Equipment Type

Bertha's BFF Back to Work After Injury

After a six week recovery, TBM 'Pamela' is back to work on the Sound Transit 3 (ST3) project after hitting glacial sediment.

February 15, 2016

Seattle's metro light rail expansion project is employing another tunnel boring machine (TBM) nicknamed Pamela for a $1.9 billion four-mile extension from Northgate to Husky Stadium.

The damage to Pamela was first thought to involve just one motor but as the contractor disassembled more components it became clear the bull gear had hairline fractures and cracks.

Pamela came to a halt December 28 after striking unusually hard material. Six of the TBM's electric motors were damaged on the bull gear - the huge circular ring that spins the drive shaft and rotary cutterhead. Pamela endured limited repairs and returned to work last week, now running at reduced torque and speed to finish the tunnel segment she was working on when she was injured. If Pamela fails again, the Sound Transit contractor says another TBM, 'Brenda', could be used or the work could be completed using the sequential excavation mining method which used conventional digging machines to scrape dirt out a few feet at a time, then spraying the walls with reinforcing concrete. When the ST3 contractors reach the nearby unfinished underground station they hope to use the area for repairs in much the same way Seattle Tunnel Partners did when Bertha broke down last month.

Sound Transit’s contract also requires a third TBM be available for backup.

Pamela isn't as large as sister Bertha, with a 21-foot cutting face - one-seventh the size of Bertha's 57-foot face - and is one of two light-rail boring machines. Her sibling, Brenda the TBM, required cutter-teeth replacement last year but recovered and is continuing to work long hours.

Bertha is still on downtime after drilling was suspended on the SR 99 project in downtown Seattle until the pier at Terminal 26 is repaired. Watch Bertha's progress on WSDOT's live webcam here.

Why are tunneling machines given names?

According to Linea Laird, WSDOT’s tunnel project administrator, the tradition and practice of naming tunneling machines dates back to the earliest mining traditions.

“Originally, it was part of the patron saints of protection of underground workers,” she said. “There would be even a little shrine that would be established there for the workers.”

Laird says the name of the saint gave the miners something personal that they could relate to as they did their dangerous work. Paying homage to their saints evolved into naming tunneling machines.

Source: Seattle Times; WSDOT

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