December 15, 2015

Crews start to refill Bertha's pit with sand

Photo courtesy of WSDOT [enlarge]

Say bye bye to Bertha for a year.

Last weekend Seattle Tunnel Partners started filling the 120-foot-deep pit that was used to fix the broken tunnel boring machine named Bertha. That operation is expected to continue through the week, and the latest schedule calls for the machine to resume mining Dec. 23.

Information posted online by WSDOT says STP placed gravel under and beside Bertha to fill gaps between the machine and the concrete cradle it was driven onto for repairs. The contractor then added a low-strength concrete and sand mixture in front of the cradle to give Bertha a firm surface to mine through.

On Sunday crews began funneling sand into the pit through a hopper connected to a flexible pipe that is used to move sand into specific areas. They will continue until Bertha is covered by about 4 feet of sand, then add material originally excavated from the pit on top.

STP will incrementally turn off dewatering wells as the pit is filled, then engineers will conduct load tests on the rebuilt machine.

If Bertha passes the tests, it will mine 430 feet north to an underground safe haven for a checkup before traveling below the viaduct and the remaining 8,000 feet of its route.

Bertha overheated two years ago and has moved little since then. Muck got through the original seals and damaged the main bearing.

Hitachi Zosen, Bertha's manufacturer, made a new seal system that is more accessible for future maintenance or repair. Bertha also was fortified with additional steel and more robust operating systems.

The opening date for the tunnel that will replace the Alaskan Way Viaduct has been pushed to April 2018.