Seeing the Unseen

Located in the heart of central North Carolina and home to some 25,000 students, the University of North Carolina at Chapel Hill is one of the oldest and most respected public institutions in the country. Much like a small city, the university relies on its web of underground infrastructure to keep it up and running. And while the university is now in its fourth century of operation, it has by no means ceased to grow. In fact, the university recently built a new steam plant on campus to add capacity and redundancy to its heating and cooling capabilities.

In conjunction with the new plant, a 4,200-ft cut-and-cover tunnel was needed to deliver the steam, cooling lines and other utilities across campus. Not surprisingly given the age and expanse of the campus, successfully navigating existing subsurface utilities was a top priority.

To be certain that it had the best information possible, the university commissioned a subsurface utility engineering (SUE) program in advance of the excavation. Carter and Burgess, the design firm for the steam plant and tunnel, commissioned KCI Technologies to complete the SUE.

What Is SUE?

Subsurface utility engineering is an engineering process for characterizing the quality of information regarding subsurface utilities, the process of identifying and verifying the location of utilities and managing the information during a project. According to the Federal Highway Administration (FHWA), SUE began in the 1980s as a way to streamline construction projects. Previously, it was common for projects to be designed without considering existing utilities. Consequently, redesigns or relocations could bring construction to a halt, as well as escalate the cost of a project.

It has been shown that use of SUE in advance of a project can result in construction savings. A Purdue University report published in 2000 shows that every $1 spent on SUE results in $4.62 in avoided costs.
Although SUE grew up in the highway sector, its use in underground construction projects is becoming increasingly common. The university learned from past projects that SUE is a useful tool. “Our experience has shown us that there are always unknowns and that there is a benefit to locating existing utilities as best we could before we actually started digging,” said John Masson, project manager for the university’s Facilities Planning Department.“The more documentation we have, the easier it is for the contractor. There are fewer surprises and fewer unforeseen conditions.”

According to the FHWA, there are four different quality levels pertaining to utility information.  

Quality Levels

A subsurface utility engineering program breaks down utility information into four different quality levels — Quality Level D (least information) through Quality Level A (best information). The following descriptions of the quality levels were summarized by R. Mark Pitchford during the Underground Infrastructure Advanced Technology Conference in 2002:

Quality Level D is the most basic level of information, based solely on existing records that are often inaccurate, unreliable and/or out of date. By providing a general overview of utility congestion, this level would be appropriate only to support decisions made very early in the project planning process.

Quality Level C has been the most commonly used level of information for engineering projects. It involves supplementing Quality Level D information with a visual ground survey of utility features such as manholes and valve boxes. Studies by the Federal Highway Administration have found, however, that it is not unusual that 15 to 30 percent of the underground utilities are either not discovered or are plotted more than 2 ft from their actual location when using Quality Level C techniques. Because of the age and construction history of the utility systems at federal facilities, it would not be surprising to find at least this degree of error when performing projects at military installations.

Quality Level B is the first level where the SUE designating activity is introduced into the utility location process to provide two-dimensional horizontal mapping information along the entire length of utility lines. This level addresses problems caused by inaccurate utility records, abandoned or unrecorded facilities and lost references, and, in the end, eliminates the need for Quality Level C information. Quality Level B is particularly useful during the design phase of a project in that slight adjustments in the design can produce substantial cost-savings by avoiding utility conflicts and eliminating utility relocations.

Quality Level A represents the highest level of informational accuracy available to final design decisions. In this case, SUE locating information is added to Level B designating information to provide the exact location of a utility in the vertical direction and a precise three-dimensional mapping of the utilities and related structures. Quality Level A information is obtained through small test holes, just 8 in. in diameter at the top, which are completed using specially designed nondestructive vacuum excavation equipment.

In planning a construction project in which existing utilities may be encountered, Masson says the university evaluates whether SUE will be used, as well as the Quality Level to be achieved, on a case-by-case basis, but that it is common on utility projects such as the utility tunnel. “On most of the utility projects we find that SUE is a big benefit because the lines go through so many different areas. There are even some building projects on which we use SUE because we know there are utilities in the area that will have to be relocated,” Masson said.

Masson said the university’s approach to SUE use was a gradual process. “We have been using SUE increasingly over the past 10 years,” he said. “We had done some utility projects without SUE and found unforeseen conditions that we then have to deal with in the field, rather than design around them. If we have to deal with it as a change order it can be much more expensive.

“SUE allows us to confirm the exact location of utilities that we know about, as well as find some utilities that we didn’t know existed, like abandoned water or sewer lines.”

On the Job

For the 4,200-ft utility tunnel being built by the University of North Carolina, KCI was called in to perform the SUE. The tunnel, roughly 12 ft by 12 ft with the crown 8 ft below grade, traversed a largely residential area. KCI crews found 11 separate utilities, some owned by the university and others by private companies. In some cases, private companies are not even aware of the location of their own utilities. Finding the unmarked utilities ahead of time almost certainly averted significant increases in construction costs.

Jim Gellenthin, KCI vice president in charge of surveys and SUE, said that SUE should be incorporated early on, with higher quality levels needed as the designs progress. “What you will typically see is a Quality Level C done in the planning stages,” he said. “This step would consist of collecting utility records and visually identifying surface components like fire hydrants or electrical cabinets.

“For the initial design, we recommend that Quality Level B information be collected, and as you proceed to final design, you should have Quality Level A information. All it takes is for one utility hit to recognize the value of a subsurface utility engineering program.”

Gellenthin said he is seeing more public agencies using SUE, but its use still lags on the private side. UNC has its own examples to cite when justifying the upfront cost of SUE.

“We had a steam line installation job about 10 years ago where we encountered a sewer line,” he said. “We had to work through a change order and eventually it cost us an additional $100,000. After that, it was easy to see the value of SUE.”

Once a project is complete, there is an added benefit of having robust data to input into the agency’s GIS. The results of the SUE program provide accurate, three-dimensional data of buried utilities systems that will be available for future generations to easily access.

James W. Rush is editor of Trenchless Technology.