Solving Stray Current Mitigation In Portland’s Rail System

By Aaron Eder, P.E. | August 2014, Vol. 69, No. 8
Rendering of the Portland-Milwaukie light rail system. (photo courtesy of TriMet)

On previous light rail projects, the city of Portland Water Bureau developed a 10-foot separation criterion between the track and water mains, which combined the requirements for access, corrosion control and cost/benefit. 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 (see Figure 4). All told, over 8,000 feet of the Bureau's water mains have been relocated, ranging in size from six to 36 inches.

Existing service lines and hydrant runs crossing under the track slabs have been replaced with PVC-encased DI or copper, depending on the existing pipe size.

To mitigate the impacts from stray electric currents, 30-pound magnesium anodes have been placed every 100 feet along the pipeline. Further, insulating joints, consisting of two flanged pipe ends with a di-electric gasket in between, have been placed at connections to existing mains, as well as at each end of the steel-encased mains. These insulating joints prevent stray currents from traveling too far along the pipeline. Test stations are 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 (see Figure 5). As an additional measure, all ductile iron pipe will be wrapped with 4-mil polyethylene encasement (see Figure 6).

Transmission main improvements

The Bureau maintains several large diameter transmission mains that convey water from the city's main supply in the east into downtown Portland. Two of these, an existing 30-inch cast iron pipe in SE Division Place and an existing 36-inch ductile iron pipe in SE Caruthers Street, were replaced and installed in 48-inch and 54-inch steel casings, respectively (see Figure 7).

In addition, the new light rail tracks will cross the Bureau's Washington County Supply Line (WCSL). This 60-inch pipe delivers up to 60 million gallons of water per day from Powell Butte to wholesale customers in the Tualatin Valley and Raleigh Water Districts and the city of Tualatin. Options for maintaining this water supply included replacing it with a new 60-inch steel pipe or protecting the existing concrete pipe in place. In the end, it was decided to protect it in place with a concrete box culvert (see Figure 8), as new steel pipe that size could not be supplied and installed during the off-season window of time that the WCSL could be taken off-line.