Verizon-LR 981030NOU-02-1998 15:12 O$'~N ENGINEERING INC ? 3034236071 P.002/016
ENGINEERING, INC.
NONDESTRUCIIVE TESTING INVESTIGATION
CONCRET~ DP./L2LED PIER LENGTI~
AND INTEGRITY EVALUATION
GTE SHARED SERVICES PROJECT
COPP~'r .r.. TEXAS
Prep~d
The Beck Group
750 Corporate Park Blvd
Coppell, Texa~ 75019
Arm: Mr. Lonni¢ Mahoney
Tx: 972-304-0503
Fax: 972-30~3288
Olson Engineering lob No. 711
October 30, 1998
Phone: 303/423-1212 F~: 303/423.6071
~A'eb Page: ww, w. olmonrnglAeerlng, com
Branch Office o Ar./anta, Georgia 770/350o9611
NOU-02-1998 15:12 OL~''~ ENGINEERING INC ~m~ ~0~4236~71 P.003/016
TABLE OF CONTENTS
1.0 SCOPE .................................................................. 1
2.0 INVESTIGATION SUMMARY AND CONCLUSIONS ...................... · ......1
3.0 INVESTIGATION BACKGROUND AND SITE CONDITIONS ..................... 2
4.0 NONDESTRUCTIVE TEST (NDT) METHODS .................................. 2
4.1 Sonic Echo (SE) and Impulse Response (IR) Methods ....................... 3.
5.0 NONDESTRUCTIVE INVESTIGATION TEST RESULTS .................. . .......5
5. i Sonic Echo/Impulse Response (sE/n;D Test Records and Results ................ 5
6.0 CLOSURE ................................................ , ............... 7
TABLE I SONIC ECHO (SE) RESULTS
Figs. 1-6 - Representative SE Results
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1.0 SCOPE
This report presents the results of a nondestructive testing (NDT) investigation of 6 drilled
pier foundatio~ which are used for GTE Shared Services Project, Coppell, Texas. The testing was
requested to evaluate the length and integrity of these six concrete drilled piers. One of these six
concrete drilled piers, i,e, Pier H-15, has a known depth with sound concrete quality from inspection
observations. Pier H-15 was tested as a reference pier and was used to backcalculate the stress wave
velocity of the concret:, which was used to predict the depths of the other five piers. The other five
piers were tested for quality assurance purposes.
The NDT was performed with the Sonic Echo/Impulse Response (SE/I~) test methods. The
SF_JIR tests involved hitting the top of each pier with an instrumented hammer and recording echoes
of the compression wave energy (sound wave) with an accelerometer mad a geophone mounted to
the top of the pier from changes in impedance (velocity times density) that are indicative of breaks,
necks, bulbs, soil/bedrock interfaces, and pier bottoms.
This report presents the investigation summary and conclusions, background information
on the project., descriptions of the NDT methods, representative SE msults and discussions of the
NDT results.
2.0 INVESTIGATION SUMMARY AND CONCLUSIONS
The Sonic Echo (SE) results are summarized in Table L As seen in Table L the SE results
show the predicted SE reflector depths of between 30 feet to 53 feet that correspond to expected
depths of the pier bottoms. Some piers showed additional echoes appeared at a shallower depth of
about 1.5 feet below the top of concrete. This shallower depth of about 15 feet corresponds to the
depth to bearing stratum, according to an I-IBC Engineering, Inc. (I-IBC) field note. The IR results
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were dominated by reflections from the bearing stratum and are not presented herein. As a result,
0nly representative results from SE tests for these six tested piers arc presented in Figs. 1-6.
3.0 INVESTIGATION BACKGROUND AND SITE CONDITIONS
Olson Engineering, Inc. was contracted by The Beck Group to provide nondestructive quality
investigation services for the subject piers. The field work was performed on October 28, 1998 by
Mr. Ming JAu of our firm with help from Mr. Todd Scallorn of The Beck Group. These six concrete
drilled arc 42 inches in diameter with shale bedrock penetrations of between 14 feet to 37 feet. The
depth to the shale bearing stratum is about 15 feet from the top of concrete.
4.0 NONDESTRUCTIVE TEST (NDT) METHODS
Tho Sonic Echo (SE) and Impulse Response (IR) tests were used to determine the length of
the tested piers and to evaluate the integrity of concrete of these piers. The SE and IR methods are
discussed in Section 4.1 below.' -
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4.1 Sonic Echo (SE) and Impulse Response
(IR) Methods
Sonic Echo (SE),,Test Method. The
SE method is a iow strain integrity test
conducted from the top of the pier as shown
below. Test equipment hacluded a 3-lb
impulse hammer and receivers (accelerometer
md/or geophone) mounted on the pier top (or
side if the top was inaccessible) and a PC-
based data acquisition/analysis system. The
impulse hammer has a built-in load cell that
can measure the force and duration of the
impact. The test involves hitting the pier top
with the hammer to generate wave energy that
Sonic EchoBmpulse Response
So~c Echo ~d ~puNe Response Test
travels to the bottom of the foundation. The wave reflects off irregularities (cracks, necks, bulbs,
soil intrusions, voids, etc.) and/or the bottom of the foundation and travels back along the foundation
to the top. The receivers measure the vibration response of the foundation to each impact. The
signal analyzer processes and displays the hammer and receiver outputs, Pier length and integrity
of concrete'are evaluated by identi~ng and analy~ing tla~ arrival times, direction, and amplitude of
reflections measured by the receivers in time. The echo depth (D) is calculated by multiplying the
reflection time (t) by the compression wave velocity (V) and dividing ~_b_is quantity by 2 to account
for the fact that the wave has gone done and reflected back, i.e.. D = V't/2.
Impulse.Response(IR) Test Method. The IR method is also an echo test and uses the same
test equipment as the SE nmthod. The test procedures are similar to the SE test procedures, but the
data processing is different. Thc IR method involves frequency domain data processing, i.e., the
vibrations of the foundation measured by the receivers are processed with Fast Fourier T~n~£orm
(FVI') algorithms to generate transfer functions for analyses. Th~ coherence of the impulse hammer
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impact and aecelerometer receiver response da~a versus frequency is calculated to indicate the data
c~uality. A coherence near !,0 indicates good quality data. For piers in air or in relatively soft soils,
the coherence will typically only be near 1.0 at frequencies for which the mobility is non-zero. In
the IR records the linear transfer function amplitude is in inches/second/pound force on the vertical
axis (mobility) and frequency in Hz on the horizontal axis. Because of the md-like shape of a deep
foundation, reflections are indicated by equally spaced resonant peaks that correspond to modes of
vibration associated with the depth of the reflector. The inverse of the SE reflection time, t, is equal
to the change in frequency, nf, between the resonant peaks in the IR mobility plot. The reflector
depth is then calculated as: D = V/(2*,~f),
SE/IR Analyses, Analysis of the length determination and the integrity evaluation of a
foundation for both the SE and IR methods is based on the identification and evaluation of
reflections. However, test results are analyzed in the time domain for the SE and in the frequency
domain for the IR method. The reflections are shown as resonant frequency pe~k~ in the frequency
domain for IR test data. The two methods complement each other because the identificatidns of
reflections are sometimes clearer in either the time or the frequency domain.
The SE and IR test methods are sensitive to changes in the pier impedance (pier concrete area
* velocity * mass density where mass density equals unit weight divided by gravity), which cause
the reflections of the compression wave energy. Compression wave energy (h~mrner impact energy)
reflects differently from increased pier impedance than from decreased pier impedance. This
phenomenon allows the type of reflector to be identified as follows. Soil intrusions, honeycomb,
breaks, cracks, cold joints, poor quality concrete and similar defects (referred to herein as a neck)
are identified as reflections that correspond to a decrease in the pier impedance. Increases in the pier
cross-section or the competency of surrounding materials such as bedrock and stiffer soil strata
(referred to herein as a bulb) are identified as reflections corresponding to increases in the pier
impedance. A decrease in impedance is indicated by a downward initial break of a reflection event
in an SE record and frequency peaks positioned in a record such that a peak could be extrapolated
to be near 0 Hz in the mobility plot. Conve~ely, an increase in pier impedance is identified by an
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upward initial break for an SE r~flector and frequency peaks posi~ion~ct in an IR record such that a
trough could be extrapolated to be near 0 Hz in the mobility plot. When length to diameter ratios
exceed 20:1 to 30:1 for piers in stiffer soils/bedrock, the attenuation of compression wave energy
is high and bottom echoes are weak or unidenfifiable in SE/IR test results.
5.0 NONDESTRUCTIVE ~TIGATION TEST RESULTS
The velocity of the compression wave is used in predicting the d~pths of SE/IR reflectors,
Pier H-15 was used as thc reference pier with a known depth of 45.5 feet from inspection
observations, we backcalculated that a compression wave velocity of 15,000 feet per second (fps)
would result in a length of about 45.2 feet. Our experience has shown that most foundation concrete
has compression wave velocities around 12,000 to 15,000 feel: per second (fps), so the
bsckCalculated velocity of 15,000 fps for Pier H-15 is reasonable. Consequently, thi.~ value was ~¢d
to analyze the SE/IR results for ail 6 piers and it is estimated that predict, ed depths are accurate to
within about 5-10% of actual values. The Sonic Echo records are discussed below followed by a
discussion of the NDT results.
5.1 Sonic Echo/Impulse Response (SF_dIR) Test Records and Results
Representative SE test records arc presented in Figs. 1-6. Review of the SE results for Piex
H- 15 in Fig. 1 shows the top trace is a single acceleration record and the bottom trace is the average
velocity trace of 6 hammer blows exponentially amplified (the amplification curve is shown by the
smoor, h line that rises to the right) all plotted versus a time of 10 milliseconds (ms, 10~-3 seconds).
A comparatively clear bottom echo is shown at a time of 7,03 ms (0.00703 seconds) from the initial
response of the receiver to the hammer blow near 1.0 ms (0.001 seconds). This echo event
corresponds to a reflector depth of 45,2 ft which is about the construction length as discussed above.
All SE results were integrated from acceleration to velocity units, digitally filtered and amplified by
exponentially with time with a tau factor of 100 to enhance the identification of any weak bottom
echoes. Breaks and significant defects occurring above the pier bottoms would result in much
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stronger echoes at earlier times as compared to the expected bottom echoes. It is believed that the
small oscill~,tions shown in l::i§s. 1-8 were due to the effect of the beating stratum to the stress wave
propa§ation in concrete piers. The bearing stratum was located at about 15 feet below the top of
concrete, according to an I-IBC field note.
CLOST.
The field portion of this NDT investigation was performed in a~cordanc~ with generally
accepted test. hag procedures. If we can provide additional information or services on this project, or
additional information becomes available that would be usef'ul to this investigation, please call,
Respectfully submitted,
01son Engineering, Inc.
Ming Li.u
Project Engineer
~arry D.Nblson' ·
Principal Engineer
(1 copy foxed and 3 copies mailed)
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TABLE I
SON'IC ECHO (SE) RESETS
Shaft
Number
Pier H-15
Pier H-17
Pier H- 18
Pier E- 18
Pier N-9
Pier C-4
Predicted Construction
SE Depth Depth Reflector
,ft. (Fiz.) : ~ (from HBC ficld note) ~
45.2 45.5
(Fig. i)
Bottom
51.7 52.4 Bottom
(Fig. 2)
37.2 38,9 Bottom
(Fig. 3)
33,7 36.2
(Fig. 4)
Bottom
31.6 30.5 Bottom
('Fig. 5)
52.3 52.7 Bottor~
(Fig. 6)
Remarks
Reference Pier
Note:
The above predicted SE depths were from the top of concrete and were calculated based
on an average velocity of 15,000 fps which was obtained by using Pier H-15 as the
reference pier with a known depth of 45.5 feet, accordin§ to an I-IBC field note.
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