Ground Penetrating Radar to Detect Shallow Foundation Defects of Bridges
Summary
As
an important part of
the bridge, the bridge foundation is always prone to
defects, cracks and other bridge foundation diseases due to long-term
geological action and external factors. Considering that the traditional
ground-penetrating radar detection method—Common-offset method is difficult to
solve the problem of bridge foundation disease, a special Common-source method
is proposed to detect bridge foundation disease. Using GprMax software to
simulate the detection process of the bridge foundation; comparing the forward
modeling results of the normal bridge foundation and the defective bridge
foundation, it can be found that there are obvious differences between the
two. These differences can be used to identify bridge foundations with
diseases in the actual detection process, which proves that it is feasible to
use ground penetrating radar to detect bridge foundation diseases.
Key words : ground
penetrating radar ; bridge shallow
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Abstract
Due
to the long-term geological process and external factors, the bridge foundation
as a key component of bridge always tends to get some diseases. Considering
that the traditional common-offset method cannot detect bridge foundation
defect very well, this paper proposes the use of a special common-source method
to detect the bridge foundation defect. The forward modeling of the bridge
foundation detection is realized by GprMax. The differences can be found easily
by comparing the result of normal bridge foundation and that of defective
bridge foundation. The result obtained by the authors is helpful to identify
the defective bridge foundation, which proves that GPR is an effective method
for detecting the defect of bridge foundation.
Keyword : GPR ; shallow
bridge foundation defect ; GprMax ; forward modeling ; defect
detection
0 Preface
Bridge
safety is a very important topic in engineering construction. As a
non-destructive electromagnetic wave detection technology, ground penetrating
radar has also been widely used in bridge disease detection. In foreign
countries, the electromagnetic wave exploration technology started earlier,
after a long period of development, it is now very mature, and there have been
many successful cases in the detection of pipelines and steel bars in concrete
media and road safety detection ; Although it started late in China,
it has developed very rapidly, and good progress has been made in the inspection
of bridge quality and the detection of geological conditions under
bridges . In terms of forward modeling, GprMax software has
been widely used in the detection of steel bars and voids in concrete
media . Most of the bridge foundation is composed of concrete, which
is an important part of the bridge. In actual use, the bridge foundation is
easily damaged by water erosion, erosion and shear stress, and defects or
cracks appear. In the work of disease detection, due to the influence of
the side pier and the horizontal distribution range of the bridge foundation is
very small, it is difficult to detect useful underground information if the
Common-offset method is used to detect along the direction of the
foundation. Therefore, this paper proposes to use the Common-source method
to detect bridge foundations, and interpret the forward modeling results in
combination with wave field analysis, so as to understand the applicability of
ground-penetrating radar in detecting bridge foundations. Using GprMax
software, the process of ground penetrating radar detection of bridge
foundation is simulated based on time domain finite difference
method. There are many types of bridges, and there are also many types of
foundations. Here we only take the shallow foundation of small bridges as an
example to carry out corresponding research, hoping to benefit practical engineering
applications.
1. Forward modeling and detection methods
Combined
with the actual situation, the bridge foundation studied in
this paper is shown in Figure 1. The longitudinal length of the
bridge is 9 m, the width of the pier is 1.1 m, and the upper surface of the
bridge foundation under the pier is located 0.9 m underground. , the foundation
thickness is 0.6 m. The pier body and foundation are made of concrete with
a dielectric constant of 6.0 and an electrical conductivity of 0.001
S/m. The surrounding medium is wet sand with a dielectric constant of 20.0
and a conductivity of 0.1 S/m.
In
the forward simulation, the common-offset method commonly used in ground
penetrating radar detection is firstly considered to detect the target
body. However, due to the special structure of Qiaodun, we found that the
detection using the Common-offset method cannot well reflect the structural
characteristics of the underground target. For example, when the survey
line is arranged perpendicular to the direction of the bridge pier, the
Common-offset method cannot detect the foundation directly below the bridge
pier well; Moreover, the measured results are seriously affected by the
reflected waves from the side piers, making it impossible to identify useful
signals. In view of this situation, it is proposed to use the
Common-source method to detect the target body, and the survey line is arranged
perpendicular to the direction of the bridge foundation, so that the
transmitting and receiving antennas can be located on both sides of the bridge
pier, which increases the flexibility of detection.
 |
Figure 1 Perspective
view of bridge shallow foundation model |
2 Forward modeling results and analysis
Two
layouts of common-source survey lines are adopted. The first arrangement
is that the transmitting antennas are placed close to the pier, and the
receiving antennas are arranged at equal intervals on the other side of the
pier; the second arrangement is that the antennas are close to the pier, and
the receiving antennas are arranged at equal intervals on the same side of the
pier; The measuring lines are all perpendicular to the direction of the pier
and arranged in the middle of the pier. According to the structure and
disease of the bridge foundation to be detected, different detection methods
are selected. The frequency of the transmitting antennas used in this
paper is 300 MHz, and the wave form is the Reker wavelet.
2.1 Normal bridge model and forward modeling results
Two
different methods were selected to detect the normal bridge model in order to
compare with the simulation results of different bridge foundation diseases.
2.1.1 The receiving antenna and the transmitting antenna are not on the same side of the pier
Figure
2a shows the first measurement method, the measurement line is
perpendicular to the direction of the pier and arranged in the middle of the
pier. In the figure, O represents the transmitting antenna, which is
located on the left side of the bridge pier, and is placed close to the pier;
G1–G4 represent the receiving antennas, which are placed close to the pier at
the beginning, and then move to the right by 0.06 m for each measurement, with
a total of 51 measurements. The profile of the obtained forward modeling results is
shown in Fig. 2b.
The
abscissa rx in Fig. 2 b
indicates the distance between the receiver and the first receiver ( Fig.
3 ~ Fig. 6 are the same). The events in the figure have
different meanings. The apparent velocity of event 1 is the fastest, and
the arrival time is the earliest, so it represents the air direct wave
propagating along the air; the time of event 2 at the first receiving antenna
is the same as that of event 1, and it is a straight line, then event 2
Indicates the direct wave propagating along the surface; the propagation paths
corresponding to events 3~7 have been marked in Fig . — The
event formed by the diffracted wave generated at the interface of the ground
and the air propagates downward, is reflected at the base interface and is
finally received; event 5 indicates that the electromagnetic wave is reflected
at the base interface and directly The received reflected wave; the event axis
6 represents the reflected wave generated by the electromagnetic wave first
reflected at the lower interface of the foundation, and then secondly reflected
at the right side of the foundation. It can be seen from Figure 2a that its
horizontal propagation direction is different from other The
directions of the broken lines are opposite, therefore, the inclination
direction of event 6 is also opposite to that of other events; event 7
represents the reflected wave that is reflected multiple times when the
electromagnetic wave propagates to the lower left corner of the foundation and
is finally received.
 |
Fig.2 The
forward modeling model and results when the transmitting and receiving
antennas are not on the same side of the bridge pier |
2.1.2 Transmitting antenna and receiving antenna are on the same side of the bridge
The
second detection method is used to simulate the normal bridge model, and the
detection method shown in Figure 3 is adopted . O in the figure
indicates that the transmitting antenna is placed close to the right side of
the pier, with an offset distance of 0.06 m. G1~G4 indicate the receiving
antennas. The initial position of the receiving antenna is 0.06 m from the
right end of the transmitting antenna, and then it moves to the right by 0.06 m
every time a measurement is made. , measured 50 times in total, and the forward
modeling results are shown in Fig. 3b.
In
Fig. 3b , the apparent velocity of event 1 is the fastest, and the arrival
time is the earliest, so it represents the air direct wave propagating along
the air; the time of event 2 at the first detector is the same as that of event
1, and it is a straight line , then event 2 represents the direct wave
propagating along the surface; the other events correspond to the
propagation path in Fig . Event 5 represents the emitted wave from
the lower interface of the foundation, event 6 represents the reflected wave
generated by the electromagnetic wave passing through the lower interface of
the foundation and the right surface of the foundation in turn, and event 7
represents the multiple waves generated by the electromagnetic wave between the
upper interface of the foundation and the ground .
 |
Fig.3 The
forward modeling model and results when the transmitting and receiving
antennas are on the same side of the bridge pier |
2.2 Forward modeling and analysis of missing bridge foundations
2.2.1 Defect model and forward modeling results at the right end of bridge foundation
Figure
4 shows the schematic cross-section of the model when the foundation is
defective. Due to the influence of various geological effects, the bridge
foundation is prone to missing and incomplete diseases. In the figure, there is
a serious defect in the lower right corner of the bridge foundation. From
the bridge defect model, when the transmitting antenna and the receiving
antenna are on both sides of the bridge pier, the electromagnetic wave emitted
by the transmitting antenna may be well received after reflection, and if the
transmitting antenna and the receiving antenna are on the same side of the
bridge pier, the electromagnetic wave may be well received. Not getting good
results. Therefore, for the defects and diseases of the bridge foundation,
the first detection method is adopted, and the arrangement of the transmitting
antenna and receiving antenna is the same as that of the normal model
in Fig. 2a .
|
|
|
 |
Figure 4. Forward
modeling and results when defects appear in the lower right corner of the
bridge foundation |
Comparing Fig.
4 with Fig. 2 , it can be clearly found that events 5 and 6
in Fig. 4 disappear, and a new event 8 appears. The reason is that
due to the lack of the lower right corner of the foundation, the events 5 and 6
generated by the reflection of the electromagnetic wave at the lower boundary
of the foundation also disappear, and the electromagnetic wave is reflected at
the new boundary generated by the defect, and finally received to generate the
event 8, Its propagation path is shown in Figure 4a.
2.2.2
Defect model and forward modeling results at the left end of the bridge
foundation
Since
the foundation of the bridge is buried underground during the actual detection,
its damage is unknown, so it is necessary to simulate the model (the source and
the defect are on the same side) when the defect occurs at the left end of the
foundation. The cross section of the model and the layout of the survey
line are shown in 5 .
 |
Figure 5. Forward
modeling and results when defects appear in the lower left corner of the
bridge foundation |
By
comparing Figure 5 and Figure 2 , it can be found that the
events 1~6 in the two figures are the same. This is because the right end
of the bridge model in Fig. 5 is not damaged, so it does not affect
the propagation path of electromagnetic waves of events 1-6; however, due to
the obvious defect in the lower left corner of the model, event 7 disappears,
and the electromagnetic wave propagates to the bridge foundation The reflection
at the boundary of the defect, that is, the event 9, is a new reflection of the
electromagnetic wave at the boundary of the defect, and its propagation
path is shown in Figure 5a.
2.2.3 Analysis and summary of forward modeling results when the foundation is defective
Table
1 shows the comparison of the forward modeling results of the two
defective bridge foundation models and the normal model.
Through
the comparison and analysis of the above three forward modeling models, it can
be found that when ground penetrating radar is used to detect defects and
diseases of the bridge foundation, by observing the reflected waves generated
by the interface under the foundation, it can be extracted from the comparison
of Figure 2 b and Figure 4 b However, it should also
be noted that the similarity between Figure 2 b and Figure
5 b is actually very high, and it is difficult to extract useful
information. In the actual situation, we don't know which side the disease
appears on, so we need to detect the bridge foundation twice. Assume that
the bridge foundation has a disease as shown in Figure 4a, that is, a
defect appears in the lower right corner of the bridge foundation. For the
first detection, the transmitting antenna is on the left side of the
foundation, and the receiving antenna is on the right side of the
foundation, and the detection results shown in Fig. 4 b are obtained; The
detection results are practically the same as in Fig. 5b
. Comparing Figure 4b and Figure 5b , it can be seen
that the two detection results actually have very obvious differences, so
although Figure 2b and Figure 5b are very similar, we can
use Figure 5b to complete the b, and analyze the defects of
the underground bridge foundation.
2.3 Forward modeling and analysis of cracked bridge foundation
Bridge
foundations are not only susceptible to incompleteness due to geological
effects, but also prone to cracks and cracks under the action of shear
stress. The medium in the fracture is air, and the
cross section is shown in Fig. 6a, where the fracture width is 0.02
m. Considering the angle of the crack in Fig. 6a , if the
previous detection method with different sides of the transmitting antenna and
receiving antenna is used, useful signals may not be received. Therefore,
the survey line is rearranged, and the common-source method with the
transmitting antenna and receiving antenna on the same side of the pier is used
for numerical simulation.
Figure
6a is the model when cracks appear in the bridge. The layout of the survey
line during the forward modeling is the same as that in Figure 3 ,
where O represents the transmitting antenna, and G1~G4 represent the receiving
antenna. Figure 6b shows the forward modeling results of the cracked
bridge foundation.
Comparing Fig.
3 and Fig. 6 , it can be found that the events 1-7 are roughly
the same, while Fig. 6b generates an extra event 8, and the anomaly
appears at the horizontal position of 0.2-1.2 m and the propagation time of
38-42 ns. It can be judged that the event 8 is the reflected wave
generated by the electromagnetic wave at the crack, as shown by the
broken line 8 in Fig. 6a. At the same time, it is also noticed that the
low-frequency signal may not be sensitive to this slightly subtle structure due
to the small width of the crack. , the information carried by the event 8 is
mostly high-frequency information, so the width of the event 8 is relatively
narrow, and its strength is relatively weak; but in Figure 6b, it is enough
to judge the information of cracks in the bridge foundation. In
actual work, when cracks similar to those shown in Figure 6 appear in
the bridge foundation , the ground-penetrating radar Common-source method can
also be used to measure the cracks in the bridge foundation.
3 Conclusion
The
above two common-source detection methods are used to carry out numerical
simulations on the normal model, the bridge foundation defect model, and the
shear stress failure model. By analyzing the distribution of abnormal waveforms
in the profile and the differences in the electric field waveforms of each
channel, the bridge foundation can be determined. and the precise location of
the disease occurrence. By comparing the forward modeling results obtained
by the disease model and the normal model, it can be seen that there are
obvious abnormal signals in the bridge diseased area, which proves that GPR is
a feasible and effective detection method in detecting bridge foundation
diseases. However, in order to apply this method in practice, more
theoretical research and practical experience are needed to provide a basis for
future detection work.
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