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Discovering Voids and Delamination (sewer pipes)


Background

A contractor retained multiVIEW to carry out a ground penetrating radar survey in two underground sewer pipes. 

The primary objectives of the survey were to map the thickness of the pipes and to determine if any voids had developed in the soil/rock surrounding the pipe adjacent to the areas of recent pipe repair.


Methodology

These sites were within a poured in place concrete pipe (2.5 m inside diameter) in shale bedrock.
At each site the interior of the pipe wall was surveyed with a set of grid lines consisting of longitudinal lines along the length of the pipe and with a set of circumferential lines perpendicular to the longitudinal lines. 
GPR data were acquired with a 1 cm spatial sampling interval along the lines with a total time window of 25 ns, sufficient to image through the concrete and into the surrounding shale.
 

Findings

Figure 1 shows evidence of steel mesh at the back wall of the pipe or just outside the pipe and scattering hyperbolas at the surface consistent with cracks on the edge of the repaired section.



Figure 2 shows that the reflection event from the back of the pipe wall within the repair patch is relatively high amplitude (on line 1E). This could be related to a void at the back of pipe wall.