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Years
ago, comic-book hero Dick Tracy's radio wrist watch represented
the latest in detective technology for tracking down unsavory underworld
characters. Today detectives in the excavation business use much
more sophisticated tools, with such features as wands resembling
fictional X-ray guns, microprocessors, and low-frequency radio wave
transmitters, to hunt down potentially dangerous underworld targets.
But instead of hardened criminals, they're looking for hardened
objects, such as pipes, cables, fuel storage tanks, and all kinds
of other items buried during previous construction projects. Their
goal is to locate these objects before an excavating crew unwittingly
finds them with a bucket, a blade, or an auger. Depending on what
they strike, the consequences can range from a loss of electrical
service in a residential neighborhood to a flooded street intersection
to much more costly impacts, such as a communications outage that
cripples an entire city or a ruptured high-pressure gas line that
leads to a deadly explosion.
"Locating underground
objects has been an ongoing problem in the excavation field since
the first person tried to dig up a buried pipe," says Ron Davenport,
president of SubSurface Instruments Inc. The Purcellville, VA, company
manufactures underground and underwater magnetic pipe- and cable-locating
equipment and magnetic-field measuring and reporting equipment.
"It's a serious business, and it's not an easy one,"
Davenport continues. "It requires good equipment and support
from the manufacturer. Locating devices are great safety tools,
but they're only as good as the operator."
When done properly, it's
a smart form of insurance. "What we charge is a whole lot cheaper
than knocking out a power plant that costs $2 million in customer
losses every hour it's off-line," says Jerry Price, president
of DES Resources Group Inc., a locating contractor based in Jackson,
TN.
Usually the job of locating
buried objects falls to the experts, such as locating contractors,
who are dispatched by one-call utility locating services. The National
Utility Locating Contractor's Association (NULCA), formed in
1994 and based in Tallahassee, FL, represents the locating industry,
including suppliers. "Our mission is to define, establish,
and maintain high standards and practices in the underground-utility-locating
contractor industry," says Jamal Masumi, president of NULCA.
"Our top priority is the safety of the general public and people
at the job site."
Because of legal requirements
to mark utilities prior to excavating a site, the demand for the
services of professional locators tends to reflect the level of
activity in the excavating market, notes Masumi, who is also president
of STS LLC, a utility-services company based in Phoenix, AZ. Even
with consolidation in the industry, he sees a steady growth in the
demand for locating contractors. "Because of the blackout in
the Northeast this past August, it's uncertain that utilities
will increase use of in-house locating services," he says.
"Also, acceptance of locating contractors by excavating companies
continues to increase. During the initial stages of the locating
contractor industry, a number of excavating firms were concerned
that these contractors weren't as efficient or as accurate
in marking the location of utilities as the utility people were.
But now locating contractors are well accepted by excavators. NULCA
represents locating contractors as professionals who can produce
the quality of work that excavating companies want."
Some grading and excavating
firms, utilities, and facility owners employ their own locating
crews. It's a way to double-check other locating services.
"As a grading or excavating contractor, you still may be responsible
for any liability due to damage to utilities even if you pay someone
else to do the locating," he says. "Regardless, avoiding
injuries is as important as the liability issue."
Hidden Obstacles
Regardless of who does it, locating buried objects accurately and
without missing any can be a challenging task. For one thing, a
lot of stuff has been buried over the years. "Pipes, cables,
you name it," says Davenport. "When you get into big cities,
that's where it really gets difficult. In some places, you
still find clay, terra cotta, cement, and other pipes and ducts—even
some old wooden ones."
Often, they're not
where they're supposed to be either. "They seldom seem
to be where the maps drawn years ago say they were placed,"
he notes. "Over time, the job has been made more difficult
by placing below ground many different types of materials, including
plastic, that are harder to locate."
Locating professionals
don't always know what's buried beneath their feet when
they start their work. "In some cases, there are no good records
and the guys who were around when the utilities were installed have
retired," says Locating Contractor Mike McGarry, president
of Worksmart Inc. in Paw Paw, MI. "When they leave, that knowledge
base leaves with them. It takes more time to locate objects if you
don't know what's there to begin with because you have
to pay attention to everything you locate. About 80% of the time,
we find unknowns."
Just what kind of unknown
objects may be lurking underground at a former construction site?
"They could include sewer and electric lines and [have] phone
cables installed to support trailer camps and work yards [that were]
never removed," answers Price. "Someone dropped a trenching
machine, ran a line to a temporary facility, and never drew a map.
Some of those lines are still active. In fact, I've been on
projects where we've found live 440-volt power cables that
weren't enclosed in conduit. We treat every underground line
as if it weren't in conduit. We don't want any surprises."
Masumi notes another
challenge for locating contractors: completing the job on time.
As he points out, just about every state requires a locating job
to be completed within 48 hours of notification. "An excavating
contractor can call in any amount of work at any time, and the locating
contractor has to meet that 48-hour deadline," he says. "Completing
every locating project within an arbitrary time frame and with no
limitations on the scope and size of the job becomes tasking. In
our judgment, eliminating this requirement would lead to better
scheduling of jobs that have higher priorities."
Detection Devices
Various technologies are available for locating things hidden in
the ground. The oldest and least accurate method is dowsing. Much
more art than science, it uses a divining rod—maybe a forked
hazel twig or a similar device—held horizontally in both hands.
Practitioners claim the rod will dip toward the ground when it's
passed over water or minerals. Witching rods—two welding rods
or pieces of copper wire held horizontally and parallel to each
other—are another variation. When the rods sweep across a buried
pipe or other object, say proponents, the two wires will cross.
"It seems to work for some people," says Jim Walton, vice
president of sales for Radiodetection, a locating-equipment manufacturer
in South Lake, TX. "Apparently, digging up the ground to install
a pipe or a cable disturbs the magnetic field, and the wire is reacting
to that change. Even when it does appear to work, you still don't
know what you've located or how deep it is."
Locating equipment more
recently developed offers much greater reliability and accuracy.
Each has its own advantages and limitations.
Electromagnetic
Locators
Used to locate buried pipes, cables, and sewers, this type of device
detects the alternating magnetic fields that surround a conducting
metallic line. As a result, it can't locate nonmetallic lines,
such as plastic pipe, unless it is installed with tracer wires.
This technology works in all soil conditions, even under water.
In addition to locating buried lines and blockages and collapses
in ducts or pipes, it can locate cable faults, monitor rust and
corrosion of pipeline surfaces, and identify the position of joints
in iron gas pipe.
Radiodetection is one
of several major manufacturers of electromagnetic locators. The
company's line of products features a self-contained transmitter,
which is placed in a drain or a duct, and a transmitter that generates
an AC signal that creates an electromagnetic field around a buried
conductor. These signals are picked up by a handheld receiver.
By setting a switch,
an operator can use these locators to detect both passive and active
signals. Passive signals include those emitted by power cables,
telephone lines, power-system return currents, and radio-frequency
currents from long-wave radio transmissions that penetrate the ground
and flow along buried pipes and cables. Locating a line using passive
signals requires only a receiver. Passive signals can't identify
a specific line, however, if multiple lines are present. That's
where the ability to detect active signals produced by the transmitter
pays off. It allows operators to identify lines more precisely in
terms of depth and signal strength. By varying the transmitter frequencies,
operators can use this type of electromagnetic locator to positively
identify and trace a single line in a congested area, such as below-street
services in a city.
"This feature allows
you to use active signals to locate a particular line to be excavated
and passive signals to locate other nearby lines that have a detectable
signal on them and may be at risk," says Walton. "Normally
our equipment is used to locate lines buried up to about 15 to 20
feet deep. However, it has been used to detect lines as deep as
35 to 60 feet below the ground surface."
These electromagnetic
locators also can be used with a global positioning system (GPS)
to produce maps of the lines. The company's electromagnetic
locators include patented features, such as Current Direction (CD),
that determine the direction of current flowing through a line as
well as the strength of the current or signal. While this feature
isn't necessary to locate lines for an excavation project,
it makes troubleshooting electrical problems faster and easier.
"Current strength, alone, isn't enough to positively identify
a line," Walton explains. "Let's say two power lines
are buried next to each other. One is a transmission line from a
transformer, the other a feeder line to a house. With CD, you can
apply a current to the transmission line and, by measuring the direction
of that current, identify which of the two lines is the transmission
line."
Magnetic Locators
Unlike metal detectors, which can be used to spot any kind of metal—including
coins, tin cans, bottle caps, and pop-tops—buried no deeper
than about 2 ft., ferrous-metal locators will detect steel and iron
objects much farther below the surface. These buried objects include
valve and curb boxes, surveyor corner markers, well casings, manhole
covers, energized electric cables, steel drums, septic-tank handles,
and unexploded bombs.
The effective depth of
magnetic locators depends on the type of object and its orientation,
either horizontal or vertical. The magnetic field and the ability
of the device to locate an item are greater when the object is vertical,
notes Davenport. "Our equipment can locate a 4-inch-long piece
of iron or steel lying horizontally as deep as 4 to 6 feet and pinpoint
the location of a survey nail accurately enough to hit it with a
half-inch star drill," he says. "We've found a mortar
round at about 9 feet in the ground and vertical pipes buried 20
feet deep or more."
Radio-Frequency
Locators
This technology uses radio signals to locate a continuous piece
of metal, such as iron, steel, or copper water lines; copper or
aluminum electrical wires; metal gas lines; steel cables; and telephone
and TV cables. The key word is continuous. The metal, whether
a wire or a pipe, has to be long enough to act as an antenna to
broadcast radio signals, which are picked up by a receiver. While
this technique will locate these types of lines, it cannot identify
the specific type of metal it finds.
Surface-Penetrating
Radar
By sending electrical pulses into the ground and detecting the electrical
echo when these pulses strike an object, surface-penetrating radar
can identify both natural materials, such as rocks, dirt, and gravels,
and manmade materials, such as concrete, brick, and steel. As a
result, it can detect a variety of metallic and nonmetallic pipes,
sewers, cables, voids, rebar in concrete, steel drums, and storage
tanks, among other buried objects.
The equipment includes
the radar unit, which resembles a high-tech lawn mover, including
the antenna, which must contact the ground surface for best results
as the unit sweeps over the ground. A video screen displays the
image—similar to the way an MRI or an ultrasound does—generated
as the radar signal bounces off of an object.
Each type of material
produces a different type of radar signature. That, plus the fact
that radar signals traveling through soil are constantly hitting
something, means a trained operator is required to properly interpret
these images. Surface-penetrating radar cannot detect objects in
salt water because the electrical charges in this type of water
cause a whiteout on the image display screen. Also, dense, wet clays
limit how deep the radar signal can penetrate into the earth.
The size of objects detected
by surface-penetrating radar depends on the type of antenna and
the depth of the object. The greater the depth of the radar signal,
the less resolution it produces. For example, at a penetration depth
of about 8 ft., a 500-MHz antenna can detect objects as small as
about 2 in. That's enough to locate buried foundations, pipes,
and cables and determine the thickness and location of backfill.
On the other hand, a 1,000-MHz antenna can resolve targets as small
as about three-eighths of an inch as long as it's no deeper
than about 3 ft. below the surface.
When first introduced
to the commercial market about 10 years ago, surface-penetrating
radar commonly was used in geophysical applications, such as determining
the depth to bedrock and water tables and mapping rock and boulders.
Its use has since expanded into other areas, including structural
analysis, environmental cleanup, and forensic investigations. In
terms of excavation projects involving utilities, surface-penetrating
radar is a very user-friendly system, says Ron LaBarca, president
of USRadar in Matawan, NJ. "It offers crystal-clear resolution
and superior depth capability. You can use it to locate adjacent
storm drains buried beneath another pipe and multiple layers of
asphalt and to find a fiber-optic cable alongside a road. Other
applications range from locating buried streambeds to measuring
the dimensions of a backfilled swimming pool. No other readily available
technology can do these tasks."
USRadar's products,
geared toward professional locators, are based on equipment developed
by ERA Technology Ltd. In Leatherhead, Surrey, England, under a
British Ministry of Defence contract to locate plastic land mines,
LaBarca notes. "That system had to work. There was no room
for error. Failing to detect even one of those mines is not acceptable."
USRadar's equipment
also can be integrated with GPS technology to map buried objects
at a site. "When it's wired into GPS, you just click a
button every time you detect an object, and when you're finished
you have an accurate map of the site," LaBarca says.
Poor performance of early-generation
surface-penetrating radar has hurt the image of this technology,
LaBarca admits. "Many people were disappointed with it. You
almost needed a degree in rocket science to use it, and it didn't
do everything proponents claimed it would do. But like any technology,
it has evolved and improved. Our biggest hurdle is trying to convince
people to take another look at surface-penetrating radar. There
is no one best technology for all underground detection applications.
But surface-penetrating radar can be a very useful tool in the right
applications."
How the Pros Do It
McGarry with Worksmart uses surface-penetrating radar made by USRadar.
"I can find things other guys can't," he says. Although
based in Michigan, his work takes him to various parts of the country,
and locating projects have ranged from locating silver veins at
silver mines and utilities at nuclear power plants to mapping a
tile drainage system installed at an automobile factory eight decades
before. Once he was even called to locate a plastic time capsule
buried in front of a high school nearly 30 years earlier.
To make sure he doesn't
miss an area and to avoid covering the same area twice, he searches
in a grid pattern. Most of the time, clay soils don't limit
his ability to detect cables and pipes. "Even when utilities
are installed in clay, they're usually backfilled with sand
and we can find them," McGarry says. "Usually we don't
look much deeper than about 10 feet, but we can go 40 feet deep
or deeper. Our system has a computer, so we can print out results
of our search, if that's what the customer wants, when we get
back to the office. Usually, though, we use flags with the depth
written on them to identify the location of buried objects as we
find them.
"You can learn how
to operate this system in just a few days. But it can take several
months to learn how to read the radar signatures correctly. Interpreting
the data is the most important part. So you need to know how utilities
and other things are put in the ground."
DES Resources Group uses
a variety of technology for locating buried objects, including seismic,
resistivity, radio frequency, and surface-penetrating radar. The
Tennessee company specializes in precisely locating objects and
mapping their location. "We can locate something within 4 inches
of where it actually is and then prepare a three-dimensional map
using GPS," he says. "Unlike a painted mark on the ground,
which can wash off in a few days, or stakes, which can be removed,
these maps provide a permanent record."
In addition to GPS coordinates,
these maps show the location of objects in relation to a curb line,
a light pole, a sewer inlet, or other type of permanent physical
feature. "Not all of our customers have a GPS receiver to locate
coordinates," Price explains.
Although many of the
firm's projects are in the Southeast, Price works from coast
to coast. Projects include mapping routes of existing utilities
and planning routes for new pipes and cables in congested areas.
One project at a nuclear power plant involved locating pipes that
had been installed above other piping installed years earlier when
the site was the location of a coal-fired power generation facility.
On another job, he spent a day locating utilities along a 10-ft.-wide,
0.5-mi.-long corridor at a hydroelectric dam and another day preparing
maps of the surveyed area. At another linear project, this one 30
ft. wide and 0.5 mi. long at a coal-fired power plant, he located
buried high-pressure lines, ammonia, and other chemical and electric
lines. "We found some surprises that didn't get put on
the as-built drawings," he says.
Even though Price has
been locating buried objects, both onshore and offshore, for more
than 20 years, he has yet to complete his education in detection
technology. "I still learn something every day," he says.
Future Prospects
Manufacturers continue to tweak performance of existing detection
equipment and image software as they develop even better technology
for locating buried objects that's both easier to use and more
accurate.
"Because the types
of materials used to make pipes, cables, and other materials keep
changing, the technology for finding all these things has to improve
too," says Davenport with SubSurface Instruments.
Price would like to see
a unit that would enable him to simultaneously find a buried object—within
6 in. of its actual depth and within 2 in. of its horizontal position—and
map its location.
"We're right
on the verge of breaking into better locating equipment," says
Davenport. "Technology has come a long way, but the best we
can do right now is to give our best educated guess as to what's
down there in the ground. What we really need is a handheld unit
that provides a three-dimensional image showing the outline of the
object and how deep it is. That would be the ultimate locator. It's
around the corner, and it's where we have to go."
In the meantime, he reports,
his company is developing a prototype locator that combines magnetic
and reflective technology with a form of ground-penetrating radar
in one unit. "I've seen it work, and I'm really enthused
about it," says Price. "It will revolutionize the locating
business."
Greg Northcutt is
a frequent contributor to Grading
& Excavation Contractor
magazine.
GEC
- January/February 2004
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