Behind the walls in the I-90 tunnel

Drivers on I-90 will once again be squeezed into the Express Lanes this weekend for another round of construction work.

And once again, drivers will not see much work going on because it’s all being done inside the Mount Baker and Mercer Island tunnels.

Once in the Mount Baker Tunnel’s dark and noisy ventilation shafts, you realize there’s almost as much room behind the tunnel walls as there is inside. It’s like a maze of dark hallways and giant chambers.

Here you find massive fans used to ventilate the tunnels and remove any bad air or smoke. Those fans are being upgraded to handle the new HOV lanes inside the tunnels and to support future light rail trains. But the only access to the ventilation rooms is from the shoulders.

Project engineer Zak Griffith said that’s why crews have to close the lanes.

“We’ll have a semi-truck parked in the roadway and it will be loading that stuff through the wall here, through the air duct and into this room,” he said. “There’s really no room for vehicles to go through when you’re doing that.”

While the fans are being upgraded, workers are also retrofitting more than 1,800 tunnel lights.

“A bunch of trucks up there [will be] taking them down, opening them up,” Griffith said. “They’ll be working on them. They’ll be re-hanging them. There’s a lot of work associated with that, but you won’t see that during the weekend because it’s in the tunnel.”

The 36 fire suppression systems are also being upgraded. Workers need the roadway to get to them too.

The I-90 eastbound closures start Friday night and last through Monday morning. The Rainier Avenue on-ramp to I-90 will also be closed.

This is the fourth of 30 directional closures on the I-90 bridge over the next two and a half years. The state is adding HOV lanes to the outer roadway and converting the existing Express Lanes into a light rail bridge.

Sound Transit will begin its construction in 2017.