Tacoma Power Taps Jensen for Problem Project

In the 1970s, as Washington State constructed State Route 16 (SR-16), a portion of Tacoma’s overhead power lines were moved underground. To accomplish this move, a standard precast vault was installed as a transition point on both sides of the freeway, tying two distribution feeders together and maintaining grid flexibility.

But over time roadway construction introduced major grade changes leaving one of the vaults in a vast hole roughly 16 feet deep. A rockery retaining wall was built to keep access, but the vault being in a basin made it extra difficult to maintain. Plus, the area was isolated and secluded attracting urban campers which created additional safety concerns for Tacoma Power.

So, they knew they needed to make a change. But how?

The vault itself remained structurally sound. The challenge was how to eliminate the hazardous hole, improve access, and keep feeder flexibility without disturbing the original underground crossing.

Tacoma Power, a division of Tacoma Public Utilities, explored several paths forward. Reboring a new crossing under SR-16 would be costly and disruptive. Converting the line back to overhead would involve complex permitting and coordination. Another option involved tying new PVC conduit into the existing underground crossing.

Still, that approach presented its own complications. The original installation used coreduct conduit containing asbestos and is encased in concrete. Disturbing the concrete to access the conduit risked damaging the line and exposing workers to nasty asbestos.

“If we chipped at the concrete, the coreduct would be falling apart,” says Joseph Rempe, Lead Distribution Engineer for Tacoma Power. “And we don’t know how far we’d have to chase that conduit line before we could find an edge we could use.”

At the same time, nearby property owners were planning redevelopment, which would require an increase in distribution capacity. The feeder tie at this location was too important to abandon, yet too complicated to rebuild conventionally.

Rather than demolish a functional vault or disturb hazardous materials, Tacoma Power approached Jensen Infrastructure with a different idea: stack a custom precast structure over the existing vault and bring the installation to grade.

Jensen’s engineering team worked closely with Tacoma Power engineers to design a modular solution consisting of six precast sections. When assembled, the structure rises nearly 17 feet above the existing vault, creating a total depth of approximately 22 feet from surface to bottom.

To ensure the original vault would not be compromised during backfill, Tacoma Powers’s civil engineering team drove pin piles around the perimeter of the existing structure. This reinforcement mitigated sidewall pressure and allowed crews to leave the asbestos-encased conduit undisturbed.

The new vault includes cast-in ladder rungs for safe entry and an access opening aligned with the original vault below. Tacoma Power plans to install a removable false floor and a working platform, creating a safe environment for workers performing maintenance. The installation was also designed to accommodate a pad-mounted switchgear, allowing the site to serve both as a pull vault and a switching location for adjacent development.

“Jensen is rather new in this area, and we’ve been pleased with their performance,” says Rempe. “I approached Jensen first kind of as a test project to see how responsive Jensen was and how technically competent they are. Like most utilities, you have the oddball event that occurs, which means you must use something that’s not quite standard. So I wanted to see how Jensen responded and they did great.”

The completed installation eliminates the unsafe deep hole while significantly improving access and safety.

Got a custom project coming up you want to chat about? Contact Jensen Infrastructure today.