The City of Fort Collins (Colorado) recently completed construction of a tunnel underneath BNSF Railway tracks to connect more than 30 miles of trails. The prospect of tunneling under an active rail line is a pretty big deal, and the railroad had strict requirements. The original concept of the tunnel was modified by the contractor and their engineer to utilize secant pile retaining walls to allow the construction of the portals closer to the rail line and decreasing the overall tunnel length. The tunnel was constructed by BT Construction and Lithos Engineering was their design engineer. Yenter construction was responsible for the retaining walls, including segments of sheet pile wall used to start things off. Anderson Drilling constructed the shafts for the secant pile wall and for the thrust wall. Read more about the project at TBM.
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)