Loudoun County in Northern Virginia is one of the fastest-growing counties in the United States, growing 84% in the last 10 years, according to the most recent census statistics. Located approximately 25 miles (40 km) northwest of Washington, D.C., the county is a business and commercial hub, home to federal contractors such as Northrop Grumman and Rockwell Collins, high-tech companies like Verizon Business, VeriSign and AOL, as well as Dulles International Airport. The seat for this growing county is Leesburg, Virginia, a quaint bedroom community with many residents who commute to the federal capital for work.

Since the 1980s, western Loudoun has been fed by a mere four distribution lines, each approximately 15 miles (24 km) in length. In the 1990s, the regional public power utility, Dominion Virginia Power, investigated ways of reinforcing the regional power system serving this area, scoped out locations for power facilities and even purchased a site for a future transmission substation.

However, it was not until early 2005 when Dominion was able to move forward with a new transmission line. At that time, Dominion made an application to the State Corporation Commission (SCC) for a new overhead transmission line running from the Pleasant View Substation, located east of the Leesburg Municipal Airport, to a new Hamilton Substation, 12 miles (19 km) to the northwest.

This project caused a predictable stir in the community, especially among homeowners along the proposed route and users of the historic Washington and Old Dominion (W&OD) Trail that runs right through the heart of Loudoun County.

In response to the public outcry, which was buoyed by several vocal community opposition groups, Dominion and the SCC exhaustively researched alternative routes (46 options) for the 230-kV transmission line between the two substations. The final order for the line did not come until 2008, when the Virginia Legislature intervened, resulting in approximately 2 miles (3.2 km) of the 12-mile route being located underground along the W&OD.

Responding to a Predictable Stir

From the start, Dominion knew — even with a clear need and the intervention of the state legislature — it would have to engage the public and affected stakeholders like never before. Dominion staffed the project with a communications manager to reach out to the affected communities during all stages of the project to inform and educate community and stakeholder groups about the project. At the beginning of the project, Dominion reached out to the Northern Virginia Regional Park Authority (NVRPA), which maintains the W&OD Trail that is popular with runners, bikers and equestrians.

The trail originally existed as a Civil War-era railroad, but was later retired, dismantled and sold to a Dominion subsidiary in the late 1960s. Between 1974 and 1982, sections of that railroad were sold to the NVRPA for the trail's construction. Dominion retained a 100-ft (30.5-m)-wide right-of-way easement for transmission lines.

Respectful of the value of the W&OD Trail to the region, Dominion met with representatives from the NVRPA to identify its primary concerns. The NVRPA requested trail usage not be disrupted along the project route, nor safety hazards be introduced, and that the trail be restored to its original condition after construction. All of this had to be factored into both the design and the construction plans.

The W&OD Trail is relatively flat but goes through heavily forested and hilly terrain. In places, the corridor — consisting of an 8-ft (2.5-m)-wide asphalt path and a crushed-stone equestrian trail — is channeled with high, steep sidewalls; crowned with steep drops on either side; and hemmed in with trees and vegetation. On average, the corridor is around 30 ft (9 m) wide and, in some cases, necks down to less than 20 ft (6 m).

This made the new line's landscape quite different from a typical underground installation along city streets. In that type of urban environment, there is often plenty of room to work. For example, if one lane is blocked on a city street because of line installation, there are typically parallel lanes in both directions where traffic can be diverted and construction equipment can be pulled up alongside the open trench. In the narrow W&OD corridor, there was no such side lane. This meant construction would have to be performed serially up the W&OD Trail, one section at a time.

Design and Accommodation

Dominion selected cross-link polyethylene (XLPE) cable for this project and had the benefit of recent experience installing this cable type elsewhere in Virginia. During an earlier project, a capable 230-kV XLPE cable supplier was found in General Cable. The company provided cable-specific activities, including design engineering and oversight of both duct bank construction and cable installation, as well as the necessary cable splicing, terminating and testing. General Cable subcontracted the design engineering to POWER Engineers to determine the best design configuration for the narrow corridor.

The driving factor in the project's design was to give Dominion the power to balance present and future load conditions. This made for a tricky design of the duct bank, cable system and termination stations. To achieve this balance, Dominion envisioned the project as a two-step process. The first step would be to install a single-circuit, two-cable-per-phase 230-kV transmission line. Step two would involve the later installation of a second 230-kV circuit when necessitated by regional load growth. To accomplish this, Dominion decided to build both duct banks at the same time, so the majority of the most disruptive construction activity would be performed just once.

Several challenges were associated with this approach. The first was the issue of manhole siting. The NVRPA had requested manhole lids be placed off of the paved portion of the trail, which meant Dominion would have to pay attention to how the manholes were positioned. Because of the width of the manholes and the narrow working area of the trail, Dominion had to site the manholes in an offset position. On top of that, because Dominion could not get cranes and other heavy equipment into certain portions of the trail, the utility was forced to locate the manholes near construction access points that originated along Dry Mill Road. This is a narrow, rural road without shoulders that runs roughly parallel to the W&OD Trail.

Another challenge was the issue of what to do about all the W&OD travelers. Laying two duct banks along the entire 2 miles of underground route meant the existing path would have to be demolished — there was no way all the trail users would be able to safely access the path with two open trenches and construction equipment along its length. Fortunately, the NVRPA and a few adjacent property owners were willing partners in solving this concern. They worked with Dominion to identify a route for a temporary bypass trail, which Dominion designed, built and opened to the public prior to the start of line construction.

Flexibility for the Future

Because Dominion elected to install two duct banks, it gave the utility flexibility to install cables in a counterintuitive manner. Typically, all cables associated with the first circuit would be installed in a single duct bank. By installing half the cables in each bank, mutual heating effects were reduced and the capacity of the installed lined increased. This will allow Dominion to delay the installation of the second circuit and its associated cost. When this circuit is installed, existing cables will be re-arranged at the terminal stations and the remaining cables will be pulled into open conduits, so one complete and independent circuit will run through each duct bank.

This two-step process made for a complicated transition station design, as shown on the plan diagram of the northernmost station, Breezy Knoll. The present aboveground configuration consists of an 80-ft by 180-ft (24-m by 55-m) fenced yard with a 60-ft (18-m) H-frame takeoff structure for the incoming overhead conductor. Disconnect switches are mounted on either side of the H-frame. At the base of the structure are two 10-ft (3-m)-high stands for the cable terminations and arresters.

In this present configuration, three-phase cables run right to left from manhole 1B, travel through conduit along the northeast side of the yard and end at the northernmost part of the existing termination structure at terminal A. The second set of three cables runs from manhole 1A through conduit to a point just beyond the edge of the future terminal B. Beyond that point, the cables are direct buried in thermal sand all the way to the southernmost side of terminal A. Conduit is in place for the future configuration, emanating from both manholes 1B and 1A. The conduit leaving manhole 1B ends just beyond the northernmost side of future terminal B, and the conduit exiting manhole 1A stops just inside the fence line.

In the future configuration, Dominion will install a second H-frame structure at terminal B, along with two termination stands similar to the original structures. The stubbed-out conduit from manhole 1A will be extended to the new south termination stand at terminal B. The thermal backfill surrounding the direct-buried cable running to the existing south termination stand at terminal A will be removed and the cable relocated to the new north termination stand at terminal B. Subsequently, the stubbed conduit from manhole 1B will be extended to the existing south termination stand at terminal A. After conduit installation, new cables will be pulled through the entire line length to create the ultimate build-out for these circuits.

Construction Approach and Challenges

POWER Engineers completed design activity at the beginning of 2009. Dominion spent most of the next year working with the community and stakeholders to acquire construction permits, develop traffic and community action plans, and devise a construction strategy.

Interestingly, Dominion structured the construction activities to have a civil firm act as the prime contractor instead of an electrical outfit as is commonly done for underground projects. Because the vast majority of this work involved civil construction, having a civil contractor act as the prime allowed Dominion to avoid the markups associated with this activity. C.W. Wright Construction Co. Inc. was selected as the lead civil contractor, and New River Electrical performed cable installation.

Construction kicked off along the southern half of the line in January 2010, and crews worked their way north from the new Dry Mill South transition station one section at a time. During each stage, Dominion would tear out the existing asphalt trail, excavate trenches, install duct bank and manholes, and later reinstall W&OD Trail asphalt to an improved width. Because the path of W&OD Trail forms a natural drainage channel, Dominion installed silt fence along both sides to minimize drainage effects and, in some cases, had to clean out clogged culverts and install new drop inlets and culverts.

In February 2010, a winter storm dropped a record amount of snow in the Northern Virginia-Washington, D.C. metro area. The storm, referred to as Snowmageddon by Dominion operations and maintenance personnel, dumped more than 30 inches (76 cm) of snow in the area, which halted construction activities along the Pleasant View-Hamilton construction route while crews dug out.

The project experienced further delays when equipment at the site was redeployed to other locations around the state to restore power in the region. In addition, the project was interrupted by two more winter weather events during construction, which put additional pressure on an already aggressive construction schedule.

Weather was not the only construction challenge during the project. On the north half of the project, the line route veers off the W&OD Trail and cuts across adjacent Dry Mill Road. Although a narrow country road, it has a lot of commuter traffic and is the only access road for many area residents. It also is an alternate emergency route if nearby highway Route 7 is closed.

The Virginia Department of Transportation stipulated that traffic could not be unduly affected, and it approved a detour plan that closed a portion of the road for five days. In the weeks before construction began, Dominion's communication team alerted the public about planned activities through radio and newsprint, and equipment was at the ready if emergency service vehicles needed to cross the work area. Due to the attention to detail in planning this effort, the road work was completed a day ahead of schedule.

Energized on Schedule

Despite delays during construction, and a lot of moving parts — from working with the community to build an alternative bike path to catching up after a series of serious weather events — Dominion hit all of its construction milestones, and the line was energized two days ahead of its Oct. 31, 2010, scheduled energization date.

Now that it has been installed and is delivering reliable power to residents of Loudoun County and the greater region, the project has scored several notable successes in the form of awards. It was a finalist for the Edison Electric Institute's 2011 Edison Award and the winning project in the transmission category for Southeastern Electric Exchange's 2011 Industry Excellence Awards program.

Accolades aside, the real value of the project is that it offers insight into the field of underground transmission, specifically that an underground line can be designed and built in the narrowest of corridors with a number of design and construction constraints. It also offers instruction on how a communications manager can implement effective dialog with local communities and affected stakeholders to build trust and acceptance of a project that is necessary but not always popular. Last but not least, it proves that all of this work can be done while still maintaining a good safety record.

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David Witt (david.c.witt@dom.com) is a senior engineer with Dominion Virginia Power. He has gained a wide range of experience with transmission and substation projects while working in several engineering, permitting and project management groups. Witt earned a BSEE degree from the Virginia Polytechnic Institute, and he is trained and practiced in the areas of Six Sigma, underground line design and project management. He works in Dominion's electric transmission planning group.

Dennis Johnson (dejohnson@powereng.com) has worked for POWER Engineers as a senior project engineer since 2001 on a wide range of underground transmission projects ranging from 69 kV to 500 kV. He served as the project engineer on this project for Dominion. He holds a BSEE degree from Brigham Young University and is a voting member of IEEE and a committee member of CIGRÉ. He is a registered professional engineer in 13 states.

W&OD Trail

The Washington & Old Dominion Trail has been called “the skinniest park in Virginia.” But, it also is one of the longest parks, with 45 miles (72 km) of paved trail for walking, running, bicycling and skating and 32 miles (51.5 km) of adjacent gravel trail for horseback riding. Built on the roadbed of the former Washington & Old Dominion Railroad, the trail runs through the urban heartland and countryside of northern Virginia.

Companies mentioned:

C.W. Wright Construction Co. Inc.
www.cwwright.com

Dominion
www.dom.com

General Cable
www.generalcable.com

New River Electrical
www.newriverelectrical.com

POWER Engineers
www.powereng.com