimates total bearing capacity of a pile or shaft (b) Soil resistance parameters (c) Resistance distribution along the shaft and at the toe (d) Static load–settlement curves from the measured force and velocity data (e) Total computed soil capacity – sum of Skin Friction and Toe Bearing (f) Computed load against settlement curve
32
(g)Stresses at any point along the shaft
Static pile load testing: it involves the direct measurement of pile head
displacement in the response to a physically applied test load. It is the most
fundamental form of pile load. Testing has been performed in the load range 100 kN to
12,000 kN. The SLT may be carried out for the following load configurations:
(a) Compression (b) Lateral (c) Tension (i.e. uplift)
For the Static Load Test the load is most commonly applied via a jack acting
against a reaction beam, which is restrained by an anchorage system or by jacking up
against a reaction mass (“kentledge” or dead weight ). The anchorage system may be in
the form of cable anchors or reaction piles installed into the ground to provide tension
resistance. The nominated test load is usually applied in a series of increments in
accordance with the appropriate Code, or with a pre-determined load testing
specification for a project. Each load increment is sustained for a specified time period,
or until the rate of pile movement is less than a nominated value. Static load testing
methods are applicable to all pile types, on land or over water, and may be carried out
on either production piles or sacrificial trial piles. Trial piles are specifically constructed
for the purpose of carrying out load tests and therefore, are commonly loaded to failure.
Testing of production piles however, is limited to prove that a pile will perform
satisfactorily at the serviceability or design load, plus an overload to demonstrate that
the pile has some (nominated) reserve capacity.
Loading is applied to the test pile using a calibrated hydraulic jack, and where
required a calibrated load cell measures the load. During the SLT, direct measurements
33
of pile displacement under the applied loading are taken by reading deflectometers (dial
auges reading to 0.01mm) that are positioned on glass reference plates cemented to
the pile head. The deflectometers are supported by reference beams that are founded a
specified distance away from both the test pile and any reaction points. Although SLT is
generally held as the most reliable form of load testing a pile or pile group, it is important
that interaction effects are minimized. These may result from interaction between the
test pile and the anchorage systems, or between the measuring system and reaction
points. For this reason, careful attention is given to performing the test in accordance
with proper procedures.
Lateral load test: Lateral load test in one of the good means of estimating lateral
capacity of pile. Piles are generally used to transmit vertical and lateral loads to the
surrounding soil media. Piles are sometimes subjected to lateral loads due to wind
pressure, water pressure, earth pressure, earthquakes, etc. when the horizontal
component of the load is small in comparison with the vertical load (say, less than 20%),
it is generally assumed to be carried by vertical piles and no special provision for lateral
load is made. Piles that are used under tall chimneys, towers, high rise buildings, high
retaining walls, bridges & other concrete elevated structures etc. are normally subjected
to high lateral loads. These piles or pile groups should resist not only vertical
movements but also lateral movements. Some of the measured are: 1) Efficiency of the
pile group loads; 2) Soil stiffness degradation; 3) Bending moments 4) Lateral pile
response; 5) Pile deflection and soil response; 6) Ultimate lateral resistance; 7)
Acceptable deflection at working lateral load.
34
Pull-out load test: many structures are constructed using deep piled foundations in
order to transfer structural dead load through unstable ground to a solid stratum. Action
of horizontal wind or wave forces on the structure and the behavior of the piles under
these loads are much less well documented. The resistance of the concrete piles to
pull-out comes from two major sources, skin friction between pile and soil and suctions
generated at the base of the pile as movement occurs. Both of these effects are greatly
affected by the generation of excess or suction pore pressures in the soil due to
movement of the pile. Suctions are generated at the base of the pile in all soils owing to
the opening up of a void as the pile moves. At the sides of the pile, un-drained shearing
of the soil when the pile is pulled quickly will result in excess pore pressure generation
in loose soils and suctions being generated in dense soils. These pore pressures will
alter the effective stress state of the soil and will hence have a great impact on the
force-displacement behavior of the pile. Pull-out tests are the ideal alternative because
of their low cost, relative rapid execution, and reliability of results. The actual skin
resistance between concrete and in-situ soil can be measured at different elevations
within the soil profile. The greater certainty achieved from pullout testing eliminates
overly conservative design values, which in turn reduces as-constructed costs.
Experience has shown that these savings far exceed the cost of pullout testing.
3.4 Dynamic Forces vs. Static Forces
3.4.1 Dynamic Forces Recorded by PDA
Usually performing a static load test after end of driving is a cost of time and
money, needing additional equipment to install loads and measurement of movement
and force. Sometimes pile load test frame’s installation may have some disturb to the
soil. But the estimation of pile tip force is very important for testing pile integrity and
35
prediction of pile performance. Static load test can directly measure pile end bearing
capacity, where provide civil engineers with open-and-shut results. Comparing to taking
long time performing static load tests, people want to measure the static shear forces
and tip resistances more quickly and economically. So getting static results from
dynamic tests is what usually researchers prefer. Since dynamic tests do not disturb soil
around pile during driving and feed back test data simultaneity, it is easy for engineer s
to monitor whether pile driving is smooth or not as well as inspecting pile capacity at the
same time. The need to predict and better understand the ultimate loads to which a
cast-in-place pile foundation is capable is critical for pile design and optimization, as
well as for quality assurance of such elements. The use of high strain dynamic testing of
cast-in-place piles and drilled shafts has become a more frequent routine for bearing
capacity evaluations in many countries around the world, increasing the levels of
standardization and codification (Beim et al. 1998). To accurately predict static capacity
from dynamic pile testing is always being researched by many geotechnical engineers,
and has been the focus of dynamic pile tests on many project sites. Signal matching on
the data seems to be the key of getting more accurately determine capacity.
Figure 3-3. Pile installed with PDA strain gage and accelerometer. Photo courtesy of Michael C. McVay
36
Figure 3-4. Instrumentation at 18 in from pile tip: PDI (strain and accelerometers).
Photo courtesy of Michael C. McVay
Figure 3-5. PDA Data Acquisition Systems. Photo courtesy of Michael C. McVay
37
Figure 3-6. PDA analyzer
A new system of multiple dynamic strain sensors and accelerometers embedded
in test piles provides direct synchronous measurement of force and acceleration at
various locations within a pile during high strain dynamic pile testing. By timing the
strain gage data with the elastic modulus and cross area of pile, forces act on the pile
could be get very easily. From accelerometers, acceleration could be calculated from
raw data, accordingly by integrating the acceleration, both velocity and movement could
be reached. And all these data can be transmit to operators immediately after pile
driving, giving people a direct idea of pile situation. Strain gages and accelerometers
are often instralled near the top of the pile, usually away from the top in the distance of
one diameter. They measure and record the instantaneous pile velocity and force
generated by each hammer blow driving in the pile. Nowadays, PDA is a commonly
used equipment of receiving dynamic force measurement from site. For each hammer
blow, the PDA displayed time traces of the force and velocity on an osillloscope. Except
for those measurements, it also calculate a number of other parameters such as
maximum tension or compression forces, energy put into the pile and check the pile
integrity. If use dynamic tests to get static forces after EOD, researchers need to wait a
38
similar time as performing static load test, waiting until pore water pressure dissipates
and pile set-up increases.
3.4.2 Match Calculated Forces to Measured Forces With CAPWAP
After PDA collects raw data from test field, recorded force and velocity waves from
selected hammer blows will be analyzed using the CAPWAP® computer program to
separate the skin friction and tip resistance from the total force. CAPWAP® (CAse Pile
Wave Analysis Program) is a software program that estimates total bearing capacity of
a pile or shaft, as well as resistance distribution along the shaft and at the toe. The
program takes as input the force and velocity data obtained with a Pile Driving
Analyzer® (PDA). This instrumentation system creates an opportunity to explore side
and end bearing pile resistance distribution with confidence and reliability than from top
measurements alone. Because PDA measures the dynamic forces during pile driving, it
is difficult to know exactly how much the static force is. Along with the popularization of
CAPWAP®, it is considered a standard procedure for the capacity evaluation from high
strain dynamic pile testing data. CAPWAP® separates static and damping soil
characteristics and also allows for an estimation of the side shear distribution and the
pile’s end bearing. CAPWAP® is based on the wave equation model, which analyses
the pile as a series of elastic segments and the soil as a series of elasto-plastic
elements with damping characteristics, where the stiffness represents the static soil
resistance and the damping represents the dynamic soil resistance. Typically the pile
top force and velocity measurements acquired under high strain hammer impacts can
be analyzed utilizing the signal matching procedure yielding forces and velocities over
time and along the pile length. Using one pile top measurement which is easy to install
and protected during pile driving from damage, recording both the downward stress
39
wave and upward stress wave, CAPWAP® alters the soil model to calculate and obtain
a match with the complimentary wave.
What this program does is adjust the model of the surrounding soil originally
entered by the person analyzing the data until the calculated results for the test match
those measured. In other words, the model is adjusted until the signals match, and the
program used for this analysis can be CAPWAP® or DLTWAVE or similar programs. All
these programs have one thing in common: they help the person analyzing the test
results in developing a possible solution. CAPWAP® models the pile into a number of
segments, and by drill the segments downward into the soil, there are soil resistance
activated because of the movement. Users who operate this program will have to give
the information of pile properties, static soil resistance, soil quake and the soil damping
ratio. Then by fitting the downward and upward waves transmitted in the pile which have
been recorded by the PDA, CAPWAP® try to get the most possible forces and assume it
is the real case. The calculated wave is compared to the measured wave, assigned a
quantitative match quality, to increase the accuracy. So for this case, CAPWAP®’s
results will not be exclusive. It depends on the experience of engineers to obtain a best
wave match. It should be noticed that this program adjusts the model of the surrounding
soil originally entered by the person analyzing the data until the calculated results for
the test match those measured. It just helps the person analyzing the test in order to
developing a possible solution. The final outcome is only a possible solution instead of
the real solution, as there is no unique solution for this process. So it is highly possible
that two engineers analyze the same data could lead to two totally different results. For
that reason it is very important that people who run CAPWAP® should have
40
experience.
Figure 3-7. CAPWAP® analyze procedure
3.4.3 Dynamic Tests Recorded by SmartPile Review
SmartPile ReviewTM (EDC) also records dynamic forces when pile is driving into
the soil. Through sensors embedded in the pile, the SmartPileTM system obtains
accurate information on stress levels in a concrete pile from the moment it is cast. This
provides the system with the unique ability to measure residual stresses during
installation and provide an accurate assessment of the true conditions in the pile.
Multiple embedded sensors also collect accurate wave speed measurements, allowing
a higher level of pile integrity monitoring. Consequently, accurate dynamic data on the
shaft friction and tip resistance is available, so that an estimate of the ultimate static
resistance (i.e. capacity of the pile) can be made. To enhance safety and ease of use,
its patented design allows monitoring and recording of data from up to 500 feet from the
pile, with no wires to connect. Powerful PC‐based software generates DOT‐formatted
41
reports, provides multi‐user access with password control, and allows data review from
both current as well as past projects.
Figure 3-8. EDC software window
3.4.4 Wave Theory
When hammer hits the pile top, any point on pile has possibility of two waves
passing up or down at any time. The passing waves will generate compression and
tension forces in the pile, they are called the downward force (fd) and upward force (fu).
Figure 3-9. WaveUp and WaveDown Forces passing along the pile
Fd = Z V
down
Fup
= -Z Vup
(-) = -Z (+)
Fup
= -Z Vup
Fup
= -Z Vup
(+) = -Z (-) F
d = Z V
d
(-) = Z (-)
42
Wave has its own velocity ( C = wave velocity), usually around 13000 ft/sec in
concrete piles. It can be calculated by pile properties
C = sqrt (E / ) (3-1)
where E = pile elastic modulus;
= pile desity
Another velocity needed to be noticed is the particle velocity which is different from
wave velocity. It can be measured directly from accelerometer, as it is the real velocity
where the pile’s particle moves. Pile impedance Z is calculated from wave velocity
, Z = EA/C (3-2)
Where A is the pile cross-section’s area.
Figure 3-10. Wave travelling in the pile
Compression Wave
V (particle velocity)
C=Wave Speed = sqrt (E / )
Downward Traveling Tension
Downward Traveling Compression
Fd = Z V
d
(+) = Z (+)
43
As soon as collecting the field data after hammer blow, Data Acquisition System
will supply operators with raw data as strain gage values and accelerometer. Pile total
force (P) calculated from strain gage data is composed of two forces - Fd and Fu forces.
P = E Across = Fd + Fup (3-3)
By integrating the acceleration, both particle velocity and pile movement can be
reached:
VT = a dt = Vd + Vup (3-4)
Where a = pile acceleration;
VT = pile total velocity; Vd = downward wave velocity; Vup = upward wave velocity
Also: P = Fd + Fup = Z Vd – Z Vup (3-5)
So wave passing up and Down pile a given point and time is calculated using the
following two equations:
Fup = ( P – Z VT ) / 2 (3-6)
Since PDA and SmartPileTM Review are both dynamic testing equipment, total
force contains static resistance ( Rs ) and dynamic resistance ( Rd ) showing in the
following equation:
P = Rs + Rd (3-7)
And dynamic resistance is composed of damping force and inertial force. Damping
is associated with particle velocity. Due to remolding effects, the major soil damping
occurs at pile tip. SmartPile ReviewTM also installs a pair of strain gages and
accelerometer at the pile tip, so it can directly calculate the total tip resistance:
Ptip = m a + c VT + K (3-8)
44
Where Ptip = the total tip force measured from tip strain gage;
M = pile tip mass; a = tip acceleration measured from tip accelerometer; c = damping ratio; K = pile tip stiffness;
= pile tip movement, also measured from tip accelerometer. 3.4.5 Unloading Point Method
Full-scale testing can be an integral component of quality control/quality assurance
for projects involving construction of deep foundations. Rapid load tests are being used
in the deep foundation industry as a method for assessing the axial static behavior of
deep foundations. Since rapid load tests involve dynamics, inertial and damping forces
must be considered in analyzing measured pile response to estimate the static pile
response.
The Statnamic load test is based on Newton’s second and third law, considering
that force is equal to mass times acceleration. Every reaction has an equal and opposite
reaction. Loads ranging from 5 tons to 5000 tons are generated by propelling a reaction
mass upward off the foundation. The force associated with propelling of this mass acts
equally and oppositely on to the pile. Statnamic load testing requires no reaction piles,
no reaction beam,and no hydraulic jack. The statnamic device is set up on the pile top
and includes a calibrated load cell and displacement measuring system. During a
Statnamic test, a high-speed data acquisition system scans and records the load cell,
displacement transducers, accelerometers and embedded strain gages. Because the
duration of the axial Statnamic test is adequately longer than the natural period of the
foundation element, the foundation thus remains in compression. The measured
Statnamic force is not simply the pile capacity but including both inertia and damping
forces.
45
Figure 3-11. Steps of Statnamic Test
Figure 3-12. Real field Statnamic Test
In the early years of Statnamic, a variety of methods were used to analyze the
results, mostly leading to incorrect results. The Unloading Point Method (UPM)
developed by Pter Middendorp in 1993 was a breakthrough in deciding pile capacity
from Statnamic test. Now SmartPile ReviewTM, Statnamic and others employ the
“Unloading Point Approach” developed by Middendorp to assess static resistance from
dynamic measurements. The approach uses force equilibrium and assumes that the
46
static resistance has reached a peak when the velocity trace passes through zero,
going negative, resulting in small incremental displacements. For the force equilibrium
equation:
Fstatic = Ftotal – ma – cv (3-9)
Where Fstatic = static pile resistance;
Ftotal = measured total Statnamic force (load cell); ma = measured inertia force; cv = soil damping force
For zero velocity, the damping force is zero and because the mass is known along
with the applied dynamic force, the static resistance may be assessed [Ftotal – ma =
Fstatic ]. Subsequently, assuming that the static resistance is constant for later times, the
damping is assessed based on force equilibrium. A major concern related to the
approach is the assumption of constant Static resistance, Fstatic, when calculating
viscous damping, c.
Figure 3-13. Statnamic measured load and calculated static force
47
CHAPTER 4 ENERGY METHOD
4.1 Theories of Energy Method
4.1.1 Newton's Three Laws of Motion
Before introducing energy method implemented in pile tip for accessing tip static
resistance, it’s better to review Newton's Three Laws first which is the core of this brand
new approach. They are three physical laws that form the basis for classical mechanics.
They describe the relationship between the forces acting on a body and its motion due
to those forces. They have been expressed in several different ways over nearly three
centuries, and can be summarized as follows:
(a) First law: Every body remains in a state of rest or uniform motion (constant
velocity) unless it is acted upon by an external unbalanced force. This means that in the
absence of a non-zero net force, the center of mass of a body either remains at rest, or
moves at a constant speed in a straight line.
(b) Second law: A body of mass m subject to a force F undergoes an acceleration
a that has the same direction as the force and a magnitude that is directly proportional
to the force and inversely proportional to the mass, i.e., F = ma. Alternatively, the total
force applied on a body is equal to the time derivative of linear momentum of the body.
(c) Third law: The mutual forces of action and reaction between two bodies are
equal, opposite and collinear. This means that whenever a first body exerts a force F on
a second body, the second body exerts a force −F on the first body. F and −F are equal
in magnitude and opposite in direction. This law is sometimes referred to as the action-
reaction law, with F called the "action" and −F the "reaction". The action and the
reaction are simultaneous.
48
For the pile tip model, the body is the pile mass under the strain gages, all the
forces are acted to the pile mass. The pile tip obeys Newton's Three Laws of Motion at
any time during the pile driving.
4.1.2 Force and Energy Equilibrium
Focusing on the tip mass, the only unknowns at the pile tip are m (mass), c
(viscous damping) and k (stiffness). In physics and engineering, damping may be
mathematically modeled as a force synchronous with the velocity of the object but
opposite in direction to it. If such force is also proportional to the velocity, as for a simple
mechanical viscous damper (dashpot), the force F may be related to the velocity v by
F = c v (4-1)
where c is the viscous damping coefficient, given in units of Newton-seconds per
meter. This force is an approximation to the friction caused by drag. Generally, damped
harmonic oscillators satisfy the second-order differential equation:
(4-2)
where ω0 is the undamped angular frequency of the oscillator and ζ is a constant
called the damping ratio. For a mass on a spring having a spring constant k and a
damping coefficient c,
(4-3)
and
(4-4)
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The value of the damping ratio ζ determines the behavior of the system. A damped
harmonic oscillator can be:
(a) Overdamped (ζ > 1): The system returns (exponentially decays) to equilibrium
without oscillating. Larger values of the damping ratio ζ return to equilibrium slower.
(b) Critically damped (ζ = 1): The system returns to equilibrium as quickly as
possible without oscillating. This is often desired for the damping of systems such as
doors.
(c) Underdamped (ζ < 1): The system oscillates (with a slightly different frequency
than the undamped case) with the amplitude gradually decreasing to zero.
The damped natural (angular) frequency ωd, i.e., the frequency the oscillation
occurs when the system is underdamped (ζ < 1) and under free vibration, with regards
to the damping factor ζ and the undamped natural (angular) frequency ω0 is given by:
(4-5)
Back to the tip model, generally m may be assumed as the mass of pile below the
tip gages. The stiffness, k, of a body is a measure of the resistance offered by an elastic
body to deformation. Stiffness is the resistance of an elastic body to deformation by an
applied force along a given degree of freedom (DOF) when a set of loading points and
boundary conditions are prescribed on the elastic body. For an elastic body with a
single Degree of Freedom (for example, stretching or compression of a rod), the
stiffness is defined as:
(4-6)
Where: F is the force applied on the body;
50
δ is the displacement produced by the force along the same degree of freedom
(for instance, the change in length of a stretched spring).
In the International System of Units, stiffness is typically measured in newtons per
metre. In English Units, stiffness is typically measured in pound force (lbf) per inch.
However, the stiffness, k, is generally not constant (i.e. nonlinear, varies with
displacement) and the damping, c is assumed a constant. Assessing c and the variable
k at the pile tip uses force equilibrium, or
tPxkFxcFxmF staticdampinginertia (4-7)
which must be satisfied for any time.
Figure 4-1. Mass-Spring-Damper model
A major improvement of the “Unloading Point Approach” which conserves force
equilibrium is to also conserve energy or work of the single degree of freedom system.
For the typical dynamic pile problem, the energy going into the system may be
51
assessed by the Force measured by tip EDC strain gage and accelerometer. The input
energy must be balanced by the inertia, damping and static energy that occurs as result
of tip acceleration, or
dxtPdxxkxcxm (4-8)
Where x is displacement and is velocity and is acceleration.
To assist with the implementation of the integration as well as improve accuracy
(accelerometer measurements), the integration variable can be changed to time (Liang
& Feeny, 2006) or,
Tt
t
Tt
t
dtxtPdtxxkxcxm
(4-9)
4.1.3 Process of Energy Method
To perform energy method, first thing needed is to generate SmartPile ReviewTM’s
raw data sheet of each blow. As the figure shown below, SmartPile ReviewTM can
provide users with pile information such as pile number, blow number, modulus of
elastic and so on, as well as simultaneous measured driven information such as top/tip
force, top/tip acceleration and so on.
Figure 4-2. Excel data sheet from SmartPile Review of one blow
52
Figure 4-3. Excel configuration sheet from SmartPile Review
SmartPile ReviewTM generates excel sheet containing pile information
(configuration sheet), raw data (data sheet) for each blow. Energy method exerts these
two sheets to calculate parameters and finally plots each force.
53
Figure 4-4. Excel sheet from Energy Method 1
From Figure 4-4, C1 labeled Wt (kips) is pile weight, divided by B1 refered to g
(ft/s2) can get D1 which is pile mass M ( kips-s2/ft), that is used to calculate damping
force. G1, H1 and I1 are stiffness coefficient, reflecting pile tip static resistance is
changing along with time. Cells A5 to I21 are data copied from SmartPile ReviewTM’s
raw data.
Figure 4-5. Excel sheet from Energy Method 2
54
From Figure 4-5, it shows all the forces calculated using the raw data from Figure
4-4. As mentioned before, forces acting on the pile tip model are forces input from
hammer (in the energy method sheet, it is directly got from SmartPile ReviewTM called
tip force), damping force (Column M, equals to velocity times damping ratio), static tip
force (Column L, equals to stiffness times tip mass), inertia force (Column K, equals to
acceleration times tip mass). From Column Q to Column T, they are energy measured
from each force. Except for force equilibrium, another equilibrium of pile tip is energy
balance. After calculating all these forces, it’s ready to draw all the forces or energy
together and minimize the error force/energy.
To see an example of the approach consider EDC Blow 777 (Figure 4-6) which
was a restrike blow after static load test on Dixie Highway Pier 8. The Purple line is the
applied force, P(t) (Equation.4-7), dark blue is inertia force, Finertia,(Equation.4-7). The
damping force, Fdamping, was found by multiply a viscous damping, c, 30 kip-sec/ft by the
measured tip velocity. The static resistance, FStatic, (Equation.4-7) was found by
multiplying a tangent stiffness, k, by tip displacement, Figure 4-6.
55
Figure 4-6. EDC Blow 777 Forces vs. Time at Pile Tip of Pier 8
The assessment of c and k begins where the damping and inertia forces are zero
(blue and green lines) in Figures 4-6 (at 0.175 sec 0.023sec and 0.0314sec) and 32 (at
0.0359 ft, 0.0523 ft and 0.0693 ft). For these times and displacements the applied force,
P(t) must equal the static resistance, Fstatic, from equilibrium eq.4-7. Knowing the Fstatic,
the value of tangent stiffness, k may be assessed (slopes of red line Figure 4-6). Finally,
the value of the viscous damping, c (30 kip-sec/ft) may be determined through the
energy balance at the pile tip from Equation 4-9. Shown in Figure 4-8 is the computed
energy for each component (applied, inertia, damping, and stiffness) as well as the error
(Eapplied – Einertia – Edamping – Estatic).
56
Figure 4-7. EDC Blow 777 Forces vs. Displacement at Pile Tip of Pier 8
Evident from the new energy plot (Figure 4-7), the inertia component is small and
dies out with time. This is because the pile tips generally undergo significant harmonic
motion (i.e. positive and negative motion – Figure 4-6). Consequently, the static energy
plus the damping energy must equal the applied energy (Figure 4-8) especially for later
times. Since points on the static force vs. displacements are known (Equilibrium), the
viscous damping coefficient may be very accurately assessed.
The new combined Equilibrium and Energy Balance Equations at the pile tip
results in very accurate assessment of the nonlinear static resistance vs. displacement
(Figure 4-7) at the bottom of the pile. Based on Figure 4-7, the static tip resistance at
0.07 ft (0.84 in) was 320 kips or 160 tons. The latter compares very favorably with the
57
measured static tip resistance of 336 kip at 0.9 in of movement.
Figure 4-8. EDC Blow 777 Energy vs. Time at Pile Tip of Pier 8
Another example here is blow 779 of Dixie Highway Pier 8, which also shows
energy method performs well.
Figure 4-9. EDC Blow 779 Forces vs. Time at Pile Tip of Pier 8
58
Figure 4-10. EDC Blow 779 Forces vs. Displacement at Pile Tip of Pier 8
Figure 4-11. EDC Blow 779 Energy vs. Time at Pile Tip of End Bent1
59
4.1.4 Examples of Energy Method
Figures 4-12 to 4-29 below are examples using energy method. First example
shown is blows analyzed from Dixie Highway Project.
4.1.4.1 Site ---- Dixie Highway
Introduction: Dixie Highway (CR-818) from South of Hillsboro Boulevard (SR-810)
to North of the Hillsboro Canal (Design-build Project). Pile End Bent NO.1 and Pier
NO.8 have been performed with compression load test, Pier NO.4 has been performed
with tension load test. Test pile installation work at the above referenced site was
performed by Cone and Graham, Inc.
4.1.4.1.1 Compression load test – End Bent NO.1
Test Pile Installation: One 24-inch-square prestressed precast concrete
compression test pile at End Bent Number 1 was installed about 45 feet below the
ground surface at a non-production pile location on April 21, 2010. The pile was driven
using and ICE (I-46), single acting diesel hammer.
Pile Load Test: The compression test pile was load tested to failure in
accordance with ASTM D1143 (quick test). The load was applied against a frame
anchored by four, 24-inch-diameter Auger Cast In-Place (ACIP) reaction piles.
Test pile deflection were monitored using: 1) dial gauges with accuracy of 0.001
inch, 2) piano wire 1/64 scale system, and 3) a survey level to read scale with accuracy
of 1/64 inch attached to the tension test pile.
Compression loads were applied using two 500 ton hydraulic jacks. Cone and
Graham, Inc. set-up the load test reaction frames, provided the pre-calibrated jack,
reference beams, pumps, and necessary personnel to run the equipment.
60
Compression Load Test Results: The compression load test was performed on
April 29, 2010. The test loads were applied in two cycles. The pile was loaded in 45 ton
increments to the maximum load of 298 tons for the first cycle. Each load was held for
at least 10 minutes. The maximum gross pile butt deflection, under the maximum test
load of 298 tons after 10 minutes of sustained loading, measured 1.78 inch. The pile
was subsequently unloaded in 90 ton decrements. For the second cycle, the pile was
again loaded in 45 ton increments to a maximum load of 317 tons. Each load was held
for at least 5 minutes. The maximum gross pile butt deflection, under the maximum test
load of 317 tons after 5 minutes of sustained loading, measured 2.97 inch. The pile was
subsequently unloaded in 90 ton decrements. The result of this load test is summarized
in the attached Compression Load Vs Settlement plot.
Geokon 4911 vibrating wire sister-bar strain gauges were installed in the
compression test pile. Eight (8) strain gauges were installed at four levels within the pile.
Two (2) strain gauges were installed at each level. The strain gauges were installed to
monitor under the applied loads. This data was used to infer load transfer along the test
pile. Based on the strain gauge data, at the maximum test load of 298 tons, about 77
percent of the load was transferred to the bottom of the pile. But temperature
corrections and gage factors have not been applied while utilizing the strain gage data,
and therefore, these load distribution values should not be taken as absolute values.
Figures 4-12 to 4-14 shown below are energy method performed on blow 740
which is one of end of driving blows. As pile set-up occurs, comparing the EOD and
BOR blows’ tip resistance and skin friction could reveal some information of how much
these capacities change.
61
Figure 4-12. EDC Blow 740 Energy vs. Time at Pile Tip of End Bent1
Figure 4-13. EDC Blow 740 Forces vs. Displacement at Pile Tip of End Bent1
62
Figure 4-14: EDC Blow 740 Energy vs. Time at Pile Tip of End Bent1
Blow 765 was driven one week later, which is BOR of Dixie High End Bent1.
Figure 4-15. EDC Blow 765 Energy vs. Time at Pile Tip of End Bent1
63
Figure 4-16. EDC Blow 765 Forces vs. Displacement at Pile Tip of End Bent1
Figure 4-17. EDC Blow 765 Energy vs. Time at Pile Tip of End Bent1
64
For Dixie highway End Bent1, 10 re-strike blows just before and after the load test
for comparison with one another as well as with the measured static tip resistance from
the load test. The specific results of one of the re-strike blow are presented here in
detail for discussion. Energy method performed for other 9 blows will be presented in
Apendix A.
4.1.4.1.2 Compression load test – Pier NO.8
Test pile installation: One 24-inch-square prestressed precast concrete
compression test pile at Pier Number 8 was installed about 49 feet below the ground
surface at a non-production pile location on May 6, 2010. The pile was driven using an
ICE (I-46), single acting diesel hammer.
Pile Load Test: The compression test pile was load tested to failure in accordace
with ASTM D1143 (quick test). The load was applied against a frame anchored by four,
24-inch-diameter Auger Cast In-Place (ACIP) reaction piles.
Test pile deflections were monitored using: 1) dial gauges with accuracy of 0.001
inch, 2) piano wire 1/64 scale system, and 3) a survey level to read scale with accuracy
of 1/64 inch attached to the hydraulic jack.
Compression loads were applied using two 500 ton hydraulic jackes. Cone and
Graham, Inc. set-up the load test reaction frames, provided the pre-calibrated jack,
reference beams, pumps, and necessary personnel to run the equipment.
Compression Load Test Results: The compression load test was performed on
May 12, 2010. The test loads were applied in two cycles. The pile was loaded in 45 ton
increments to the maximum load of 255 tons for the first cycle. Each load was held for
at least 10 minutes. The maximum gross pile butt deflection, under the maximum test
load of 255 tons after 10 minutes of sustained loading, measured 1.69 inch. The pile
65
was subsequently unloaded in 90 ton decrements. For the second cycle, the pile was
again loaded in 45 ton increments to a maximum of 285 tons. Each load was held for at
least 5 minutes. The maximum gross pile butt deflection, under the maximum test load
of 285 tons after 5 minutes of sustained loading, measured 3.12 inch. The pile was
subsequently unloaded in 90 ton decrements.
Geokon 4911 vibrating wire sister-bar strain gauges were installed in the
compression test piles, Eight (8) strain gauges were installed at four levels within the
pile. Two (2) strain gauges were installed at each level. The strain gauges were
installed to monitor the strain at different depths within the pile under the applied loads.
This data was used to infer load transfer along the test pile. Based on the strain gauge
data, at the maximum test load of 255 tons, about 65% of the load appeared to be
transferred to the bottom of the pile.
Figures 4-18 to 4-21 shown below are energy method performed on blow 751
which is end of driving blows.
Figure 4-18. EDC Blow 751 Forces vs. Time at Pile Tip of Pier 8
66
Figure 4-19. EDC Blow 751 Forces vs. Displacement at Pile Tip of Pier 8 Pile
Figure 4-20. EDC Blow 751 Energy vs. Time at Pile Tip of End Bent1
67
Blow 779 was driven five days later which is BOR of Dixie High End Bent1.
Figure 4-21. EDC Blow 779 Forces vs. Time at Pile Tip of Pier 8 Pile
Figure 4-22. EDC Blow 779 Forces vs. Displacement at Pile Tip of Pier 8
68
Figure 4-23. EDC Blow 779 Energy vs. Time at Pile Tip of Pier 8
4.1.4.2 Site – Caminada Bay
Caminida Bay is in Louisiana, 70 km south of New Orleans. The site consists of 10
m of silty fine sand with clay (SPT N ~ 14) followed by 10 m of fine sand with silt (SPT N
~ 24); and high plasticity (40 < PI < 70) clays. The first pile (pile 1) analyzed was a 0.76-
m-quare precast prestressed concrete pile driven 21 m below the ground surface (1m
into clay) using a single acting diesel hammer. Re-strikes were conducted 7 days after
installation, and the static compression load test was conducted 2 days after the re-
strikes.
69
Figures 4-24 to 4-26 below show blow 630 which is beginning of drive at Caminida
Bay Bent1.
Figure 4-24. EDC Blow 630 Forces vs. Time at Pile Tip of Caminida Bay Bent1 Pile1
Figure 4-25. EDC Blow 630 Forces vs. Disp. at Pile Tip of Caminida Bay Bent1 Pile1
70
Figure 4-26. EDC Blow 630 Energy vs. Time at Pile Tip of Caminida Bay Bent1 Pile1
Figure 4-27. EDC Blow 660 Forces vs. Time at Pile Tip of Caminida Bay Bent1 Pile1
71
Figure 4-28. EDC Blow 660 Forces vs. Disp. at Pile Tip of Caminida Bay Bent1 Pile1
Figure 4-29. EDC Blow 660 Energy vs. Time at Pile Tip of Caminida Bay Bent1 Pile1
72
CHAPTER 5 OBSERVATIONS OF ENERGY APPROACH
5.1 EOD vs. BOR Predicted Static Response of Dixie Highway
The predicted tip resistance for both EOD and BOR blows are shown below
respectively. Figure 5-1 represents the static tip resistance for 5 blows at EOD and
figure 5-2 are for 5 restrike blows ( BOR ), also combined with static load test result.
Figure 5-1. Estimated tip resistance of Dixie Highway Bent 1 Pile1 blows before the
load test. Photo courtesy of Yipeng Xie
Figure 5-2. Estimated tip resistance of Dixie Highway Bent 1 Pile1 blows after the load
test. Photo courtesy of Yipeng Xie
73
Figure 5-3. Estimated tip resistance of Dixie Highway Pile8 blows before the load test
Figure 5-4. Estimated tip resistance of Dixie Highway Pile8 blows after the load test
74
5.2 EOD vs. BOR Predicted Static Response of Caminida Bay
Next is the predicted tip resistance for both EOD and BOR blows of Caminida Bay
respectively. Figure 5-5 represents the static tip resistance for 3 blows at EOD and
figure 5-6 are for 5 restrike blows ( BOR ), also combined with static load test result.
Figure 5-5. Estimated tip resistance of Caminida Bay1 blows before the load test
Figure 5-6. Estimated tip resistance of Caminida Bay1 blows after the load test
75
Figure 5-7. Estimated tip resistance of Caminida Bay7 blows before the load test
Figure 5-8. Estimated tip resistance of Caminida Bay7 blows after the load test
76
To better compare the static load test results with estimated tip resistance, it is
necessary to calculate Davisson’s Capacity first, to see how much displacement the pile
tip moves under such load. Davisson method is a graphical method which defines the
pile capacity as that load corresponding to the movement which exceeds the elastic
compression of the pile by a value of 0.15 inches plus a factor equal to the diameter of
the pile ( in inches ) divided by 120. Figures 5-9 to 5-12 below are Davisson’s capacity
found in each pile:
Figure 5-9. Davisson’s Capacity of Dixie Highway Bent1 Pile1
77
Figure 5-10. Davisson’s Capacity of Dixie Highway Pier8
Figure 5-11. Davisson’s Capacity of Caminida Bay Bent1
78
Figure 5-12. Davisson’s Capacity of Caminida Bay Bent7
From Figure 5-1 to Figure 5-8 it is observed that the estimated tip resistances
compares very favorable to the load test results. When calculating Davisson’s capacity
on the static load test, it is found that pile tip movement is about 0.3 to 0.5 ft, less than
the maximum observed displacement which is around 0.5 to 0.8 ft. For Dixie Highway
End Bent1 and Pier 8, in the displacement range of Davisson’s capacity movement,
estimated tip resistance compares favorably to the load test. Dixie Highway End Bent1
the two capacities are around 300 kips, and Dixie Highway Pier8 they are about 220
kips. In the case of Caminida Bay, both beginning of drive blows compares favorably to
the static load test. For Caminida Bay Bent1 they are both around 180 kips, for
Caminida Bay Bent 7 they are around 80 kips.
79
Considering pile freeze occurred after EOD, comparison of EOD and BOR blows
for the Dixie Highway piles were not considered as the static load test used augured
cast piles at 3D spacing from the tested piles which may have influenced the results
between EOD and BOR. Refer to Caminida Bay piles, tip resistance of Bent1 piles
drops 10%, Bent7 increases 20%. The increase in tip resistance may be associates with
pile freeze, which is due to the dissipation in pore water pressure in clay soils.
Another interesting behavior between EOD and BOR tip response is shown in
Figures 5-13, 5-14, and 5-15 for the case of Florida Department of Transportation
Design Build US192 over I-95 (24”x24” x 105ft), and Figures 5-16, 5-17, and 5-18 for
the case of Florida Department of Transportation I-95 Eau Gallie bent1 pile1. Figure 5-
13 shows five EOD blows analyzed with force/energy tip approach. From the figure 5-
13, EOD shows 400 kips of end bearing at 0.04 ft (0.48”) of tip movement.
Subsequently, after 2 days the pile was restruck, and Figure 5-14 shows the measured
BOR response for 5 blows. The restrike analysis shows a mobilized tip resistance of
300 kips at 0.025 ft (0.3 in). Notice, the maximum tip movement of the BOR blows are
smaller (0.3”) vs. the EOD value (0.48”). A comparison of the mean EOD vs. mean BOR
tip resistance is given in Figure 5-15. Evident of from figure 5-15, the EOD and BOR
stiffness are quite similar, but the difference in tip resistance is due to the mobilized tip
displacements. A possible explanation of the different mobilized displacement at EOD
vs BOR strikes may be due to the rated energy of the hammer and the fact that the
same hammer and energy is being delivered to the pile for the EOD and BOR blows.
However, in the case of EOD driving, there is a loss of skin friction (i.e. excess pore
pressure, etc.) and more of the hammer energy arrives at the pile tip, mobilizing more
80
tip displacement and resistance, Figure 5-13. After 2 day wait time, pile freeze occurs,
and the same hammer delivers the same energy to the BOR blows; however more of it
is dissipated in mobilizing skin friction and less arrives at the pile tip to mobilize tip
resistance, Figure 5-14. Another example of this effect is shown from the FDOT EDC
database of Eau Gali Bridge on I-95 in Figure 5-18 for mean EOD and BOR (18 days)
resistances. Again, the tip resistance of pile at EOD went from 280 kips at 0.043ft (0.5”)
to 180 kips at 0.025 ft (0.3”) for the 18 day BOR restrikes. Obviously for the US 192 and
Eau Gali Bridge over I-95, the BOR blows may not identify the full Davisson capacity, if
movements greater 0.025 ft (0.3”) are needed. To address the latter, the recent NCHRP
Synthesis Report “Developing Production Pile Driving Criteria from Test Pile Data,” has
suggested using predicted tip resistance at EOD with the skin friction assessed from
BOR blows. Of course, this assumes that same error in predicting total capacity applies
to estimation of skin or tip resistance. With tip monitoring and proposed force/energy
analysis the latter may be viable.
Figure 5-13. Predicted Tip Resistance at EOD for pile 5 at US 192
81
Figure 5-14. Predicted Tip Resistance at BOR after 2 days for pile 5 at US 192
Figure 5-15. Predicted Tip Resistance at EOD and BOR for pile 5 at US 192
82
Figure 5-16. Predicted Tip Resistance at EOD of I95 Eau Gallie bent1 pile1
Figure 5-17. Predicted Tip Resistance at BOR of I95 Eau Gallie bent1 pile1
83
Figure 5-18. Predicted Tip Resistance at EOD and BOR I95 Eau Gallie bent1 pile1
Tables below are summation of each pile’s tip resistance calculated by energy
method and total capacity recorded by SmartPile ReviewTM.
Table 5-1. EOD and BOR Tip Forces of Dixie Highway Pile No. Energy
Method
Static Tip
Resistance
(kips)
UF method
Tip
Resistance
(kips)
UF method
Total
Capacity
(kips)
Pile No. Energy
Method
Static Tip
Resistance
(kips)
UF method
Tip
Resistance
(kips)
UF method
Total
Capacity
(kips)
Dixie
Highway End
Bent 1
Blow No. 740
300 350 449 Dixie Highway
Pier 8
Blow No. 754
177 250 522
Dixie
Highway End
Bent 1
Blow No. 741
331 374 447 Dixie Highway
Pier 8
Blow No. 755
228 273 505
Dixie
Highway End
Bent 1
Blow No. 742
305 405 456 Dixie Highway
Pier 8
Blow No. 756
243 270 471
84
Table 5-1. Continued Pile No. Energy
Method
Static Tip
Resistance
(kips)
UF method
Tip
Resistance
(kips)
UF method
Total
Capacity
(kips)
Pile No. Energy
Method
Static Tip
Resistance
(kips)
UF method
Tip
Resistance
(kips)
UF method
Total
Capacity
(kips)
Dixie
Highway End
Bent 1
Blow No. 743
260 398 436 Dixie Highway
Pier 8
Blow No. 758
176 300 454
Dixie
Highway End
Bent 1
Blow No. 744
300 395 434 Dixie Highway
Pier 8
Blow No. 759
229 290 432
Dixie
Highway End
Bent 1
Blow No. 760
300 410 482 Dixie Highway
Pier 8
Blow No. 777
194 410 430
Dixie
Highway End
Bent 1
Blow No. 765
280 450 450 Dixie Highway
Pier 8
Blow No. 778
240 418 425
Dixie
Highway End
Bent 1
Blow No. 778
273 406 437 Dixie Highway
Pier 8
Blow No. 779
256 400 403
Dixie
Highway End
Bent 1
Blow No. 779
308 438 438 Dixie Highway
Pier 8
Blow No. 780
226 383 416
Dixie
Highway End
Bent 1
Blow No. 780
280 425 425 Dixie Highway
Pier 8
Blow No. 801
267 340 451
Time interval between EOD and BOR of Dixie Highway End Bent 1 is 12 days.
Time interval between EOD and BOR of Dixie Highway Pier 8 is 8 days.
Table 5-2. EOD and BOR Tip Forces of Caminida Bay
Pile No.
Energy
Method
Static Tip
Resistance
(kips)
UF method
Tip
Resistance
(kips)
UF method
Total
Capacity
(kips)
Pile No. Energy
Method
Static Tip
Resistance
(kips)
UF method
Tip
Resistance
(kips)
UF method
Total
Capacity
(kips)
Caminida Bay
Bent1 Pile1
Blow No. 625
172 233 567 Caminida Bay
Bent 7
Blow No. 313
60 139 422
Caminida Bay
Bent1 Pile1
Blow No. 629
171 192 584 Caminida Bay
Bent 7
Blow No. 315
51 76 367
Caminida Bay
Bent1 Pile1
Blow No. 630
170 210 582 Caminida Bay
Bent 7
Blow No. 316
60 57 355
85
Table 5-2. Continued
Pile No.
Energy
Method
Static Tip
Resistance
(kips)
UF method
Tip
Resistance
(kips)
UF method
Total
Capacity
(kips)
Pile No. Energy
Method
Static Tip
Resistance
(kips)
UF method
Tip
Resistance
(kips)
UF method
Total
Capacity
(kips)
Caminida Bay
Bent1 Pile1
Blow No. 660
185 67 585 Caminida Bay
Bent 7
Blow No. 317
60 53 352
Caminida Bay
Bent1 Pile1
Blow No. 661
210
70 595 Caminida Bay
Bent 7
Blow No. 318
54 58 362
Caminida Bay
Bent1 Pile1
Blow No. 664
192 60 573 Caminida Bay
Bent 7
Blow No. 331
70 44 502
Caminida Bay
Bent1 Pile1
Blow No. 665
190 30 530 Caminida Bay
Bent 7
Blow No. 336
60 57 539
Caminida Bay
Bent1 Pile1
Blow No. 666
195 40 502 Caminida Bay
Bent 7
Blow No. 343
62 43 539
Caminida Bay
Bent 7
Blow No. 345
80 45 538
Caminida Bay
Bent 7
Blow No. 350
60 75 499
Time interval between EOD and BOR of Caminida Bay Bent1 Pile1 is 6 days. Time interval between EOD and BOR of Caminida Bay Bent 7 is 7 days.
Table 5-3. EOD and BOR Tip Forces of I-95 DEsign Build US 192 bent3 pile5 & I-95
Eau Gallie bent1 pile1
Pile No.
Energy
Method
Static Tip
Resistance
(kips)
Pile No.
Energy Method
Static Tip
Resistance
(kips)
Pile No.
Energy
Method
Static Tip
Resistance
(kips)
Pile No.
Energy
Method
Static Tip
Resistance
(kips)
I95 DEsign Build
US 192 bent3
pile5
Blow No. 953
270 I95 DEsign Build
US 192 bent3 pile5
Blow No. 972
287 I95 Eau Gallie
bent1 pile1
Blow No. 1694
160 I95 Eau Gallie
bent1 pile1
Blow No. 1716
202
I95 DEsign Build
US 192 bent3
pile5
Blow No. 954
258 I95 DEsign Build
US 192 bent3 pile5
Blow No. 973
300 I95 Eau Gallie
bent1 pile1
Blow No. 1696
160 I95 Eau Gallie
bent1 pile1
Blow No. 1717
200
86
Table 5-3. Continued
Pile No.
Energy
Method
Static Tip
Resistance
(kips)
Pile No.
Energy Method
Static Tip
Resistance
(kips)
Pile No.
Energy
Method
Static Tip
Resistance
(kips)
Pile No.
Energy
Method
Static Tip
Resistance
(kips)
I95 DEsign Build
US 192 bent3
pile5
Blow No. 956
270 I95 DEsign Build
US 192 bent3 pile5
Blow No. 976
288 I95 Eau Gallie
bent1 pile1
Blow No. 1700
167 I95 Eau Gallie
bent1 pile1
Blow No. 1719
203
I95 DEsign Build
US 192 bent3
pile5
Blow No. 960
270 I95 DEsign Build
US 192 bent3 pile5
Blow No. 977
300 I95 Eau Gallie
bent1 pile1
Blow No. 1702
178
Time interval between EOD and BOR of I95 DEsign Build US 192 bent3 pile5 is 2 days. Time interval between EOD and BOR of I95 Eau Gallie bent1 pile1 is 7 days.
Table 5-4. EOD and BOR Tip Forces Comparison
Pile No.
Energy
Method
Static Tip
Resistance
EOD (kips)
Energy
Method
Static Tip
Resistance
BOR (kips)
Static Tip
Resistance
Increase
Pile No.
Energy
Method
Static Tip
Resistance
(kips)
Energy
Method
Static Tip
Resistance
(kips)
Static Tip
Resistance
Increase
Dixie Highway
End Bent 1 N/A N/A N/A I95 DEsign
Build US 192
bent3 pile5
267 290 8.61%
Dixie Highway
Pier 8
N/A N/A N/A I95 Eau Gallie
bent1 pile1 166.5 197.5 18.61%
Caminida Bay
Bent1 Pile1
171 194 13.45% US1 over St
Sebastian
River I75
Bent2 Pile2
196 217 10.70%
Caminida Bay
Bent 7
57 66.4 16.4% SR21 over
Black Creek
Bent5 Pile6
250 300 20%
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Table 5-5. EOD and BOR Skin Friction Comparison
Pile
No.
Skin
Friction
EOD
(kips)
Skin
Friction
BOR (kips)
Skin Friction
Increase
Pile No.
Skin Friction
EOD (kips)
Skin Friction
BOR (kips)
Skin Friction
Increase
Dixie
Highway
End Bent 1
N/A N/A N/A I95 DEsign
Build US 192
bent3 pile5
251 377.33 50.33%
Dixie
Highway
Pier 8
N/A N/A N/A I95 Eau Gallie
bent1 pile1 311.5 562.5 80.51%
Caminida
Bay Bent1
Pile1
N/A N/A N/A US1 over St
Sebastian River
I75 Vanderbilt
Bent2 Pile2
N/A N/A N/A
Caminida
Bay Bent 7
314.6 457 45.2% SR21 over
Black Creek
Bent5 Pile6
N/A N/A N/A
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CHAPTER 6 CONCLUSION
6.1 Summary
In this study, a completely new method of assessing pile tip static resistance called
energy method has been raised, and by given different test sites for investigation,
energy method is proved to be feasible and efficient. Advantages of this method are
obvious:
It could be done by researchers manually. It based on excel sheet and easy to
learn. The more experience researcher gains, the shorter operating time it takes. Or, it
also could be done automatically by computer once the coding is finished.
Energy method’s results are shown both in the format of numbers and figures,
researchers can read the forces directly from plots without confusion.
Comparing to static load test, dynamic test is convenient and less cost. To get
static tip resistance from dynamic test is always reseachers’ preference. Energy method
uses the data collected from dynamic testing, providing with a guaranteed static tip
force.
As the theories of energy method is force equilibrium and energy equilibrium, and
those equilibrium should be satisfied any time of driving, this method could be applied at
each blow, which means it can calculated static tip resistance either from EOD or BOR.
As pile set-up observed and validated from more and more researchers
nowadays, energy method’s finding can lead to a deeper understanding of it. From the
piles analysed using energy method, static tip resistance seem not to change very much
compared to the skin friction, which most geotechnical researchers agree that is the
main reason contributing to the increase of the pile total capacities. Although pile set-
89
up’s mechanism is still under investigating, results from energy method can provide
some clues of assessing the set-up factor. What we choose here is blows from EOD
and BOR period, usually the time interval between these two phases are several days
long. So it guarantees most of the pile set-up will develop during this time interval. At
the same time, the time when hammer stops hitting is less than one hour is also
considered, and pile set-up is still observed for most cases. Data performed with energy
method compared to static load test results are quite agreeable, which means this
method could be used in future studies.
6.2 Recommendations
For this thesis energy method’s raw data sources are all from SmartPile
ReviewTM’s excel sheets. PDA nowadays is common dynamic testing equipment around
America, so performing energy method with PDA’s outputs will be put into
consideration. Also, to accelerate the calculation, computer automatically run will surely
reduce the time cost, and also provide with more standardized results.
90
APPENDIX A
EXAMPLES OF ENERGY METHOD
Figure A-1. EDC Blow 740 Forces vs. Time at Pile Tip of Dixie Highway Bent1
Figure A-2. EDC Blow 740 Forces vs. Disp. at Pile Tip of Caminida Bay Bent1 Pile1
91
Figure A-3. EDC Blow 740 Energy vs. Time at Pile Tip of Caminida Bay Bent1 Pile1
Figure A-4. EDC Blow 778 Forces vs. Time at Pile Tip of Dixie Highway Bent1
92
Figure A-5. EDC Blow 778 Forces vs. Disp. at Pile Tip of Dixie Highway Bent1
Figure A-6. EDC Blow 778 Energy vs. Time at Pile Tip of Dixie Highway Bent1
93
Figure A-7. EDC Blow 756 Forces vs. Time at Pile Tip of Dixie Highway Pier8
Figure A-8. EDC Blow 756 Forces vs. Disp. at Pile Tip of Dixie Highway Pier8
94
Figure A-9. EDC Blow 756 Energy vs. Time at Pile Tip of Dixie Highway Pier8
Figure A-10. EDC Blow 779 Forces vs. Time at Pile Tip of Dixie Highway Pier8
95
Figure A-11. EDC Blow 779 Forces vs. Disp. at Pile Tip of Dixie Highway Pier8
Figure A-12. EDC Blow 779 Energy vs. Time at Pile Tip of Dixie Highway Pier8
96
Figure A-13. EDC Blow 625 Forces vs. Time at Pile Tip of Caminida Bay Bent1
Figure A-14. EDC Blow 625 Forces vs. Disp. at Pile Tip of Caminida Bay Bent1
97
Figure A-15. EDC Blow 779 Energy vs. Time at Pile Tip of Dixie Highway Pier8
Figure A-16. EDC Blow 660 Forces vs. Time at Pile Tip of Caminida Bay Bent1
98
Figure A-17. EDC Blow 660 Forces vs. Disp. at Pile Tip of Caminida Bay Bent1
Figure A-18. EDC Blow 660 Energy vs. Time at Pile Tip of Caminida Bay Bent1
99
Figure A-19. EDC Blow 313 Forces vs. Time at Pile Tip of Caminida Bay Bent7
Figure A-20. EDC Blow 313 Forces vs. Disp. at Pile Tip of Caminida Bay Bent7
100
Figure A-21. EDC Blow 313 Energy vs. Time at Pile Tip of Caminida Bay Bent8
Figure A-22. EDC Blow 313 Forces vs. Time at Pile Tip of Caminida Bay Bent7
101
Figure A-23. EDC Blow 313 Forces vs. Disp. at Pile Tip of Caminida Bay Bent7
Figure A-24. EDC Blow 313 Energy vs. Time at Pile Tip of Caminida Bay Bent7
102
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BIOGRAPHICAL SKETCH
She was born on May 21, 1986. She finished her undergraduate study and got her
B.S. in Hohai University, Nanjing, China. In the August, 2008 She came to America to
pursue her master’s degree in University of Florida, majoring in Geotechnical
Engineering. Here she met her favorite Professor Dr. McVay who she followed to do
research and under his instruction finished this master thesis. She admires his profound
knowledge of geotechnical engineering and pure-hearted character, determine to
become a eligible engineer like him in the future.