This 57-km tunnel under the Swiss Alps has nearly broken through after 11-years of work. This tunnel will create a ‘flat’ rail path through the Alps and become the world’s longest rail tunnel. [Source: World Tunnelling. Image: Alp Transit]
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)
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To speed the project, intermediate shafts were sunk to allow for simultaneous tunnelling on several faces, these points (Erstfeld, Amsteg, Sedrun, Bodio and Faido) to become integrated in the finished structure for servicing and emergency access.