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Proceedings of Fifth International Symposium on Deep Foundation
on Bored and Augered Piles (BAPV), Ghent, Belgium, 8-10th Sept,
2008
1 INTRODUCTION
Cross-Hole Sonic Logging has been a common method of assessing
cast in place concrete founda-tions since the 1960s. It is now used
extensively throughout the world and on many significant
con-struction projects.
Its popularity can be attributed to two main factors firstly,
there is no depth limitation to the method and secondly, the
apparent ease of interpreting results, compared with low strain
type integrity tests.
However, if the equipment operator is not able to view all of
the raw data and select appropriate fil-ters, it can be possible to
come to the wrong conclu-sion and interpret results incorrectly. Is
has not been unknown for piles to be condemned and replaced,
because of problems with tube installation, such as:
Tube debonding Poor tube joints Joint wrapping Bent tubes
These can all appear to alter the first arrival time of the
signal, even though the true velocity of the signal in concrete
between the tubes is normal.
This paper investigates the ways of differentiating between true
pile shaft defects and tube defects when interpreting sonic logging
results.
2 SONIC LOGGING INTERPRETATION
Figure 1. Simple CSL Schematic
The principle of cross-hole ultrasonic logging is very simple,
in that it measure the time taken for a signal to travel from one
transducer to another, between tubes cast into concrete. The time
will depend on the distance between transducers and the material
be-tween the transducers. The further apart the trans-
Interpretation and Misinterpretation of Sonic Logging Test
Results
Williams, H.T. & Jones, I. Testconsult Limited, Warrington,
Cheshire, United Kingdom
ABSTRACT: Cross-Hole Sonic Logging (CSL) is one of the most
powerful methods of assessing the integ-rity and quality of cast in
place foundations. It offers many advantages over low strain
methods, in particular the ability to determine the vertical and
lateral extent of anomalies at any depth. In recent years 2 and 3
di-mensional tomography is being applied to results to present a
graphical visualisation of results, which are easy for engineers to
understand. In addition first arrival times can be automatically
picked from response signals. However, without understanding how
these new developments are created, there is a real danger that
results can be misinterpreted. This paper explores the causes and
effects of real and apparent defects in cast in place piles on
cross-hole sonic logging results.
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ducers and the lower the density of material, the longer the
transit time. In homogenous concrete, free of defects, the velocity
of sound is constant and in the order of 4000m/sec. Concrete
containing soil inclusions, bentonite, hon-eycombing etc has a
lower sound propagation veloc-ity. This means that measurements of
wave speed or transit time can be used as a non destructive method
of assessing the quality of buried concrete founda-tions. Anomalies
in the concrete are indicated by a change in signal arrival time or
amplitude. A typical signal is shown in Figure 2.
Figure 2: Typical CSL signal More recently cross-hole tomography
techniques have been used to produce both two and three
di-mensional images of the pile shaft. Anomalies are shown as
different shaded coloured area on a visual representation of the
pile shaft.
A waterfall plot is usually produced, which is effec-tively a
profile built up from modulated signals, taken from each test
level, see Figure 3.
Figure 3: Typical CSL waterfall plot
The wave speed of ultrasonic waves in concrete is given by :
V2 = E (1-)____ (Equation 1)
(1+ ) (1 2 ) Where: V = wave speed, = density, E = dynamic
modulus & = Poissons ratio
In practice, the transducers are placed in water filled tubes,
cast into the concrete. So the signal actually has to pass through
water/tube and tube/concrete in-terfaces twice on its journey.
First Cautionary Note: Each interface has the po-tential to
alter the quality of the signal, independ-ently from the quality of
concrete between the tubes the very thing we are trying to
assess!
2.1 What is a significant change in first arrival time
(FAT)?
Correlations between test results and exca-vated/cored defects
indicate that an increase in FAT of 20% or more is significant.
This corresponds to a 17% reduction in apparent signal velocity.
So, con-crete with a normal velocity of 4000 m/sec would reduce to
3320m/sec.
Reductions in FAT of less than 10% are not consid-ered to be
significant. This corresponds to a 9% re-duction in apparent signal
velocity. So, concrete with a normal velocity of 4000 m/sec would
reduce to 3640m/sec.
Reductions in FAT between 10-20% are of interme-diate
significance and the total number of profiles should be taken into
consideration. 2 and 3D tomo-graphy can be of assistance in
visualising the lateral extent of anomalies.
Figure 4: Significant mid-shaft defect found in a retaining wall
in New Zealand.
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Proceedings of Fifth International Symposium on Deep Foundation
on Bored and Augered Piles (BAPV), Ghent, Belgium, 8-10th Sept,
2008
Figure 4 shows 3 profiles from a test result showing a
significant increase in transit time. The FAT has increased between
57 76% indicating a significant defect over a vertical zone of
approx 1m. By pre-senting results alongside each other, it is
visually apparent that the anomalies are connected. Signal energy
has also decreased by approx 20 24 dB.
It is important to view the individual signal within defective
zones, to ensure that the automatic FAT calculation is taken from
the correct first arrival. Figure 5, below, shows a damped signal,
however the first arrival is still visible. It may be appropriate
to re-test a signal such as this with a higher signal
amplification.
Figure 5: Damped signal
2.2 Base Defects Figure 6 shows a result from a pile with
contamina-tion at the base. This type of defect tends to occur with
tremied piles cast under bentonite, when it is difficult to clean
the base.
The signal does not disappear suddenly, but gradu-ally increases
in transit time. This could indicate pe-ripheral contamination;
however with an increase in excess of 150% it is probably
significant enough to affect the whole of the pile section. This
was con-firmed by the other 5 profiles.
Figure 6. CSL profile of base defect in London
2.3 Measurement of Concrete Velocity and Bent Tubes
It is not uncommon for CSL to be used to determine concrete
velocity and hence give an indication of concrete modulus. However,
the test does not meas-ure velocity directly, it measures the
transit time be-tween the probes. Converting this to velocity
in-volves assuming a path length. The path length is only known
accurately at the top of the pile, where the distance between tubes
can be accurately meas-ured. In practice the tubes can bend over
the length of the pile, giving rise to gradual changes in
trans-mission time. If the path length is assumed to be constant,
then velocity calculations will be incorrect and misleading. If the
tube spacing is known, an ap-parent velocity can be calculated by
dividing the tube spacing by the transit time. It must be
remem-bered however, that this apparent velocity includes the water
and the tubes. It should also be noted that a signal travelling
around a void could yield the same velocity as one travelling
through a zone of low modulus material.
Figures 7a & 7b, show the results for a pile with bent
tubes. Figure 7a shows a maximum increase in transit time of 43%.
Assuming that the tubes are straight this would correspond to a
reduction in con-crete velocity of 30%, i.e. to 2800m/sec if normal
velocity is 4000m/sec. This would be comparable to much weaker
concrete. This is clearly misleading and could lead to the pile
being condemned incor-rectly. The corresponding profile shown in
Figure 7b shows a matching reduction in transit time. It is
unlikely that concrete properties have increased so
dramatically!
Figure 7a. Tubes bend out Figure 7b. Tubes bend in
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As a general guide if tubes are bent, signal transit time will
tend to change gradually. If the tube is se-verely bent or kinked,
it is unlikely the transducer will pass anyway. The increase may
also be matched by an opposing decrease by other profiles.
Increases in transit time caused by voids or contamination, tend to
appear more abruptly.
Second Cautionary Note: Calculation of concrete velocity from
sonic logging results should be treated with caution, and clearly
state they are ap-parent.
2.4 First Arrival Time versus Signal Energy Most modern CSL
systems have the ability to view not just the individual signal,
but also a modulated waterfall plot. A first arrival time (FAT) can
then usually be determined and plotted out against depth (on some
systems this is all that is displayed). The energy in the signal
can also be calculated by meas-uring the area under the curve and
plotted out against depth.
First Arrival time is generally considered to be the most
important measurement with CSL. For this rea-son it is important to
understand exactly where it is being measured. On modern digital
systems, the FAT is measured automatically. It does this by
set-ting two signal amplitude thresholds. The lower threshold is
set to ignore background signal noise. The upper threshold is set
to catch the first signifi-cant signal arrival.
A problem can occur if you are testing a large di-ameter pile or
diaphragm wall unit with a large path length, especially if the
system is not sensitive enough or emitter strength is insufficient.
Because of the higher signal to noise ratio, the selection of a
correct threshold is imperative for a correct FAT measurement. If
it is set too high, it will miss the true first arrival and falsely
indicate a problem.
Another reason for a low signal to background noise ratio is
tube debonding. Where tube debonding oc-curs, the actual path
length of the signal through concrete is unchanged. A small gap is
introduced be-tween the tube and the concrete, which effectively
reduces the amplitude of the signal.
Signal amplitude is therefore of secondary impor-tance to FAT
and cannot be relied upon on its own, as a measure of concrete
quality. It can however be used to back up FAT measurements. Figure
8 shows a sonic logging test result from a pile with tube debonding
over the upper 5m of pile shaft. Signal amplitude is clearly
significantly reduced, however the first arrival can be seen,
albeit very faintly on the waterfall plot.
Figure 8a. CSL result for pile with de-bonded tubes over the
upper 5m of pile.
Figure 8b shows the FAT and Energy plots using a correctly
selected low threshold, however Figure 8c shows the same result
with the threshold set too high. The signal at its most damped part
has been in-correctly interpreted as having a 72% increase in
signal transit time. On its own it could have lead to an incorrect
interpretation. For this reason it is much more reliable to assess
anomalous areas by viewing the waterfall plot and also the
individual signal.
Figure 8b. Good Threshold Figure 8c. High Threshold
Plastic tubes have a tendency to de-bond from con-crete more
readily than steel. They are also more prone to damage during
breading out. For this reason metal tubes generally give better
results. On deeper piles, plastic tubing may also suffer from heat
of hy-dration or pressure and collapse.
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Proceedings of Fifth International Symposium on Deep Foundation
on Bored and Augered Piles (BAPV), Ghent, Belgium, 8-10th Sept,
2008
Third Cautionary Note: Do not rely purely on plots of first
arrival against depth
2.5 Vertical Resolution Distance apart of readings
This varies from system to system. The smaller the vertical
interval between readings, then the smaller the defect you will be
able to detect, without stag-gering probes. Some systems take a
reading every 20cm, whereas other take readings every 1 or 2cm.
Whilst it is quite difficult to detect horizontal cracks in
concrete piles with CSL, due to signal skipping, with 1cm spacing
it is more likely you will detect some change in signal. With the
power and memory of current computers, testing and storing results
at 1cm intervals is no longer an issue and is probably best used as
standard.
2.6 Poor Joints? Screwed and socketed steel tubing is the best.
Welded joints can lead to transducers becoming stuck (very
expensive!) or unable to pass. Another problem that can be
encountered are wrapped joints. Site engineers in good faith may
wrap joints with densotape type material to ensure a waterproof
joint, unaware that the signal find it difficult to pass through
this interface. Figure 9 shows a CSL result from such a case. This
was clearly identified how-ever by the precisely spaced anomalies
coinciding with the joint spacing!
Figure 9. Pile with lagged joints at 3m intervals
2.7 What is signal skipping? As you would expect, signals tend
to take the short-est and easiest route wherever possible. If the
trans-ducers are aligned exactly on the same level as a very thin
crack, then the signal will simply travel up the tube a little and
through the good concrete above or below. Even if the probes are
staggered this will occur, although a slight shift in FAT and
signal en-ergy may be observed.
2.8 So how can thin cracks be detected? It is recommended that
low strain integrity testing is used if cracks are suspected. The
signal from this type of test is unable to pass cracks and is
travelling in the vertical plane rather than horizontally. This
does presume however that there is good access to the top of the
concrete.
2.9 When to Test? 7 days is the recommended minimum time that
con-crete should be left to cure before testing. However, assuming
that you are not relying on the test to measure concrete velocity
(which would be inadvis-able as discussed above), CSL can be used
as a comparative test and used to test concrete piles at 3 days.
This would be purely to check that signal tran-sit times are
constant and no changes exist. If that is the case then anomalous
areas are not likely to sud-denly appear. If however, an area of
increased FAT is measured, it would be advisable to re-test the
pile again after at least 7 days, during which time con-crete
strength may have improved.
2.10 Tube Layout what are you missing? The main drawback of CSL,
is the requirement to pre-install tubes in foundations during
construction (although in emergencies it is possible to core or
drill holes in concrete for testing). The layout and number of
tubes must therefore be chosen to suit the information that is
required by the engineer. For ex-ample, if 3 tubes are used and
attached equidistant to the reinforcement cage, then only 3
profiles are pos-sible and it is impossible to take a measurement
across the centre of the pile. This may be critical if the pile is
tremied, when core defects are more likely to occur. With 4 tubes,
6 profiles are possible, around the periphery and across the cores
which is why it is the most widely used configuration.
On diaphragm walls, the tube layout will again de-pend on panel
dimensions. It is recommended how-ever that tube spacing does not
exceed 1.5m to en-sure good strength signals.
The more tubes, the better the lateral extent of de-fects can be
determined.
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2.11 Tomography how useful is it? As a quick overview of where
anomalous areas are, tomography software is a useful tool. 2D
tomogra-phy can also give you a clearer idea of the lateral
ex-tent. However, they do not give an actual measure-ment of change
in FAT. To do this, you must be able to ideally view each
individual signal or if not a good waterfall plot. A simple FAT
plot is no good unless you are confident it picked correctly see
section 2.4 above.
First arrival time is king and should be the main ba-sis of all
interpretation and used to quantify the se-verity of defects along
with the number of profiles affected at the same depth.
2.12 Operator Error The most common operator errors are:
Starting tests with slack cables Carrying out tests without
transducers level Not topping up tubes with water on long piles
Pulling transducers up too fast
When commencing tests, particularly on longer piles, the slack
should be taken out of cables and held just in tension before
taking data. Figure 10 shows a result with approx 400mm of cable
slack, which is evidenced by a perfectly aligned modulated signal
at the base. The winch is moving but the transducers are not
lifting. This will produce a test profile longer than the actual
pile.
Figure 10: cable slack at base of pile
To ensure that the transducers are level, they should either be
lowered together to the base, or when at the base, the signal
should be viewed, and one trans-ducer raised and lowered until the
transit time is at a minimum (this is best done say 2m from the
base, in case of base contamination.
Many systems have a maximum speed that transduc-ers can be
raised. The signal acquisition time should be such that data is
processed and stored before the next signal is triggered. This is
governed usually by warning lights, however with much faster
computers being available, this is less of a problem. If warning
lights are ignored however, signals may not to stored or processed,
before the next one is acquired. This would lead to shorter
profiles than expected.
On long piles, the cables will displace quite a lot of water. It
is therefore necessary to top up the tubes being tested, before the
transducers reach the top. The signal will not transmit through air
and part of the pile profile will be lost if the water level is
low.
To ensure that the full length of the pile is tested, we would
recommend that the tube length is plumbed with a tape measure (if a
metal weight the same size of the transducer is used, it may
prevent jammed transducers). The tube top level and pile toe level
should also be determined. By comparing all read-ing it is possible
to confirm that the tubes go to the base of the pile, that the
tubes are not blocked and that the tested length is correct.
3 CONCLUSIONS
In this paper, the interpretation of cross-hole sonic logging
results has been discussed and potential pit-falls have been
explored. Incorrect interpretation can be caused by many factors if
care is not taken. Users should not rely too heavily on calculated
val-ues such as apparent signal velocity, automatically picked
first arrival time plots, signal energy plots and tomography
profiles. Whilst these do give valu-able additional information,
the severity of any anomaly should always be assessed mainly on the
change in first arrival time, so the original signal should always
be available for interpretation after testing. Where anomalies are
suspected, the possibility of this being caused by the tube
bonding, joints, loss of water, lagging, or bending should also be
consid-ered.
REFERENCES
Stain, R.T. and Williams, H.T. (1991) Interpretation of Sonic
Coring Results: a research project, Proceedings of the 4th
International Conference on Piling and Deep Foundations, Stressa.
Vol. 1, pp633-640.
Turner, M.J. 1997, Ciria Report 144, Integrity testing in piling
practice