Stray current mitigation in Portland's Transit Mall
Aaron Eder, P.E.
Project Manager/Civil Engineer
From its modest beginnings in the early 1870s when horse-drawn rail cars plied First Avenue, light rail in Portland flourished, declined, and has enjoyed a resurgence. By 2030, planners forecast a million new residents in the Portland metropolitan area. The Portland Mall Light Rail project is a key component in the planning effort connecting Clackamas County, one of the region's fastest-growing areas, with Portland State University (PSU), the number one destination in its transit system. However, light rail systems such as Portland's impose unique impacts on utilities within the light rail corridor. This fast-tracked project mitigates those impacts while reducing the duration and cost of construction and minimizing the impacts to downtown businesses.
When the Portland Mall was completed in 1978, it received immediate international attention as a model for transit and downtown redevelopment. The Portland Mall revealed a prototype for redeveloping a downtown using a transit project as the catalyst. Today the Portland Mall—commonly referred to as the Transit Mall—serves many purposes. It fronts on office buildings and retail businesses. It is a transit facility with the highest concentration of bus service in the city, and it is an important public space comprising 17 total blocks in downtown.
After more than 25 years of service, time has taken its toll on the Transit Mall. The most noticeable problem is that it looks worn out. The bricks and granite pavers are cracked in many places and the benches and other furnishings need repair or replacement. But there are also other problems. Several businesses have found that the mall is a poor front door for their businesses. Some have even closed their entrances fronting on Fifth and Sixth Avenues.
History of Light Rail in Portland
In the 1870s, Portland's first trolleys were horse- and mule-drawn, operating on First Avenue from NW Glisan Street to SW Caruthers Street. In the 1900s, the Portland metropolitan area had the first interurban electric rail service in the nation. By 1912, the electric rail system saw its high point with 28 electric rail lines, serving a city population of 250,000. However, after World War I, electric rail lines declined with the emergence of the automobile. During the Depression in the 1930s, streetcars began to be replaced with buses, and in 1950, the last streetcar was retired.
By the 1960s and 1970s, Portland was violating federal clean-air standards on a consistent basis. Concurrently, a controversial new freeway, the Mt. Hood Freeway, was in the planning stages. This freeway would have sliced through southeast Portland, forcing thousands to relocate. Grassroots efforts around the city called attention to the general distaste for new freeways and affected local elections. These efforts resulted in the election of anti-freeway politicians to city and county offices, and these new administrations successfully lobbied against the Mt. Hood Freeway plan.
The Tri-County Metropolitan Transportation District of Oregon (TriMet) was established in 1969 after the previous transit provider, Rose City Transit, failed to generate ridership while meeting federal air quality standards and went bankrupt. TriMet quickly absorbed many of the outlying suburban bus services over the next few years. TriMet also began preparing for rapid transit solutions, opening the Portland Transit Mall to buses in 1975 and studying light rail. After the decision was made to abandon the Mt. Hood Freeway in the 1970s, interest in light rail in Portland went to the forefront to improve the traffic problems in the Portland Metro Area. Momentum increased when legislation was passed in the U.S. Congress to allow funds from abandoned freeway projects to be used for urban transit projects. Residents voted in favor of constructing what was then called the Banfield Light Rail Project, named for the freeway (I-84) which the majority of the alignment followed.
In September 1986, the Eastside Metropolitan Area Express (MAX) line opened for revenue service, spanning 15 miles from Gresham to downtown Portland. One of the first light rail systems in America, Eastside MAX helped set the standard for the future of American light rail design. It also marked a critical point in Portland's history, as the region abandoned automobile-focused urban design to become a civic leader in land use and transportation.
In 1998, the Westside MAX line, TriMet's first extension of its light rail network, opened for revenue service, spanning 18 miles from the fast-growing high-tech corridor in Hillsboro to downtown Portland. One of the largest public works projects in Oregon's history, it included twin tunnels, each tunnel 21 feet in diameter and spanning three miles through Portland's West Hills area.
Portland International Airport (PDX) had seen steady growth for many decades, becoming America's fastest growing airport in the late 1990s. Additional access was needed to address the increase in traffic congestion at and around the airport. The Airport MAX line, the first "train to plane" connection on the west coast, was also the first to take passengers directly to an airport terminal. September 10, 2001 was the day on which TriMet celebrated the opening of this 5.5-mile extension from Portland's Gateway District to PDX. The celebration was short-lived, however, as the tragic events of 9/11 unfolded the very next day.
In April 2004, four months ahead of schedule and $25 million under budget, the Interstate MAX line opened for revenue service. Fred Hansen, TriMet's General Manager, refers to this line as the "phoenix line," rising from the ashes of a previous and more extensive light rail project. In 1998, voters defeated a ballot for a north-south light rail from the southern-most portion of Portland to North Portland. However, a majority of the residents in North Portland voted in favor of the line, as this area was long in need of revitalization and would benefit substantially from the presence of light rail. With the community's backing, TriMet decided to construct the Interstate MAX line, extending 5.8 miles from Portland's Rose Garden Arena to the Expo Center.
Portland Mall Revitalization
The City of Portland (City) has joined with TriMet to undertake the Portland Mall Revitalization Project. Portland Mall Light Rail is a key element in the planning effort connecting Clackamas County, one of the area's fastest-growing areas, with Portland State University (PSU), the primary destination in TriMet's transit system. Because planners forecast a million new residents in the region by 2030, this extension of the region's light rail system is a key element in the long-range regional transportation plan. The extension will position the region for future growth of high-capacity light rail to the southeast and southwest portions of the metro area.
The goal of the Portland Mall Revitalization Project is to add light rail service to the mall and to use this opportunity to revitalize the mall so that it better serves its multiple functions. This project and the Interstate 205 light rail project are jointly considered to be part of the "South" portion of the North-South light rail concept. Without the Portland Mall segment, the light rail system will not be able to expand due to limited capacity on the existing cross mall system.
Shoring can prove to be challenging in streets that are congested with other utilities.
Unique Impacts to Utilities
Light Rail Transit (LRT) system operations impose two unique impacts to utilities within the LRT corridor. The first and most obvious impact is that, unlike cars or buses, light rail trains cannot be detoured around obstructions in the road when, for instance, a water line breaks. Utilities that lie beneath the tracks must be either relocated or improved to minimize the possibility of breaks and mitigate the need to tear up the road and shut down LRT operations during routine or emergency utility maintenance. Due to safety constraints, construction crews are restricted from working too closely to moving trains or the high-voltage catenary power lines installed overhead. Furthermore, excavation beneath or bordering the track is restricted due to the potential for undermining the track as well as reducing the load on the existing utility. Construction parallel to rail systems can be more challenging than crossing under the track because these excavations warrant trench safety systems such as shoring to support the trench and protect crews. In streets that are congested with other utilities, such as Portland's Transit Mall, shoring can prove to be a challenging task in itself. An additional challenge of excavations parallel to rail systems is the physical limitations of the heavy machinery used to install and maintain utilities. Construction machinery generally spans the excavation, which may infringe upon the rail system right-of-way and impede train movement.
The second, and less obvious, impact of a new light rail system is stray current corrosion. Light rail trains are powered by electricity. Electrical current travels in a circuit from the power station to the train via the overhead wires above the tracks, through the train's electric engine, and back to the power station via the rail and ground. Because the soil serves as a parallel conductor to the track, a portion of the current will return to the power station through the ground. The current returning through the ground is known as a "stray current," due to the fact that it follows a path other than that intended. Electric current in the ground will use the most conductive medium to return to the power station. Iron and steel water pipes are often good conductors. However, when the current leaves the pipe, it takes electrons with it and corrodes the pipe.
Why Metallic Pipes Corrode
The basic fundamental cause of corrosion can be explained in terms of energy. It is more natural for a metal to exist in the form of a compound, since compounds such as oxides contain less energy than metals and are therefore much more stable. When metallic pipes are made, iron is separated from its associated oxygen in a blast furnace. This produces a large amount of energy in the form of heat. As long as they remain metallic, steel and iron pipes retain this energy, bound up within itself, with a natural tendency to corrode back to the ore from which it was derived. It is this energy that drives the corrosion process.
In order for corrosion to occur, there must be a complete electrical circuit, including an anode and a cathode. The anode is the location where the current leaves the metal, and the cathode is the location that receives the flow of current. In the case of electric light rail systems, the current goes out from the substation along the overhead wire until it reaches the train, passing through the motors and theoretically returning along the rails to the substation. In reality, the majority of the current does return along the rails to the substation. However, if a reasonably low-resistance parallel path exists, whereby the current may follow pipelines and cables to return to the negative ground of the substation, a portion of the current will take this route. In these cases, the current follows a parallel metallic path, such as a water main. Portions of water mains can be electrically continuous due to typical construction methods such as leaded joints on cast iron pipes and mechanically restrained joints on ductile iron pipes. When the current jumps from the rails to the water main, the rail serves as the anode and the water main serves as the cathode. In these cases, the water main is protected, because electrons are not being removed from the pipe. When the current leaves the water main to enter the lead cable, the water main serves as the anode and the cable serves as the cathode. In these cases, electrons leave the pipe with the current, and the pipe corrodes. Table 1 presents a galvanic series of selected metals commonly used in construction. In a galvanic cell of two dissimilar metals, the more active metal will act as the anode and be corroded. The more noble metal will act as the cathode and be protected.
The Solution to Corrosion
The previous example was simple, but in the complex environment of a metropolitan district, the path of stray currents can be difficult to follow. To reiterate the important point, anodes corrode; cathodes do not. To prevent pipeline corrosion, it is only necessary to make it sufficiently cathodic. If a current is passed from the earth to a pipeline, the incoming currents will nullify any outgoing currents from the anodes of local corrosion cells. Further, if the pipe receives current over its entire area, it will be immune from corrosion. A current through the earth can be easily produced by galvanic action from the energy of corrosion of a magnesium anode. In this case, a piece of magnesium is connected to the pipe with wire and buried, away from the pipe. Because magnesium is much more active than the iron or steel in the pipe (as demonstrated in Table 1), a considerable voltage is established between the magnesium and the pipe. Current will flow from the magnesium (anode), through the earth, to the pipe (cathode). The electrical circuit is completed when the current flows from the pipe to the anode through the wire.
Today, the vast majority of buried pipelines are coated with some kind of organic coating. Corrosion can be prevented completely by either 1) maintaining a perfect coating, or 2) impressing a protective cathodic current density on a bare line. Maintaining a perfect coating is impossible, as a perfect coating does not exist. Impressing a protective cathodic current density is too costly due to the high current demand. Somewhere between these two extremes lies the economic optimal combination of a good coating supplemented by cathodic protection to protect the inevitable imperfections in the coating.
The Portland Water Bureau's Approach to Corrosion Control
On previous light rail projects, the Portland Water Bureau (Bureau) developed a 10-foot separation criterion between the track and water mains, which combined the requirements for access, corrosion control, and direct construction conflicts. The requirement for a 10-foot clearance provides access for operation and repair and corrosion control, all of which are components of risk assessment. With this "Electric Rail Design Criteria" (ERDC), all water mains within 10 feet of the proposed track slab were to be relocated. Further, all mains crossing under the track were to be steel-encased ductile iron (DI) pipe with the casing extending 10 feet from each side of the track slab. In streets that are congested with other utilities, such as Portland's Transit Mall, there is simply not enough unoccupied space to construct a new pipeline 10 feet away from the track or extend a steel casing 10 feet beyond the track slab. In these cases, the design criteria are supplemented by secondary insulation of the track slab and/or an upgrade of pipeline materials and construction.
TriMet's design criteria for corrosion control take active measures to prevent stray currents in the first place. The embedded track design sets a resistance standard of 500 ohms per thousand feet of track. Di-electric rail boots and di-electric geomembrane under the track slab meet this standard. Di-electric rail boots are essentially rubber spacers, placed between the rail and surrounding track slab, which allow dynamic movement of the rail and provide electrical isolation. Di-electric geomembranes are essentially impermeable membranes that are resistant to the transfer of electrical currents. Problems occur only if the rail boot is punctured. To help track this, Tri-Met monitors "real time" returns at the substation ground mat. The presence of a stray current outside a set range triggers an automatic response, and the leak can be located and repaired.
Cathodic protection of the pipe is cost-based. As such, these di-electric features allowed some existing water mains located within 10 feet of the track slab to remain, depending on their age and condition. However, water mains parallel to the track and located within five feet of the track slab were relocated outside of this corridor to eliminate the need to shut down LRT operations during routine or emergency utility maintenance. All told, over 12,000 feet of 12-inch water mains will be relocated.
The Bureau allows an additional measure to reduce the construction schedule and cost of the water main relocations. Distribution main crossings can be replaced with concrete-encased high density polyethylene (HDPE) pipe. The Bureau identified four large-diameter critical supply main crossings that are crucial to the water supply in the downtown area. These four crossings will be replaced with steel-encased DI pipe.
Existing service lines and hydrant runs crossing under the track slab will be replaced with PVC-encased DI or copper, depending on the existing pipe size.
Bonding wires installed across each pipe joint creates a continuous electric circuit between corrosion test stations.
To mitigate the impacts from stray electric currents, 30-pound magnesium anodes will be placed every 100 feet along the pipeline. In addition, insulating joints, consisting of two flanged pipe ends with a di-electric gasket in between, will be placed every 500 feet along the pipeline, as well as at each end of the steel-encased critical supply mains. These insulating joints prevent stray currents from traveling too far along the pipeline. Test stations will be located at these locations to monitor corrosion. A continuous electric circuit will be created between corrosion test stations by installing bonding wires across each pipe joint. As an additional measure, all ductile iron pipe will be wrapped with 8-mil polyethylene encasement.
Maintaining business and pedestrian access was critical to the success of the project. TriMet has been working closely with its contractors to speed the pace of construction and ensure that customers always have access to businesses. To ensure that construction impacts are minimized, crews will be working in 3-5 block work zones for up to eight weeks, and then moving to the next work zone.
This innovative engineering project will provide a water system that will not impact LRT operations during routine or emergency utility maintenance. Further, the project will provide cathodic protection from stray electric currents that are common with LRT projects. Measures will be taken to reduce the duration and cost of construction while minimizing the impacts to downtown businesses and their respective customers.
Aaron Eder can be reached at (503) 423-4016 or AaronEder@KennedyJenks.com.