Structural retrofits in WSDOT tunnels on I-90


Washington State Department of Transportation (WSDOT) is adding high occupancy vehicle  lanes (HOV) to the existing Mercer Island Tunnel (MIT) and Mount Baker Ridge Tunnel (MBRT) on Interstate 90 between Seattle and Bellevue Washington. 

The WSDOT project information can be found here

Ashley & Vance Engineering is proud to have provided structural engineering to accommodate the addition of the larger supply and exhaust fan motors. With the addition of traffic within the tunnels it was necessary to increase fresh air supply and increase air exhaust for the tunnels.  The general contractor IMCO General Construction is currently working on the 12 fan motor pedestal retrofits. Below are some photos of the supply fan motor pedestals in the Mercer Island Tunnel showing form work, reinforcing steel, and a recently completed pedestal. The new motors are scheduled to start being installed in the next few weeks. Each new motors range in size from 300 hp to 450 hp. The size of the venting, motors, and scale of the overall tunnel project is absolutely staggering.

supply fan motor pedestals in the Mercer Island Tunnel

To add to the challenge of this the existing motors had to remain in service allowing only 2 to be offline during the switch over for the addition of the new motors.  Many of the pedestals are built on top of existing reinforced concrete decks that are over active traffic lanes. Getting the new materials into the project areas was also a big challenge considering these tunnels are thousands of feet in length and are only accessible by a few key entry points.

reinforced concrete decks

rebar inside reinforced concrete decks

The photo below shows a newly constructed motor pedestal. A new motor will soon be installed on this new larger foundation. 

engineer in front of newly constructed motor pedestal

engineers under WSDOT tunnels on I-90

More information about each tunnel can be found using the following links.

Mount Baker Ridge Tunnel

Mercer Island Tunel