June 1, 2008In any utility installation, accurate prior designation of existing utilities is a critical component to properly costing and safely executing the work. If you have the appropriate level of information on the existing utilities in your path, the job is most likely to succeed on time, on budget and safely.
This is why ground penetrating radar (GPR) is an excellent tool to enhance the financial performance and safety on your jobs.
In recent years, use of GPR for utility designation has steadily gained traction and this trend will continue. With GPR’s ability to identify many types of metallic and non-metallic structures in favorable conditions, utility installation companies, private locators and subsurface utility engineering (SUE) firms are successfully deploying GPR and improving the utility construction market. In most cases, and as recommended, GPR scanning is being done in conjunction with other utility designation methods such as the electromagnetic and radio frequency locating that is being provided through private locators, in-house locating crews and state and local one-call arrangements.
The following are two instances of potentially devastating utility hits that were avoided by using GPR to double-check the marks left by local utility owners. In both cases, GPR provided the horizontal directional drill (HDD) crew with critical information to avoid a potentially explosive natural gas line strike and an otherwise costly and disruptive sewer main break.
Not a Lucky Shot
X-Zach GPR Services, a Florida-based locating firm headed by William Morgan, regularly provides GPR locating to some of the area’s HDD crews. The project was a seemingly simple one: Shoot a conduit across a four-lane road. The local utilities had already been out and their lines were marked. Everything seemed in order.
A few days before the job was scheduled to begin, Morgan was working his typical routine of clearing the driller’s entry, exit and path with GPR. After the entry and exit points were cleared, Morgan focused in on the drill path, checking the existing marks and noting depths with GPR.
While most of the existing marks in the road checked out, Morgan noticed an unmarked target near the center of the road. Suspicious of this unmarked target, Morgan proceeded with a few more GPR scans and made preliminary marks, which revealed the target as a consistent linear feature. Because the GPR data indicated reflections typical of a pipe line, Morgan notified the drill crew of the potential conflict. After conferring with the project lead, who again contacted the utility companies with lines in the drill path, the local gas utility was brought back out to investigate further. Sure enough, the unmarked target was positively identified as a gas line that was not noted on any drawings and untraceable with standard electromagnetic locating tools.
In this case, GPR was essential to avoiding a potentially devastating situation in which the unmarked line was directly in the path and at the same depth of the proposed drill shot. By using GPR to enhance the locate data provided through the standard channels, this job was not a lucky shot, but an excellent example of due diligence and foresight by the drill team.
The Bus Stops Here
In a second example involving X-Zach GPR Services, GPR was used to identify the horizontal and depth positioning of an untraceable sewer main. The job was a conduit installation under a road and the staging area for the school buses in front of the local public school. Again, a seemingly simple job, but there were complications.
When Morgan arrived at the jobsite to locate with GPR for the drill crew in the days before the job, locators from the sewer department were just leaving. Locating the sewer had been little more than guesswork because the sewer was untraceable with the electromagnetic instruments. The sewer department did mark a line according to its drawings, but as the GPR quickly revealed, there was more to the story.
Working through his routine, Morgan began scanning the drill path. As he approached the painted marks left for the sewer main, the GPR picked up not one, but two lines at different depths and more than 3 ft from the sewer department’s marks on the ground. With this new information, Morgan notified the drill crew, which had the sewer department confirm the placement of both of their lines — the second line had been installed many years after the first and was never noted on any drawings — and revised the drilling plan to avoid the conflict.
While avoiding this particular incident seems innocuous, consider the potential problems that could have resulted had the sewer been breached and left undetected for a period of time. There are a number of potential health and safety problems that could have arisen, but the point is that they were avoided by using GPR.
Another Tool, Another Job Done Right
As demonstrated in these two examples, GPR was essential in successfully executing the installations. By itself, GPR will not solve all of the potential conflicts on any given job. But when used in conjunction with existing locating tools and services, it does have a positive impact and that is worth investigating.
Peter Masters is an application specialist with Geophysical Survey Systems Inc. (GSSI), based in Salem, N.H.