Cross-hole sonic logging (CSL) has over recent years become the standard method for evaluating the integrity of bridge drilled shafts. The CSL method is based on measuring the speed of ultrasonic waves traveling between probes in parallel tubes placed inside the drilled shaft. Several existing studies have proposed methods that rely on the arrival time and wave speed to evaluate concrete integrity of drilled shaft foundations such as cross-hole tomography. In this study, a processing method for a three-component wide band CSL data is presented. This method named frequency tomography analysis (FTA) is based on the change of the frequency amplitude of the signal recorded by the receiver probe at the location of anomalies. The signal's time domain data are converted into frequency domain data using fast Fourier transform (FFT); the distribution of the FTA is then evaluated. This method is employed after a CSL test has determined a high probability of an anomaly in a given area and is applied to improve location accuracy and to further characterize the features of the anomaly. Two drilled shaft samples were built in Florida International University (FIU)'s Titan America Structures and Construction Testing (TASCT) Laboratory. Cubic foam pieces were placed inside the rebar cage before casting of concrete and throughout the length of the shaft. FTA was then utilized after the CSL tests to detect their location. The technique proved to have a very high resolution and was able to clarify the location of any artificial or planed discontinuities through the length of the drilled shaft.

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1

Cross-hole Sonic Logging and Frequency Tomography Analysis of Drilled

Shaft Foundations to Better Evaluate Anomalies Locations

Masood Hajali1, Caesar Abishdid2

1Doctoral Candidate, Dept. of Civil and Environmental Engineering, Florida International

University, Miami, FL 33174, E-mail: masood.hajali@gmail.com

2Assistant Professor, Department of Civil Engineering, Lebanese American University, Byblos

Campus, Lebanon, E-mail: caesar.abishdid@lau.edu.lb

Abstract

Cross-hole Sonic Logging (CSL) has over recent years become the standard method for

evaluating the integrity of bridge drilled shafts. The CSL method is based on measuring the

speed of ultrasonic waves travelling between probes in parallel tubes placed inside the drilled

shaft. Several existing studies have proposed methods that rely on the arrival time and wave

speed to evaluate concrete integrity of drilled shaft foundations such as cross-hole tomography.

In this study, a processing method for a three-component wide-band CSL data is presented. This

method named Frequency Tomography Analysis (FTA) is based on the change of the frequency

amplitude of the signal recorded by the receiver probe at the location of anomalies. The signal's

time -domain data is converted in to frequency-domain data using Fast Fourier Transform (FFT);

the distribution of the FTA is then evaluated. This method is employed after a CSL test has

determined a high probability of an anomaly in a given area, and is applied to improve location

accuracy and to further characterize the features of the anomaly. Two drilled shaft samples were

built in FIU's Titan America Structures and Construction Testing (TASCT) laboratory. Cubic

foam pieces were placed inside the rebar cage prior to casting of concrete and throughout the

length of the shaft. FTA wa s then utilized after the CSL tests to detect their location. The

technique proved to have a very high resolution and was able to clarify the location of any

artificial or planed discontinuities through the length of the drilled shaft.

Keywords: Drilled Shaft, Cross-hole Sonic Logging (CSL), Frequency Tomography Analysis

(FTA), Time-domain, Frequency-domain, Fast Fourier Transform (FFT), Anomalies.

Introduction

The development of drilled shafts, more or less independently, in various parts of the world led

to the different construction methods that exist today (O'Neil and Reese, 1999). The wet method

is the most widely used method whereby shafts are cast under wet conditions using slurry in

order to keep the borehole open during drilling and casting of the concrete. Sometimes a casing

is installed to make the drilling process easier and keep the borehole from caving in during the

drilling process. In many cases, a casing is used in only a part of the shaft length. The casing

maybe left in place, but usually is temporary and is removed after the concrete placement

because of its high cost. During all this process, different types of anomalies such as necking,

soft-bottom gap at the base, voids or soil intrusions, poor quality concrete, and dry chunks of

concrete can occur. These anomalies may result from concrete that is too dry, water flow into

2

the excavation, collapse of soft soil or rock, excavation spoils falling into the hole, tightly-spaced

reinforcing, or poorly mixed or designed concrete.

Drilled shafts have inherently large factors of safety and can handle minor discontinuities

without impacting their intended performance. They usually carry large loads since they have

larger diameter and length. For this reason they need to be built with a high level of quality

assurance and control applied to each in-place constructed deep foundation element. Integrity

testing using Nondestructive Test (NDT) methods in deep foundations centers on checking the

structural condition of the foundations. Several methods are available to perform this type of

testing. CSL has become a common and reliable method among the most usual methods of NDT

testing. ASTM D6760 (2008) provides a complete guidance for the CSL test.

The first use of the CSL method in the Americas was by the Hertlein in 1986. This method was

discussed by Baker, et al. (1993) and O'Neill (1999). CSL is a common type of NDT that is

currently used to check the integrity of placed drilled shafts based on the propagation of

ultrasonic waves between two or more access tubes inside the reinforcing cage. CSL has been

shown to be the most reliable technique for assessing the integrity of in-place constructed deep

foundation elements such as drilled shafts. Sarhan et al. (2003) showed that flaws occupying up

to 15% of the drilled shaft's cross section could remain undetected. Camp et al. (2007) found

that, out of 441 drilled shafts tested on multiple projects in South Carolina, approximately 75%

of the projects had at least one shaft containing an anomaly and 33% of all shafts tested

contained at least one anomaly.

Crosshole Sonic Logging Tomography (CSLT) method is a velocity imaging method used for

detecting anomaly zones (Jalinoos, et al. 2005). CSL data can be collected by initially offsetting

either the emitter or the receiver and then pulling the two probes together while maintaining a

constant non-zero angle between them. In the CSLT method, data is collected by running a zero-

offset log in combination with several positive offset (receiver is shallower) and negative offset

(source is shallower) logs. This procedure is repeated for all possible access tube combinations to

form a three-dimensional tomography dataset. CSLT needs specialized analyses software for true

3-D imaging.

Several variations of the CSL test have been developed while still using the same

instrumentation as that used in the CSL test. These tests are typically conducted after the CSL

test has determined the high probability of an anomaly in a given area, and are applied to

improve location accuracy and to further characterize the features of the anomaly. One of the

most popular of these tests is the Cross-hole Tomography (CT) test. A CT test is performed by

keeping the receiver at a fixed position and raising the hydrophone while the hydrophone is

producing sonic pulses. As in the CSL test, the arrival times from the hydrophone to the receiver

probe are recorded. This procedure produces ray-paths that make the three dimensional modeling

of the suspect shaft possible.

Olson and Hollena (2002) illustrated the use of the CT velocity imaging method of concrete

anomalies in drilled shafts. They used the CT method to produce colored velocity tomograms of

anomalies in actual bridge shafts. They showed the ability of CT to provide 2-D and 3-D velocity

images of a potential anomaly and to provide highly accurate information on the shape and

3

severity of the CSL anomalies. CT is an analytical technique which is increasingly used in

hydrological and geological studies. Some applications of CT have been reported by Tronicke

(2002) in hydrological applications, and by Fullagar, et al. (2000) in mining application.

Haramy (2006) presented a timely comprehensive study on the performance monitoring of

concrete mixes during the hydration process, CSL detection of anomaly locations, tomographic

imaging of the anomaly, and the effects of anomalies on drilled shaft capacity. When anomalies

occur, the NDT methods can assist in detecting their locations and sizes. Chang and Nghiem

(2006) have shown that anomalies near the top of a drilled shaft will significantly affect the

structural capacity of drilled shafts. In this study, one of the shaft specimens constructed had a

20% anomaly at the top of the shaft, and the proposed method was used to pinpoint its exact

location.

Anomalies throughout the length can significantly reduce the axial load capacity of the drilled

shaft, and their effect becomes severe after the installation thus leading to an unsatisfactory

performance of the drilled shafts. For this reason anomalies need to be detected accurately in

order to quantify their influence on the drilled shaft axial load carrying capacity. The purpose of

this study is to present a new method to detect the location of the anomalies after performing the

CSL test. To accomplish this objective, two drilled shaft samples with prefabricated anomalies

were built at the Florida International University's (FIU) TASCT laboratory. Samples were

tested seven days after concrete placement using CSL, and the CHA results were evaluated using

signal processing. An improved detection method is proposed that considers not only the

traditional arrival time changes but also the signal strength and frequency amplitude of the signal

reduction to improve the location accuracy.

Cross-hole Sonic Logging (CSL)

Cross-hole Sonic Logging (CSL) CSL is the most widely accepted and used integrity testing

method for drilled shaft foundations. It is currently used to check the integrity of placed drilled

shafts based on the propagation of ultrasonic waves between two or more access tubes inside the

reinforcing cage. CSL establishes the homogeneity and integrity of the concrete, such as

anomalies or soil intrusions, by recording the time and computing the velocity of the signals

from an emitter to a receiver probe (Lew et al., 2002). The CSL method is used to measure the

speed of ultrasonic waves between water-filled access tubes. A number of access tubes (PVC or

steel galvanized) are installed as guides for the sensors inside the reinforcing cage and prior to

concrete placement. To carry out the test, the probes with 8.5 inch (215 mm) length and 1 inch

(25 mm) in diameter are lowered down to the toe of the tubes. The transit time of an ultrasonic

compressional wave (p-wave) signal from a signal source in one access tube to a receiver in

another access tube is measured from the bottom to the top of the shaft (Figure 1). The ultrasonic

transmitter and receiver probes are capable of producing data records at a minimum frequency of

40,000 Hz with a good signal amplitude and energy through good quality concrete. The first

arrival time can be used to determine the ultrasonic pulse velocity (C) if the distance between

tubes is measured.

The velocity of P waves in a medium is related to the dynamic modulus of material, E, density of

material, ρ , and Poisson's ratio, μ , as follows:

4

(1)

In homogeneous, good quality concrete, the ultrasonic wave speed is around 12000 to 13000 ft/s

(3658 to 3962 m/s) (it can be lower or higher), in water is 4800 ft/s (1463 m/s) and in air is 1100

ft/s (335 m/s). Normal density of concrete would be about 150 lb/ft3 (2403 kg/m3). The dynamic

modulus of concrete varies from 4060 to 5800 ksi (28 to 40 GPa) and the Poisson's ratio of

concrete is between 0.1 to 0.2.

After performing the CSL test between two access tubes, the Cross-Hole Analyzer (CHA)

software can be used to analyze the results. CHA evaluates the quality of the concrete of deep

foundations by the CSL method. The CHA detects the arrival time by locating the peak value of

a signal, then by using a relative percentage of that peak as a threshold for locating the leading

edge. This value can be monitored visually by the horizontal blue dotted line on the signal trace

graph. The data acquisition signal graph includes 250 data points, sampled at 500 KHz. The

graph represents data points versus strength of the received signal by the receiver probe. The

strength or amplitude of the signal ranges between -10 to +10 volts. The energy can be also

calculated and curved base on Equation 2.

Energy = [received signal strength (volts)] x [arrival time (millisecond)] (2)

Figure 1: CSL Test Equipment

Testing Program

Two drilled shaft samples were tested at FIU's TASCT laboratory using Cross-hole Sonic

Logging (CSL) equipment. Each sample has four galvanized tubes outside the shaft and four

tubes inside the shaft. Therefore, six CSL tests were carried out between the tubes inside the

shaft and four CSL tests between the tubes inside and outside the shaft. The length and diameter

of the shafts, stirrup spacing, size of the formwork around the shaft, number of the CSL tubes,

and steel reinforcement were kept constant in both samples.

5

1. Test Samples

The test specimens used in this study to perform the CSL through the length of the shaft are

designed to simulate a drilled shaft foundation. Two drilled shaft samples were built at FIU's

TASCT laboratory (Figure 2). The diameter of each drilled shaft specimen was 20 inches (50.8

cm), with a length of 4 feet (122 cm). The formwork had a length of 48 inches (122 cm), a width

of 48 inches (122 cm), and a height of 48 inches (122 cm) as shown in Figure 2. A sona-tube

with a diameter of 20 inches (50.8 cm) and a length of 4 feet was used to serve as a casing

around the drilled shaft. The shaft was longitudinally reinforced with six (6) No. 10 steel bars

that were equally spaced around the perimeter. This amount of steel corresponds to 1.06 percent

of the gross cross-sectional area of the shaft. In this study however, the amount of longitudinal

bars will not affect the NDT results. The longitudinal bars were Grade 60 with the nominal yield

strength of 60 ksi (414 MPa). The horizontal ties used were No. 4 and were spaced along the axis

of the shaft at 4 inches on center. A total of twelve No. 4 ties were used throughout the shaft

samples. The clear cover on all steel was 1 inch (Figure 3). The CSL tubes that were installed

inside and outside the cage were galvanized tubes with 2 inches (5 cm) inside diameter.

Figure 2: Drilled Shaft Specimens

6

Figure 3: CSL Tubes Arrangement – Top View

The number of access tubes required in order to conduct the CSL tests is very important. The

specimens used in this study had four access galvanized tubes inside the cage and four tubes

outside the cage. Each galvanized tube was fixed at the end and at third points throughout the

length of the shaft (Figure 4). For all the specimens, the outside galvanized tubes were installed 3

inches (7.5 cm) away from the edge of the shaft. The specimens were tested one week after

concrete placement.

(a) (b)

Figure 4: (a) Installing the Outside Galvanized Tubes, (b) Installing the Sona-tube

For each specimen, eight cubic yards of dry limestone with a unit weight of 80 lb/ft3 were used

for the soil outside the sona-tubes. While pouring limestone inside the formwork, the soil wa s

compacted in three lifts. Concrete with 5,000 psi compressive strength was placed inside the

sona-tubes (Figure 5a). The sona-tubes were then removed using an overhead crane (Figure 5b).

The concrete was then vibrated inside the drilled shaft to have drilled shaft specimens with

varying diameters through their corresponding lengths. Vibration causes the concrete to have a

better contact between the soil and concrete. The galvanized tubes were aligned to have uniform

distance of 3 inches from the edge while the concrete is fresh.

7

(a) (b)

Figure 5: (a) Concrete Placement inside the Sona-tube, (b) Taking out the Sona-tube using Crane

The casting form for the concrete specimens consisted of a cardboard sona-tube with an inside

diameter of 20 inches (50 cm). Before the steel cage was positioned inside the sona-tube, eight

plastic spacers with 1 inch (2.5 cm) length were installed throughout the length of the cage to

keep the rebar cage at the center of the sona-tube and for ensuring the 1 inch concrete clear

cover. A wood formwork was built and placed at the bottom of the sona-tube to ensure that the

steel cage was aligned properly and to secure the fluid concrete during casting.

The drilled shaft specimens were cast separately. Concrete from mixer trucks was pumped

vertically inside the sona-tubes for all the specimens to ensure uniformity. Concrete placement

was continued in one operation to the top of the shaft. The concrete for the drilled shaft

specimens was also designed and placed in such a manner that it could be pumped, and could

flow easily using gravity through the rebar cage and to the bottom of the shaft without the need

for any vibration.

2. Anomaly Types and Locations Considered

Iskander, et al. (2003) studied drilled shafts constructed with built-in anomalies located in

various areas within the shafts and included anomalies and soil inclusions occupying 5-45% of

the cross section. Iskander, et al. (2003) concluded that down hole methods such as CSL and

cross hole tomography are generally able to identify anomalies exceeding 10% of the cross

sectional area. The built-in anomalies that were placed in the shaft specimens in this study

occupy close to 20% of the gross cross sectional area of the shaft specimen. This was based on

the common anomalies percentage in full-scale drilled shafts used in real life.

8

One type of anomaly was considered, which was installed inside the rebar cage. For the case of

anomalies inside the caging, two drilled shaft specimens were considered: one sample with

prefabricated anomalies at the top and the bottom of the shaft, and another sample with a built-in

anomaly at the middle of the shaft's length. These anomalies will be detected by performing a

CSL test between the inside galvanized tubes. Foam pieces were used to represent the anomaly

shapes inside the cage, and were secured at the center of the steel cage before casting. Steel wire

was used to tie the foam pieces to the reinforcement bars and keep the foam at the center of the

cage.

Table 1 shows the characteristics of the drilled shaft specimens that were tested with built-in

anomalies inside the rebarcage. Anomaly type, percentage and their locations, and the number of

the CSL tubes are listed in this Table 1 for both specimens. Specimens 1 and 2 have four CSL

galvanized tubes inside the cage and four CSL tubes outside the shaft and inside the surrounding

soil. Anomalies are occupying about 20% of the gross cross-sectional area and they are located

inside the caging. In specimen 1, one anomaly has a length of 7 inch (17.8 cm) and it is located

between depths of 5 inches and 12 inches (12.7 cm to 30.5 m) from the surface. The other

anomaly is 7 inches (17.8 cm) long and is located at the bottom of the shaft between depths of 36

inches and 43 inches (91 cm to 109 cm) as shown in Figure 6(a). In specimen 2, the anomaly has

a length of 7 inch es (17.8 cm) and is located at the middle of the shaft specimen between depths

of 24 inches and 31 inches (61 cm to 79 cm) from the surface. In specimen 2, the anomaly is

inside the caging (Figure 6b). All the anomalies have the same cross-sectional shape and

occupying 20% of the gross cross-sectional area of the drilled shaft.

Table 1: Characteristics of Tested Drilled Shaft Specimens with Built Anomalies

Top and Bottom

of the Shaft

9

(a) Specimen 1 (b) Specimen 2

Figure 6: Constructed Anomalies inside the Caging Through the Length of the Shaft Specimen

3. Materials

The concrete used in this study was normal weight concrete (150 lbs/ft3 or 23.565 kN/m3).

Standard concrete cylinder samples with 4-inch (10.16 cm) diameters and 8-inch (20.32 cm)

lengths were tested using the Concrete Compression Machine in the laboratory at FIU. The

average measured axial compressive strength for three standard cylinders was 62,832 lbs (280

KN) at 28 days. Therefore, the cured concrete cylinders had a 28-day compressive strength of

5000 psi (34.5 MPa). The concrete slump was measured to be 4 inches (10.16 cm) at the time of

casting, and the maximum coarse aggregate size (rounded river gravel) was 0.5 inch (1.27 cm).

Fine aggregate used was based on ASTM C33 natural sand with a fineness modulus of 3.0. The

cement used was type I Portland cement and comprised about 24 percent of the weight of the

mix. The water to cement ratio varied between 0.4 and 0.42, depending on the moisture content

of the aggregate. Sona-tube with inside diameter of 20 inches (50.8 cm) was used as a formwork

for the concrete. All shaft specimens were cast in a vertical position without vibration after

concrete placement to simulate actual conditions.

A No. 4 bar was tested using a Universal Tensile Testing Machine in the laboratory at FIU. The

tensile test loading ratio used was 100 lbs/sec (445 N/sec). The longitudinal steel and lateral ties

in all the tested specimens were Grade 60, with yield strength of 60 ksi (414 MPa). The actual

10

yield strength was more than the nominal value (65 ksi, 448 MPa), and the modulus of elasticity

was 29×106 ksi (2×108 MPa).

4. Test Procedure

The procedure used to detect the anomalies inside the drilled shaft was to perform the CSL test

between the inside tubes and apply a signal processing on the results to analyze the data from the

CHA software. Two drilled shaft specimens were tested (Figure 7a). Before testing a shaft, the

access tubes were checked for free access and filled with water to obtain good acoustic coupling.

Each tube was fit with a watertight shoe on the bottom and a removable cap on the top. Tubes

were be secured to the interior of the reinforcement cage at third points. Two probes connected

to pulleys were then inserted into the tubes, at least one of which was equipped with a depth

meter (Figure 7b). The emitter and receiver probes were then lowered into the tubes. After

reaching the bottom of the tubes, the probes were then pulled simultaneously upwards with

smooth motion until they reach the top. During this time the emitter produces a continuous series

of pulses, sending waves in all directions. For each profile, the First Arrival Time (FAT) and

data acquisition signal of each pulse is plotted every two inches.

In specimens 1 and 2, six CSL tests were performed between the tubes inside the concrete (tubes

5-6, 5-7, 5-8, 6-7, 6-8 and 7-8) to detect the anomalies inside the caging, one week after concrete

placement. Also, four CSL tests were performed between the inside and outside tubes (tubes 1-5,

2-6, 3-7, and 4-8). Figure 3 shows the CSL tube arrangements for Specimens 1 and 2 with four

inside tubes and four outside tubes.

Based on ASTM 6760, Standard Test Method for Integrity Testing of Concrete Deep

Foundations (ASTM D6760, 2008), CSL testing can be performed any time after concrete

placement when concrete has obtained sufficient strength, which is almost 66% of the ultimate

concrete compressive strength. Because the concrete strength and quality generally increase as

the concrete cures, longer wait times are usually desirable particularly if minimum pulse wave

speeds are specified or to reduce result variability between drilled shafts, or even the variability

as a function of depth in a single drilled shaft. However, if PVC tubes are used for wet cast

shafts, long wait times increase the tube de-bonding which is detrimental to the accuracy of the

test results. Production drilled shaft installation and subsequent construction influence the dates

of CSL testing.

11

(a) (b)

Figure 7: CSL Test on the Drilled Shaft Samples

Signal Processing On the CSL Test Results

After performing the CSL test, CHA can be used to record the data acquisition signal graph,

which includes 250 data points at each depth. Time domain data can easily be obtained from the

data acquisition signal. The horizontal axis in the data acquisition signal is the data point, and it

should be divided by the frequency to determine time as in Equation 3.

(3)

where t is the time, n is data points, f is frequency, and T is the period. The most important part

of the CHA software is the data acquisition signal graph, which is based on the time domain

data. Figure 8 shows the data acquisition signal for two separate CSL tests on the drilled shaft

sample at the height of 31.5 inches. Both tests are between two tubes inside the concrete (C-C).

Figure 8 compares the time domain for two C-C tests, and it can be seen that both tests have

exactly the same FAT. Figure 9 shows the frequency domain curve for the same tests after de-

noising and fitting with a tenth degree polynomial. It shows that they both have almost the same

maximum amplitude of signal. It also shows that the maximum amplitude of the signal is around

7.0×10-4 when both tubes are placed inside the concrete. Since limestone material is not as dense

as concrete, it can be said that the FAT for a concrete-soil test is much higher than that in the

concrete-concrete test; this is because the ultrasonic wave travel time is much higher.

12

Figure 8: Time-Domain (5-6 and 6-7)

Figure 9: De-noised and Fitted Frequency Domain Curve (5-6 and 6-7)

Drawing the Time domain contour does not allow for the detection of the exact location of the

anomalies. This indicates that the time domain data has to be converted to frequency domain data

in order to obtain the frequency domain tomography in an effort to pinpoint the exact location of

the anomaly. Fast Fourier Transform (FFT) was be used to perform this signal processing.

13

Discrete Fourier Transform (DFT) is a specific kind of discrete transform, used in Fourier

analysis. It transforms one function into another, which is called the frequency domain. DFT is

widely used in signal processing to analyze the frequencies, because it takes a discrete signal in

the time domain and transforms that signal into its discrete frequency domain representation. The

DFT transforms time-based data into frequency-based data. The DFT of a vector x of length n is

another vector y of length n :

(4)

where

is a complex n th root of unity and is defined by:

(5)

FFT is an efficient and faster algorithm to compute the DFT and its inverse with a significant

speed increase. The functions Y=FFT(x) implement the transform using Equation 6.

n

j

kj

n

jxkX 1

)1 )( 1(

)()(

(6)

where

is an n th root of unity.

(7)

It is difficult to identify the frequency components by looking at the original signal. Converting it

to the frequency domain, the discrete Fourier transform of the noisy signal y is found by taking

the FFT of the signal. The FFT utilizes an algorithm to perform the same function as the DTF,

but in much less time.

The frequency domain curve after de-noising and fitting with a tenth degree polynomial is

sufficient to clearly obtain the maximum amplitude of the signal. The maximum amplitude of the

signal was 7.2×10-4 for C-C test and 2.8×10-4 for C-S test with a concrete thickness of 7 inches

for the two different CSL tests. Figure 9 compares the frequency domain for two C-C tests, and it

can be seen that both tests have almost the same frequency domain data. Also, they have the

same maximum amplitude of signal. Results from two C-S tests show that for one test with

concrete thickness of 5 inches, the maximum amplitude of the signal is about 8×10 -5, and for

other test with concrete thickness of 4 inches, it is 6.5×10-5. All these results from the

preliminary sample show that the concrete thickness can be correlated to the maximum

amplitude of the signal. This means that with the change in the concrete thickness due to

presence of anomaly inside the cage the frequency domain and maximum amplitude of the signal

will change. All results show that the maximum amplitude of signal occurs at 500 kHz. This is

because the data acquisition signal graph includes 250 data points, sampled at 500 KHz.

Once an anomaly is identified after the CSL test, some actions are required for frequency

imaging of the anomalous zones. This task is achieved by using a Matlab program provided in

this study that uses the CSL data for anomaly identification. This allows a site engineer to

identify the exact location and size of the anomaly, and to apply necessary action to save the

drilled shaft for the considered project. The following process was carried out using Matlab to

14

obtain the de-noised frequency domain tomography of the received signal from the data

acquisition in CHA.

In this program, the measured data acquisition signals obtained from CHA is initially plotted in

time domain. Although in time domain, the peak and strength of the signal can be observed, but

in order to have a better understanding of the signal characteristics, an effective approach is to

convert the time domain signal into the frequency domain. FFT process was used to convert the

time domain signal into the frequency domain. After taking the FFT of the signal, the frequency

spectrum of the signal is plotted to observe the characteristics of the signal. To have a better

illustration of the results, all the results are plotted in contour format. Also, the frequency domain

data needs to be de-noised to obtain the best estimate of the function. The signal de-noising was

done based on wavelet decomposition. Finally, the obtained signal in frequency domain is fitted

into a curve to show the similar behavior. This task is performed in order to reduce the time

consuming calculation performed on discrete data sets.

Figure 10 shows the frequency domain graph for one CSL test between two inside tubes through

the shaft length, which is obtained from the time domain plot using FFT. This graph shows that

for each depth, the frequency domain graph will change and it depends on the concrete thickness

and presence of any kind of anomaly between the tubes. Drawing different parameters such as

FAT, energy, velocity, strength of signal, and amplitude of signal shows that the frequency

imaging of the signal has the best capability to show the anomaly location. Also, the maximum

amplitude of signal is obtainable from the frequency domain data. Therefore, time domain data

has to be converted to the frequency domain data and the maximum amplitude of the signal can

be obtained from the frequency domain data.

Figure 10: Frequency domain graph through the shaft length

15

Test Results

A three-dimensional (3D) tomographic image helps to better evaluate the extent of local

anomalies. Tomography is a mathematical procedure that operates on the measured data where

the shaft is modeled as a grid with each given node point assigned as one property of the wave.

The existing CSL tomographic methods are based on wave speed, which is based on the change

in the FAT since the distance between the CSL tubes is known. FAT of all data points in all tube

combinations with known probe locations is used to solve for the wave speed at each node point.

This study used a novel idea of converting the information of FTA to information of frequency

received by the receiver probe in the tomography method using the FFT of the signal. Each point

in the drilled shaft grid is assigned signal frequency amplitude in the FTA method in order to

more accurately pinpoint location of the anomaly. Uniform signal frequency amplitude generally

produces straight ray travel paths, but variable frequency amplitude produce curved ray paths.

The amplitude of the signal in time domain and frequency domain were compared to determine

which one can better identify the anomaly location. Amplitude means how much of something

and in this case it represents strength or voltage, or some number that measures those values.

Figures 11 and 12 show the result of CSL test between the inside tubes (tubes 5 and 7) in

Specimen 1. Figure 11 is a 2-dimensional visualization of the CSL results of one single path

between two tubes, which is based on time domain data. In this figure, the horizontal axis

indicates the length of the shaft and the vertical axis is data points. The third dimension which is

shown by color is to indicate the amplitude or voltage of the signal. This plot shows the

amplitude of the signal at different points in time. Low wave speed or FAT indicates concrete

with poor quality. In Figure 11, it can be seen that in uniform concrete the strength of the signal

is around 6 volts, and in anomaly locations the strength decreased to -8 volts. The exact location

of the anomaly cannot be identified using time domain characteristics.

Figure 12 is a 2-dimensional visualization of the CSL results of one single path between two

tubes, which is based on frequency domain data. In this figure, the horizontal axis indicates the

frequency of the signal, the vertical axis is length of the shaft and the third dimension shown by

color contours indicates the frequency amplitude of the signal. This plot shows the amplitude of

the signal at different points in frequency. Low frequency amplitude indicates the location of the

anomaly. It can be seen that for areas of normal concrete, the frequency amplitude is around

16×10-4 , and this number around the anomaly drops to 10-4. It can be concluded that the exact

location of the anomaly can be identified using frequency tomography. The major anomalies in

the shaft were at the depth where the frequency amplitude of the signal decreased significantly.

One anomaly at the depth of 5 inch and other one at the depth of 36 inch were determined using

the FTA method.

Figures 13 and 14 show the result of the CSL tests between inside tubes (Tubes 6 and 8) in

Specimen 2. Figure 13 shows the time domain results and Figure 14 shows the frequency

tomography after the CSL test in Specimen 2. It can be seen that using frequency domain results,

the exact location of the anomaly at about 24 inches depth below the surface can be clearly

identified. The "area of concern" can be seen in both time domain and frequency domain images,

16

but the frequency domain depicts the exact anomaly location and size while the time domain

does not.

Figure 11: 2-D Visualization of the CSL Results of one Single Path between 2 Tubes in Time

Domain (Specimen 1)

17

Figure 12: 2-D Visualization of the CSL Results of one Single Path between 2 Tubes in

Frequency Domain (Specimen 1)

Figure 13: 2-D Visualization of the CSL Results of one Single Path between 2 Tubes in Time

Domain (Specimen 2)

18

Figure 14: 2-D Visualization of the CSL Results of one Single Path between 2 Tubes in

Frequency Domain (Specimen 2)

Conclusions

A novel method was proposed for determining the location of anomalies in drilled shafts after

performing the CSL test between inside tubes. This method rely on the arrival time and wave

speed to detect the anomalies in drilled shaft foundation. This method was developed using the

application of signal processing on the CSL test results. The proposed method is based on a color

change in the frequency amplitude of the signal recorded by the receiver probe at the location of

anomalies, and it is named Frequency Tomography Analysis (FTA).

In order to determine the location and size of anomalies, the proposed technique plots the data

acquisition signal from the CSL test in time domain. The proposed technique then converts the

time domain signal into the frequency domain using Fast Fourier Transform. The frequency

domain data is then de-noised and fitted into a tenth-degree polynomial to obtain the best

estimate of the function. The technique then converts the frequency-domain plot into a

tomography plot by assigning frequency amplitude to each and every point in the drilled shaft,

and using a colored contour to unveil the location and size of the anomalies.

The technique has a very good resolution and shows the depth location and approximate size of

foam blocks through the length of the drilled shaft. A sufficiently large frequency amplitude

reduction from a large anomaly would define an anomaly even if the time domain information

were normal. In cases of local anomalies which include only part of the cross section, frequency

tomography analysis is very helpful to visualize and quantify the extent and location of the

19

anomaly. Such information is critical for construction quality control on the jobsite, and is

extremely helpful for the structural engineer who must assess the adequacy of the drilled shaft to

resist the applied load.

Acknowledgement

The authors are very grateful to Tally Engineering Company for providing the CSL equipment.

Also, the authors thank Mr. Amir Mostafa Sotoodeh for assisting in performing the CSL tests.

The authors also express sincere gratitude to the staff of the Titan America Structures and

Construction Laboratory of the Florida International University.

References

1. ASTM D6760, 2008. Standard Test Method for Integrity Testing of Concrete Deep

Foundations by Ultrasonic Crosshole Testing.

2. Baker, C.N., Drumright, E.E., Briaud, J-L., and Mensah, F., 1993. Drilled Shafts for

Bridge Foundations. FHWA Publication FHWA-RD-92-004, Federal Highway

Administration Office of Engineering and Highway Operations, McLean VA.

3. Camp, W. M., Holley, D.W., Canivan, G.J., 2007. Crosshole Sonic Logging of South

Carolina Drilled Shafts: A Five Year Summary. Deep Foundations, American Society of

Civil Engineers, pp. 1-11.

4. Chang N., Nghiem H., 2008. Drilled Shaft Axial Capacity Due to Anomalies. Report No.

FHWA-CFL/TD-08-008. Federal Highway Administration Colorado.

5. Fullagar, P.K., Livelybrooks, D.W., Zhang, P., Calvert, A.J., Wu, Y., 2000. Radio

tomography and borehole radar delineation of the McConnel nickel sulfide deposit,

Sudbury, Ontario, Canada Geophysics. 65, pp. 920–1930.

6. Haramy K. Y., 2006. Structural Capacity Evaluation of Drilled Shaft Foundations with

Defects, MS thesis, University of Colorado Denver.

7. Iskander, M., Roy, D., Kelley, S., and Ea, C., 2003. Drilled Shaft Defects: Detection, and

Effects on Capacity in Varved Clay. 10.1061/ASCE,1090-0241, 129:12,1128

8. Jalinoos, F., Mekic, N., Hanna, K. 2005. Defects in Drilled Shaft Foundations:

Identification, Imaging, and Characterization. Publication No. FHWA-CFL/TD-05-003,

Federal Highway Administration.

20

9. Lew, M., Zadoorian, C.J., Carpenter, L.D., 2002. Integrity Testing of Drilled Piles for

Tall Buildings, Structure. A joint publication of: National Council of Structural Engineers

Associations: Council of American Structural Engineers, October, pp. 14-17.

10. O'Neill, M. W., Tabsh, S. W., and Sarhan, H. A. 2003. Response of drilled shafts with

minor flaws to axial and lateral loads. Engineering Structures, 25(1), 47-56.

11. Olson, D. L., Hollema, D. A., 2002. Crosshole Sonic Logging and Tomographic Velocity

Imaging of a New Drilled Shaft Bridge Foundation. Structural Materials Technology

Topical Conference, Cincinnati, Ohio.

12. Reese, L. C., O'Neill, M. W., 1999. Drilled Shafts: Construction Procedures and Design

Methods. Publication No. FHWA-IF-99-025, Federal Highway Administration.

13. Hajali, M. and Abi Shdid, C., 2012"Behavior of Axially Loaded Drilled Shaft

Foundations with Symmetric Voids outside and inside the Caging". Deep Foundation

Institute Journal, Vol. 6 , pp.25-34.

14. Amiri, S., N., 2011, "A comprehensive study on soil consolidation: The pore pressure

development/dissipation during and effect of variable permeability and compressibility

on the consolidation behavior", Publisher VDM Verlag Dr. Müller, pp. 124.

... Hajali and Abishdid (2014) [4,5] investigated that how many percent of the maximum applied load will be shed in side friction and how much will be transferred to the base in drilled shaft foundations. The axial capacity of the drilled shaft foundation is influenced by the size of the drilled shaft, and soil characteristics. ...

... However, flaws occupying up to 15% of the drilled shaft's cross section have remained undetected with CSL (O'Neill et al. 2003;Sarhan et al. 2002). CSL measures the speed of ultrasonic waves between water-filled access tubes to detect anomalies and establish the homogeneity and integrity of concrete (Rausche 2004;Hajali and Abi Shdid 2014). ...

The widespread use of drilled shafts and the large and critical nature of loads they carry make post-construction quality control a rather important aspect of inspections. For this reason, states have implemented specific procedures to follow when performing inspections of deep foundations. The current accepted approach is to compare, at discrete points of each production shaft, the theoretical concrete volume to that actually placed. Only those shafts with a difference that exceeds a certain prespecified percentage are subjected to additional scrutiny using nondestructive testing methods. This paper reports on an innovative and comprehensive method proposed for the quality control of drilled shafts that overcomes several limitations of conventional methods. The method is developed and validated using experimental data, and is based on applying the fast Fourier transform mapping algorithm on data obtained from cross-hole sonic logging. Tomographic imaging and regression analysis are subsequently used to improve the accuracy of the integrity testing to a degree that facilitates the determination of both the diameter of the shaft and the exact location and size of anomalies.

... Corrosion rate of the steel reinforcements depends on certain parameters such as moisture content, organic content, level of compaction and grain size, pH, chloride concentration and resistivity of the soil (Amiri, 2011). Although the ability of engineers and facility managers to detect such corrosion-causing voids during construction is possible using a new Frequency Tomography Analysis method developed by Hajali and Abi Shdid (2014), the subsequent ability to measure the residual capacity of the shaft during its lifespan due to rebar corrosion is not possible due to the inaccessible subterranean nature of drilled shafts. ...

Drilled shafts are a common type of deep foundations used to support a wide array of infrastructure facilities such as bridges, high mast lighting, tanks and communication towers. The deep inaccessible subterranean nature and the construction procedures of drilled shafts result in various types of anomalies; most critical of which being necking and voids. This necking or voids translates to loss of concrete cover around the longitudinal bars and its subsequent exposure to the surrounding soil, leading to corrosion. An experimental study using large-scale drilled shaft samples is presented that evaluates the effect of different percentages of rebar corrosion on the axial load capacity of drilled shaft foundations. Results show that the presence of corrosion in the longitudinal bars affects both the strength and buckling capacity of a shaft, and that a 25% necking or void area will result in more than 60% of axial capacity loss of the shaft over a time period of 20 years.

  • Aleksei Churkin Aleksei Churkin

The purpose of the work was to: systematize information about the causes of defects in monolithic buried structures; describe the capabilities of geophysical methods for quality control of such structures; obtain results that develop the method of using the geophysical complex. To achieve these goals, experimental research methods were used: fieldwork, numerical and physical modeling. Among the results obtained are: - a set of methods is proposed for solving the problem of quality control of "hidden works", applicable for pile foundations and diaphragm and pile walls; - information was collected on the appearance of defects in buried structures; - the thermal integrity profiling technique was used in quality control of piles and diaphragm walls for the first time in Russian testing practice; - the method for assessing the contact conditions of the "pile-soil" system was tested on the basis of the dynamic attributes of low strain testing signals. Цели работы состояли в: систематизации информации о причинах возникновения дефектов в монолитных заглубленных конструкциях; описании возможностей геофизических методов при контроле качества данных конструкций; получении результатов, развивающих методику использования геофизического комплекса. Для достижения поставленных целей использованы экспериментальные методы исследования: опытные работы, численное и физическое моделирование. Среди полученных результатов можно выделить: — предложен комплекс методов для решения задачи контроля качества «скрытых работ», применимый для свайных фундаментов и «стен в грунте»; — собрана информация о появлении дефектов заглубленных конструкций; — термометрический метод использован для контроля качества свай и траншейных «стен в грунте» впервые в российской практике; — проведена апробация метода оценки контактных условий системы «свая-грунт» на основе динамических атрибутов акустических сигналов.

  • Nien-Yin Chang
  • Hien Nghiem Hien Nghiem

Drilled shafts are increasingly being used in supporting critical structures, mainly because of their high-load supporting capacities, relatively low construction noise, and technological advancement in detecting drilled shaft anomalies created during construction. The critical importance of drilled shafts as foundations makes it mandatory to detect the size and location of anomalies and assess their potential effect on drilled shaft capacity. Numerical analysis was conducted using Pile-Soil Interaction (PSI), a finite element analysis program to assess the effect of different anomalies on the axial load capacities of drilled shafts in soils ranging from soft to extremely stiff clay and loose to very dense sand. The investigation included the affect of anomalies of various sizes and lengths on both structural and geotechnical capacities. The analysis results indicate that the drilled shaft capacity is affected by the size and location of the anomaly and the strength of the surrounding soil. Also, nonconcentric anomalies significantly decrease the structural capacity of a drilled shaft under axial load. The resulting drilled shaft capacity then equals the smaller one of the two capacities: structural or geotechnical.

Drilled shaft foundations usually carry very high design loads, and often serve as a single load-carrying unit. These conditions have created a need for a high-level of quality assurance during and after construction process. During the construction process, different types of anomalies such as necking, soft-bottom gap at the base, voids and soil intrusions can occur. Anomalies throughout the length can significantly reduce the axial load capacity of the drilled shaft. This paper studies the effect of voids inside and outside the reinforcement cage on the strength and structural capacity of drilled shafts. The objective of this research is to quantify the extent of loss in axial strength and stiffness of drilled shafts due to presence of three different types of symmetric voids throughout their lengths; also, to evaluate the potential for buckling of longitudinal bars within the various types of voids. To complete these objectives, fifteen large-scale drilled shaft samples were built and tested using a hydraulic actuator at the Florida International University's (FIU) Titan America Structures and Construction Testing (TASCT) laboratory. During the static load test, load-displacement curves were recorded by the data acquisition system (MegaDAC). Results show that the presence of symmetric voids outside the rebar cage (void Type C) that occupy 40% of the cross sectional area of the drilled shafts cause 27% reduction in the axial capacity, while the symmetric voids that penetrate inside the core (void Type B) cause 47% reduction in the axial capacity. The findings indicate that the voids Type B decrease the capacity and stiffness of drilled shafts more than other types due to the resulting inadequate confinement of the concrete and reinforcement.

  • M. O'Neill
  • Sami W. Tabsh Sami W. Tabsh
  • Hazem A. Sarhan

The use of drilled shafts as foundations for bridges and other structures has greatly increased in recent years due to the advent of routine non-destructive evaluation (NDE) methods. The availability of the NDE methods has given engineers and contractors the false impression that any defects that may have been produced during construction due to problems in concreting, drilling, casing, slurry and rebar cage placement can be identified and repaired before the bridge is opened to traffic. In reality, there is a lower limit on the size of defects that can be detected by current NDE techniques, which recent studies have identified it to be roughly between 10 and 20 percent of the cross-sectional area of the shaft. In this study, eleven 1/3-scale concrete shafts with minor void flaws were tested in the lab to determine the effects of minor void flaws on the stiffness, structural capacity, mode of failure, cracking pattern and ductility of concrete shafts, without the confining soil. Three different loadings were considered for the shafts: (a) pure flexure, (b) pure axial compression, and (c) combined axial compression and flexure. The size of the void was taken equal to 15 percent of the cross-sectional area of the shaft. The shape of the void resembled a wedge with the outer arc length and length of the void along the centerline of the shaft being both variables. The results of the lab tests showed that void flaws that penetrate the concrete core of a shaft are more critical than those that are located within the concrete cover. Further, the presence of a void affects the ductility of a shaft subjected to axial compression much more than it affects the axial strength. Computational methods that are based on strain compatibility and force equilibrium can predict the strength of a defective shaft with reasonable accuracy. Ultimately, the study will be helpful in developing rationally based structural resistance factors for drilled shafts that account for possible presence of minor void flaws.

  • Peter K. Fullagar
  • D. Livelybrooks D. Livelybrooks
  • Ping Zhang
  • Yiren Wu

In an effort to reduce costs and increase revenues at mines, there is a strong incentive to develop high-resolution techniques both for near-mine exploration and for delineation of known orebodies To investigate the potential of high-frequency EM techniques for exploration and delineation of massive sulfide orebodies, radio frequency electromagnetic (RFEM) and ground-penetrating radar (GPR) surveys were conducted in boreholes through the McConnell massive nickel-copper sulfide body near Sudbury, Ontario, from 1993-1996. Crosshole RFEM data were acquired with a JW-4 electric dipole system between two boreholes on section 2720W. Ten frequencies between 0.5 and 5.0 MHz were recorded. Radio signals propagated through the Sudbury Breccia over ranges of at least 150 m at all frequencies. The resulting radio absorption tomogram clearly imaged the McConnell deposit over 110 m downdip. Signal was extinguished when either antenna entered the sulfide body. However, the expected radio shadow did not eventuate when transmitter and receiver were on opposite sides of the deposit. Two-dimensional modeling suggested that diffraction around the edges of the sulfide body could not account for the observed held amplitudes. It was concluded at the time that the sulfide body is discontinuous; according to modeling, a gap as small as 5 m could have explained the observations. Subsequent investigations by INCO established that pick-up in the metal-cored downhole cables was actually responsible for the elevated signal levels. Both single-hole reflection profiles and crosshole measurements were acquired using RAMAC borehole radar systems, operating at 60 MHz. Detection of radar reflections from the sulfide contact was problematic. One coherent reflection was observed from the hanging-wall contact in single-hole reflection mode. This reflection could be traced about 25 m uphole from the contact. In addition to unfavorable survey geometry, factors which may have suppressed reflections included host rock heterogeneity, disseminated sulfides, and contact irregularity. Velocity and absorption tomograms were generated in the Sudbury Breccia host rock from the crosshole radar. Radar velocity was variable, averaging 125 m/mus, while absorption was typically 0.8 dB/m at 60 MHz. Kirchhoff-style 2-D migration of later arrivals in the crosshole radargrams defined reflective zones that roughly parallel the inferred edge of the sulfide body. The McConnell high-frequency EM surveys established that radio tomography and simple radio shadowing are potentially valuable for near- and in-mine exploration and orebody delineation in the Sudbury Breccia. The effectiveness of borehole radar in this particular environment is less certain.

Input data of georadar tomographic inversion can be contaminated by systematic errors involved in data acquisition and preparation. Such errors are caused by mislocations of the antennas (e.g., unrecognised borehole deviation), traveltime delay problems or unconsidered anisotropy of the medium. If they are not recognised and not eliminated they may produce severe artifacts in the tomograms leading to misinterpretation of the results. However, specific errors produce significant anomalies in certain plotting schemes and certain artificial structures in the reconstructed geophysical parameter distributions. This is demonstrated using purposely contaminated synthetic data sets. These synthetic data examples illustrate how certain systematic errors, e.g., unrecognised borehole deviation, can be identified and distinguished from the expected real subsurface structure. Furthermore, a field experiment is presented carried out for fracture and void detection in a limestone formation, where such consequent data analysis helped to identify and remove discrepancies in the data. Tomographic results were clearly improved showing a more reliable and consistent behaviour. The field and synthetic data examples and calculations show that careful pre- and post-inversion analysis and handling of tomographic georadar data lead to a quality increase and to more reliable results.

Since 2001 the South Carolina Department of Transportation (SCDOT) has regularly used crosshole sonic logging (CSL) to evaluate the integrity of the majority of drilled shafts installed on state bridge projects. The authors have been responsible for approximately half of the logging performed to date. The CSL results are summarized for the purpose of simple statistical analyses in an effort to identify trends. To date more than 400 shafts have been logged on more than 42 projects. The shafts have been installed by 10 different drilling contractors, in a variety of soil conditions using various construction methods. The most pertinent findings of the evaluation are that 1) about 33% of the tested shafts contain an anomaly (or anomalies), 2) nearly 90% of the anomalies are located within the top two diameters or bottom two diameters of the shafts, 3) the vast majority of the anomalies are attributed to concrete placement issues and 4) most anomalies are not considered defects.

  • Magued Iskander Magued Iskander
  • Douglas Roy
  • Shawn Kelley
  • Carl Ealy

This paper presents the results of nondestructive integrity tests (NDTs) and axial static load tests on drilled shafts constructed in varved clay at the National Geotechnical Experimentation Site in Amherst, Mass. The shafts were constructed with built-in defects to study: (1) the effectiveness of conventional NDT methods in detecting construction defects and (2) the effect of defects on the capacity of drilled shafts. Defects included voids and soil inclusions occupying 5-45% of the cross section as well as a soft bottom. Nine organizations participated in a blind defect prediction symposium, using a variety of NDT techniques. Most participants located defects that were larger than 10% of the cross sectional area. However, false positives and inability to locate smaller defects and multiple defects in the same shaft were encountered. Static load tests indicated that (1) minor defects had little or no effect on skin friction: (2) a soft bottom resulted in a 33% reduction in end bearing relative to a sound bottom; and (3) reloading resulted in a 20-30% reduction in the geotechnical capacity.

  • Khamis Y. Haramy

Thesis (M.S.)--University of Colorado at Denver and Health Sciences Center, 2006. Includes bibliographical references (leaves 361-364).

Drilled Shafts: Construction Procedures and Design Methods. Publication No. FHWA-IF-99-025

  • L C Reese
  • M W Neill

Reese, L. C., O'Neill, M. W., 1999. Drilled Shafts: Construction Procedures and Design Methods. Publication No. FHWA-IF-99-025, Federal Highway Administration.

Defects in Drilled Shaft Foundations: Identification, Imaging, and Characterization. Publication No. FHWA-CFL

  • F Jalinoos
  • N Mekic
  • K Hanna

Jalinoos, F., Mekic, N., Hanna, K. 2005. Defects in Drilled Shaft Foundations: Identification, Imaging, and Characterization. Publication No. FHWA-CFL/TD-05-003, Federal Highway Administration.