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On March 22, 2002, a
construction crew from B&H Construction of Eunice, NM, was working
to relocate a natural-gas line near the Phoenix, AZ, Loop 101 freeway.
According to reports in the Arizona Republic newspaper, the
earthmoving equipment hit and ruptured a high-pressure gasoline
pipeline the crew hadn't known was there, and the puncture began
gushing fuel. Before it could be contained, 35,000 gal. of premium
unleaded gasoline escaped, with 30,000 gal. seeping into the ground.
In order to remove the
contaminated soil, workers had to dig a hole 200 ft. wide by 55
ft. deep on one side of the road and 100 ft. wide by 40 ft. deep
on the other side. Also a 200-ft. section of the road itself had
to be dug up to remove contaminated soil. The closure forced drivers
to make detours of up to 2 mi. with major backups at all intersections,
and the road remained closed for three and a half weeks. The Republic
did not report the total losses from the incident, stating only
that "the company that owns the gas line … may be left
to haggle over damages with B&H Construction."
Although this might seem
an extreme case, the fact is that prior to excavation, contractors
often do not know the exact location, or sometimes even the existence,
of underground utilities. The Department of Transportation's
Damage Prevention Quality Action Team states the problem forcefully:
"The United States
has a vast underground infrastructure of pipelines, conduits, wires,
and cables that affect every individual. This underground infrastructure
is critical to our way of life, constantly providing oil and natural
gas, telecommunications, electricity, water, sewage, cable TV, and
other vital products and services constantly. Disruption of any
of these underground facilities could affect the safety of the public,
the environment, and continued service reliability that could impact
our entire economy.
"One of the leading
causes of disruption to our country's underground facilities
is external force damage (sometimes called ‘third-party damage')
that occurs during excavation activities. This has been recognized
by both industry and government. Although such damage occurs far
too frequently, it is usually preventable. Responsibility for preventing
excavation damage is shared by all stakeholders. Advanced planning,
effective use of one-call systems, accurate locating and marking
underground facilities, and the use of safe-digging practices can
all be very effective in reducing underground facility damage. In
most states, increased and mandatory use of the state's one-call
system has significantly reduced the incidence of excavation damage.
However, damage still occurs."
One-Call Systems
One-call systems, funded
by members consisting of public utilities and other underground
facility owner-operators, provide an excavator with a single toll-free
number to call in order to alert the affected member(s) about his
plans to excavate in a specific area. The one-call system serves
as a message-handling service, taking information about each planned
excavation and distributing this information to its member utilities.
It is then the responsibility of each utility to mark the location
of its underground facility or facilities at the excavation site.
Excavators are required by law in most states to notify the one-call
organization at least two working days before starting a project.
The call and service are free to the contractor. Member utilities
support the organization based on their message volume.
There is at least one
one-call organization in every state and in the District of Columbia,
Canada, Australia, Finland, Scotland, and the Republic of China.
These organizations can be quite comprehensive. Consider JULIE (Joint
Utility Locating Information for Excavators), also known as the
Illinois One-Call System. JULIE was formed in 1974 by the owners
and operators of underground facilities as a means of reducing damage
to those facilities. In 1991, the Illinois General Assembly enacted
a law that requires anyone excavating to contact the Illinois One-Call
System before digging and requires owners of underground utilities
to be members of the system. There are approximately 976 member
companies in JULIE.
JULIE logged almost a
million calls in 2001. And because one call from an excavator can
result in communication with multiple utility companies, these calls
resulted in more than 6.4 million notification messages being transmitted
to member utilities in 2001.
Beyond One-Call
Unquestionably, one-call
systems are the cornerstone of underground facility damage prevention.
But the one-call system is not foolproof; accidents still happen.
Brian Brooks, president of Dawn Companies in Frankfort, IL, cites
some of the problems his company has encountered. "Probably
90% of the underground facilities are covered by the one-call systems,
but this is not precision location. They try to get you within 18
inches in either direction, which often is not close enough, and
they seem to miss by more than that. Worst of all, one-call just
deals with public utility lines; their responsibility ends at the
meter [or the transformer]. Once the primary hits the transformer,
the owner's engineers run lines to distribute the utility medium
throughout the facility. By the time the facility has been sold
once or twice, the current owners don't know exactly where
these underground facilities are located. And since the one-call
utilities don't have the data to locate these underground lines,
they certainly don't want to assume the responsibility for
locating them."
Vance Green, owner of
Detection Specialties, a Phoenix-based firm that specializes in
locating underground utilities and other hazards on customer-owned
properties, agrees, and he has a case study to back his contention.
"In July 1999, an existing building was to be demolished and
rebuilt at a large downtown Phoenix hospital complex. The building
had been constructed in 1957 and had been almost continuously modified
and remodeled since then. Little if any information existed concerning
the utility systems that might conflict with the demolition project.
And, of course, prior to demolition of the building, it was necessary
to disconnect the building from these ‘lost' utility systems
that must remain active and serve the remainder of the hospital
campus. One-call could provide some insights, but their public utility
providers claimed only a few publicly owned utility lines in the
area.
"This was not a
trivial job. After locating and mapping the utility systems encircling
the building to be demolished, we determined that approximately
12 separate lines were at risk, including the medical oxygen line
feeding the main hospital building and two 36-inch chilled water
lines also supplying the main hospital building. None of these lines
would have been discovered by the one-call utility locating program.
"However, using
our underground utility location equipment and techniques, we were
able to obtain this information in advance of any construction activity
and thereby protect these critical hospital systems during the demolition.
As the project proceeded, the general contractor had the information
needed to knowledgeably and intelligently disconnect those parts
of the underground systems feeding the building to be demolished
while maintaining service to the main hospital building."
To augment the one-call
service, therefore, a niche industry has developed to locate underground
utilities. Companies such as Radiodetection, Metrotech, and Rycom
have developed radio frequency (RF)—based instruments that
allow owners and contractors to pinpoint a broad range of underground
utilities, including:
- any accessible (at
least one end available) electrically continuous metallic utility
line;
- any energized line
drawing current;
- any metallic utility
line radiating an electromagnetic field detectable by line-locating
equipment;
- any drain, sewer,
or waste line into which a small radio transmitter can be inserted
with a push rod;
- any empty PVC electric
or communications conduit with at least one end available;
- pressurized nonmetallic
(PVC) water system lines, under certain conditions.
In addition, these companies
offer metal and ferrous metal detection instruments that permit
location of metallic valve covers, metallic manhole covers, electric
junction boxes, and other metallic system components.
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| A
typical 1- x 1-ft. "pothole" created during Level
A vacuum excavation activities |
Travis Leintz, Western
territory sales manager for Rycom Instruments Inc., adds fiber-optics
tracing instruments to this list of products on the market. Rycom's
Fiber Optic Tracing System uses a remote transmitter at the telephone
company office. When that transmitter turns on a line at a known
frequency, a technician with a receiver in the field can precisely
locate the position of that line and provide the information to
the home office.
"Fiber optics are
very expensive to repair," Leintz points out, "and there
are very few access points in fiber optics with which to hook up
tracing equipment. Therefore, fiber-optics tracing instruments represent
a valuable tool to augment our line of cable and pipe locators,
metal locators, and sheath fault locators."
Locating Underground
Utilities Made Easier
Most suppliers offer
a full line of underground location instruments that are fundamental
to the work of underground detection contractors. "With all
these instruments, we can locate underground utilities in several
different ways," Green explains. "The principal methods
are passive-sweep locating and active-sweep locating. Passive-sweep
locating takes advantage of the fact that many underground lines
radiate a detectable electromagnetic field either through inductive
coupling from power distribution systems or from broadband noise
impressed on the line from electromagnetic equipment attached to
it. Such a field can be received by pipe and cable locators, and
the path of the line can be marked. Lines located using this method
can be detected by walking the instrument receiver over the subject
area in a grid pattern.
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| The
RD400 pipe and cable locator in action |
"In active-mode
locating, a signal of a known frequency is applied either directly
or through inductive coupling to an electrically continuous underground
utility line. A receiver tuned to this frequency is used to track
the path of the line. As both passive-sweep locating and active-mode
locating have their strengths and weaknesses, both methods must
be used within an underground utility survey area to ensure that
all locatable items have been detected."
The underground utility
detection instrument industry is quite competitive and, according
to Casey Pelton of Metrotech in Sunnyvale, CA, is constantly upgrading
its instruments to meet user demands. "Five years ago, users
were satisfied with single-frequency, low-cost instruments,"
he recalls. "But now the demand is for a single multifrequency
RF instrument that can locate pipes as well as electrical lines.
That multipurpose instrument enables contractors to cover the great
majority of applications they will face, and of course it reduces
their investment. The multipurpose design requirement threatened
to overcomplicate instruments for the technicians who use them in
the field. However, we have been able to limit the number of controls
on the instrument panels and incorporate left/right guidance so
that, with minimum training, technicians simply pick the instrument
up, turn it on, dial in the appropriate frequency, and go."
Pelton says Metrotech
sells most of its underground locating instruments to utilities,
governmental agencies, and underground-location contractors. But
occasionally general contractors will buy instruments and locate
underground utilities with their own crews. Dawn Companies is such
a contractor because it believes it can do the job with fewer delays
than would be possible by contracting with a third party. "We
had a job at Joliet [Illinois] prison that convinced us," recalls
Brooks. "Because of the prison's security measures, our
guys found themselves sitting around waiting for the utility locater
contractor to get clearance to come inside. As a result of experiences
like this, we bought four Metrotech instruments: two metal detectors
and two RF instruments for conductible and traceable lines. They
have really saved us a lot of time, and it's rare that we can't
do an entire construction project with these four instruments."
Subsurface Utility
Engineering
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| The
multiple-frequency, digital Verifier detects and identifies
underground utilities |
Many contractors and
utilities share this point of view, whether they subcontract the
location work to underground utility locating contractors or not.
The combination of one-call services and crews equipped with RF
instruments usually will do the job and therefore is the dominant
approach to underground utility location today. An increasing number
of government and construction people are tired of "hitting
things with yellow equipment," however, and favor a more proactive
approach called Subsurface Utility Engineering (SUE).
According to Project
Engineer Scott Smith of General Engineering in Charleston, SC, SUE
is "an engineering process utilizing state-of-the-art technology
to accurately identify, characterize, and map underground utilities
prior to the design of highways or new structures or the
installation of new underground assets." Developed a decade
ago by the Federal Highway Administration (FHWA), SUE results in
major cost savings over the life of a construction project. A recent
study of 71 construction projects by Purdue University on behalf
of the FHWA determined that "$4.62 was saved for every dollar
spent on SUE.… Qualitative savings were non-measurable, but
it is clear that those savings are also significant and may be
many times more valuable than the quantifiable [$4.62 to $1] savings"
(emphasis added).
"This cost benefit
results from the avoidance of conflicts by predetermining the path
of least disruption of the proposed utility, preventing unnecessary
relocations for existing utilities, and minimizing the likelihood
for damage to existing infrastructure," Smith explains. "Traditionally
many design teams simply sketch in a new building foundation or
pipeline realignment, having little or no prior knowledge of the
existing subsurface utilities. During the construction phase, then,
change orders occur due to conflicts or major relocations that are
preventable with the SUE approach. There is also the added safety
benefit, which is immeasurable in terms of human health and welfare
and liability. Even if a contractor intercepts a dead utility, the
presence of this alone usually results in project delays until the
exact nature of the utility is investigated and identified."
SUE Elements
Identifying, characterizing,
and mapping underground utilities prior to design of new facilities
requires an accurate and complete underground survey of the site.
In addition to conventional RF utility-locating equipment operated
in either the active or passive mode, SUE has higher-tech detection/location
equipment modules at its disposal. One of these mobile modules features
ground-penetrating radar (GPR), an electromagnetic (EM) method that
detects interfaces between underground materials having different
dielectric constants.
"The GPR transmitter
radiates repetitive, short-duration EM signals into the earth from
an antenna moving across the ground surface," Smith says. "Electromagnetic
waves are reflected back to the receiver by the interfaces. Subsurface
features that might cause such reflections are (1) natural geologic
conditions, such as changes in sediment composition, bedding and
cementation horizons, voids, and water content, or (2) such man-introduced
materials or changes to the subsurface as soil backfill, buried
debris, tanks, pipelines, and utilities. The profiling recorder
receives the signal from the antenna and produces a continuous cross-section
of the subsurface interface reflections. No spurious reflection
events are generated on the GPR data by aboveground features, which
could lead to false interpretation of subsurface anomalies."
An even more powerful
SUE equipment module is General Engineering's Computer-Assisted
Radar Tomography (CART). The mobile CART system consists of a 16-antenna
multichannel GPR system that enables several GPR transmitters and
receivers to operate in tandem over the same area, thereby providing
images of underground objects that could not be achieved with conventional
single-channel systems.
"The premise of
multichannel systems is that GPR energy can be beamed at an object
underground from several different angles with several different
antennas," Smith explains. "Subsequent processing of the
data with software that can focus the reflected energy back to a
point underground, or to an object the energy was reflected from,
creates a reconstructed image of that object. The framework for
the software used to process the data originated in the oil industry,
was developed primarily at Schlumberger, and was later refined by
Witten Technologies.
"The CART system
was developed because current technology does not allow for very
accurate data acquisition and is seldom mapped to digital file.
Most important, no underground mapping technology other than GPR
exists that can detect non-metallic infrastructure such as PVC,
ACP [asbestos concrete pipe], concrete, or terra cotta. Several
thousands of miles of plastic gas lines alone are installed throughout
the United States with no viable means for nondestructive detection
other than GPR systems. Coupled with advanced positioning systems,
the CART is able to map buried objects to within a few centimeters
horizontally and typically provides exact depths to buried objects."
A lower-tech but quite
useful equipment module is a high-pressure air or vacuum evacuation
rig that is used to expose buried utilities for verification of
their type, size, and depth. The trailer-mounted rig "potholes"
utilities using a high-pressure air lance to loosen the soil while
a vacuum hose simultaneously takes the loosened soil to a canister
mounted on the rig. After the survey crew records the exact location
and depth of the exposed utility, the soil in the canister is placed
back in the hole. The rig also has equipment to open and subsequently
close a 1-ft.2 opening in asphalt, and it can use water
rather than air to loosen clayey soils.
Each of the SUE equipment
modules has strengths and weaknesses that dictate which module or
combination of modules is used on a specific project. The final
product of the SUE process, whichever equipment complement is used,
is a thorough and accurate location of utilities and other underground
hazards provided in the form of computer-aided design—compatible
electronic maps for use in the subsequent facility design process.
Does SUE pay off? The
University of North Carolina thinks so. General Engineering performed
SUE services to assist project architects in planning for a major
addition at the Medical Science Research Building. The firm performed
extensive GPR and RF surveys over the 8-ac. site to locate underground
utilities that had been installed as far back as the 1800s. Ten
previously unknown utilities were identified with GPR and later
verified with vacuum excavation. The project cost was $50,000.
The payoff? By accurately
mapping a major chilled water line to the site of the new building
addition, the survey precluded the need to relocate the line during
construction at a savings the university estimates at $640,000.
What's more, when the presence and location of the 10 previously
unknown utilities were discovered, the initially planned sewer line
route was abandoned before expensive design and construction costs
were incurred. Armed with this knowledge, planners are selecting
a more advantageous and cost-effective sewer line route.
The university project
was one that the FHWA defined as Quality Level A because it included
vacuum excavation to provide an exact three-dimensional location
and positive identification of the underground utility/utilities.
The SUE process also includes Quality Levels B, C, and D. Although
Level D involves only a database search and Level C consists of
just a visual inventory of the above-ground features of the site,
Level B employs geophysical methods to designate the existence and
horizontal location of utilities in the target area using electromagnetics,
GPR, and now CART.
Thus, underground utility
location today ranges from a technician walking a site with a $500
instrument and looking for visual and electromagnetic clues, to
a full-blown Level A survey that typically costs about 10% of the
total engineering budget or about 1% of the total project cost.
There is value in both approaches for contractors who want to control
the risk of excavating each new site.
Charles D. Bader is
with Dateline II Communications in Los Angeles, CA.
GEC
- September/October 2002
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