Road work continues in tunnel after Bertha finishes digging

Apr 17, 2017, 5:34 AM | Updated: 8:41 am
sr 99 tunnel...
(WSDOT image)
(WSDOT image)

Left in Bertha’s wake, crews have been constructing a living, breathing SR 99 under Seattle.

“It’s really a large mechanical plant,” said Brian Russell with HNTB Corporation. “Within that facility there’s also people, facilities for monitoring the tunnel, control of the tunnel, for maintenance purposes, and for staff to be housed.”

RELATED: Watch video of Bertha finally emerging into the open

“What people will see — they will drive into the tunnel and see two lanes,” he said. “… in one direction they’ll see a rectangular tunnel going northbound, like the Mount Baker Tunnel on Mercer Island. When they go southbound, there will be a curve at the top; they will actually see the profile of the rings that were built.”

But looks can be deceiving. The SR 99 Tunnel will be more than meets the eye. To provide this two-minute drive under downtown Seattle, an entire operation is required, utilizing control centers and mechanical systems for air, water, communication, power supply and more. The entire tunnel will, therefore, be a systematic machine.

Russell, along with other HNTB colleagues, designed the systems that will run throughout the SR 99 Tunnel. So while Bertha may be done tunneling, work remains to finish the job.

The SR 99 tunnel machine

Take the security system, for example. It’s composed of video cameras and signage inside the tunnel. The cameras are monitored by employees inside one of two buildings, where the signs are also controlled.

“It allows the operator to make decisions remotely,” Russell said. “There’s also an intelligent transportation system that includes variable message signage which can change based on what’s going on in the tunnel.”

But that’s just one piece.

Control centers

There will be two buildings from where the tunnel will be operated – one at each end. They’re like two hemispheres of the tunnel’s brain.

“The primary purpose of the buildings is to house the big ventilation equipment,” Russell said. “There are some fairly large fans in each of the buildings – four large fans. There’s the electrical supply, and backup systems, fire protection systems, and other normal mechanical ventilation.”


Breathing air through the tunnel is actually pretty easy — it’s done via a “piston effect” of vehicles moving in and out, Russell notes. But there are situations when something more powerful is needed.

“…your vehicle stops and you may build up carbon monoxide and things we need to get out of the tunnel,” Russell said. “And it’s designed for an event like a fire.”

When turned on, the ventilation system sucks air out of the tunnels. Fresh air will enter through the portals.

Pump stations

When drivers enter through the tunnel, they will travel downward and then upward. The deepest point of the tunnel is beneath Madison Street. That means water will flow down into the tunnel, and as we know, it rains a lot in Seattle.

When that happens, the tunnel’s pumps will kick in, removing water and sending it into the city’s sewage and stormwater system at each end — at King Street and in South Lake Union.

RELATED: What happens to Bertha next?

“They’re designed to handle the calculated stormwater that comes through the portals, and what comes off of the vehicles,” Russell said. “And they are designed to pump water that comes from the (fire suppression) sprinklers, in case they do discharge. They will pump that water out. We are not talking millions and millions of gallons per day, but it will keep up with any storm event or sprinkler event.”


The power in the SR 99 Tunnel is not just for pumps, fans signs, etc. There are lights and backup emergency lights, for example.

“We are connected to two substations — one at the north and one at the south,” Russell said. “It has the capacity, in case one substation went down, to be fed from the other. There’s a redundancy in the power, as well as a generator in case both systems go down.”

“It’s a large power supply, directly connected to the City Light substations … it’s certainly enough to power the pumping equipment, the ventilation equipment, the lighting,” he said.

The SR 99 Tunnel is expected to open in early 2019 — three years after initially planned.

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Road work continues in tunnel after Bertha finishes digging