An Overview of the SR99 TBM Access Shaft,Seattle, Washington

摘要

The SR99 bored tunnel will replace the seismically vulnerable, double-deck Alaskan Way Viaduct. "Bertha," at 57.3-ft in diameter, was, at the time, the largest EPB-TBM in the world. The TBM launched in July 2013 and stopped after mining approximately 1000-ft when it was discovered that the outer seals had been damaged. An access shaft was installed in front of the TBM to facilitate removal and repair of the cutter head assembly. Access shaft design and construction overcame difficulties including buried structures, archaeological concerns, limited site access, complex geology, adverse groundwater conditions, and proximity to the adjacent viaduct that remained in service. The project required installation of deep secant piles up to 10-ft diameter, multi-stage dewatering, structural and hydraulic grouting, structural and geotechnical modeling, and real-time instrumentation. The TBM successfully mined into the access shaft in early 2015 and the 2000-tonne cutter head was removed from the shaft via modular lift tower supported on the shaft.