For decades the University of Washington has been home to world-class engineering and groundbreaking innovation.
Starting October 2, Link light rail riders will glide over another example of engineering breakthroughs from 70 to 140 feet below campus when the new 1 Line extension and new stations in the U District, Roosevelt and Northgate neighborhoods open.
The underground tracks through this area include about 7,500 feet (3,800 feet in each tunnel) of highly-specialized rail infrastructure resting on what’s called a “floating slab.”
The tunnels pass under campus research buildings that are extremely sensitive to physical and electromagnetic vibrations. We’re talking about electron microscopes and other extremely sensitive equipment.
This equipment, and the billions of dollars in research conducted at UW, are very sensitive to vibrations and electromagnetic interference that can come from operating electric trains 20 hours a day.
Sound Transit engineers had to make sure the trains did not disturb these buildings and the research conducted there.
Shankar Rajaram, the Sound Transit Rail Vehicle Engineering Manager, led the team tasked with making sure the trains rumbling at up to 35 miles-per-hour beneath campus do not disturb the work happening above.
With a Ph.D in Materials Science (specializing in noise control), Shankar knew the task would not be simple. But with the right people working toward a common goal, they delivered a unique world-class solution.
“We all knew from the get go that we were part of something very challenging but also very special and unique,” he said. “It was a dream team, at least in my career, because everyone was in lockstep supporting and encouraging each other.”
How to ‘float’ rails
The tracks sit on about 1,600 extra-dense concrete slabs reinforced with steel rebar that each weigh more than 11,000 pounds.
The concrete used to make the slabs includes a mixture of the mineral hematite, which has its own energy-absorbing qualities.
And each slab rests on custom-built rubber pads sourced for their durability and vibration-absorbing properties.
We also used what’s called “ultra-straight rail” that is common for high-speed trains in Europe to help smooth out the ride.
The floating slabs and rail have proven to absorb the vibrations and other potential interference at the source as trains pass over.
Every train that passes through the area is recorded by 40 monitors at 300 feet intervals. Extensive testing in the area shows we’re meeting all the criteria for quiet operations.
Check out the photo gallery below for details of how all the pieces came together from scratch.
A look at the extensive rebar framework that is inside of every floating slab. This helps add density to make each one weigh almost 12,000 pounds.
Mounds of hematite ready to be added to the concrete mixture for each floating slab. The mineral, sourced from Minnesota, adds density and energy-absorbing properties to each slab.
Cross section of a floating slab - note the round hematite pieces throughout.
Moises Gutierrez, left, and Shankar Rajaram from the Sound Transit engineering team inspect the specially-sourced rubber pads that each floating slab rests upon. Some properties of the pads are on rubber samples from the London Underground, which at 100+ years old are still going strong!
A construction crew poses beneath the special machinery (gantry truss) that lowered each of the 1,578 slabs into place in the tunnels.
Some of the 1,578 floating slabs awaiting delivery to the tunnels beneath the University of Washington
The Sound Transit engineering team thanks our consultant team for their invaluable contributions to the project. We couldn’t have done it without the great collaboration with these organizations:
Thank you for following The Platform and this window into the amazing engineering at work in the Northgate tunnels.
If you're new here, check out these other articles that have tracked the 1 Line extension construction.