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Seattle tunnel contractor releases detailed plan for repairing Bertha

The contractor building the new Seattle tunnel has released details for its plan to repair the SR 99 tunneling machine known as Bertha and get the world's biggest boring machine moving again.

The plan by Seattle Tunnel Partners (STP) and manufacturer Hitachi Zosen calls for a number of steps to access Bertha 60-feet below the Seattle waterfront, remove its massive cutter head and repair the machine.

The work plan calls for four major repair elements as well as a series of enhancements, according to the Washington State Department of Transportation. Crews will replace the damaged seal system with a more robust system, replace the main bearing, install enhanced monitoring systems, and add steel to strengthen the machine and accommodate the new seal system.

Crews will access Bertha through a 120-foot-deep circular pit to be dug in front of the drilling machine from above. Work has been underway since May, with STP installing 73 concrete columns and other reinforcements before the walls of the pit are constructed. Excavation work is scheduled to begin in August.

Once the pit is completed, a huge crane rising above the Alaskan Way Viaduct will be erected, providing access to the drilling machine. Crews will disassemble the cutter head and use the crane to lift the cutter head, drive axle and bearing in one piece some 2,000 tons altogether and set the massive pieces on the surface. The plan calls for that work to take place in October.

The proposed schedule calls for repair work to be completed by the end of the year, with testing to get underway in February 2015. Officials say drilling on the Seattle Tunnel is scheduled to resume in March 2015.

Along with the repairs, the plan calls for STP to widen the openings at the center of the cutterhead, improve the soil conditioning injection system, install bit- and wear-resistant steel on the cutterhead, and extend the length of what are called the agitator arms in the mixing chamber that handle soil dug by the machine.

WSDOT says the contract with STP requires the tunnel contractor to prove, through tests and detailed analyses, that the repaired machine can stand up to all of the loads from the surrounding ground and operation and can handle the rest of the project before it can resume drilling next year.

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