Tunneling for University Link
The first step in building tunnels is to dig a hole where the station will eventually go. The tunneling crews use this hole as a starting point to launch the tunnel boring machines.
The tunnels for University Link were built by three tunnel boring machines. Two TBMs were launched from the University of Washington and traveled side by side for nearly 2 miles south to Capitol Hill. A third TBM was launched from Capitol Hill and traveled a little less than a mile to the end of the existing Downtown Seattle Transit Tunnel. That machine was then disassembled, brought back to Capitol Hill and re-launched to dig the second tunnel.
A few facts:
- Each TBM is 21 feet in diameter, about 330 feet long (the length of a football field) and weighs at least 679,000 pounds.
- The TBMs excavated an average of 50 to 75 feet of tunnel per day.
- For safety reasons, the tunneling operations must continue around the clock. Sound Transit receives special variances to the city's noise ordinance to allow this round the clock construction.
- The two TBMs that are digging from UW to Capitol Hill were named Togo and Balto after famous Huskies - the four-legged kind. "Togo" and "Balto" were canine heroes of a grueling sled dog relay to deliver medicine 674 miles from Anchorage to Nome, Alaska during a diphtheria outbreak in 1925. This amazing journey is commemorated each year with the Iditarod sled dog race.
- As they traveled south, Togo and Balto reached depths of up to 300 feet underground and had to withstand up to almost five times normal air pressure!
- For more info on U-Link TBMs, check out the fact sheet.