Since the 1970s, electric distribution cable has been installed underground in residential developments, business and office complex campuses, and government and educational facilities.
Water, sewer and gas pipes had always been buried, but placing electric, telephone and later television cabling underground and out of sight became a sales point for real estate agents to promote “all utilities underground” neighborhoods.
Putting the “dry” utilities underground was made possible and economically feasible by the evolution of compact trenching equipment that could quickly dig narrow, easy-to-refill trenches for small-diameter cable and conduit as well as the development of cables that could withstand being buried in the ground.
In the early 1990s, the first horizontal directional drilling (HDD) machines designed for utility work began appearing on job sites, providing a mostly “trenchless” method of burying cable and conduit. Cobb EMC, Marietta, Ga., acquired two of the first machines sold to replace old electric cable in residential neighborhoods.
While HDD has significantly impacted the way underground electrical distribution cable and other utilities are installed, much of the nation’s electrical distribution system is overhead, and it may surprise some that the primary methods of installing underground electric cable is trenching.
Following is a brief overview of the options available to install underground segments of electrical distribution systems:
Trenchers for installing electric cable range from compact walk-along models to riding machines up to around the 30-horsepower class. Compact excavators and conventional backhoes can dig trench for conduit or cable, but progress is slow and the trenches are much wider than necessary to accommodate the product, which also makes restoration much more time consuming.
HDD units have a “size” that is generally stated in pounds of push and pullback. An HDD installing power cable in a residential area generally would have 15,000–30,000 pounds of pullback. The elimination of most excavation, the capability of going under surface obstructions and the ability to steer the pilot bore are valuable benefits of directional drilling.
One of the most significant advances in HDD equipment is a dual-pipe mechanical drilling component able to drill and steer in hard-rock conditions.
Larger HDD models can make installations of thousands of feet, including crossings under bodies of water, large paved areas and other surface obstructions.
Other options include disc-type trenchers for cutting through pavement and frozen ground, and vibratory plows that install pipe or conduit either through a chute in the plow blade or attached to a pulling blade. Product is buried as the unit moves forward with the shaker box activated.
Unguided pneumatic piercing tools pound their way through the ground under sidewalks, drives or streets, and the product is pulled through the bore hole. Microtrenching systems include a compact disc trencher and apparatus to feed cable or conduit into the trench produced. This method has been used for installing fiber optic cable.
Representatives of three electrical service providers shared information about underground segments of their systems.
Christopher Laird, executive director for electric operations at Consumers Energy, Jackson, Mich., explained that approximately 15% of the entire low-voltage distribution system is underground.
“Most of the underground cable is on the distribution primary system. There has been no significant increases in underground, and none is projected. Consumers Energy does not own transmission lines,” he said.
Trenching and directional drilling are the most used methods for underground cable installation. Trenching is typically used unless there are obstacles such as roads, driveways, significant landscaping, etc. Directional drilling accounts for less than 20% of the installations for new construction. There is a significantly higher cost for directional drilling compared to trenching, but restoration costs change significantly if roads, driveways or other obstructions are involved.
Vibratory plowing is used occasionally, but not piercing tools or microtrenching. Hydrovac “soft” excavation with high-pressure water is also used occasionally where needed.
Underground construction is performed primarily by company personnel, but contractors are used as well. For areas where One-Call has no responsibility, the company uses contract locators.
Consumers Energy has no undergrounding program to replace aerial cable and place it underground.
At Dominion Energy, Richmond, Va., Steve Eisenrauch, manager of electric distribution, said easily a third of Dominion Energy’s distribution facilities currently are underground. That amounts to approximately 20,000 miles for Dominion Energy Virginia and approximately 5,700 miles for Dominion Energy South Carolina.
“Since Dominion Energy Virginia’s Strategic underground program was implemented in 2014, we have converted to underground over 1,500 additional overhead distribution facilities. This program is designed to replace the overhead electric distribution lines that are most prone to outages and that can be cost-effectively converted to underground,” Eisenrauch said.
The majority of the underground cable in Virginia’s and South Carolina’s distribution systems is across all segments of the company’s distribution facilities.
For new construction, open trenching is the typical installation method (used 34% of the time). When necessary, due to environmental factors such as soil conditions, wetlands or tree-heavy areas, or when requested by the customer, HDD is also used (66% of the time).
For most new construction installations, restoration after open trenching simply includes backfilling and tamping. Significant restoration such as seed, straw, sod and landscaping is not typical unless specifically requested by the customer or required by the guidelines for special projects and programs. Drilling can be up to seven times more expensive than trenching.
In cable replacement projects, the typical installation method used is HDD, followed by property restoration with seed and straw in places where digging is required.
“A piercing tool is utilized when there is no room to set up a drill. In situations where driveways or sidewalks are present along the cable route, piercing tools may be used to install cable along those small sections of the route. Limited plowing is done in some of our rural areas; we do not utilize microtrenching,” Eisenrauch said.
Before excavation, existing facilities in the public right-of-way are located and marked by a contractor. In areas not serviced by One-Call, the property owner or tenant of the property is required to mark their own private facilities. Within certain special programs, the company has authorized its contractors to hire third-party locators to help locate private facilities beyond the 811 marking responsibility.
Dominion Energy Virginia has installed some 230 kilovolt transmission facilities underground, though the majority of its transmission facilities are installed overhead.
Eisenrauch ended with a final note about the strategic underground program: “Thanks to being in conjunction with other reliability programs, Dominion Energy Virginia has eliminated thousands of outage events over the past two years. While new programs have been implemented in the last two years to help reduce outages, many long-term, established programs have contributed to the outage reduction as well.”
Dean Sherrick is distribution superintendent for Edmond Electric, Edmond, Okla., a community-owned utility. He said all residential and commercial new construction projects are installed underground. It’s really only the new main feeder distribution lines that are installed overhead on poles. They run along roadways, and because of the amount of load they carry, it is cost-prohibitive to put those underground for any distance greater than roughly 200 feet.
Roughly 90% of the underground electric lines are trenched, with 10% installed using HDD.
Open trenching is more common for new residential and commercial development installations where obstacles are not in place. HDD is most often used in existing landscaped areas (i.e., established residential yards, underneath existing roadways, etc.).
When aerial cable is replaced, it generally goes underground.
“In our ‘5-Year Plan and Budget,’ we have identified locations where we will be converting older/aging overhead lines to underground residential distribution as part of our preventive maintenance program. The main/feeder lines running along roadways are typically installed overhead on poles due to the cost associated with installing larger cable,” Sherrick said. “We developed a plan to convert overhead to underground, but its implementation is dependent on time constraints of all the new construction projects and manpower needed for higher demanding projects. There are currently no active projects of this sort, but it is still in the plan.
“In addition to trenching and HDD, we have used vibratory plowing only for streetlight wire. Plowing other types of electric distribution wire could damage the insulation, creating a potential cable failure,” he continued.
“Our company’s in-house crews work closely with contractors to ensure proper installation depths and minimal surface damage is achieved. Our crews will inspect the work performed, and we have the option to have the contractors install the cable in conduit or to do it ourselves,” Sherrick said.
Edmond Electric has also recently started using cable-in-conduit as a way to shorten the duration of a project, which seems to balance the cost/satisfaction in the long run.