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Damage to world’s largest tunnelling machine worse than anticipated

Investigations into problems with the world’s largest tunnelling machine have revealed that more repairs than initially anticipated are needed.

The SR 99 tunnelling machine, known as Bertha, was put in place two years ago to tunnel beneath Seattle to create a 3km stretch of highway to minimize road closures while the Alaskan Way Viaduct is constructed.

However Bertha’s cutterhead clogged in late 2013, approximately 330m along the route. This week the main contractors, Seattle Tunnel Partners (STP), confirmed that disassembly of Bertha is complete, and that it is continuing to assess the damage.

STP is replacing the main bearing and outer seals of the machine as expected, however it will also replace the inner seals to make them more compatible with the new outer seals and to provide easier access. The new inner seals will arrive later this month.

Bertha, Seattle

Source: Washington State Department of Transportation

The SR 99 tunneling machine, known as Bertha, before disassembly

A statement from Washington State Department of Transportation said: “Damage to the machine was more extensive in some areas than anticipated and some minor damage occurred during disassembly. For example, the outer seals and the steel retainers that hold them in place were destroyed. There was also damage to the cutter drive motor pinions and the main bearing bull gear.”

As yet, a date hasn’t been set for work to resume and one won’t be confirmed until the full scope of repairs to Bertha is understood. STP and Hitachi are responsible for the repair effort.

Meanwhile, reports have also come out after investigations into ground settlement of approximately an inch near Bertha’s tunnel access pit last November. They concluded that although there were no simple answers, causes include natural influences and dewatering. Settlement levels have been steady since the initial detection.

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