Evolution of In-Pipe Ground Penetrating Radar
The defects discovered by the CCTV inspection were of the liner and not the actual concrete pipe. There was concern raised about the condition of the concrete pipe and reinforcements “behind” the liner. Subsequently, a need was recognized for utilizing a form of non-destructive method testing for evaluating the condition behind the liner. Morrison (2004) recommended the use of secondary assessment technologies during a pipe assessment program.
Several non-destructive evaluation methods were examined for suitability including: sonar; thermography; X-ray; laser profilometry; and ground penetrating radar. It was determined that ground penetrating radar (or GPR) was the most promising of the applications for looking behind the liner and into the concrete pipe. Furthermore, it was decided that it would be best if the GPR unit were placed on the CCTV robot and transported within the sewer line providing close contact with the lined pipe.
Field testing of a prototype unit was conducted in Tskuba, Japan in the fall of 2003 to determine whether readings could be made. Defects were manually created in the concrete behind sections of lined pipe. The first prototype, illustrated in Figure 5, performed well in the trials and was brought to Phoenix for a pilot project.
Figure 5. Prototype inspection robot with gpr unit
Due to the pipe size and potential flow conditions, it was determined to use a wheel robotic body rather than the track body used in the Japanese field tests. The second generation robot, illustrated in Figure 6, capture data behind the liner at pre-positioned liner defect positions along several pipeline sections. Unfortunately, a limitation was the fact that information could only be captured at the 12 o’clock position due to the fixed GPR unit.
The third generation robot, illustrated in Figure 7, solved any problems associated with clock face positioning through the use of multiple gpr units attached to two arms. This enables the unit to capture defect information anywhere along the 9 o’clock to 3 o’clock positions of a pipe section. An example of the corresponding ground penetration radar results is shown in Figure 8. The top results show the defect areas in relation to a folded view of the pipe, while the bottom results show a front view of the liner tears.
Tears in liner
Figure 8. In-pipe ground penetrating radar results