November 2008: Feature Story

Locating Mission: Impossible
Utility Mapping Solutions to Aid the One-Call System
By Glenn Fox and Morgan Abele

It was clear at the beginning of the project in Aberdeen, Md., that locating this seemingly-undetectable fiber-optic utility would be no easy task. In fact, with only sporadic pieces of broken tracer wire left, it was impossible for the contract locate company to mark the duct for the One-Call system and ongoing construction in the area. The poor quality of the installation and materials of the duct — low-grade plumbing pipe — excluded the blowing or pushing of a new cable or trace wire through the duct. The conduits had become broken and caved in throughout this section of the system.

Initially, the telecommunication company hired a contractor to try to locate and repair the duct run. The contractor was never able to push a 200-ft rodder, a fiberglass snake used to trace out empty conduits or duct systems, or to push a wire or cable in a conduit, with the tracer wire attached to the outside of the fiberglass more than 100 ft due to the condition of the duct. The attached trace wire, which was connected to a piece of electromagnetic locating equipment, did not work well because it would get caught or disconnected from the rodder due to the condition of the inside of the duct.

The contractor would excavate and expose the duct at the point where the signal was lost and break into the duct to try the same technique to continue locating it. The point of signal loss was not always an exact location of the duct because, as it was later determined, there was an undocumented and unmarked electric cable in the ground paralleling and crossing the telecom duct, which affected the contractor’s locating. This was causing the contractor (who did not have an experienced locator on site) to hunt and peck for the duct. This method worked to a degree, but it was very slow and, consequently, very expensive. In many instances, they had to dig every 50 ft or so and begin the process again.

In an effort to speed up the process, reduce the cost and provide the owner with a locatable line, as well as supply them with an accurate map of the line, a representative from the telecommunication company consulted with Private Utility Locating Solutions Inc. (PULS) to see if it could offer any solutions for these unique circumstances. A meeting with all parties — the utility owner, the contractor, PULS and the One-Call locate company — was arranged, followed by a site inspection. A plan was developed by PULS that proved to significantly increase the speed at which the duct was located and mapped for future use. They proposed using a combination of technologies to determine the location of the telecommunications duct, including electromagnetic locating equipment (used with a traceable rodder) and Ground Penetrating Radar (GPR) supplemented with vacuum excavation.

With electromagnetic locating, there is a transmitter and receiver. Typically, locators connect the transmitter to a structure on the target utility — a phone pad, gas valve or water meter, for example. It sends a signal out onto that target utility and then, depending on the conductor, it will determine how far the signal will carry across the line.

“If everything is grounded correctly and it’s a good conductor — like a 1-in. copper pipe or phone cable — the signal will travel a good distance,” says Glenn Fox, Regional Director of PULS, Baltimore, Md. “But if the system isn’t grounded properly, the line is too deep, the soil is too dense, or if there are multiple utilities crowded into an area, it’s hard to track the signal. Then there is the frequency — low frequency is better on cables and high frequency is better on pipes, generally.”

The electromagnetic location portion of the investigation entailed pushing a traceable rodder into one of the four conduits until it could be pushed no farther. They had much better success with this type of rodder. The distance the rodder traveled varied from manhole to manhole, but sometimes the rodder could be pushed up to 500 ft. They were also able to induce a signal from the traceable rodder to damaged portions of the tracer wire in the duct, in some cases, allowing the location of the entire pathway between manholes. When an electromagnetic signal was lost, GPR and vacuum excavation techniques were employed. The terrain, soil and surface conditions dictated which technology was used, but typically, GPR investigation was the next step.

Electromagnetic won’t pick up plastic or concrete, so PULS switched to GPR, which is typically used for non-metallic utilities. GPR sends a signal into the ground and then relays the information back to the instrument that determines if there is something buried; with GPR, the bigger the pipe, the better the results. But the results of GPR are also dictated by the soil conditions. The system works well in sandy, loamy soil, but has trouble in clay and harder soils and areas with a high water table. The clay limits the depth of the signal’s penetration and the signal can also reflect off water.

In the locations where finding the duct exceeded the limitations of electromagnetic locating, GPR units located the duct. The technicians operating the GPR unit calibrated it in areas where the duct had been exposed, giving the technicians a signature signal to use as a guideline.

“None of these locating methods tell you that there is a gas line that’s 4-ft deep. It just tells you that something is there,” says Fox. “We have to trace it from known point to known point — like tracing a line from water valve to water valve. And then we can be relatively sure that we’re on the right line. The only way to know for sure is to dig and expose it. Typically, PULS only excavates when it’s required by the client.”

PULS used vacuum excavation as an exploratory method in this case. When the electromagnetic signal was undetectable and the GPR couldn’t penetrate the ground, PULS dug a slot trench to expose the line. It’s generally safer for the utility to use air vacuum excavation as opposed to hydro-vacuum excavation. Sometimes the water is so powerful it can cut a cable or cut the coating off a gas line.

“There are certain parts of the country where the soil is so unfriendly that hydro-vacuum excavation is the only choice,” explains Fox. “We’ve hit soil in central Virginia where the clay is so nasty that air didn’t do anything. So we’ll use water as much as we can and when we get closer to the utility, we’ll back off and use air.”

A slot trench was dug perpendicular to the expected path of the duct run. When the duct was found, the depth and location were recorded on a test hole form, which included information about soil conditions, condition of the utility, material type and size. PULS provided the telecommunications company with copies of all test hole forms. This data could now be incorporated into their permanent records.

To ensure that the duct run could be located quickly and easily in the future by a One-Call locator (or the utility owner), multiple means of finding the duct were integrated into the project. First, technicians mapped the location of the duct by taking measurements at regular intervals. A utility pole line paralleled the duct run on one side and a concrete jersey barrier wall in the centerline of the road paralleled the other side of the duct. These two linear structures were used as baselines and assigned stationing so that distances and offsets could be measured from them to the duct. Hand-held lasers accurately measured these distances from the poles and the jersey wall.

Next, locatable marker balls were installed at strategic points along the route via vacuum excavation. A core drill was used in locations where the duct was under the roadway to minimize the impact to the roadway.

This was also more aesthetically pleasing to the Highway Department. In locations where it was not practical to set a marker ball, flush-mounted surface markers were installed in the roadway and on the edges of the road at crossings.

PULS completed this project by transferring the information to an aerial photography/bird’s-eye view image of the area. These bird’s-eye view maps provided a quick, visual summary of the route of the duct and served as an easy reference map for future utility locators. This type of mapping provides a large amount of information in an easy-to-use format. For example, an 11- by 17-in. map summarized a 12,000-ft section of the project.

The big winner in this project was the telecommunications company. The original method employed by the contractor, which included digging blindly with backhoes in areas where they could not get a signal on the duct, was time consuming, expensive and dangerous to the safety of the duct run. The telecommunications company estimated it saved over 50 percent in direct costs, avoided additional cost resulting from damage to the duct, as well as other utilities in the area.

The key to the success of this project was not one single technology or process, but the result of careful planning and creative application of a series of different technologies and methods. PULS took the time to understand the problem, analyze available options and then formulate a plan that would meet the utility owner’s needs. This project proves that solutions to unusual problems can be resolved by a group effort, when utility companies, construction contractors and locating firms (both private and One-Call) all work together. The accurate mapping of underground utilities and the means to find the lines is often a combined endeavor and needs to continue along this path.

Glenn Fox is a Regional Director of Private Utility Locating Solutions (PULS) Inc., Baltimore, Md. He can be reached at glenn.fox@pulsinc.com.

Morgan Abele is Vice President of Private Utility Locating Solutions (PULS) Inc., Bethlehem, Pa. He can be reached at morgan.abele@pulsinc.com.