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Evaluation of unknown foundations

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Title:
Evaluation of unknown foundations
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English
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Florkowski, Ronald W
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University of South Florida
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Tampa, Fla.
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Subjects / Keywords:
Sonic echo / impulse response
Bridge inspections
Scour critical
Erosion
Non-destructive testing
Dissertations, Academic -- Civil Engineering -- Masters -- USF   ( lcsh )
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bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Abstract:
ABSTRACT: In recent years, bridge foundations have been in the spotlight throughout the nation. Bridges built over running water are susceptible to erosion or scour around their foundations. The reduction in load capacity to piers and abutments pose a safety risk to highway motorists. It has become necessary for engineers to examine and monitor these "scour critical" bridges. The difficulty arises with subsurface foundations of which very little is known about their construction. Hence, the methods applied to analyzing "Unknown Foundations" have become a necessary topic of research. This thesis explores a method to determine foundation lengths. Similar to Sonic Echo / Impulse Response, this procedure measures reflected shock waves sent through concrete pilings. The technique is non-destructive in nature and is performed near the surface of the foundation. The test is performed on the side of the exposed piling. Current methods are limited by the fact that the tops of most pilings are inaccessible due to pilecaps or beams. Often times, pilings are embedded in stiff soils, which have a dampening effect on the stress waves. This thesis employs a method of analysis that will overcome such limitations and provide engineers with another tool to determine subsurface foundation lengths.
Thesis:
Thesis (M.S.)--University of South Florida, 2007.
Bibliography:
Includes bibliographical references.
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Mode of access: World Wide Web.
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by Ronald W. Florkowski.
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Title from PDF of title page.
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Document formatted into pages; contains 60 pages.

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aleph - 001915686
oclc - 180705326
usfldc doi - E14-SFE0002002
usfldc handle - e14.2002
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SFS0026320:00001


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Evaluation of Unknown Foundations by Ronald W. Florkowski A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Civil Engineering Department of Civil & Environmental Engineering College of Engineering University of South Florida Major Professor: Gray Mullins, Ph.D., P.E. Rajan Sen, Ph.D., P.E. Abla Zayed, Ph.D. Date of Approval: March 27, 2007 Keywords: sonic echo / impulse response, scour cri tical, erosion, non-destructive testing, bridge inspections Copyright 2007, Ronald W. Florkowski

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Table of Contents List of Figures ................................... ................................................... ..............................ii Abstract .......................................... ................................................... ..................................v Chapter 1 Introduction........................... ................................................... ......................... 1 1.1 Introduction ......................... ................................................... .........................1 1.2 Project Objective ......................... ................................................... .................3 1.3 Organization of Thesis .................... ................................................... .............3 Chapter 2 Background ............................ ................................................... .......................4 2.1 History of Unknown Foundations .......... ................................................... ......4 2.2 Current Testing Methods ................... ................................................... ..........4 Chapter 3 Methods to Obtain Data ................ ................................................... ..............12 3.1 Introduction .............................. ................................................... ..................12 3.2 Equipment and Procedure ................... ................................................... .......12 3.2.1 Lab Scale Trials ....................... ................................................... ..12 3.2.2 Field Scale Testing .................... ................................................... .16 Chapter 4 Data Analysis & Results ............... ................................................... ..............19 4.1 Stress Wave Propagation .............. ................................................... .............19 4.2 Frequency Analysis ................ ................................................... ....................20 4.3 Results & Conclusions .................... ................................................... ...........21 Chapter 5 Summary & Recommendations.............. ................................................... .....34 5.1 Summary ............................... ................................................... .....................34 5.2 Recommendations ....................... ................................................... ...............34 References ....................................... ................................................... ..............................36 Appendices ........................................ ................................................... .............................37 Appendix A Results from Laboratory Tr ials .............................................. ........38 Appendix B Results from Field Testing .................................................. ...........56 i

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List of Figures Figure 1.1: I-90 over Schoharie Creek (Courtesy of Northwestern University Infrastructure Technology Instit ute [6])........................................... .................1 Figure 2.1: Borehole Radar Method (Courtesy of FHWA – Geophysical Imaging Resource Website [3])............ ................................................... .........................5 Figure 2.2: Cross Borehole Seismic Tomography ..... ................................................... ......6 Figure 2.3: Mapping Between Boreholes (Courtesy of FHWA – Geophysical Imaging Resource Website [3])............ ................................................... .........................6 Figure 2.4: Parallel Seismic Method (Courtesy of F HWA – Geophysical Imaging Resource Website [3])............ ................................................... .........................7 Figure 2.5: Parallel Seismic Data (Courtesy of FHW A – Geophysical Imaging Resource Website [3])............ ................................................... .........................8 Figure 2.6: Induction Field Method (Courtesy of FH WA – Geophysical Imaging Resource Website [3])............ ................................................... .........................9 Figure 2.7: Sonic/Echo Impulse Testing Equipment (C ourtesy of Mullins G., et al, “Thermal Integrity Testing of Dri lled Shafts,” [5])................................. ........10 Figure 2.8: Sonic/Echo Impulse Test Results (Courte sy of Mullins G., et al, “Thermal Integrity Testing of Drilled Shaf ts,” [5]).......................................... ...............11 Figure 3.1: Concrete Piling in Lab 6”x 6”x 9’ ..... ................................................... ..........13 Figure 3.2: Strain Gage PL-60-11 .................. ................................................... ................14 Figure 3.3: Piezoelectric Accelerometer SN45519.... ................................................... .....15 Figure 3.4: Capacitive Accelerometer............... ................................................... .............15 Figure 3.5: Pre-stressed Concrete Piling 12”x 12”x 40’ ............................................... ....17 Figure 3.6: Strain Gage Positions 12”x 12”x 40’ Pil ing ............................................... ....17 Figure 3.7: Piezoelectric Accelerometer SN45519 Mo unted to Bracket..........................18 ii

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Figure 3.8: Capacitive Accelerometer Mounted to Br acket.............................................1 8 Figure 4.1: Piezoelectric Accelerometer Unfiltered in Time Domain ..............................23 Figure 4.2: Piezoelectric Accelerometer in Frequenc y Domain .......................................24 Figure 4.3: Piezoelectric Acceleration Filtered to 1000 Hz............................................ ...25 Figure 4.4: Top Impact Strain & Accelerometer Data ................................................... ...26 Figure 4.5: Top Impact Strain 1 Filtered to 2500 Hz ................................................ .....27 Figure 4.6: Top Impact Strain Gages Filtered to 250 0Hz ............................................... ..28 Figure 4.7: Center Impact Strain Gages Filtered to 2500Hz ............................................ 29 Figure 4.8: Bracket Impact Strain Gages Filtered to 2500Hz ........................................... 30 Figure 4.9: Top Impact Accelerometer Data ......... ................................................... ........31 Figure 4.10: Center Impact Accelerometer Data ..... ................................................... ......32 Figure 4.11: Bracket Impact Accelerometer Data .... ................................................... .....33 Figure A.1: Laboratory 6”x 6” Pile Accelerometer Pl acement ........................................38 Figure A.2: Midline Bracket Impact Accelerations T rial 001 .........................................3 9 Figure A.3: Midline Bracket Impact Accelerations T rial 002 .........................................4 0 Figure A.4: Midline Bracket Impact Accelerations T rial 003 .........................................4 1 Figure A.5: Bottom Lateral Side Impact Acceleratio ns Trial 004 ...................................42 Figure A.6: Top Bracket Impact Accelerations Trial 005 .............................................. .43 Figure A.7: Bottom Lateral Side Impact Acceleratio ns Trial 006 ...................................44 Figure A.8: Midline Bracket Impact Accelerations T rial 007 .........................................4 5 Figure A.9: Top Lateral Side Impact Accelerations Trial 008 ........................................4 6 Figure A.10: Bottom Lateral Side Impact Accelerati ons Trial 009 .................................47 Figure A.11: Top of Piling Impact Accelerations Tr ial 010 ........................................... .48 iii

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Figure A.12: Top Lateral Side Impact Accelerations Trial 011 ......................................49 Figure A.13: Top of Piling Impact Accelerations Tr ial 012 ........................................... .50 Figure A.14: Midline Lateral Side Impact Accelerat ions Trial 013 ................................51 Figure A.15: Bottom Lateral Side Impact Accelerati ons Trial 014 .................................52 Figure A.16: Laboratory 6”x 6” Pile Accelerometer a nd Strain Gage Placement ............53 Figure A.17: Top of Piling Impact Accelerations Tr ial 015 ........................................... .54 Figure A.18: Top of Piling Impact Strain Data Tria l 015 ............................................. ...55 Figure B.1: Accelerometer and Strain Gage Placement 12”x 12”x 40’ Piling .................56 Figure B.2: Top of Piling Impact Accelerations T est 2 ............................................. ....57 Figure B.3: Top of Piling Impact Strains Test 2 ................................................... .........58 Figure B.4: Bracket Impact Accelerations Test 2 ................................................... .......59 Figure B.5: Bracket Impact Strain Data Test 2 .. ................................................... ........60 iv

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Evaluation of Unknown Foundations Ronald W. Florkowski ABSTRACT In recent years, bridge foundations have been in t he spotlight throughout the nation. Bridges built over running water are suscep tible to erosion or scour around their foundations. The reduction in load capacity to pier s and abutments pose a safety risk to highway motorists. It has become necessary for engi neers to examine and monitor these “scour critical” bridges. The difficulty arises wit h subsurface foundations of which very little is known about their construction. Hence, th e methods applied to analyzing “Unknown Foundations” have become a necessary topic of research. This thesis explores a method to determine foundat ion lengths. Similar to Sonic Echo / Impulse Response, this procedure measures re flected shock waves sent through concrete pilings. The technique is non-destructive in nature and is performed near the surface of the foundation. The test is performed on the side of the exposed piling. Current methods are limited by the fact that the tops of mo st pilings are inaccessible due to pile caps or beams. Often times, pilings are embedded in stiff soils, which have a dampening effect on the stress waves. This thesis employs a m ethod of analysis that will overcome such limitations and provide engineers with another tool to determine subsurface foundation lengths. v

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Chapter 1 Introduction 1.1 Introduction There are a number of risks associated with highway travel. Throughout the United States, there have been reports of foundatio n failures ranging from sinking half a foot to causing total structural collapse of roadwa ys. Figure 1.1 shows one such bridge in New York where an entire bridge span was lost resul ting in 10 fatalities. The Federal Highway Administration and state DOT’s have recogni zed the need to examine our current roadways and correct deficiencies that have the potential to be very harmful. Figure 1.1: I-90 over Schoharie Creek (Courtesy of Northwestern University Infrastructure Technology Institute [6]) 1

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Further, as more is learned in the area of extreme event loading scenarios, re-evaluating existing bridge foundation elements has become a pa ramount concern. This thesis looks at bridge surveys, with a focus o n assessing the capacity of subsurface foundations. It is known that pilings lo se strength as the soil around them erodes away. A problem arises when trying to calcul ate capacities on pilings, of which, very little is known. The challenge is to develop a n economical method to evaluate and characterize the length and condition of existing b ridge piles below the surface. One such method is Sonic Echo / Impulse Response (S E/IR). This procedure had evolved as a means to test the integrity and determ ine length of drilled shafts and driven piles. It is based on the principle that stress wav es created by an impact will travel through a foundation and reflect back to the surfac e when there is a change in stiffness, cross-section or density. As this is a Non-Destruct ive Test (NDT) it does not require coring or drilling through bridge components. The t est is conducted at the ground surface or top of foundation element. However, there are li mitations to the SE/IR method. One limitation is that testing performed through a n existing pile cap or beam is difficult. The large cross-section and interface wi th the pile or shaft creates reflections that are difficult to interpret. For this case, col umnar shaped piles or shafts need to be exposed above the ground or water where testing can be performed on the side. Studies have shown that pilings embedded in stiff soils ten d to absorb wave energy, therefore preventing identifiable reflections. In theory, a l arger impact delivered to a side-mounted bracket will provide the energy necessary to perfor m the test. Should, at some point, the required energy approach the driving energy (e.g. p ile driving hammer), then it would not be reasonable for existing structures. 2

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1.2 Project Objective The goal of this thesis is to introduce several im provised methods of examining stress wave propagation delivered to concrete piles for determining unknown foundation lengths. Typically, impact forces originate from th e top of piles. A comparison is made to waveforms developed by lateral impacts to the side and then to a mounted bracket on the pile to produce axial impacts at various locations along the length of the pile. The generated waves are measured by a set of accelerome ters. Sets of strain gages have also been utilized to augment data collection and wavefo rm analysis. 1.3 Organization of Thesis This report is organized into four subsequent chap ters. Chapter 2 begins with the history of “Unknown Foundations” and then gives an overview of current testing methods in use today. Chapter 3 describes the equip ment used for this project and explains the procedure for data collection. Chapter 4 discusses stress wave propagation and includes the concept of frequency analysis. Fin ally, chapter 5 summarizes the results of trial studies. 3

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Chapter 2 Background 2.1 History of Unknown Foundations Out of a half million bridges nationwide, over 1 00,000 have unknown foundations. More than 20,000 of these are consider ed “scour critical” and susceptible to collapse. Years and years of erosion have taken a t oll. The loss of foundation embedment has resulted in reduced loading capacities; some mo re serious than others. The problem is, deciding which roadway is worse. Many older bri dges lack information on design or actual as-built construction records. Without knowl edge of foundation dimension, material, depth or condition, a proper assessment c annot be made. Engineers must rely on subsurface testing to characterize structural eleme nts below ground. 2.2 Current Testing Methods A number of methods have been developed to investig ate the unknown foundation depth below ground. Applications vary de pending on the type of foundation; that is, timber, steel or masonry. Labor and cost a re also important factors. Ground excavation, although very reliable, is prob ably the most expensive and least practical method. The process requires heavy equipment, dewatering, closing roadways and is generally dangerous work. Probing with a rod and hammer is less dangerous, b ut also physically demanding. It may be helpful with shallow elements, but still leaves a lot of uncertainty. For example, hammering a rod into bedrock could easily be confus ed for a solid foundation element. 4

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The method of coring may be applied to exposed foun dations. Drilling into the element creates the need to repair damage. Similar to rod & hammer, it is used for shallow elements. It is not useful in determining p ile depths. The borehole radar method may be used to determine pile geometry and depth. The technique involves installing a PVC-encased bor ehole in close proximity of the foundation. Radar energy is transmitted into the su rrounding soil. A receiver records reflection produced at interfaces of material with different dielectric properties (Figure 2.1). The reflection of electromagnetic wave energy off steel is strong, which makes it particularly useful in identifying reinforced concr ete pilings. Figure 2.1: Borehole Radar Method (Courtesy of FHWA – Geophysical Imaging Resource Website [3]) 5

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Cross Borehole Seismic Tomography is a similar tech nique that incorporates multiple boreholes through which signals may be tra nsmitted (Figure 2.2). By varying the depth of transmitter and receiver, a two or three d imensional velocity image may be produced (Figure 2.3). Figure 2.2: Cross Borehole Seismic Tomography Figure 2.3: Mapping Between Boreholes (Courtesy of FHWA – Geophysical Imaging Resource Website [3]) 6

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Both methods have limited use in certain environmen ts, such as conductive clays or salt-water saturated soils where signal interfer ence would be expected. Another disadvantage is that it requires installation of on e or more access casings, which is associated with higher time and cost. Parallel seismic is a method that also requires a b orehole in close proximity to the foundation (Figure 2.4). An impact is delivered to exposed structural elements creating seismic wave energy that travels down below the sur face. Figure 2.4: Parallel Seismic Method (Courtesy of F HWA – Geophysical Imaging Resource Website [3]) 7

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Receivers in the adjacent borehole track changes in wave energy along the piling or shaft. When the receiver is lowered below the to e, the signal weakens. This indicates the depth of foundation. Figure 2.5 is an example o f parallel seismic data. Figure 2.5: Parallel Seismic Data (Courtesy of FHW A – Geophysical Imaging Resource Website [3]) The induction field method also requires a PVC-enca sed borehole through which a sensor is lowered (Figure 2.6). An oscillating el ectric current is sent through the conductive material of a foundation, such as reinfo rcing rebar, to produce a magnetic field. As the sensor is lowered below a foundation, the magnetic field strength drops. This indicates the foundation depth. 8

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Figure 2.6: Induction Field Method (Courtesy of FHWA – Geophysical Imaging Resource Website [3]) The most important requirement with this method is electrical conductivity. Metal within a pile or shaft must extend the full length and be accessible from the surface. There are limitations in certain environments. Cond uctive materials in bridge components or soils may interfere with signals. Sonic echo/impulse response measures the reflection of stress waves created by an impact to an element. It would appear to be the most economical means to determine pile depth. The testing procedure is far less invas ive then the aforementioned methods. Figure 2.7 shows a typical application of the test. Two positions on top of the shaft are cleaned and prepped. Tapping the surface occurs at the center. An accelerometer is 9

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secured to the second position, which is off-center but within the perimeter of the reinforcement cage. A laptop is commonly used to ac quire data and process signals. Figure 2.7: Sonic/Echo Impulse Testing Equipment (C ourtesy of Mullins G., et al, “Thermal Integrity Testing of Drilled Shafts,” [5]) When a hammer strikes the structure, it causes a di stortion and sends a compressive wave down the shaft. The time it takes for the stress wave to reflect off the toe and return to the surface is a function of wave velocity and length of the shaft. Any alteration is due to a change in impedance, such as a change in cross-section, material density or possibly damage to the shaft. Mechanical impedance, Z, is a measure of how much a structure resists motion when subjected to a given force. The impedance is a ratio of the force, F, to the velocity, v, at a point and is a function of frequency, : F() = Z()v() 10

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Part of the stress wave is reflected back to the su rface when there is a change in impedance, while the remaining wave continues to th e toe and back. Figure 2.8 is an example output showing the initial impact and retur n wave from which a shaft length can be calculated. Signal changes in between indicate a change in impedance. There is generally a response limit for stress wave reflecti on based on a length / diameter ratio and characteristics of the soil. Stiff soils will absor b all energy traveling down shafts with L / D ratios greater than 30:1. Soft soils will allow energy travel in shafts with L / D ratio as high as 50:1. Figure 2.8: Sonic/Echo Impulse Test Results (Courte sy of Mullins G., et al, “Thermal Integrity Testing of Drilled Shafts,” [5]) 11

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Chapter 3 Methods to Obtain Data 3.1 Introduction Although sonic/echo impulse testing has proven reli able in certain circumstances, methods to remove inherent limitations for unknown foundation applications could prove fruitful. Figure 2.6 shows a typical application of the sonic/echo impulse test. This thesis introduces a similar application, but with the addi tion of multiple accelerometers and/or strain gages. The study looks at axial impacts deli vered from the side of pilings and lateral impacts as well as the traditional top-of-p ile impact. Tapping the surface of a specimen produces compression waves that produce ch aracteristic reflections that are typically recorded by accelerometers, but in this c ase wave direction, speed and foundation length can also be determined. Two test programs were conducted; (1) lab scale, where equipment and methods were explored an d (2) field scale, where full-length piles were tested. 3.2 Equipment and Procedure 3.2.1 Lab Scale Trials Experiments were conducted in the lab to study the effectiveness of utilizing strain gages in addition to accelerometers to recor d compression waves. Impacts were delivered to the lateral face of the pile, a mounte d steel bracket and the top surface. Figure 3.1 shows the 9-foot, concrete pre-stressed piling positioned vertically in the lab. 12

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Two steel brackets were attached to the 6” x 6” pil ing. One was bolted at the top. The second was secured midway; a distance 4 feet from t he top. Figure 3.1: Concrete Piling in Lab 6”x 6”x 9’ Four PL-60-11 strain gauges were mounted equally sp aced 2 feet apart and centered on the same face of the piling. They were sequentially numbered beginning with number 1 mounted 1 foot from the top. A bonding adh esive was used to secure the gages directly centerline on the piling (Figure 3.2). The PL series gage is a standard 120 wire strain gage with a transparent backing impregnated with a polyester resin. The wire is a Cu-Ni alloy and has an operating temperature range of –20 to 80C. The gage is 60mm x 13

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1 mm in size and the polyester backing is 74mm x 8m m. The size and durability of the gage makes it easy to apply. Figure 3.2: Strain Gage PL-60-11 Two accelerometers were utilized; each with a disti nct fundamental principle of operation. A piezoelectric accelerometer consists o f a quartz crystal. A mass within the component and applied accelerations create forces t hat deflect the lattice of the crystal. Displaced electrical charge accumulates on an elect rode. The signal is conditioned then analyzed. A capacitive accelerometer senses a change in elect rical capacitance. A diaphragm within the component flexes in response t o accelerations. This alters the distance between parallel plates. The resulting cha nge in capacitance varies the output of an energized circuit. 14

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Figure 3.3: Piezoelectric Accelerometer SN45519 Figure 3.4: Capacitive Accelerometer 15

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The accelerometers were secured to a clean surface of the steel bracket with a wax medium. The piezoelectric accelerometer was attache d to the top bracket (Figure 3.3). The capacitive accelerometer was secured to the bra cket midway (Figure 3.4). The advantages of each type were reviewed. In gener al, capacitive devices are more sensitive. Piezoelectric devices tend to be mo re robust, yet less sensitive. In this application, both types were considered due to unce rtainty in the amount of signal that might be generated. A moderate strike with a 12-ounce hammer was used t o create compression waves in the piling. Data collected showed that instrumentation and data acquisition systems selected, worked sufficiently for field data application (See Appendix A). 3.2.2 Field Scale Testing Studies in the field were conducted in similar fash ion. Figure 3.5 shows a 40-foot, concrete pre-stressed piling positioned horizontall y. Two steel brackets were attached to the 12” x 12” piling. One was bolted 3.5” from the top. The second was secured at a point, 7 feet from the top. Four PL-60 strain gauges were mounted symmetrically spaced 2 feet apart with a 3’ spread, midway, between the second and third gag e (Figure 3.6). They were sequentially numbered beginning with number 1 mount ed 2 feet from the top. Two accelerometers were utilized. A piezoelectric a ccelerometer was mounted on the top bracket (Figure 3.7) and a capacitive accel erometer to the bracket midway (Figure 3.8). Both were found to give reasonable informatio n from the trial study. 16

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A 12-ounce hammer striking the top, lateral side an d mounted bracket of the piling generated compression waves. Figure 3.5: Pre-stressed Concrete Piling 12”x 12”x 40’ Figure 3.6: Strain Gage Positions 12”x 12”x 40’ Pil ing 17 Top impact Bracket impact Lateral side impact

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Figure 3.7: Piezoelectric Accelerometer SN45519 Mo unted to Bracket Figure 3.8: Capacitive Accelerometer Mounted to Br acket 18

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Chapter 4 Data Analysis & Results 4.1 Stress Wave Propagation Upon impact to a solid, stress waves are generated which radiate in all directions. Generally, there are three types of waves. Compress ion waves propagate in the direction of the impact or distortion. Shear waves travel per pendicular to the direction of propagation and will only be present in materials t hat have a shear stiffness. The third type are called Rayleigh surface waves. These waves propagate over the surface of the solid. The velocity of stress wave propagation is a funct ion of the material properties. It depends on the elastic modulus, E, Poisson’s ratio, and the density, The compressive wave velocity is given by the equation: vp = [ E(1-)/(1+)(1-2)] Shear wave velocity is a function of the shear modu lus, G, and material density. The shear modulus is given by the equation: G = E/2(1-) Shear wave propagation is given by the equation: vs = [G/] 19

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Typical compression wave velocities in concrete ran ge from 11484 ft/s to 14765 ft/s, depending on composition, condition and age. Shear wave velocity is generally slightly more than half the compression wave veloci ty. For concrete, a typical shear wave velocity is 0.61 to 0.54 times the compression wave velocity. vs = vp x [ (1+)(1-2)/2(1-)2] Rayleigh surface waves range from 0.862vs to 0.955vs relative to poisson’s ratio of 0 to 0.5. When a compression wave is generated b y an impact, it travels down the structure until is meets a change in impedance. Thi s may be a change in composition, cross-sectional area, or a break. The wave is then reflected back to its origin where it can be recorded. 4.2 Frequency Analysis Data collected from a sonic/echo impulse test is ve ry complex. Waveforms represent multiple sinusoids from many frequencies (Figure 4.1). Common practice in signal processing is to convert data from a time do main to a frequency domain for filtering. In the time domain, amplitude is valued at each time interval. In the frequency domain, the output represents the amount each parti cular frequency or sinusoid is present in the whole signal. The mathematical formula to co nvert data to the frequency domain is called The Fast Fourier Transform (FFT). Data repre sented in Figure 4.1 is converted by a FFT to the frequency domain in Figure 4.2. In thi s form, it is easier to identify particular frequencies. Unwanted interference or no ise can be filtered out. Finally, the signal is converted back to the time domain by an i nverse FFT (Figure 4.3). 20

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4.3 Results & Conclusions The field study performed on a 40’ pre-stressed con crete piling was successful. All instrumentation responded well as shown in Figu re 4.4. Initial observations, without filtering, show the accelerometers in complete disa rray, but there appears to be a wave pattern in the strain gages. Filtering data was bas ed on trial and error. Figure 4.5 shows filtered data in the first strain gage. Care must b e taken not to cut out too much. As shown, 2500 hertz appears ideal. Filtering at small er frequencies, like 500 hertz, results in waveforms that are heavily altered and difficult to interpret. Figure 4.6 shows filtered strain gage response to a top impact. The sequence of strain gage response clearly shows departure and re turn of a stress wave. The distance between gage 1 and 4 is 7 feet. With a time of 0.00 0565682 seconds between peaks, the calculated stress wave velocity is 12,374 feet/seco nd. The time between compression peaks for strain gage 1 is 0.006191042 seconds, whi ch works out to a distance of 76.6 feet. The margin of error is 4.25%. It is an advant age to have multiple gages to calculate or verify standard stress wave velocities. But, it is difficult to be accurate with such a short distance between gages and very short time fr ame without first filtering. Figure 4.7 shows filtered strain gage response to a center impact. The pile is experiencing flexural stresses instead of an axial direction. Waveforms are present, but not as distinct. As expected, gages 2 and 3 react s imultaneously; as do gages 1 and 4. The slight offset between peaks indicates a slight offcenter impact. A pre-trigger time of 0.001 seconds captured the response of gage 1 and 4 but cut off the initial strains of gage 2 and 3. With a calculated shear wave velocity of 3 500 to 4000 feet/second, a 0.001 sec. pre-trigger would suffice. The delay was due to the accelerometer set for pre-trigger on a 21

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positive direction. It should have triggered in eit her case, positive or negative; something to be cognizant of in future tests. It is difficult to see reflections from the toe, but it appears to be between 0.009 and 0.010 seconds. Ther e appears to be a lot of overlap of multiple waves. Figure 4.8 shows filtered strain gage response to a bracket impact. The complete lack of response is due to the position of the brac ket. The impact was on the adjacent face of the pile, 90 from the position of strain gages. This coincides with the neutral axis of the pile. Accelerometer data was difficult to analyze. Even a fter filtering out higher frequencies, a stress wave could not be identified. The accelerometers were secured to the steel brackets, which may have created an excess of unwanted vibrations. Since the piling was horizontal, contact with the ground may have in terfered with axial wave travel. 22

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-60.000 -40.000 -20.000 0.000 20.000 40.000 60.000 0.0000.0020.0040.0060.0080.0100.0120.0140.0160.0180 .020 Time (seconds)Acceleration (g's) piezoFigure 4.1: Piezoelectric Accelerometer Unfiltered in Time Domain 23

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0.00 2000.00 4000.00 6000.00 8000.00 10000.00 12000.00 14000.00 16000.00 0.00500.001000.001500.002000.002500.003000.003500.0 04000.004500.005000.00 frequency (Hz)AmplitudeFigure 4.2: Piezoelectric Accelerometer in Frequenc y Domain 24

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-60.00 -40.00 -20.00 0.00 20.00 40.00 60.00 0.0000.0020.0040.0060.0080.0100.0120.0140.0160.0180 .020Tine (seconds)Acceleration (g's) piezo acceleration vs time Piezo filtered Figure 4.3: Piezoelectric Acceleration Filtered to 1000 Hz 25

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-60.00 -40.00 -20.00 0.00 20.00 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration g's strain1 strain2 strain3 strain4 CAPAC PIEZO Figure 4.4: Top Impact Strain & Accelerometer Data 26

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-15.000 -10.000 -5.000 0.000 5.000 10.000 15.000 20.000 0.000 0.005 0.010 0.015 0.020Time (seconds)microstrains unfiltered 500 Hertz 2500 Hertz Figure 4.5: Top Impact Strain 1 Filtered to 2500 Hz 27

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-10 -5 0 5 10 15 200.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .0100.0110.0120.0130.0140.0150.0160.0170.0180.0190. 020Time (seconds)Microstrains strain1 strain2 strain3 strain4 -30 -20 -10 0 10 20 30 400.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010Time (seconds)Microstrains strain1 strain2 strain3 strain4Figure 4.6: Top Impact Strain Gages Filtered to 250 0Hz 28

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-10 -5 0 5 10 15 20 25 300.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .0100.0110.0120.0130.0140.0150.0160.0170.0180.0190. 020Time (seconds)Microstrains strain1 strain2 strain3 strain4 Figure 4.7: Center Impact Strain Gages Filtered to 2500Hz 29

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-2 0 2 4 6 8 10 12 14 16 180.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .0100.0110.0120.0130.0140.0150.0160.0170.0180.0190. 020Time (seconds)Microstrains strain1 strain2 strain3 strain4 Figure 4.8: Bracket Impact Strain Gages Filtered to 2500Hz 30

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-55.000 -35.000 -15.000 5.000 25.000 45.000 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010Time (seconds)Acceleration (g's) CAPAC PIEZO PIEZO 1085Hz CAPAC 2500Hz Figure 4.9: Top Impact Accelerometer Data 31

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-50.00 -40.00 -30.00 -20.00 -10.00 0.00 10.00 20.00 30.00 40.00 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) CAPAC PIEZO PIEZO 1085Hz CAPAC 2500Hz Figure 4.10: Center Impact Accelerometer Data 32

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-85.00 -65.00 -45.00 -25.00 -5.00 15.00 35.00 55.00 75.00 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) CAPAC PIEZO PIEZO 1085Hz CAPAC 2500 Hz Figure 4.11: Bracket Impact Accelerometer Data 33

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Chapter 5 Summary & Recommendations 5.1 Summary Methods of non-destructive testing have become nece ssary in identifying unknown foundations. Developing instrumentation is vital to such an endeavor. In this thesis, strain gages and accelerometers were examin ed in the arena of impact loading of concrete pilings. Measured strains identified stres s wave velocities and direction, which was used to determine pile length. Axial loading pr oduced stress waves consistent with compression wave propagation. Surface and shear wav es were prevalent under flexural bending associated with lateral impacts. Both the c apacitive and piezoelectric accelerometers were successful in capturing data in all trial and testing scenarios. 5.2 Recommendations There were several types of impacts observed (i.e. top, side & bracket). The best performance was from the top. A method to deliver p ure axial loading to the side of a piling would be ideal. The first recommendation is, instead of a hammer, a weighted ring or sleeve wrapped around a piling may be dropped on to a bracket fastened to the side. The weight of the ring may vary depending on projec ted pile depth or stiffness of the soil. The use of multiple gages helped determine wave ve locities and direction. It is recommended that gages be spread apart as far as po ssible on the same side of the piling. The further apart, the more accurate it is when cal culating wave speed. It took several 34

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hours for the strain gage epoxy to set. It might be more cost effective to use a gage that fastens quicker and is reusable. Accelerometers that are used as trigger mechanisms should be set to activate under both positive and negative accelerations. It is important to capture an adequate amount of pre-trigger time. Total sampling time can be estimated based on wave speed through a projected pile depth. When setting a samp ling rate, consider the number of data points needed to do a Fourier transform. 35

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References 1. U. Meier and R. Betti, “Recent Advances in Bridg e Engineering,” Printing Office Columbia University New York, 1997. 2. Baldev Raj, T. Jayakumar and M. Thavas imuthu, “Practical Non-Destructive Testing,” Narosa Publishing House, 2002. 3. Federal Highways Administration – Geophysical Imagi ng Resource Website< http://www.cflhd.gov/agm/engApplications/BridgeSyst emSubstructure/2 12BoreholeNondestMethods.htm >Last Accessed on 15 March 2007. 4. Federal Highways Administration Unknow n Foundation Summit, Lakewood, Colorado, November 15-16, 2005. 5. Mullins G., Kranc S., Johnson K., Stokes M., Win ters D., “Thermal Integrity Testing of Drilled Shafts,” University of South Flo rida, 2007. 6. Northwestern University Infrastructure Technolo gy Institute WebsiteLast Accessed 15 March 2007. 7. Petros P. Xanthakos, “Bridge Substructure and F oundation Design,” Prentice Hall PTR, 1995. 36

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Appendices 37

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Appendix A Results from Laboratory Trials Included in this appendix are trial impacts conduct ed in the lab. Piezoelectric and capacitive accelerometers were both located on the midline bracket. Impact was created with a 12-ounce hammer. Strikes were made to the to p and mid-line bracket. Impacts were also on the lateral face of the pile midway an d at the bottom. Figure A.1: Laboratory 6”x 6” Pile Accelerometer Pl acement 38 Top face of pile Top bracket Piezoelectric accelerometer Midline bracket Top lateral face Midline lateral face Capacitive accelerometer Bottom lateral face

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Appendix A (Continued) -60.00 -40.00 -20.00 0.00 20.00 40.00 60.00 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) PIEZO CAPAC Figure A.2: Midline Bracket Impact Accelerations T rial 001 39

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Appendix A (Continued) -80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) PIEZO CAPAC Figure A.3: Midline Bracket Impact Accelerations T rial 002 40

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Appendix A (Continued) -80 -60 -40 -20 0 20 40 60 80 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) PIEZO CAPAC Figure A.4: Midline Bracket Impact Accelerations T rial 003 41

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Appendix A (Continued) -40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0 40.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) piezo capac Figure A.5: Bottom Lateral Side Impact Accelerati ons Trial 004 42

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Appendix A (Continued) -60 -40 -20 0 20 40 60 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) piezo capac Figure A.6: Top Bracket Impact Accelerations Trial 005 43

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Appendix A (Continued) -25.0 -20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Accelerations (g's) piezo capac Figure A.7: Bottom Lateral Side Impact Acceleratio ns Trial 006 44

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Appendix A (Continued) -80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Accelerations (g's) piezo capac Figure A.8: Midline Bracket Impact Accelerations T rial 007 45

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Appendix A (Continued) -80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Accelerations (g's) piezo capac Figure A.9: Top Lateral Side Impact Accelerations Trial 008 46

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Appendix A (Continued) -80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) piezo capac Figure A.10: Bottom Lateral Side Impact Accelerati ons Trial 009 47

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Appendix A (Continued) -100 -80 -60 -40 -20 0 20 40 60 80 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Accelerations (g's) piezo capac Figure A.11: Top of Piling Impact Accelerations Tr ial 010 48

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Appendix A (Continued) -80 -60 -40 -20 0 20 40 60 80 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Accelerations (g's) piezo capac Figure A.12: Top Lateral Side Impact Accelerations Trial 011 49

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Appendix A (Continued) -80 -60 -40 -20 0 20 40 60 80 100 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) piezo capac Figure A.13: Top of Piling Impact Accelerations Tr ial 012 50

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Appendix A (Continued) -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) piezo capac Figure A.14: Midline Lateral Side Impact Accelerat ions Trial 013 51

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Appendix A (Continued) -80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) piezo capac Figure A.15: Bottom Lateral Side Impact Accelerati ons Trial 014 52

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Appendix A (Continued) Strain gages were included in the following trial. The Piezoelectric accelerometer was located on the top bracket. The capacitive acce lerometer was located on the midline bracket. Impact was created with a 12-ounce hammer strike to the top face of the pile. Figure A.16: Laboratory 6”x 6” Pile Accelerometer a nd Strain Gage Placement 53 Top face of pile Top bracket Midline bracket Capacitive accelerometer Piezoelectric accelerometer Strain gage 1 Strain gage 2 Strain gage 3 Strain gage 4

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Appendix A (Continued) -100.0 -80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Accelerometer (g's) piezo capac Figure A.17: Top of Piling Impact Accelerations Tr ial 015 54

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Appendix A (Continued) -60.0 -50.0 -40.0 -30.0 -20.0 -10.0 0.0 10.0 20.0 30.0 40.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Microstrains strain1 strain2 strain3 strain4 Figure A.18: Top of Piling Impact Strain Data Tria l 015 55

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Appendix B Results from Field Testing Included in this appendix are field studies perform ed on a 12”x 12”x 40’ prestressed concrete piling. Strain gages were include d in the following data. The Piezoelectric accelerometer was located on the top bracket. The capacitive accelerometer was located on the midline bracket. Impact was crea ted with a 12-ounce hammer strike to the top, lateral side and mounted bracket of the pi le. Figure B.1: Accelerometer and Strain Gage Placement 12”x 12”x 40’ Piling 56 Top impact Lateral side impact Bracket impact Strain gage 4 Strain gage 3 Strain gage 2 Strain gage 1

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Appendix B (Continued) -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Acceleration (g's) piezo capac Figure B.2: Top of Piling Impact Accelerations T est 2 57

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Appendix B (Continued) -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Microstrains strain1 strain2 strain3 strain4 Figure B.3: Top of Piling Impact Strains Test 2 58

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Appendix B (Continued) -100.0 -80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0 100.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Accelerations (g's) piezo capac Figure B.4: Bracket Impact Accelerations Test 2 59

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Appendix B (Continued) 0.0 5.0 10.0 15.0 20.0 25.0 0.0000.0010.0020.0030.0040.0050.0060.0070.0080.0090 .010 Time (seconds)Accelerations (g's) strain1 strain2 strain3 strain4 Figure B.5: Bracket Impact Strain Data Test 2 60


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Evaluation of unknown foundations
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ABSTRACT: In recent years, bridge foundations have been in the spotlight throughout the nation. Bridges built over running water are susceptible to erosion or scour around their foundations. The reduction in load capacity to piers and abutments pose a safety risk to highway motorists. It has become necessary for engineers to examine and monitor these "scour critical" bridges. The difficulty arises with subsurface foundations of which very little is known about their construction. Hence, the methods applied to analyzing "Unknown Foundations" have become a necessary topic of research. This thesis explores a method to determine foundation lengths. Similar to Sonic Echo / Impulse Response, this procedure measures reflected shock waves sent through concrete pilings. The technique is non-destructive in nature and is performed near the surface of the foundation. The test is performed on the side of the exposed piling. Current methods are limited by the fact that the tops of most pilings are inaccessible due to pilecaps or beams. Often times, pilings are embedded in stiff soils, which have a dampening effect on the stress waves. This thesis employs a method of analysis that will overcome such limitations and provide engineers with another tool to determine subsurface foundation lengths.
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