Construction / Underground

Mapping Subsurface Utilities Requires Detective Work

December 18, 2013
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The slogans “Call before you dig” and “Know what’s below” are familiar reminders of the importance of verifying the location of all kinds of subsurface utility infrastructure before excavation takes place. It only takes a moment for a pipeline or conduit to be damaged by heavy equipment, potentially causing public safety issues and disruption of service; thus the need for subsurface utility engineering (SUE).

Upfront research can be time consuming but is well worth the effort to avoid unseen hazards that may delay the construction process and lead to even larger costs in the long run. An excerpt from the New York State Department of Transportation website states, “Conservative estimates based on the Department's experience, as well as the experiences of other states, indicate that for every $1 spent on SUE, overall project savings can average between $5 and $10. Other benefits include time savings during both design and construction, elimination of unexpected utility conflicts and unnecessary relocations, and enhanced safety.”

The task of locating utilities would be fairly straightforward if accurate up-to-date records were available; however, there are often many gaps in the data and the quality varies widely in the information that is accessible. An extreme example is “the Big Dig” in Boston, which involved turning a six-lane elevated highway into a tunnel under a city that was founded in 1630. Engineers not only encountered unmapped pipelines and utility lines, they had to deal with in-use subways, buried house foundations and sunken ships. There were significant cost overruns and schedule delays due to the complexity of this engineering feat, but it could have been even worse without preliminary analysis.

“On a subsurface utility mapping project, we are managing and minimizing the risk of all parties involved,” says Scott Croshaw, PLS, Vice President at Wilson & Company. “We take into consideration the public’s health and safety, the owner’s liability in case of accidents, and the rights of other entities that may have title to land that is impacted by the project. The potential for unplanned costs and schedule slips associated with redesigns and relocating utilities is very high if the subsurface mapping is not thorough and accurate.

 “We get to play detective and assemble facts from every possible source, rather than only relying on as-builts,” Croshaw adds. “We start with records provided by the utilities, but we look at everything, including licensing agreements, existing roadway plans, above ground utilities, joint use agreements on poles and visual inventory of pole usage. Supplementing this research with the surface locating accuracy provided by an SUE consultant ensures a complete investigation. A typical project can require mapping gas lines, fiber optic cable, water transmission lines, sewer lines, electricity conduits — there is a variety of subsurface infrastructure, particularly along highway corridors.”

New Technology for Underground Locating Techniques

3D Radar Tomography technology locates subsurface features using a multi-channel radar array to achieve a highly precise image of underground infrastructure, whether metallic or non-metallic. Accuracy rates of +/- 1-2 inches horizontal and +/- 4-5 inches vertical are attainable without excavation.

Airborne Ground Penetrating Synthetic Aperture RADAR (GPSAR) uses multiple transmit and receive antennas to detect subsurface metallic or non-metallic objects; however, depth of penetration varies with soil conditions. In addition to military applications, such as locating land mines, the technology is appropriate for locating underground pipelines and buried archeological features.

Radio Detection Sondes are self-contained signal transmitters that can be attached to a flexible rod for insertion or pushing through pipes, while smaller sondes can be used with jetting machines and ‘blown’ through pipes. A locator device on the surface receives the signals from the sonde. The operator marks the location and notes the depth indicated by the signals. 

 

Some of the challenges faced during the mapping process include abandoned utilities, unmarked and deeply buried utilities, non-metallic conduits and layered utilities. The search continues for the perfect tools. Conventional underground locating techniques are being supplemented by new technology, such as 3D radar tomography, Ground Penetrating Synthetic Aperture RADAR (GPSAR) and radio detection sondes that are pulled, pumped or blown through a pipeline or conduit. However, variables such as soil type impact the accuracy of some detection methods, so there is no one answer that fits every situation. It often depends on the geology of the area being mapped and the age of the infrastructure in the area. For example, if the depth of the existing utility and geology present challenges and a high degree of accuracy is required, a device such as Geospatial’s Smart Probe might be used to obtain XYZ centerline data.

Mapping subsurface utilities is a necessary part of preliminary design work, as well as for change orders and repairs on existing structures. This professional service offered by qualified SUE engineering and mapping firms is defined by four quality levels for existing subsurface infrastructure —Level D (lowest level of detail) through Level A (highest level of detail):

·         Level D consists of an inventory of existing facilities, typically requested by the Federal Highway Administration as a preliminary analysis in its study phase before rebuilding a highway. Information such as “Are there any known existing utilities?” and “Are there existing utility maps?” is gathered.

·         Level C is a survey and plot of 2-D visible objects and above-ground utility features that correlates the survey with as-built records and adjusts the line work produced from Level D research to coincide with the above ground appurtenances.

·         Level B involves trained SUE technicians locating utilities using electromagnetic, magnetic, sonar and other non-invasive methods, and a survey plot of the designated utilities and all information obtained during Level C/D investigations. This level is the most common request, often provided to design engineers to identify possible conflicts. Horizontal conflicts with the proposed design are identified and avoided by using SUE Level B information.

·         Level A exposes and provides visual confirmation of subsurface utilities at selected positions in order to identify their precise locations and depths and obtain 3-D information. These locations are based on a detailed analysis and are specific to a potential vertical conflict. The end result is a highly accurate survey and plot on composite drawing that effectively manages the risk associated with underground utilities.

 

Utility companies have not always been held responsible for the accuracy or completeness of their records, but with the development of geographic information systems (GIS), the quality and accessibility of data has improved. Some current GIS databases include attributed pipeline data (depth, size, material, condition, etc.), and assets are inventoried and precisely geo-located.

Transportation agencies and oil and gas companies are typical clients for SUE services, but there are many non-traditional needs as well. A school may need to map utilities before it builds a new playground, or a restaurant may have to know what is under an existing parking lot before it builds an addition. It is typical for utilities to install lines directly from Point A to Point B to save money, so the next person that comes along, often decades later, may be faced with a convoluted mess of easements, storm drains, pipelines, etc.

Wilson & Company was recently involved in a highway interchange project in New Mexico, the Paseo del Norte & Interstate 25, during the 0-30 percent planning stage. The firm, along with their sub consultant Cobb Fendley & Associates, Inc., was responsible for mapping many of the existing utilities through the corridors, which required meeting with the utility companies, locating all subsurface infrastructure, identifying old easements and locating the existing rights of way.

“Coordinating with the utilities is a crucial part of any project of this kind. Many utility companies have large amounts of data such as system maps and easements, but metadata is often lacking so quality of data is unknown and missing information is typical,” Croshaw says. “It is an iterative process. We review the evidence that is obtained through our research, while our field crews map the SUE marks and visible evidence of existing utilities. We assemble a map of all the evidence that includes designated utility spots, visible utility appurtenances, GIS resource information, existing easements and permits and use our experience and judgment to create the best map possible.”

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