Clogged cutterhead, heat causing Bertha’s delay

It’s been a mystery for over two months and now we know what caused the huge drill tunneling under Seattle’s waterfront to stop: a clogged cutterhead.

The Washington State Department of Transportation and Seattle Tunnel Partners says high temperature readings also indicate there are other factors to be determined.

Bertha came to a screeching halt Dec. 6, 2013 still not reaching the one-tenth mark in the project timeline. Since then, Seattleites had been left to wonder what could be in the way. Guesses ranged from a UFO to a ice-age boulder.

On Jan. 3, the WSDOT released a photo of an 8-inch steel pipe protruding from the cutterhead.

“We believe the steel pipe is a well casing installed by WSDOT in 2002 after the 2001 Nisqually earthquake to better understand how groundwater flows through this area,” said a statement from WSDOT.

Workers spent almost two weeks in January inspecting the world’s largest drill to discover the cutterhead openings were clogged with dirt and “other material,” according to the WSDOT.

“It was determined that a major obstruction was not the cause of the mining difficulty. The more likely cause was the clogged cutterhead.”

Crews unclogged the cutterhead and moved Bertha forward two feet, but one of the head sensors picked up higher-than-normal readings. STP discovered “damage to the seal system that protects the tunneling machine’s main bearing,” which allows the cutterhead to spin.

SPT says it’s working with Bertha’s manufacturer and tunneling experts to try to determine the extent of damage and figure out the best way to fix the seal system.

It’s unclear how much the delay will cost and whether it’ll push back the timeline. Tunneling was scheduled to be completed by the end of 2014 and SR 99 opened to traffic by the end of 2015.