Contractors that install underground power telecommunications cable often must work in easements already crowded with underground infrastructure, including buried gas lines. A critical element of any construction project that involves displacement of the soil is to avoid damaging existing buried utilities. Not only do accidental utility hits knock out service, delay work and add to project costs, but also they pose the risk of serious injury, even death.
Whether cable is to be installed by open-cut methods, vibratory plowing or horizontal directional drilling, it is critical to accurately locate and mark existing utilities.
It is the job of the nation’s One-Call centers to locate and mark utilities of member organizations. A locate ticket is initiated by calling 811 at least 48 hours before work is to begin. The call connects to the nearest One-Call center, which then notifies member utility owners to locate and mark their facilities.
The following are basic locating tools:
• Electromagnetic locators are compact handheld receivers and small transmitters. They are the most widely used equipment for locating buried utilities.
• Ground penetrating radar (GPR) locators are wheel-mounted models rolled across the site to identify locations of utilities and other buried objects.
• Backhoes, compact excavators and vacuum excavation equipment can be used to make potholes, which may be necessary to uncover utilities and visibly confirm their exact locations.
Consisting of a lightweight handheld receiver and small transmitter, electromagnetic instruments are by far the most frequently used locating tool. The operator walks above where utilities are expected to be, and the receiver locates underground pipe and cable by detecting magnetic fields created by electrical current passing through the lines. Information is displayed on a window at the top of the receiver.
“For communications cable and metallic pipe, the small transmitter is connected to cable or pipe and sends current through the line to create a signal, which is detected by the receiver,” said Matt Lumbers, line product manager, the Charles Machine Works Inc., manufacturer of Ditch Witch locators and underground construction equipment.
“For PVC pipe with tracer wire, the wire is energized by the transmitter to provide a signal that the receiver can pick up,” Lumbers said. “Receivers use different frequencies and modes to help identify different types of utilities. Typically current produces frequencies between 256 hertz and 200 kilohertz.”
Electromagnetic locators are accepted throughout the industry. They are accurate when correctly used and are relatively easy to operate. Their primary shortcoming: current flow is necessary to make locates.
The locator component is mounted on a wheeled platform and pushed like a lawn mower across the work site while generating electromagnetic pulses downward. The radar waves bounce off buried objects and reflect back to a receiving antenna, which displays a graphic representation on the unit’s screen. GPR locators can find plastic and cast iron pipes and other utilities that are difficult or impossible to locate by other methods. They also locate rocks and buried debris. Interpreting the screen display requires training and experience.
However, the biggest drawback to GPR locators is they don’t work in dense, highly compacted soils, and that has limited their utility locating applications.
Potholing—uncovering a utility to visibly confirm its location—should be done when a bore path crosses another utility and in situations when it is advisable to verify the exact location of a utility. On many projects today, project owners or local regulatory agencies require potholing for specific situations.
A common-sense practice for many years, potholing is being used with increasing frequency to precisely identify locations of utilities, but it was not a method used for One-Call locates; potholing is an extra step, and someone has to pay for every pothole. Cost per pothole varies by the method used to make it.
Hand labor is perceived as the least costly; however, most crews have a backhoe excavator on-site. Frequently, it may appear to be the fastest and least expensive alternative. For many years, backhoes and compact excavators have dug deeper potholes. However, these mechanical methods come with a greater risk of damaging the objects they seek to locate. Depending on the job, shallow potholes are dug by hand, but care must also be taken with this method to prevent damage the utility being uncovered.
The recommended potholing tool today is the vacuum excavator, which displaces soil by “soft” excavation with either high-pressure air or water. Most vacuum excavators used for potholing or compact models are mounted on trailers, and many directional drilling crews have one or more on-site to clean up drilling fluids that escape from bore holes.
The dual-purpose machines can quickly shift from vacuuming to excavation. Contract potholing specialists also are available.
Who makes locates?
Utility locates made through One-Call are made either by the utility owner/operator or a contract locating company. Clearly there are advantages for using locating specialists: the utility doesn’t have to train and pay crews to do the job. Effectively finding and marking buried utilities requires training and skill, and faulty locates continue to be a primary cause of damage to utilities (see box). Some utility owners also prefer not to delegate the important locating responsibility.
Wisconsin Public Service (WPS) is a utility that makes One-Call locates for power lines with company personnel.
“WPS is committed to performing accurate locates to uphold the safety of the people who live or work around buried facilities as well as making sure the critical services we provide are maintained,” said Pete Wurl, WPS manager of customer service. Wurl said that, in the summer months, WPS receives approximately 3,500 locate requests per week. In the winter, the number drops to about 500 per week. “Most communities contain multiple buried utilities such as gas, electric, telephone, cable TV, and water and sewer lines. Some utility owners and operators hire contract-locating companies. WPS uses its own personnel.”
WPS’s basic tool is the handheld electromagnetic locator.
During the peak construction period from April through November, when locating demands are high, Wurl manages 29 seasonal locators. Other WPS employees handle winter season locating.
Typically a WPS locating crew is only one person, but some situations may require two or more, Wurl said.
“Size of the area to be located, job complexity, signal interference, and traffic or safety concerns are conditions that may require more than one locator,” he said. “Time to complete a locate varies with each site and whether an area is urban or rural. Typically a locator can make 16 to 24 locates in a workday. Most are marked with flags and paint, but wooden stakes and plastic locate ‘whiskers’ also are used, depending on field conditions.”
Experience and training are essential to performing accurate locates, Wurl said.
“Keys to effective locating are understanding the theory of locating, using quality locating equipment, knowing what facilities are to be located and the routes they take, and being able to interpret data provided by the locating tools in order to make intelligent decisions,” Wurl said. “All WPS locator personnel are screened through the interview process for key attributes needed for locating; they then complete initial classroom training, followed by on-the-job training with a qualified and experienced locator. We also hold annual refresher training to provide the latest updates and share locating best practices with each other. Our continuing challenge is organizing and managing work in an efficient and safe manner while providing 100 percent accuracy.”
No matter how the locates are made or by whom, no construction project that involves excavation, drilling, boring, or other disturbance of the earth should be initiated until all existing utilities are accurately located and marked.