Located on a constrained urban site on a bustling waterfront, the Marquam Substation replaces a 60-year-old existing substation to provide more reliable power to Portland.

The project’s compact size and construction sequencing strongly influenced site safety. The site’s proximity to traffic and the nearby International School required diligent safety practices to keep workers and the public safe. In coordination with the school, large excavation work north of I-405 occurred during the summer. During the project’s nearly 68,000 work-hours, the team tallied an OSHA recordable incident rate of zero and reported no lost-time accidents.

The 1.6-acre site was formerly occupied by a bus maintenance garage, demolished in 2012. During demolition of the garage, a retaining wall was left intact to retain soil on the site. Temporary braces supported the wall, but the braces conflicted with the new substation design.

The team built a new cantilevered retaining wall with steel piles and precast concrete panels, which allowed the temporary braces and wall to be incrementally decommissioned. According to the team, the cantilevered retaining wall solution was required because soil nails or reinforcement straps could not be used within the confines of the property.

The retaining wall design was the first of its kind used on a Portland General Electric project, says PGE project manager Ezra Richards. “A lot of the construction on this project was new to us,” he adds.

The substation’s 13-kV network feeders, which service downtown Portland, exit where the site borders the Interstate 405 freeway. To install the network feeder conduits across I-405 without affecting traffic, the team designed and constructed four 48-in.-dia by 230-ft-long steel casings with the “jack and bore” method.

Due to a 40-ft elevation difference between Marquam Substation and I-405, the casings were installed at a steep slope, ranging from 15% to 18.5%. The casings were installed over a 60-year-old sewer, beneath a retaining wall and between foundations supporting a portion of I-405.

A new 115-kV transmission line between Marquam and the Harrison substation was installed primarily underground, with a 1,700-ft-long segment that was integrated into the concrete deck of the new Tilikum Crossing Bridge across the Willamette River.

Typically, PGE would install high-voltage cables in conduits suspended beneath the bridge, but Richards says the company saw an opportunity to try a different method because the bridge was being built at the same time as the substation.

“Getting the design of the bridge to incorporate our needs was extremely challenging,” Richards says. “The people building the bridge didn’t understand our needs, so we had a full-time inspector there watching the construction every day.”