Washington State DOT and Seattle utility move transmission lines to protect against future earthquake damage.
In February 2001, a major earthquake shook Washington's Puget Sound region. Among the structures damaged in the quake was the Alaskan Way Viaduct, an elevated highway that spans about 2 miles of Seattle's downtown waterfront. The roadway, which carries about 110,000 vehicles per day and is one of the main north-south routes through Seattle, needed to be replaced. But before that could happen, crews would have to address a lesser-known but equally important function of the viaduct.
Since it opened to traffic in the 1950s, the structure has spawned a growing web of utility lines, including two 115-kV transmission lines attached to the lower deck that provide power to nearly half of downtown and five 13.8-kV network express feeder circuits. The electrical lines escaped the quake unscathed, but the damage to the viaduct illustrated the system's vulnerability. If the structure was to collapse in the future, the lines, and by extension the city's power supply, would be at risk.
Moving the lines presented a significant challenge. Because the work zone is in a high-traffic area near two sports stadiums and several key freight facilities, including two rail yards and one of the Port of Seattle's main shipping terminals, construction had to be coordinated around event, truck and rail traffic. There were also other utilities — telecom wiring, high-pressure gas, storm water and water lines — competing for space in the corridor.
Enter Seattle City Light, which forged a unique partnership with the Washington State Department of Transportation (WSDOT) to relocate the transmission lines to nearby underground locations. Between September 2008 and completion of the project in December 2009, the construction team, which included crews from WSDOT, its contractor Frank Coluccio Construction and Seattle City Light, removed 5,300 ft of line and buried two 115-kV cross-linked polyethylene (XLPE) transmission lines. The project resulted in better protection for the city's power supply and set the stage for major road and bridge construction to replace the southern mile of the viaduct beginning later this year.
While there was no debating the need to move the electrical lines, reaching consensus on the best way to replace the viaduct was another matter. The viaduct is part of State Route 99, a vital transportation corridor in the region, but it also happens to sit on Seattle's waterfront. Since the area is a popular tourist attraction, many Seattleites saw the replacement of the viaduct as an opportunity to open up the waterfront.
Over the course of eight years, the city and state looked at more than 100 different replacement options, including elevated, surface and tunnel replacements. In January 2009, after an extensive public process, elected officials recommended replacing the viaduct's central waterfront section with a bored tunnel beneath downtown and other improvements designed to bolster the city's transportation system.
Meanwhile, planners identified several related projects that would need to be completed regardless, including replacing the southern mile of the viaduct — the structure's most vulnerable section — with a new side-by-side roadway. Closely linked to the south-end replacement was the electrical line relocation project, which would require strong coordination between state and local agencies.
Spirit of Cooperation
WSDOT was the contracting agency, and Seattle City Light had the work performed under the state contractor. The utility was in an oversight position throughout the project and participated in extensive review and testing.
Before construction began, designers thoroughly studied the project area to determine the relocation component based on the cable sizes and routing. They also had to determine the proper loading of the cables and systems, and what requirements existed for removal of the lines.
Seattle City Light was tasked with creating specifications and standards for the solid dielectric cable and for the transition joints. The agencies jointly agreed on the best way to terminate into the substation and attach wiring to the viaduct. It wasn't feasible to shut down both 115-kV lines simultaneously, so they had to work with the Bonneville Power Administration to carefully schedule any power interruptions.
Crews went out ahead of time to locate underground utilities, including water, sewer, gas, electrical and communications. They based their investigations on existing records and a base map developed by a team of subsurface utilities engineers. A utility profile was created to help crews understand what each of the crossing utilities were, and the actual depth that the different utilities had in terms of designing the actual installation of the new XLPE solid dielectric cable system since a certain spacing between the new transmission cables and the existing utilities is required.
In several cases, the as-built drawings did not match what crews found during excavation. Even objects that were accurately depicted in the plans weren't immune to problems. The city of Seattle uses a steam company, and the steam logs are active lines that, in this case, were located near one of the transmission lines. Crews discovered the line had been damaged by heat from the steam line and were forced to make unforeseen repairs north of the original project area.
In addition to utilities, excavation crews found other surprises, some of which drew the interest of archaeologists. Because the area was once home to rail operations and a depression-era Hooverville, crews adhered to a rigorous Accidental Discovery Plan designed to protect any potentially historical artifact they encountered during construction. Remnants of former rail structures were found buried, as were the charred remains of the Great Seattle Fire and a brick foundation from a building that dates back to the late-1800s.
While some of these discoveries ultimately weren't of archaeological significance, crews made sure they followed the proper protocol. Every scoop with the excavator could contain artifacts, making careful excavation an important component of the project.
In addition to taking caution during the boring and trenching, the construction team also had to decide upon the best option for cable technology. The self-contained fluid-filled cable (SCFF) system that formerly hung from the viaduct was first installed in 1972. It included two different types and sizes of cable. For the lines suspended from the viaduct, 620 kcmil aluminum oil-filled cable was used. Because of issues with heat and thermal resistance, 820 kcmil SCFF cable was used in the underground portions of the system.
Designers looked to the latest technology when selecting a replacement, opting for a solid dielectric XLPE cable for the underground portion. Although the U.S. has been slow to catch on, this technology is used frequently in Europe, because it is easier to install and splice, is less expensive and requires less maintenance when installed in a vault or duct bank.
The transmission lines run between the Massachusetts Street and Union Street substations. From the Massachusetts substation at the south end of the project, the workers installed two 115-kV circuits that were 1,000 kcmil copper. The solid dielectric conductor runs from the substation to the transition splice near Dearborn Street, which is about halfway between the two substations.
Crews had to do some unique construction to deal with sewer lines and overflow lines. The workers relocated five 13.8-kV network distribution feeders, which fed a third of downtown. The feeders originally came out of the Massachusetts substation and were suspended alongside the transmission conductors.
The electrical workers terminated the XLPE cables inside of the substations with the latest technology and installed lightning arrestors. While the utility employees did the testing and phasing, the company hired contractors to take care of the splicing for the 115-kV cable at each of the two vaults and terminations at Massachusetts substation and the transition joints.
Several unique tools were used on the project, including two different degasifiers. One was manufactured by Brugg Cable, and the other was owned by Seattle City Light and manufactured by KATO. These tools helped to maintain constant pressure within the SCFF cable. Because the oil is an insulating fluid, all gas and moisture must be removed through a vacuum filter process and then be pumped under pressure back into the SCFF cable system.
Half by Half
Originally engineers planned to construct the line between the Massachusetts Street and Union Street substations, which are located about a mile-and-a-half apart. As the decision process for the waterfront portion of the viaduct lingered on, however, it became clear that removal of the electrical lines would have to be limited to the viaduct's southern mile. The team decided to move ahead with the project in the south end and splice the new lines at the halfway point to those still attached to the viaduct's central waterfront section.
The transition joints are a unique set of splices that have an oil-filled low-pressure side with special small oil tanks located at the splices, and then a transition to the solid dielectric cable side that has a special small polymer oil-filled tank at the splices as well. They are the first of their kind to be installed in the Western U.S. The splicing took four to six weeks per circuit and about four to five weeks for the transition splices alone.
Crews installed the splices from platforms suspended 35 ft above ground. Bolted to the viaduct, each platform measured about 17 ft by 25 ft and will remain in place for several years, until the second half of the project can be completed. I-beam framing with steel decking supported the workers while they assembled the splice, in addition to supporting the splice in place to prevent it from falling apart.
Safety was a major priority throughout construction. Every aspect of the project — from working with lines to installing the platforms and mounting equipment — was monitored by a continuous safety watch. While working on the underground portions of the project, crews performed inspections and had to be mindful of energized equipment. When trenching into the substation, the contractors excavated and trenched between and around energized capacitor banks and getaway feeder circuits. No metal or steel shoring or trench plates could be used inside the Massachusetts substation, as it was a working substation delivering power to the industrial end of Seattle as well as the five mass feeders and nine 13.8-kV distribution feeders that cover a good part of downtown.
Entering the new year, the viaduct program is poised to move forward. The electrical lines have been taken off of the southern mile of the viaduct, and road and bridge construction is set to begin in late spring.
Meanwhile, work on the central waterfront viaduct replacement is contingent on an ongoing environmental review that's expected to wrap up in 2011. When the time comes, WSDOT and Seattle City Light will be ready to take down the remaining lines to the north, so the viaduct can be demolished and Seattleites can be assured of even greater power reliability in the future.
Ron Paananen (PaananR@wsdot.wa.gov) is WSDOT administrator for the Alaskan Way Viaduct and Seawall Replacement Program.
Mark Anderson (AnderMA@wsdot.wa.gov) is project engineer for the utilities for the Alaskan Way Viaduct program.
Jeff Joy (Jeff.Joy@seattle.gov) is the director of operations for Seattle City Light and worked with the viaduct program for four years for Power Engineers before joining the utility company.
By the Numbers
Start date: September 2008
Completion date: December 2009
Cost: $21.8 million
Participants: Seattle City Light (www.seattle.gov/light), Washington State Department of Transportation (www.wsdot.wa.gov), Frank Coluccio Construction (www.coluccio.com), Brugg Cables (www.bruggcables.com)
- 25,022 linear ft, 115-kV 1,000 kcmil copper XLPE 1/C cable (transmission cable)
- 15,896 linear ft, 13.8-kV 1,000 kcmil copper EPR 3-1/C TX cable (distribution cable)
- Installed two transmission and four distribution panel vaults
- Installed two trifurcating/transition splices (on a steel platform bolted to crossbeams of viaduct) measuring 17 ft by 25 ft
- Installed a pressurized oil system to support the existing SCFF cable system.