press release:
Crews will close the State Route 99 Alaskan Way Viaduct this weekend to reinforce a two-block section of the structure located above the path of the future SR 99 tunnel.

Both directions of SR 99 between the Battery Street Tunnel and the West Seattle Bridge will close from 11 p.m. Friday, April 6, to 5 a.m. Monday, April 9. Drivers should expect increased congestion in and near downtown Seattle during the closure and plan accordingly.

“The weather is improving and shops and restaurants will be open for the holiday weekend,” said Matt Preedy, WSDOT Alaskan Way Viaduct Replacement Program deputy administrator. “Remember to plan ahead and allow a little extra time to get to your destination.”

This map shows recommended routes to the waterfront, Pike Place Market and Pioneer Square.

Preparing to tunnel beneath the viaduct
Crews building the SR 99 tunnel will spend the weekend reinforcing both decks of a two-block section of the viaduct between South Washington and Columbia streets in Pioneer Square. There will be one additional weekend closure of the viaduct in mid-May to complete the scheduled work, which includes filling cracks and wrapping previously damaged sections of the structure in protective reinforcing fiber. Protecting this section of the viaduct is important because crews will drive the tunnel boring machine beneath it in late 2013.

Currently being manufactured in Japan, the machine will launch from the construction area to the west of CenturyLink Field in mid-2013. It is designed specifically for the soil and groundwater conditions along the tunnel route. After digging its way north beneath Alaskan Way, the machine will pass as close as 20 feet below the viaduct’s foundations. By comparison, members of Seattle Tunnel Partners, WSDOT’s contractor for the project, successfully bored a tunnel within 6 feet of the foundations of a famous cathedral in Spain in 2010.

Although tunneling crews do not anticipate significant levels of ground settlement, they are monitoring utilities and structures above and near the tunnel route as a precaution. In addition to this weekend’s reinforcement work, crews are building underground walls beneath the viaduct’s foundations to further protect it during tunneling. The walls consist of a row of steel pipes installed at an angle between the foundations and the tunnel. This creates a barrier to limit ground movement.

For more information on the Alaskan Way Viaduct Replacement program, visit http://www.alaskanwayviaduct.org/. Real-time traffic information is available at www.wsdot.wa.gov/traffic or by dialing 511.