Knowing What’s Below: Buried Utility Location

We humans have put an awful lot of effort into our infrastructure for the last few centuries, and even more effort into burying most of it. And with good reason — not only are above ground cables and pipes unsightly, they’re also vulnerable to damage from exposure to the elements. Some utilities, like natural gas and sanitary sewer lines, are also dangerous, or at least perceived to be so, and so end up buried. Out of sight, out of mind.

But humans love to dig, too, and it seems like no sooner is a paving project completed than some joker with a jackhammer is out there wrecking the pristine roadway. Before the construction starts, though, cryptic markings will appear on the pavement courtesy of your local buried utility locating service, who apply their rainbow markings to the ground so that nothing bad happens to the often fragile infrastructure below our feet.

Call Before You Dig

By law, every public utility company in the United States must participate in a “one-call locator service.” The Federal Communications Commission established 811 as the phone number to access the once-call system, which allows excavators to request location and marking of buried services. Location services are paid for by the utility companies, so there’s no direct charge to the customer.

Anyone doing any sort of excavation is required to call (or now, submit a request online) to schedule a location service to mark the intended work area. This applies to any digging — while the damage a backhoe with 40,000 pounds of breakout force can do to a gas main can be catastrophic, a homeowner with a posthole digger for that new mailbox can be just as dangerous. If you’re going more than 15″ deep, you need to call, because if you cause any damage to underground services, you’re on the hook for it. If you live, of course.

Beep, Beep, Beep

Once a ticket is entered, a location technician will usually come mark the job site within a couple of days. The caller is required to mark the general location with white spray paint. The tech isn’t coming in blind — he or she will have maps that indicate rough locations for the major utility lines. But the world beneath our feet is surprisingly dynamic, and coupled with the possibility of there being facilities that were never recorded properly, that makes physical verification of line locations a big part of the tech’s job.

Location techs have a wide range of tools to locate that which can’t be seen. Given that most underground facilities have some kind of metallic component — older water and sewer lines, natural gas pipes, and the copper wire in electrical or telecom cables — electromagnetic tools get the bulk of the work done.

Detectors can be active or passive. Passive methods are usually used to locate facilities that already have an AC signal on them, like power lines or telecom cables. These can be trickier since the signal strength depends a lot on what’s going on in the circuit. Passive methods can also include magnetometry to detect weak magnetic fields generated by utility lines, and even ground penetrating radar to probe beneath the surface and build an image of the buried structures.

Active detection applies a current to an above-ground section of a utility line, like a water or gas meter. An AC signal is applied to the line by the transmitter, which turns it into an antenna. Utility companies sometimes even lay copper wire alongside plastic lines as an aid to active detection. Handheld receivers with highly directional antennas are swept over the ground to pick up the signal, allowing the tech to mark the location. Sometimes a transmitter is even threaded into a pipe and a receiver is used to trace the signal as the probe is advanced through the pipe.

The Consequences

So what can happen if you fail to call before digging? Chances are, nothing. But when something happens, you can bet it won’t be good:

So next time you’re trenching for a power line to your shed or even driving a ground rod for a new antenna, make sure you make the call that could just save your life.

[Featured images: Positively Naperville]

Filed under: Engineering, Featured