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 SystemsOne-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-CallUnquestionably, 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.
A typical 1- x 1-ft. “pothole” created during Level A vacuum excavation activitiesTravis 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 EasierMost 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.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 EngineeringThe multiple-frequency, digital Verifier detects and identifies underground utilitiesMany 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 ElementsIdentifying, 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.
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