SR 99 TBM Nears Breakthrough

Bertha, the TBM mining the SR 99 tunnel in Seattle, was 219 ft from its end point as of March 27, according to the Washington State Department of Transportation’s (WsDOT) Twitter feed @BerthaDigsSR99.

The breakthrough date was still to be determined as of March 24, but contractor Seattle Tunnel Partners (STP) is making final preparations for the TBM’s arrival at the disassembly pit near Seattle Center, WsDOT reported. According to a March 26 report in the Seattle Post-Intelligencer, the TBM could break through before the end of March.

Despite the fact that the TBM may be reaching the end of its journey, much work remains before traffic flows through the tunnel, which is estimated to open in 2019. According to kiro7.com, it will take three weeks to fully move the machine into the reception pit after it breaks through the 5-ft headwall. Then, the machine will be disassembled over the course of four to five months.

RELATED: SR 99 TBM Gears Up for Final Stretch

The 57.5-ft diameter Hitachi Zosen TBM’s journey has been well-document since arriving in Seattle as the largest TBM ever constructed at that time. It began mining in July 2013 before experiencing a stoppage in December 2013 that led to repairs including replacing the TBM’s main bearing and seals.

The TBM resumed mining in December 2015 and, after a brief stoppage, began mining in earnest in February 2016. Since that time, the TBM has made steady progress on the 9,270-ft tunnel, averaging between 6 and 8 rings a day, according to kiro7.com.

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