With the support and cooperation from Sound Transit, YPT Seattle had a fantastic opportunity to tour the East Link tunnel in downtown Bellevue on last Thursday. The East Link tunnel is located at the southeast edge of the city as the light rail alignment travels next to Main Street and 112th Avenue SE.
As we convened at Sound Transit’s field office, we were welcomed by Jennifer Lemus, Sound Transit’s outreach coordinator, and Atkinson Construction. Jennifer provided us a detailed presentation of Sound Transit’s progress and milestones along the East Link alignment as well as future plans to extend the alignment further into Redmond specifically into the nexus of Microsoft’s campus. For the half-hour presentation, Jennifer and her team followed-up by answering in-depth questions from our YPT attendees.
After our Q & A, Sound Transit provided us with a safety briefing as we equipped our safety gear: a construction vest, a hard hat, and safety glasses before our visit to the construction site. For safety and security purposes, the construction site was enclosed by temporary plywood walls. As we assembled at the mouth of the tunnel, a convoy concrete trucks were deployed on-site to supply the shotcrete required for the construction of the tunnel walls.
Shotcrete is simply concrete that is sprayed under high pressure. Imagine using a fire extinguisher but using one that expels concrete instead of fire retardant! If you ever swam in a large outdoor pool or dropped into your local skate park, there is solid chance that you have used a facility that was built by using shotcrete. Given by its content of aggregate and mortar, concrete is very heavy; it has a density of about 150 pounds per cubic foot which is equivalent to a block of concrete the size of a milk crate. To support the shotcrete formed around the tunnel walls, a steel lattice frame is assembled after the tunnel is burrowed with a standard construction excavator.
So why is the East Link Extension using a conventional excavator for Bellevue underground tunnel instead of a burrowing machine similar to what is used for replacement of the Alaskan Way Viaduct? The reason is dependent on the different scale of the projects and the volume excavation.
The length of the Alaskan Way tunnel is 10,560 feet whereas the East Link tunnel is about a fourth of that length, 2000 feet. Not only is the Alaskan Way tunnel longer than the East Link tunnel, but also the Alaskan Way tunnel has a larger cross-section as well. The Alaskan tunnel provides two lanes of traffic for both northbound and southbound vehicle traffic with a 57-foot diameter whereas the East Link tunnel provides 2 tracks of rail with an elliptical tunnel dimensioned with a width and height of 34 feet and 28 feet respectively. Let’s do the math to find a rough volume for how much earth that needs to be excavated for both tunnels.
The volume of excavation for Alaskan Way Tunnel is about 20 times the volume of excavation for the East Link Tunnel!