Proper separation of underground utilities, especially gas and electric, has become an important issue since an explosion in Virginia in 1999. The National Transportation Safety Board (NTSB) determined that an electric line overlaying a plastic gas line arced and melted the gas line. The gas migrated into a home and an explosion occurred, resulting in one fatality. The NTSB investigation discovered the presence of corrosion and subsequent overheating and arcing at a splice in one of the conductors of the nearby triplex electrical service line.
The NTSB concluded: "Had the gas and electrical service lines involved in this accident been adequately separated, the heat from the arcing electrical failure would probably not have damaged the gas service line, and the accident would not have occurred."
Many distribution companies are taking seriously new practices that will prevent accidents and outages to its customers. These proactive companies have developed safe separation procedures for installation of new lines in common trench situations. These procedures include the use of economical, high-density plastic spacers to ensure safe separations.
Less Expensive Than a Box of Dirt
In the early years of underground line work, several separation methods were practiced. Some company crews used 6-inch x 12-inch cardboard boxes filled with dirt to separate utilities. Boxes had to be assembled, placed in the trench, filled with dirt and positioned between the gas and electric lines. This was a time consuming chore.
"In wet, raining conditions the boxes collapsed and the fill dirt was like soup-it was a real mess," one grounds crewmember said.
The real issue was that the boxes cost nearly $12 each. Many utility companies also used the wooden stake method to keep conduit, pipe and cable separated in a joint trench. This method, however, is not only costly but also labor intensive.
One Northeast utility company recently re-evaluated its separation procedure and decided to use Common Trench Spacers, a simple, economical solution developed by Innovative Trench Solutions Inc. in Rochester, N.Y.
In the first project, the utility was to extend services to a new facility under construction. Local telephone, cable television (CATV), gas and electric were in a common trench.
"The project was approximately 1,300 feet and spacers were ideal to maintain the 12-inch separation because of the muddy and often frozen conditions," one crewmember said.
"The process is simple," a field superintendent said. "Typically, the other wire utilities, telephone and CATV are placed in the 2 1/2- to 3-inch wide trench prior to our installation of gas and electric lines. Our crews first roll out and install the gas line along one side of the trench. The electric line is then installed on the opposite wall of the trench. After all utilities are in the trench, our crews install the tracer wire, first attaching it to a stake at one end of the trench and stringing it along the center of the ditch next to the gas pipe. The spacers, which are designed to snap onto the gas line, are attached every 10 feet along the pipe-or more often if the trench is curvy. As this is being done the spacer is tucked under the other wire utilities in the trench. The spacer has a clip some 6 inches away to house the tracer wire-to comply with Federal Rule 49 CFR, part 192.321-and a vertical post 12 inches away for the electric line. Crews then attach the tracer wire at the center clip 6 inches from the gas line. Crews are careful to gently tug at the tracer wire to take out the slack and maintain a taut position. Next, the electric line, which is already in the trench, is positioned on the far side of the vertical post to ensure 12 inches consistent separation. After final inspection, the trench is backfilled. The spacer ensures there is no displacement of the utilities during the backfilling process. Not only is the Common Trench Spacer simple to install and weather resistant, it is priced under $2 per spacer-compared to the $12 cardboard boxes or the labor-intensive wooden stakes."
Similar to that service extension, the company installed 5,200 feet in a new subdivision, installing its gas line with the incumbent electric utility, telephone and cable. On both jobs, melting snow and freezing/thawing muddy conditions were addressed using Common Trench Spacer separators.
"All we want to know is that when it's covered up, it stays separated," a crew foreman said. "We would have had a difficult time without the spacers."
To address safe separation industry concerns, the Common Ground Alliance board of directors approved for inclusion in the CGA Best Practices Manual the following best practice for separation: "When installing new direct buried supply facilities in a common trench, a minimum of 12-inch radial separation should be maintained between supply facilities such as steam lines, plastic gas lines, other fuel lines and direct buried electrical supply lines. If 12 inches of separation cannot be feasibly attained at the time of installation, then mitigating measures should be taken to protect lines against damage that might result from proximity to other structures. Examples may include the use of insulators, casing, shields or spacers. If there is a conflict among any of the applicable regulations or standards regarding minimum separation, the most stringent should be applied."
The CGA best practices sub-committee delivered the recommendation to the NTSB after an 18-month, industry-wide consensus process. Al Yonkman, Detroit Edison and Electric stakeholder on the CGA board, said: "This was a highly debated issue because it impacted all utilities. We believe it will establish a safer standard for installation, save lives and mitigate damages."
Joint trench has become the preferred method in new subdivision installation. In recent field studies, several utility companies identified safety and cost benefits:
• The timeline from design to completion is shortened with multi-utility installation;
• Reduction in number of contractors working in the area improves safety;
• Shared installation cost saves participating utilities 30 percent to 40 percent;
• Developers and the community appreciate the smaller right-of-way requirements;
• The consistent design makes future excavation in the area easier, minimizing the number of mismarks; and
• The 12-inch spacing eliminates the potential for energized lines accidentally damaging plastic gas lines in the right-of-way.
Crews are quick to mention that for future line locating, it is strongly advised that as-builds include notations detailing which side of the tracer wire the gas and electric line are positioned. In cold weather, the spacers should be stored at room temperature; avoid excessive temperatures in the summer months.
The spacers ensure separation is not compromised during the backfilling process. Installation over the wooden stake method is less labor intensive and ensures the tracer wire stays where it belongs.
Acceptance of common-trench industry practices varies, depending on the market and utility you talk to. Some utilities are apprehensive about the close proximity, especially when repair and maintenance is required. Some have had problems with the logistics of coordinating multiple utility engineering. But, in recent years, a growing number of utilities have reported successful, cost-saving, joint-trench practices for new underground residential subdivision installations.