New SR 99 Tunnel will be ‘safest place in Seattle during an earthquake’
During the 2001 Nisqually earthquake, the damage to the Alaskan Way Viaduct was as telling as what wasn’t damaged: the Great Northern Railroad Tunnel that runs underneath the viaduct’s north end. Though it was built in 1905, it suffered no damage at all.
The inherent safety features of a tunnel over a viaduct were touted in a new video from SDOT, perhaps as something to think when you’re stuck in traffic during the impending Viadoom.
“The viaduct was built in the 1950s and seismic design is now light years ahead of what we knew back then,” said David Sowers, deputy administrator of the Alaskan Way Viaduct Replacement Program. “Ultimately, replacing the viaduct is all about keeping people safe and this new tunnel meets the gold standard for safety.”
Why is a tunnel safer during an earthquake?
“In an earthquake, as the waves come up from below, their amplitude increases as they get closer and closer to the surface, so the shaking that you feel on the surface is greater than the shaking you feel at some depths,” said Professor of Civil Environmental Engineering Steve Kramer.
Since the tunnel is underground, when the ground moves, the tunnel moves with it, so there’s less stress on the tunnel itself than on a structure above ground. As the video points out, it’s similar to how ocean waves work.
“If somebody is on top of the ocean wave, they get bounced around,” said tunnel expert Red Robinson. “If you’re down below the wave, you get moved around slowly and easily with the water, but you’re not getting bounced around.”
Additional safety features inside the tunnel include more than 300 cameras to monitor traffic, automatic ventilation systems, fire suppression systems, an AM/FM override communication system, and more than 100 programmable traffic signs to allow tunnel operators to communicate with travelers.
The new tunnel is designed for a 9.0 earthquake and is composed of 1,400 concrete rings which hold and yet create flexibility. After moving with the earthquake, the tunnel would return to its original shape.
“The SR 99 tunnel would be the safest place to be in Seattle during and after a big earthquake,” Sowers said.