7-foot-deep sinkhole opens near King Street as Bertha tunnels underneath
SEATTLE — A 7-foot-deep sinkhole was discovered in a restricted area south of Seattle’s King Street Thursday as Bertha, the Alaskan Way tunneling machine, was boring beneath the area, officials said.
“Located in the construction yard just south of King, the sinkhole was about 15 feet long, 20 feet wide and 7 feet deep,” transportation officials said. “The hole was quickly filled in as Bertha continued on her way, but not before it illustrated why our contractor spent $53 million building a protected area at the start of the tunnel drive. If ever there was a good spot for a sinkhole, it’s here, in a restricted area protected by underground walls.”
The Bertha blog said some of the soil near King Street is glacial till, which is good for tunneling, but a layer near the surface is fill material dumped in by Seattle’s early settlers. The fill includes loose soil, sawdust and timber piles that, if disturbed during tunneling, can create voids above the machine, it said.