SR 99 tunnel - Tunnel design

Tunnel design and safety features

  • The SR 99 tunnel is the widest single-bore tunnel in the U.S., and the longest road tunnel in the country (two miles).
  • The tunnel carries two 11-foot travel lanes in each direction, with one 8-foot shoulder and one 2-foot shoulder. 
  • There are no mid-tunnel entrances or exits. Each end of the tunnel features on- and off-ramps for northbound and southbound travel.
  • Three different types of ventilation fans keep fresh air flowing in the tunnel and make up part of the tunnel's emergency systems in case of a fire.
  • The 9,270-foot tunnel is built of more than 1,400 strong concrete and steel rings, each 6.5 feet wide. These rings are bolted together to form the tunnel, and while very sturdy, they have some flexibility to account for ground movement. This means they can move and return to their round shape. The roads inside are also designed to be flexible, allowing them to move with earthquake waves and remain functional.
  • The tunnel is designed to withstand a 9.0 earthquake, and would be a relatively safe place to be during an earthquake (video).
  • The tunnel features modern operational and safety systems that make it one of the smartest tunnels in the world (video).

Tunnel interior showing two lanes, yellow lane line at left, white walls with exit signs

How the tunnel was built

  • The SR 99 tunnel was built with the largest-diameter tunneling machine of its time (57.5 feet), nicknamed "Bertha."
  • The tunneling machine “Bertha” began tunneling beneath Seattle in July 2013, assembling the tunnel rings while digging forward (video).
  • In December 2013, approximately 1,000 feet into the tunnel drive, the machine overheated and stopped moving forward. After ruling out a blockage as the cause, Seattle Tunnel Partners discovered contamination in the machine’s seal system, which protects the main bearing. Seattle Tunnel Partners and machine manufacturer Hitachi-Zosen opted to lift the machine's cutterhead out of the ground for repairs.
  • Crews built a 120-foot deep pit to reach the cutterhead and extract it in March 2015 for repairs. Disassembly uncovered additional damage to the machine. Repairs were completed in December 2015 and tunneling resumed in early 2016. 
  • On April 4, 2017, Bertha completed the tunnel drive by breaking through into the receiving pit at the tunnel’s north end, near Seattle’s Space Needle.
  • The machine was fully dismantled within that pit and removed pice by piece, while crews completed the tunnel's roadway decks.
  • The tunnel opened to drivers on February 4, 2019.

Large pit with tunneling machine head visible after breaking through pit's far wall. Wall is braced with large metal beams.