WSDOT: Sinkhole developed in ground Bertha mined through last week

SEATTLE — A sinkhole formed on the State Route 99 tunnel project Tuesday night in ground the drill Bertha mined through last week.

Chris Dixon of Seattle Tunnel Partners said the hole measured 35 feet long, 20 feet wide and 15 feet deep.

It was filled with 250 cubic yards of concrete.

"The sinkhole's an unfortunate event but it wasn't unique or something that wasn't anticipated," Dixon said.

Sinkholes can happen on tunnel projects, especially in soft soil, which is why contractors installed a concrete wall beside the Alaskan Way Viaduct to protect it.

Dixon said the viaduct was not affected by the sinkhole, nor was anything else damaged.

Dixon said the machine would soon be in more stable soil where sinkholes are less likely.

He also said contractors plan to do more monitoring to verify the amount of soil removed from the machine as a way of measuring the potential for voids.

Dixon said that type of monitoring had been used prior to Bertha's breakdown in 2013, but was not used last week, as operators were instead focusing their attention on how the repaired and reinforced machine was performing.

"We've got a plan in place to deal with any potential ground loss," Dixon said of future efforts.

Both STP and the Washington State Department of Transportation expressed confidence that sinkholes would not form as Bertha tunnels beneath the Alaskan Way Viaduct, probably in March.

The structure will close to traffic for about two weeks during that portion of the tunnel drive.

The sinkhole formed as STP was dealing with a problem with hauling away excavated soil on barges.

On Tuesday morning, a barge being loaded with material listed to one side and struck some concrete fenders.

Some clean soil spilled into Elliott Bay, and both the fenders and the barge were damaged.

Dixon said STP is bringing in a new type of fender to resume barge operations and is also working on a parallel effort to clear off an area of a pier to hold excavated material. He expected a solution to be in place in a few days so Bertha can resume tunneling.

Mining temporarily stopped after the barge incident because there was nowhere to put the dirt.

Neither the sinkhole nor the barging problems are considered serious setbacks and Dixon said the machine is working well.

As of Tuesday morning, state officials said Bertha had mined more than 190 feet and installed 30 concrete tunnel rings since first moving forward in the access pit on Dec. 22.

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