Brooke Wisdom | September 3, 2010

What lies beneath

Calling 811 before you dig is the law of the land and a good system, but there’s no reason why you shouldn’t double check all your One Call locates.*

By Tom Jackson


In Texas this past June three construction workers were killed and 11 injured in the span of two days in two separate accidents when the heavy equipment with which they were working hit natural gas pipelines. Investigations indicated that One Call procedures had been followed.

By calling 811 and waiting for the utilities to mark their lines contractors are in most cases absolved from legal liability. But legal absolution doesn’t bring co-workers back from the dead.

Excavation and underground operations have never been safer thanks to efforts by government and industry to improve utility damage prevention. But while the risks are coming down, the fact remains that just one accidental utility strike can injure or kill employees or put your company out of business.

Guilt by association

Safety is the first and foremost reason contractors should double check their locates with their own equipment. But you should also be aware of what an accidental utility strike can do to your public image.

“If you hit a telephone or fiber optic line that cuts off service to a business, that company’s revenue is gone for that day. All they know is the contractor with his name on the truck is the guy who cut off their telephone service,” says Matt Manning, a locator specialist and product manager for electronics at McLaughlin. “They don’t understand that it wasn’t painted correctly.”

What the company calls “smart” transmitter technology along with improved noise reduction allows the McLaughlin Verifier G2 great accuracy in line location and depth calculations. Features include peak and null locating modes, four active frequencies, depth readout, current measurement index (CMI), passive radio search and new data storage and time stamp capability. The depth capacity has been increased to 30 feet.

Another reason contractors should own and use locating equipment is to avoid the delays and disruption caused by mismarked or undetected utilities, says John Bieberdorf, senior product manager for electronics at Ditch Witch. “Ultimately this efficiency is going to save them money. The recommended best practice is to pothole, or visually uncover utilities by hand digging or vacuum excavation. Designating the depth and an accurate centerline with your own equipment can often save hours of work. And in most cases the One Call locators are not going to give you depths,” he says.

The limitations of One Call

While the vast majority of people who work for locating companies and One Call centers are diligent and skilled professionals, there are limitations to what they do. Knowing what these limitations are and being able to supplement their work with your own locating equipment gives you and your crews a much bigger margin of safety.

The locating companies do their best to put the paint marks directly over the utility in question. But there are big fudge factors built into this.

“In reality what a locator does isn’t locating but designating, meaning they designate the approximate area within 18 to 24 inches of what they believe is the centerline of the pipe,” says Bob Ringwelski, a business development manager in Caterpillar Electronics. “In some states they are restricted by law from giving depths, and most locating companies aren’t interested in giving depths anyway because it increases their liability.”

0.5to1 Estimated percentage of a project’s total costs that SUE survey and mapping will require.

Note, too, that locating companies only mark the utilities for the facility owners who are members of the One Call. This sometimes leaves out a number of underground utilities. “Sewer laterals and abandoned utilities are the two big ones,” says Keith Sjostrom, senior project engineer for Vermeer. “It depends on who owns the line.”

Utilities owned by government agencies also pose special challenges. “If it’s a federal facility, it may not want people knowing where its communication line is,” Bieberdorf says. Many locating companies also only locate and mark utilities in public right of ways, not on private property, he says. So if you’re digging on private land, depending on local codes or regulations, you may need to hire your own locating company or do the job yourself.

Also be aware that in places where there is new construction, – an unfinished building for example – the utilities may not yet be activated or listed on the maps the utility companies provide to locating companies.

Contractor mistakes

Another way error can creep into the utility locating process is when contractors fail to do their part right.

The Ditch Witch 950R/T pipe and cable locator system transmits via direct connection, induction clamp, or induced broadcast signals. In the passive mode the receiver detects 31-kHz cable TV and 50/60-Hz power lines as well as re-radiated radio frequencies. Put it in beacon mode to detect signals from optional beacons to detect non-metallic service lines.

“There are complaints on both sides, but the biggest miscommunication happens when contractors don’t go to the jobsite prior to calling in the ticket,” says Manning. “Someone in the office calls in the ticket, but they don’t know what the jobsite looks like. When you are at the jobsite you can make sure you have the right address, that you’re telling them to locate on the right side of the road and you know the nearest intersection. Afterwards, you can make sure they paint the right area. If you’re working in someone’s backyard, you can make sure the locator knows this. They may assume you’re only working in the right of way.”

Contractors should also understand that locating is an art, not a science – or at best an inexact science. “That’s why they have up to a 2-foot tolerance on either side of the line,” says Bieberdorf.

3.5 million  Number of miles of buried utilities in the United States.

That’s not to say that the utility and pipeline industries aren’t trying to make this science more exact. Over the past two decades a new engineering process called Subsurface Utility Engineering (SUE) has been developed that prescribes the steps to be taken for reducing the risk on the site and depicting the level of service the contractor has asked for. On projects with a lot of underground complexity or uncertainties you can get firms that perform SUE to go over your site with an array of high tech locating devices and techniques. This will give you a much more detailed and accurate picture of the utilities and obstacles underground than any one locating device or technique.

Contractors can also help their case by establishing clear communications with the locating company. Locating companies are required to respond to 811 requests within 48 hours of the call, but if you don’t intend to start digging on a site right way, let the One Call service know so they can better prioritize their schedules.

When locates don’t match

What do you do if:

• your own locating equipment detects underground lines other than where the paint marks indicate, or

• you pothole or hand dig to expose a utility and fail to find it, or

• find it where it wasn’t marked?

Shown here in the Standard Cart Configuration, the GSSI UtilityScan GPR system is compatible with five antennas and offers depth penetration from 10 inches to 18 feet. It sports 20-inch front and 24-inch back removable wheels and an integrated survey wheel encoder. The 8.4-inch color display features 800x600 resolution and 64K colors for clear visibility in sunlight. A Rugged Cart System is also available for harsher applications.

First you call the One Call or locate company. The law doesn’t say you have to call the locate company, Bieberdorf says. “But if they’ve mislocated, they’ll want to know it. It’s their business to be as accurate as they can be,” he says.

Don’t just assume you’ve done a better job than the locate company. There could be a lot of explanations. It might be that there are several utility lines buried in a common trench and you – or the locating company – have only located one of them. It could be that you or the locating company has mistakenly identified an abandoned utility, and the live utility is still somewhere on the site.

Despite the pressure to get the job going, when presented with conflicting information you have an obligation to your crews and customers to get these questions answered before proceeding.

Selecting the right equipment

There are two basic types of locating devices – electromagnetic and ground penetrating radar. Electromagnetic locating uses a two piece system: a box-like transmitter which puts out the signal and a hand-held receiver that you wand over the estimated location to pinpoint where the signal is coming from underground. There are three ways to induce the signal:

1. Direct connection. The most accurate method is to make a direct metal-to-metal connection to the utility line with the transmitter. You first ground the transmitter with a stake pushed into the earth and then clip a wire from the transmitter to any exposed metal on the above-ground portion of the utility line. With direct connection you can use lower frequencies and power settings, which reduce the chance that the signal will bleed over to an adjacent utility or bounce off fence posts or other obstacles and lead your transmitter astray.

$4.62    Amount of money saved through productivity and safety enhancements for every $1 invested in SUE services.

2. Induction clamping. If you can’t get a direct metal-to-metal connection you can use what’s called an induction clamp, a device that wraps around the above-ground termination point of the utility and puts out a higher power, higher frequency signal you then follow above ground with your receiver. But because you’re using more power and a higher frequency the chance of bleed over is increased.

3. Passive induction. If the utility in question does not offer access for a direct connection or induction clamp, and then try passive locating, or “dropping the box.” In this method you put the transmitter on the ground above the utility line. It blasts a high power, high frequency signal into the earth that should in most cases get picked up by the buried utility. But since this method uses the most power and highest frequencies, it is the most susceptible to bleed over. Another way to passively locate objects is to tune your receiver to a naturally occurring frequency, such as the 60-Hertz (Hz) pulses electrical lines put out, and follow the utility that way. In such cases you don’t need a transmitter at all. There are also a lot of steel gas lines underground that use 120-Hz cathodic protection that can be traced passively.

“A higher frequency can get you to the point where everything lights up,” Bieberdorf says. “It can be hard to pinpoint one specific cable. Every cable in the area gets the signal. As you get into lower frequencies, that’s where you start narrowing down to find one cable as opposed to multiple cables in a congested area.”

Professional utility locating personnel carry with them an arsenal of electromagnetic, or EM locating devices. For contractors, multiple devices might prove impractical. So what’s the solution?

Schonstedt TraceMaster II offers several options. The top of the line model has four active frequencies, two passive frequencies and an optional sonde or cathodic frequency with a radio link to change frequencies from the receiver. The screen shows directional arrows, signal strength and depth readouts plus continuity or good connection indications. It has a practical operating depth of 10 to 15 feet and can locate utilities up to 20 feet underground. Lower cost versions offer a combination of features including four manual frequencies, single frequencies, passive frequencies only and no radio link.

“The first thing we ask the contractor is what utilities they want to find. Then you can hone in on specifics,” Bieberdorf says. “Are you crossing power lines, communication lines, gas lines? There are specific locators that can fine tune your ability to find those more precisely,” he says. “If you’re a general contractor who wants to find everything, then I’d recommend a multi-frequency locator that has depth estimate capability.”

“You want to make sure the locator has multiple frequencies and passive and inductive capabilities,” Manning says. Unless you’re working for a specific utility, you will not have access to power transformers or telephone and cable TV pedestals. Without this access, direct metal-to-metal connections are not possible, forcing you to rely on passive or inductive methods.

It’s also a good idea to look for a locating device that’s rugged enough to withstand being tossed around in the bed of a truck, and to make sure you have a local dealer capable of service and repair.

Prices for EM locators range from around $1,000 for a single frequency/single-purpose locator, to around $7,000 to $8,000 for top-of-the-line models. But you’ll find versatile multi-frequency, multi-function models in the $3,000 to $4,000 range, as well.

Manning cautions that going cheap may cost you money in the long run. He cites the case of a boring contractor he’d worked with who failed to find a critical utility using a $1,000 locator. “They had no problem finding the gas line and the power line, but they dug all afternoon and couldn’t locate the telephone line,” he says.

22   Number of state highway agencies that support the use of SUE

his crew standing around idle the frustrated contractor popped open the telephone pedestal to attempt a direct connect and got into trouble for that. “Had the contractor used a better locator, a $3,000 model instead of a $1,000 model, he wouldn’t have been getting those false readings and he would not have wasted a whole day,” Manning says. As it turned out, Manning located the telephone line within 30 minutes of arriving, using the proper locator.

Ground penetrating radar

The biggest limitation to EM locating is that you have to have metal – either a metal pipe, metal wires or a tracer wire in the pipe or trench – to accept the signal put out by the transmitter unit. So with EM you’re never going to detect concrete culvert pipes, PVC pipe, clay-tile or wood sewer lines. Many of the early fiber-optic lines were buried without a tracer wire, rendering them invisible to EM locating, as well.

The Vermeer Interragator EZ GPR system can survey underground utilities at speeds of up to 5 mph using a 400 MHz antenna. It’s powered by 10.8-volt lithium-ion rechargeable batteries that give you 1.5 to 2 hours of service depending on conditions. The unit is sealed against inclement weather and weighs just 10 pounds for easy portability. You can choose from five selectable depth settings, and a survey wheel provides horizontal position of data.

The solution for detecting non-metallic utilities is ground penetrating radar. GPR also has the ability to find a lot of underground anomalies that can wreak havoc on an excavation project, things like boulders, buried tree stumps, trolley tracks, old foundations, rebar and underground construction rubble.

GPR works by transmitting an electromagnetic pulse into the ground and then receiving the reflected energy and noting any subtle dissimilarity in the properties of the soil. On a GPR screen backfill in a trench gives a different reading than the undisturbed soil, concrete is different than soil, and so on. The typical unit is pushed around the jobsite, much like a lawn mower, creating a gridwork of perpendicular lines. “By understanding how to interpret these geophysical anomalies, we can with a high degree of confidence conclude the existence of linear objects and other anomalous structures as long as the soils cooperate,” says Mark Wallbom, CEO of UIT. (See photo page 38.)

30   Percentage of locates done by utility locating professionals that may be inaccurate

The above-ground detective work done before you commence a GPR scan is similar to the observations done before conducting an EM sweep of an area. “To properly calibrate the GPR system, the first thing you find is a known target, such as a nearby culvert. You could also pop a manhole cover and find the utility in it,” Sjostrom says. “Then you run down the centerline of your proposed excavation or bore path, do two passes on either side of the borepath and then run the device perpendicular to the borepath to create a grid.”

The one limitation to GPR is soil type. “As you start adding more fines, whether it’s clay or silt, the effectiveness of GPR is going to worsen,” Sjostrom says. “And with wet clay soil GPR won’t work at all.” A place like Florida, with its sandy soil, is ideal, he says. Sjostrom estimates that about one third of the county has soils suitable for effective GPR use.

GPR units also cost considerably more than EM devices. “For utility contractors there are at least a half dozen models in the $18,000 to $25,000 range that will give you a number of choices,” Sjostrom says. “What separates them from one another is the ease of use, size and software friendliness. The physics is pretty much the same,” he says.

38  Percentage of reported utility strikes that were caused by failure to notify the One Call center

At those prices not every contractor is going to carry a GPR unit around in the back of his truck. But in areas where the soils are favorable, locating companies use them frequently. And there are cities such as San Antonio, Texas, that require the use of GPR after years of utility strikes convinced city officials to force contractors to use more caution, Sjostrom says. You can also hire geophysical or advanced locating firms to do a GPR sweep of a site if you’re concerned about abandoned utilities or running into troublesome objects underground. EW

Editor’s note; This article is not intended as a substitute for utility locating training, but rather as an overview of the issues and equipment used in utility locating. If you’d like to further your training in this field, see the resources section on page 41. You may also want to seek some familiarization training from the manufacturer of whatever device you buy.




Twenty years ago underground utility locating was a hit and miss affair, with no rules, regulations or best practices. But in 2002 the American Society of Civil Engineers created Standard 38-02 to create formal quality standards for underground investigations. In a nutshell SUE describes four quality levels which aggregate as quality levels increase. For example quality level B includes all activities included in quality level C and quality level D. The levels include:

• D: is the lowest level of investigation and only involves reading existing records.

• C: involves a visual survey of above-ground facilities and correlates this with existing utility records.

• B: requires the use of geophysical techniques such as ground penetrating radar and electromagnetic devices.

• A: requires non-destructive digging to visually verify the horizontal position and vertical elevation of a utility.

The SUE standards are due for an update this year, which when finalized will be labeled ASCE Standard 38-10. For more information check out these websites:

Caterpillar goes underground with UIT

Equipment manufacturer Caterpillar has been pushing safety hard in the last few years and its purchase of UIT announced this summer is another big step in that direction.

UIT, which stands for Underground Imaging Technologies, is a service that Caterpillar will offer, rather than a specific product. What’s the service?

“We see ourselves as all things geophysical,” says Mark Wallbom, CEO of UIT. “We use whatever equipment is needed, not just our own, and we don’t have anything to sell above and beyond the fact that we’re experts in using geophysical means and methods to detect underground utilities and other linear features.”

That said, UIT also uses its own exclusive equipment that goes beyond most current technology in the market. Take, for instance, its multi-channel GPR system. Rather than a single antenna, UIT’s cart-based device uses 14 antennae that send out signals in a 5-foot-wide path. The cart is towed by a vehicle at speeds up to 5 miles per hour and the overlapping signals from the multiple array antennae compensate and correct for the pitch, yaw and roll the device experiences rolling across uneven ground.

“It collects vast amounts of data, that when you’re done looks like an MRI,” says Wallbom. “We’ll go down through the ground in 2-inch stacked slices and you will see features appear, brighten and disappear.”

UIT also deploys an EM-type device for detecting ferrous materials that is likewise a vehicle-towed, 5-foot-wide cart with multiple sensors. The company calls this technology Time Domain ElectroMagnetic Induction (TDEM). The TDEM system is induction based and does not require physical attachment to the utility. It can also locate features such as changes in rock and soil conductivity in underground trenches and other construction operations.

Both cart systems also synch up to either a GPS or a total station. Software combines the GPR or TDEM data with the positioning data from the GPS or total station to give you a 3-D map of the underground spaces scanned. That data can be plugged into a Caterpillar Accugrade, GPS-enabled excavator or other earthmoving machines. “Now the operator in the cab can see on his GPS screen the areas he needs to avoid and areas he can get high production without having to worry about hitting utilities,” Wallbom comments.

“We map the entire area of interest,” Wallbom says, and not only known linear features such as pipes and things of that nature, but we also find all the unknown features, some of them linear, some of them not – trolley tracks, brown fields, thing like that. What makes us different is that we are not a locating company. We do not designate known utilities with paint on the ground. Instead we provide the client with a 3D map of the subsurface tied to stationing with elevations, not just depth below current grade.”

For more information go to

Ridgid’s SeekTech SR-20 uses a combination of multi-direction antennas and a mapping display that shows target line direction and changes in direction as they occur, left-right guidance arrows, signal strength and a proximity number that increases as distance to the target decreases. It has four frequencies in active mode and seven sonde frequencies. In passive mode it offers broad band electric and low and high band radio frequencies.



Training videos

Locating equipment is of little use unless you know some of the science behind it, and what it can and cannot do. If you want to get better acquainted with this subject check out the series of training videos from Excavation Safety University at

ASCE Standard

Standard Guidelines for the Collection and Depiction of Existing Subsurface Utility Data

Call before you dig information

Common Ground Alliance Best Practices

Can be downloaded for free or ordered as a bound manual for minimal charge. Website is updated with new practices and information on a regular basis.

natural gas pipeline mapping crew From our partners

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