The Bertha tunnel boring machine has resumed chewing away beneath Seattle after a ‘suspension for cause’ order from WSDOT shut it down in January following two safety incidents. The BTM began tunneling with some conditions in place late in February, and those conditions were lifted when the demonstration period ended on March 7. Bertha has now reached her next scheduled maintenance stop, having tunneled a total of 1,560 feet. [Source: WSDOT Alaskan Way Viaduct Replacement Project Page. Image: WSDOT]
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The Seattle Times (hat tip to ASCE SmartBrief) has reported that seven voids have been discovered above the Beacon Hill Tunnel with one opening up at the ground surface. The tunnel is being constructed by Sound Transit, the area’s transportation agency as part of a roughly $2.6 billion (yep, billion) light-rail project connecting downtown Seattle with the University of Washington and SEA-TAC airport. The voids were a result of running sand pockets in the otherwise stable clay units that were encountered by the tunnel boring machine or TBM. These voids migrated up like a chimney with one reaching the surface, almost 160-ft above the tunnel. This void was apparently 21-ft deep and opened up in a resident’s front yard and could have easily swallowed her up as she noticed it while gardening. The other voids were discovered at a depth of 20- to 65-ft below the ground surface. More after the break. (Illustration from Seattle Times)