Locating underground utilities is becoming more of a challenge as the vast array of lines in the ground continues to increase. As the ground becomes more congested with utility lines, it’s vital to locate utilities accurately – gas, fiber, water, telecommunications and sewer – to ensure the safety of your crew and protect your bottom line.
The first step before digging is to contact your state’s One Call office and give them the location of the area to be excavated. A One Call contractor will visit the site and mark the locations of the utilities.
One Call contractors do a good job of marking existing lines and pipes. However, due to interference, the marks may not be as accurate as needed. In some cases, privately owned lines not known by the One Call contractor may not be marked.
Many contractors show up and begin excavating. That’s fine if the One Call markings are accurate. However, a mark could be 6 inches off and you may not find that line. Now you need to make a decision: Do you keep going or contact the One Call contractor and ask them to come back out?
Either way, it costs you time and money. However, if you have utility locating equipment, you can confirm the marks that are on the ground and identify underground facilities that could have been missed.
Understand methods
In locating technology, there are basically two methods: passive and active. Passive locating is used to check for unknown lines. The contractor sweeps the area with a receiver. This method detects any frequencies created by energized power and radio signals being radiated by utilities. However, it does not allow the operator to distinguish between types of lines.
Active locating involves searching for a specific line using either the direct connection or the inductive method. In the direct connection, the contractor connects a transmitter to the line or pipe to be located by hooking cable leads, similar to jumper cables, to an above-ground structure (for example, a fire hydrant in the case of a water main locate). This imparts an alternating current down the cable or pipe that can be detected by the receiver.
The only issue with this method is gaining access to facilities. For example, a user who is not a contract locator for the telephone company cannot legally open a telephone pedestal to clip onto the line.
To overcome this, contractors can use a locator with inductive capability. Here, the user sets the transmitter on the ground, and it emits a signal that induces a current in the pipe or cable. The receiver then can detect the target utility.
Where to start
The industry offers two types of locators – single- and multiple-frequency units – to accomplish active and passive locating. Each unit has its advantages and limitations. Single-frequency locators have been around for decades. These systems consist of a transmitter that is placed on the ground and induces a single high-frequency signal.
The signal is picked up by the underground line and then radiated outward so that the receiver can detect it. Single-frequency systems work well on lines and pipes in uncongested easements, but putting a high frequency into the ground in a congested area tends to “light up” everything underground, and that may produce a distorted signal.
In other words, the user cannot tell whether the signal comes from a power, gas or communications line. The other limitation is that most single-frequency locators cannot determine the depth of the line.
Since lines and pipes are constructed of different materials (copper, aluminum, iron, steel, etc.), the optimum locating frequency differs for each one. Multi-frequency systems (some offer up to five frequencies) allow users to tune the frequency being put into the ground to the type of line or pipe being located. A lower frequency reduces the chance that the frequency will jump from the target line to another nearby line. That makes it easier to distinguish, for example, a gas from a water line.
Some locators offer a current measurement index (CMI) that measures the current being put onto the line or pipe. This helps distinguish the type of utility, especially if it crosses over another line. It also helps users ensure that they are staying on the original target line and are not jumping to other lines in the area.
Most modern receivers can also estimate the depth of the line or pipe at the push of a button. The locator measures signal strength and uses an algorithm to convert the information to an estimated depth. Although interference may distort the depth estimate, the user still gets an idea of the line’s location before digging or hydroexcavating.
Changes on the horizon
While locator technology has changed little in the past five years, several manufacturers are adding enhancements.
Virtually all manufacturers are putting more output power into their utility locators. While this extra power does not enhance accuracy, it does provide a clearer and higher-quality signal, which is beneficial as more lines are being installed using horizontal directional drilling at greater depths.
Many manufacturers have also made advancements in receiver software to help filter out noise in the ground and air. The other trend is linking locators with GPS devices to enable mapping of utility lines with latitude and longitude coordinates. This provides accurate location for future reference.
While it’s important to select a utility locator that includes the technology necessary for the most common jobs, the devices also should be simple to operate. The simpler the operation, the less training users require. Ideally, operators should be able to use the unit after only a brief instruction, rather than a day-long training class. Operators may not use the unit for a week or more. In that event, they should be able to pick up the unit and recall how to use the unit without further instruction.
Durability is also important. Operators take the equipment in and out of their trucks every day and may not handle it with absolute care. A locator must be durable enough to handle jobsite conditions. Weather-proofing is a desirable feature.
Locators can cost $1,000 to $4,500. While that may seem like a large expense, it is an investment that can pay off simply by helping to prevent one bad mistake.
