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112
TRANSPORTATION RESEARCH RECORD 1546
Condition Assessment of Installed
Timber Piles by Dispersive Wave
Propagation
SHUNYI CHEN AND Y. RICHARD KIM
Timber piles are widely used for supporting bridges, piers, wharves,
and other marine structures. As they age, it becomes critical that their
in situ condition be assessed so their remaining service life can be
evaluated. Current inspection methods involving visual examinations
and sounding tests are unable to quantitatively disclose a pile’s degree
of deterioration, depth of penetration, or remaining load-bearing
capacity. Years of exposure to wood-decomposing fungi and weathering may have substantially decreased a pile’s effective cross-sectional area, so that the pile can no longer function as originally
intended. A study was conducted in which nondestructive dispersive
wave propagation tests were applied to both laboratory pile models
and field timber piles. The laboratory models consisted of acrylic
cylinders having different wall thicknesses to simulate various levels
of internal deterioration and timber posts with drilled holes to simulate damage created by marine borers. Seven installed and two uninstalled field timber piles were tested. Laboratory experiments indicated that the dispersive wave propagation test is an excellent means
for evaluating the degree of hollowness and borer damage. Field
experiments further verified the applicability of the dispersive wave
propagation in finding the condition of the installed timber piles. The
approaches found to be promising for the condition assessment were
(a) phase velocity comparison between the first pass and the return
pass and (b) wave speed versus test location. The general trend of the
dispersion field in condition assessment is the higher the wave speed,
the better the condition.
Timber pilings are one of the most commonly used structures in
bridge construction because of the relatively low cost of the raw
wood and installation. These economic advantages are partially offset by the timber’s proneness to decay and deterioration from
insects, marine organisms, fungi, and weathering. As timber piles
age, it becomes critical that their in situ condition be assessed so
their remaining service life can be evaluated.
While there are a number of schemes to protect timber
piles from biological damage, a widespread practice is to simply
replace all the piles at regular intervals. A preferable criterion for
replacement should be based on a reliable evaluation of the structural integrity of a pile. Unfortunately, the current practice of
visual inspection and sounding does not provide a quantitative
measure of the pile’s condition. Inaccuracy in these approaches
has resulted in actual failure of the piling in service (safety consideration) as well as undocumented economic loss (economic
consideration), which can be a significant amount considering the
tens of thousands of pilings in service on the coast of the United
States.
S. Chen, 2775 Lexington Ave., No. 123, Roseville, Minn. 55113. Y. R. Kim,
Department of Civil Engineering, North Carolina State University, Raleigh,
N.C. 27695-7908.
Presented here are results from two research projects conducted
at North Carolina State University to determine whether dispersive
wave propagation methods can be used for assessing the condition
of installed timber piles. The techniques involve dispersive transverse and surface wave propagation tests to determine whether the
condition can be related to wave velocities, wave intensities, or signal’s changing spectral content.
In working toward a solution to this problem, several methods
have been proposed and carried out by different organizations (1–3).
These methods use a longitudinal wave. However, the application
of the longitudinal wave method requires either open access to the
top of a pile (1) or a modified approach to create a longitudinal wave
(2). As a consequence, the method is either costly or not feasible in
most cases because of the presence of superstructure. The dispersive
wave propagation technique was used in this study. The surface and
transverse wave groups traveling along the length of a pile resulting
from a transverse blow to the side of a pile are analyzed. Tests were
conducted using both laboratory models and field timbers. Dispersive data were analyzed by both the Fourier transform method and
the short kernel method (4–10). It is hoped that the findings of this
work will lead to an inexpensive method for evaluating a timber
pile’s condition. Such evaluations could then lead to early identification of questionable piles, less frequent replacements, and the
avoidance of possible dangerous structural failures.
USING DISPERSIVE WAVES TO EVALUATE
PILE CONDITION
The wave speeds of a dispersive signal depend on a pile’s material
properties and geometry. A change in its properties, as may be
caused by deterioration or changing cross-sectional area, leads to
different wave speeds for the different frequencies in a signal.
Whereas stress waves propagating through undamaged timber will
have characteristic wave intensities and wave velocities, damaged
timber cannot support stress wave propagation in the same manner.
For this reason, wave speeds of a dispersive signal can be used to
determine a pile’s condition. Fortunately, the dispersive surface
waves and transverse waves created by a strike (thump) on the side
of a pile will contain most of the energy put into the signal and will
be the easiest to record and study.
The equipment used in the dispersive wave propagation test
included a digital oscilloscope, accelerometers, power supplies, tools
for creating the signals, and a laptop computer for storing the data.
Field test setup is shown in Figure 1.
Chen and Kim
113
each hole was 6 mm with 13 mm equal spacing in the vertical and
horizontal directions.
Field Timber Piles
Nine timber piles, located in Kure Beach Fishing Pier, North Carolina, were tested for the condition study. Table 1 shows the field
records of the tested piles. Among these piles, seven are installed
piles with varying conditions, and the remaining two are uninstalled
piles. Of the uninstalled piles, one is an uninstalled good pile and the
other is a damaged pile that had been installed in 1984 and extracted
in November 1993. This extracted pile had been damaged severely
near the ground line by shipworms. Field investigation indicates that
the extracted pile does not have enough chromated copper arsenate
(CCA) treatment. The required CCA treatment, according to the
American Wood Preserver’s Association standard, is 40.0 kg/m 3
(2.5 lb/ft3) for timber piles that are used along the coasts of the
United States.
FIGURE 1
Field test setup.
MATHEMATICAL BASIS FOR EVALUATING
PILE CONDITION
The available digital signal processing tools used in this research for
wave propagation analysis are the Fourier transform (FT) method
(11–13) and the short kernel method (8–10).
SPECIMENS
Laboratory Specimen—Acrylic Cylinders
Internal deterioration in a timber pile cannot be easily detected
under visual inspections or sounding tests. Three hollow acrylic
cylinders with different wall thicknesses (3.2, 4.8, and 6.4 mm)
were used to simulate different levels of internal deterioration (hollowness). In addition, a solid cylinder was used to simulate a sound pile. All cylinders have the same length, of 1.8 m, and
the same outer diameter, of 38 mm. Their percentage of hollowness (H) was defined by Equation 1 for use when graphing the
results.
2
i
2
0
A
d
H (%) − (100) i = (100)
A0
d
(1)
where
Ai 5 cross-sectional area of the hollow portion of each cylinder,
di 5 diameter of the hollow portion of each cylinder,
A0 5 entire cross-sectional area of each cylinder, and
d0 5 diameter of each cylinder.
FT Method
Information extracted from the Fourier transform are wave speeds
and dispersion field. A dispersion field presents the relationship
between wave speeds and wavelengths. Wave speed at a particular
frequency is computed by
∆φ
C = (GL) 
⋅ T 
 2π
T =
1
f
Df 5 (f2 2 f1) 1 (n ? 2p)
A Douglas fir timber post, 100 mm by 100 mm by 5.5 m, was used
to simulate damaged timber. Transverse holes were drilled in the
middle of the post to simulate the damage created by marine borers.
The damage was made by drilling a total of 185 holes in the middle
of the wood in an array of 5 rows by 37 columns. The diameter of
(3)
and the corresponding wave length is computed by
λ = C⋅T = C⋅
1
f
where
Laboratory Specimen—Timber Post
(2)
C 5 wave speed,
GL 5 physical distance of two accelerometers (gauges),
T 5 period,
f 5 frequency,
Df 5 phase angle difference in two signals,
f1 5 phase angle obtained from the Gauge 1 signal,
f2 5 phase angle obtained from the Gauge 2 signal,
(4)
114
TRANSPORTATION RESEARCH RECORD 1546
TABLE 1 Field Records of Tested Piles
n 5 number of 2p jumps of the phase angle traveling from
Gauge 1 to Gauge 2 at a particular frequency, and
l 5 wavelength.
Short Kernel Method
The short kernel method (SKM), a digital signal–processing algorithm developed by Douglas (5–7) at North Carolina State University, can identify the entrance of individual phases. Information
from the SKM can be used to compute wave speeds and to construct the dispersion field. The SKM is based on the cross-correlation procedure described by Bendat and Piersol (14). Mathematically, a single value of the SKM at a particular frequency can be
stated as follows:
n −1
Sk ( j∆t ) = ∑ x[( j + i )∆t ] fk [i∆t ]
j = 0, 1, 2, . . . , N − 1
(5)
i=0
where
Sk
fk
x
Dt
N
n
5 SKM output at an assigned kth frequency,
5 SKM kernel,
5 real time record,
5 time step,
5 number of data points in x,
5 number of data points in fk.
method using different frequencies. Figure 3 shows the SKM
dispersion field indicating that each cylinder possessed a different dispersion curve. Figure 4 presents the relationship between
percentage of hollowness and SKM wave speed at a selected
frequency shown in Figure 3. On the basis of the pattern in the
data, a heuristic equation could be established in the following
form:
H 5 A ? C p2 1 B ? Cp 1 D
where
H 5 percentage of hollowness;
Cp 5 SKM wave speed; and
A, B, and D 5 real scalars determined by a curve-fitting procedure.
The percentage of hollowness can also be defined as the ratio
of the volume of the hollow portion to the total volume of
each cylinder. This definition is useful in field tests because
of the existing inconsistent hollowness along a pile’s longitudinal axis. However, the models and the equations developed
herein are valid for both the area- and the volume-oriented
environments.
A detailed description of how to perform SKM transformation, calculate wave speed, and calculate overall length by SKM can be
found elsewhere (6–10).
LABORATORY TEST RESULTS
Hollowness Assessment—The Acrylic Cylinders
Each acrylic cylinder was tested in the test configuration shown
in Figure 2, and the wave speeds were calculated by the SKM
(6)
FIGURE 2
Test setup—the acrylic cylinders.
Chen and Kim
115
phase angle difference (Df) between the first and the second
accelerometer locations caused by the Fourier transform’s inability to determine a unique phase angle above 2p radians (9,10). It
may be attempting to display a different limb of the dispersion
field.
FIELD TEST RESULTS
FIGURE 3
SKM dispersion field—the acrylic cylinders.
Damage Detection—5.5-m Timber Post
Figure 5 shows the bending wave test setup, and Figures 6
and 7 show the SKM and FT dispersion fields, respectively. In
Figure 5, Tests 1 and 2 provide data that were collected from the
undamaged and the damaged wood, respectively. In both Figures 6 and 7, the solid line is the dispersion curve obtained
from the undamaged section, and the dashed line is the dispersion
curve from the damaged section. Both analysis methods indicated
the same pattern: a given frequency’s wave speed is higher when
traveling through the undamaged wood compared with the
damaged wood. It is emphasized that in the FT dispersion field,
there is a region of dramatic variation in the vicinity of 4 kHz
(see Figure 7). This phenomenon is due to the uncertain Fourier
FIGURE 4
Two types of analyses were carried out for the installed timber
pile condition assessment. The fist type is called overall structural integrity analysis. The idea is that if the created wave can
travel down to a pile’s tip (toe) and back to the pile’s head again,
then the time record will have the first pass and the return pass
of the same wave. The variation of the wave speeds between
the first and the return pass may be able to disclose the structural integrity of the pile; more specifically, the overall structural
integrity in the region between the accelerometer and the pile’s
tip. In field tests, two accelerometers were spaced at a known
distance with the thumps made above the accelerometer locations.
Heavier hammers were used to create low-frequency signals.
These tests provided data for the computations of overall condition of the pile.
The second type of analysis is called local condition analysis.
Overall structural integrity analysis can indicate the average
condition of the pile that has been tested. However, the failure
of piles is quite frequently governed by local conditions that
cannot be measured accurately using the overall condition analysis. Local condition analysis is needed to detect severe local damage. In field tests, two accelerometers were spaced at a known distance and systematically moved down along the pile length. Light
hammers were used to create high-frequency signals. This series
of tests was used to determine whether internal flaws or deteriora-
Hollowness versus SKM wave speed—the acrylic cylinders.
116
TRANSPORTATION RESEARCH RECORD 1546
(Figure 8). The trends found from these two dispersion fields are
reversed.
The reason for the increase in return wave speed for the good pile
is not clearly understood. One possible reason is the effect of the
much smaller amplitude in the return wave on the strain level dependent modulus of materials. That is, the smaller strain levels in the
return wave may have resulted in a higher modulus and therefore a
higher wave speed.
Overall Structural Integrity Analysis—Installed Piles
FIGURE 5
Test setup—the timber post.
tion existed by comparing the wave speeds at different testing
locations.
Figures 10 and 11 show the SKM dispersion fields that were
computed from the two time records of installed piles. Pile 7 in
Figure 10 was in very poor condition at the time of testing
and extracted several months after testing. Inspection after the
extraction revealed severe hourglass with a 152-mm-diameter
internal hole. Figure 10 displays 8 to 16 percent reduction in the
return wave speed compared with the first pass wave speed. On the
contrary, Pile 9, which has been identified as a sound pile, has a
wave speed increase of 0.1 to 5 percent (Figure 11). The trends in
dispersion fields found from these two piles are reversed, which
coincides with the results obtained from the uninstalled timber
piles.
Overall Structural Integrity Analysis—Uninstalled Piles
Local Condition Analysis—Installed Piles
A comparison was made between a new uninstalled good pile (Pile
3) and an extracted damaged pile (Pile 4). Test results indicate that
a severely damaged pile will have a significant wave speed reduction in the return pass. Figures 8 and 9 show the SKM dispersion
fields that were computed from both the good and the damaged
piles’ time records, respectively. The damaged pile has a wave
speed reduction in the range of 28 to 35 percent in the selected frequencies (Figure 9). On the contrary, the good pile has a wave speed
increase in the range of 0.1 to 21 percent in the selected frequencies
The local condition assessment was performed on a good pile (Pile
1) and a damaged pile (Pile 2) by moving the test area systematically
from the head to the tip. The results are presented by the SKM dispersion fields in Figures 12 and 13.
In Pile 1, the wave speed comparison between the highest and the
lowest test locations indicates a wave speed reduction of 9 to 14 percent (Figure 12). Pile 2, with an hourglass damage in the lowest test
location, indicates a wave speed reduction of 33 to 36 percent
between the highest and the lowest locations (Figure 13). Interpre-
FIGURE 6
SKM dispersion field—the timber post.
Chen and Kim
117
FIGURE 7
FT dispersion field—the timber post.
tation of the results from the installed piles is more complicated
because the lower portion of the pile is saturated with water, which
also contributed to the wave speed reduction.
Assuming that most of the wave speed reduction in Pile 1 is from
the effect of water, the additional wave speed reduction in Pile 2 can
be attributed to the effect of damage. The reduction of wave speeds
in two different test areas is mainly due to the change in diameter,
water content, and damage. To accurately represent the condition of
the tested pile due to damage, the effects of changing diameter and
water content need to be corrected. A correction procedure has been
developed by Chen (10) for adjusting wave speeds in sections with
FIGURE 8
pile.
varying diameters to the wave speed of a reference diameter. Chen
also indicates, in the laboratory, that significant wave speed loss
occurs from the presence of water. The procedure for correcting the
water effect, however, is more involved and is currently under
investigation by the authors.
CONCLUSIONS
Dispersive wave propagation tests were found to be a promising
means of evaluating the degree of hollowness by the way in which
SKM dispersion field, first pass versus return pass, Pile 3—good
118
TRANSPORTATION RESEARCH RECORD 1546
FIGURE 9
SKM dispersion field, first pass versus return pass, Pile 4—bad pile.
their individual dispersion curves shift. A heuristic equation relating the degree of hollowness and wave speed was derived. Simulated borer damage to a 5.5-m timber post was successfully detected
by the SKM dispersion field and the FT dispersion field, with the
SKM analysis demonstrating more consistent results.
Two different experimental approaches were found to be promising for field condition assessment: (a) overall condition analysis by
comparing wave speeds between the first pass and the return pass
and (b) local condition analysis by comparing wave speeds at different locations. Test results, from both uninstalled and installed
FIGURE 10
poor piles, indicated that the return wave speed decreased significantly compared with the first wave speed. The second approach
revealed that the wave speed determined from the areas with more
damage was lower than that of sound areas. The condition of the
embedded portion, however, can be detected only by an overall condition analysis approach.
The determination of the degree of damage of the installed pile
requires a correlation between the laboratory-developed heuristic
equations and the field-collected data, which is currently under
investigation by the authors.
SKM dispersion field, first pass versus return pass, Pile 7—bad pile.
Chen and Kim
119
FIGURE 11
pile.
SKM dispersion field, first pass versus return pass, Pile 9—good
ACKNOWLEDGMENTS
This paper presents results of two research projects conducted in the
Civil Engineering Department of North Carolina State University.
Principal funding was provided by the Sea Grant program of the
National Oceanic and Atmospheric Administration. The authors
FIGURE 12
good pile.
acknowledge the important contribution of Robert A. Douglas of
North Carolina State University in providing many helpful suggestions and discussions. The authors also acknowledge the valuable
assistance of J. Darrin Holt, FDH Inc.; Spencer M. Rogers, North
Carolina Sea Grant Marine Advisory Services; and Mike Robertson,
Kure Beach Fishing Pier.
SKM dispersion field, wave speeds at different locations, Pile 1—
120
TRANSPORTATION RESEARCH RECORD 1546
FIGURE 13
bad pile.
SKM dispersion field, wave speeds at different locations, Pile 2—
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