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Tracking a Storm Sewer

Survey lines over a storm sewer. Lines 1, 2 and 3 run perpendicular to the pipe. Line 4 runs along the pipe.

 

Overview

The current case study involved demonstrating how to use GPR to locate and track a concrete storm sewer. As this example shows, GPR can determine the orientation, depth, slope and diameter of a pipe. GPR's  advantage is greatest when pipes are non-metallic and not readily located with traditional locating devices.

 

Problem

The Dekalb College site in Georgia had no as-built records for many of the underground utility pipes and cables. The challenge was to ascertain where the storm sewer connected to a surface drain went. Surface observations suggested the exact opposite of what was discovered.

 

GPR Solution

During an Underground Focus Utility Workshop held at Dekalb College in Covington, Georgia utility locators were exposed to new technologies and practices in the industry.

The storm drain intake could be seen at the curb but the location of the storm sewer was unknown. Initial assumptions suggested that the pipe drained into a stream and marsh area some distance away.

GPR traverses around the intake located a large pipe. A series of GPR transects were carried out to track the pipe. Three transects perpendicular to the pipe (Lines 1, 2 and 3) are indicated on the photo at the top. The GPR response on the three transects are shown below.

The first surprise was that the storm sewer ran in exactly the opposite direction anticipated. It ran back under the driveway and not into the stream area.

To assess the drainage/slope, a transect was profiled along the pipe (Line 4 in the picture at the top). The pipe location was  determined from Lines 1, 2, and 3. The depth of the pipe along the traverse is seen on Line 4 in the figure below.

Line 4

GPR cross-sections parallel and perpendicular to the sewer. The depth of the pipe increased from right to left by about 3ft (1m). In addition, the reflections from the top and the bottom of the pipe were detected.


From the transect it was apparent that the depth of the pipe increased from right to left by about 3ft (1m). In addition, the reflections from the top and the bottom of the pipe were detected. The storm drain appeared to be concrete with no metallic structure. Using the reflection depth of the top and bottom of the pipe and assuming the pipe was air filled, the pipe diameter was estimated to be 36 inches (90mm).

The entire exercise of locating the pipe, marking the alignment, tracking the depth, and estimating its diameter took about 10 minutes.

 

Results & Benefits

The above stduy demonstrates the value of GPR for unraveling historical construction details without access to records. Some key benefits are:

  • The GPR was optimal for this application being easily maneuvered, compact and very portable for transport to site.
  • A very simple methodology was used that new users on the site very quickly learned and were able to replicate.
  • The high quality data allowed the drain's slope and diameter to be estimated.
  • The fact the drain ran in exactly the oposite direction from what was anticipated demonstrates the folly of working on assumptions.
GPR responses vary greatly depending on the target being sought and the host material. GPR response variability can be challenging to new GPR users. When learning about GPR, the best practice is to review several similar case studies to develop an understanding of variability. Check for other insightful information on the resources tab to learn more. Also use info@gprlocates.com  to tap into Sensors & Software's vast technical information knowledgebase.
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