GPR is an effective tool for underground inspection and quality control, the field of application of which is very wide and includes locating buried objects, detecting voids or voids, locating steel reinforcement in concrete slabs and also in archeology and environment. In this study, GPR is first calibrated in three different materials, concrete, clay and sand, and the behavior of radargrams is understood. So, GPR has been successfully used to determine the deformations of soil under bridge abutment slabs.
Non- Destructive Testing (NDT)
Ground Penetrati on Radar (GPR)
Using the reflections and the two-way elapsed travel time of the EM pulse, a cross-sectional reflection profile is generated. The profile can often be viewed in real time from the computer where the reflected EM data collection is taking place. Further understanding of the geomorphological background of the study area is required for completion.
Applications
Advantages and Disadvantages of GPR
Advantages
Under favorable conditions, a survey area on a football field can be measured in one day and finished the next. With GPR, the transmitter and receiver antenna can be placed on a cart or transported over the survey area, where data collection occurs immediately - requiring only one person for the entire data collection process. GPR can also provide accurate depth information, something that can only be roughly estimated with other geophysical methods.
Disadvantages
Due to survey site and soil composition, GPR data interpretation can be ambiguous and it may be desirable to obtain and integrate data from one or more other geophysical methods such as ER to determine the subsurface structure. GPR data is highly compatible with these other geophysical methods, and the ability to image the same subsurface with another geophysical method and supplement the GPR data can drastically reduce uncertainty in the interpretation of geophysical data. When maintaining access to the study area is a priority, the advantages of GPR's rapid discrete data collection is a major advantage over other geophysical methods and is likely to become a critical consideration when deciding which method to implement.
Objective
Layout of Report
As a general rule, the frequency of the antenna determines the penetration depth and the resolution - the higher the frequency, the better the resolution, but at the expense of the penetration depth. One of the important things to watch out for is the radar wave speed. This can be evaluated based on the overall dimensions of the plate (one-way or two-way plate).
There are hyperbolic reflections at the location of PVC pipe and the hyperbolic fit is done and the diameter of the pipe is determined. Also the depth at which the bottom of the pipe is located can be obtained from the radargram i.e. at 0.6m. The cross-section and top view of the test pit with the buried objects is shown in Figure 3.21.
Since the first survey was carried out at the edge of the test pit, only the reflections due to the PVC pipe are identified, as shown in Figure 3.26. The plan and cross-section of the test pit with buried objects are shown in Figure 3.28. The radagram was analyzed using the hyperbolic curve fitting method to determine the diameter of the objects.
In the radargram, the area where concrete blocks were present in the center of the test pit, no deflections/deviations were observed in the radargram, indicating that the object is very stiff like a concrete as shown in Figure 3.32. The same hyperbelt fitting technique was also used to determine the diameter of the cylindrical cube. The first survey of the setup is run along the left edge of the trial pit from west to east direction as shown in figure 3.36, where the interference of the objects is not present.
The same hyperbola fitting technique as mentioned previously was used to determine the diameter of the cylindrical concrete block. The diameter of hollow tubes can be determined by using the two-way travel time of the radar wave and the radar wave speed.
Introduction
Background
Since the plate thickness will be less than 0.3 m, a 1.6 GHz antenna was used as shown in Figure 3.13, as it gives good resolution at such a depth. It was also found that the bottom of the PVC pipe is located 0.3 m from the surface, which confirms the depth of the test pit. The objective of this study is to determine the changes in the radargram due to the change in soil density.
Calibration
Ground Penetration Radar (GPR)
- Encoder
- A/D converter
- Monitor/PC
- Control unit
- Antennas
The first step in choosing a GPR system is to understand the main components of the system. The Control Unit is the brain center of the GPR system and is responsible for coordinating the operation of the subordinate components. While the Control Unit performs the functions of the brain and Antennas are the legs, which do the job of sending out radar signals and receiving the reflected waves.
Basic working principles
The monitor or PC is used to visualize the GPR information in real time and to operate the system. Depending on the type of monitor, or if a PC is used, GPR data can be saved for later processing.
GPR antenna frequency description
Test procedure
Testing can be done by knowing the material velocity exactly from experimental investigation or by arbitrarily setting about three to four velocities given within the range, or else it can be obtained exactly if hyperbolic reflections are observed in the radargram. If hyperbolic reflections are observed, the exact speed appropriate for the test can be obtained in post-processing in software, regardless of the speed set during testing. As the project is started, a signal dialog box will appear on the right side of the monitor and the first peak of the antenna's signal should be set to zero level before start using a setting called "Adjust Signal Position" as shown in Figure 3.7 .
The radargram color palette and zero level can be adjusted by setting the display parameters as shown in figure 3.8. After testing is completed, the raw data should be copied to a removable disk from the automatically saved files as shown in Figure 3.9. Several corrections/filters related to background removal, gain function, migration, running average are implemented in the software, as shown in Figure 3.11.
The background removal filter can temporarily eliminate consistent noise from the entire profile, thereby revealing signals that were previously covered by this noise. The linear gain filter is applied to compensate for any damping or geometric dispersion losses. In addition to these filters, contrast of the image, hyperbola adjustment, color palette of the radargram, speed adjustment, depth and distance adjustments can also be done in the software.
Calibration
- Case 1
- Calculation of Rebar
- Case 2
- Case 3
Therefore, the actual diameter of the reinforcement can be determined based on whether the reinforcement is a main reinforcement or a secondary reinforcement. Later, the GPR survey was carried out 2 meters before the length of the compacted test pit to see the changes between the natural ground surface (GS) and the compacted GS using the GPR running from west to east and the data was incorporated into the software and the radargrams are analyzed. The calculated diameter of the steel pipe was 0.105 m, which is almost the same as that of the original 0.1 m pipe.
Testing was conducted from west to east of the pit using a 1.6 GHz antenna with a radar wave speed set at 0.15 m/ns. The diameters of the objects obtained from equations 1 and 2 are 13.5 cm and 14.1 cm, respectively. The test pit is divided into two halves by placing a row of concrete blocks in the middle of the pit.
A cylindrical concrete block has been placed on one side of the pit and an almost round stone has been placed on the other side. Considering the distance at which the concrete blocks were present (0.34 m), the radargram shows a sudden change in the reflectance compared to that of the sand layer. The diameter of the objects identified using GPR was successfully determined using the hyperbolic fitting technique with very minimal error.
Fig.4.1: Google Image of Bridge Site near Outer Ring Road, Hyderabad 4.2 Description of Bridge Approach Site. Along the GPR survey track, lightweight deflectometer (LWD) testing is performed to verify/quantify the modulus of the test sections.
Determination of Soil Deformations under Bridge Approach Slabs
Introduction
The GPR technique is used in determining subsurface deformations under bridge approach slabs over a section selected on NH-9 near the Outer Ring Road intersection at Patancheru, Hyderabad. The national highway (NH 9) and the bridge section are oriented east-west as shown in figure 4.2. The markers in Figure 4.2 represent the corresponding chains and can also be seen in the GPR studies.
The data obtained was post-processed in the software and the radargrams were analyzed to check for any ground deformations. There were ground deformations identified in the section from 15m to 17m and 83m to 91m at a depth of approximately 1m from ground surface. The deformation in the section 83m to 91m was identified as more compared to the latter case.
Also a settlement of 0.03m was identified in the pavement section from 0m to 17m compared to that of bridge section. The radargram showed similar kinks and distortions at the same chain locations as in the previous case, as can be seen in Figure 4.10. 271.08 MPa and 284.81 MPa at 0m and 100m respectively where no ground deformations were identified in the GPR radargrams.
Adjacent to the approach plate, where severe deformations were identified in the radargrams, very low deformation modulus was recorded, i.e.
Pavement Stretch description (from West
GPR
Deformation Modulus of Pavement
Conclusions
