SEM excavation for Seattle East Link LRT
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3 May 2017: In Seattle, SEM excavation of the 1,985ft (605m) long metro tunnel under downtown Bellevue is underway. Soon after the bid result in November 2015, a change order was processed to allow construction of the first 50% of the tunnel – through more competent over-consolidated glacial tills – using a full top heading, bench and invert sequence. For the remainder of the alignment, which passes through more highly complex geology in which disturbed clasts of till, outwash and numerous shear planes are predicted, the heading will progress in single sidewall drifts in six sections, as originally scheduled for the full tunnel route. Modeling by the design engineer confirms little difference in ground settlement using the more aggressive approach at the start, and conditions to date have confirmed the modeled results.

Snapshot

SEM excavation for Seattle East Link LRT

Sterling Noren and Peter Kenyon for TunnelTalk
Project: Seattle East Link LRT Bellevue tunnel(Contract E330)
Contractor: Atkinson
Client: Seattle Sound Transit
Contract Value: US$121,446,551
Project scope: SEM excavation of 1,985ft x 36ft wide x 32ft high tunnel with tunnel center wall;
Temporary lining: Fiber-reinforced shotcrete(min 10in thick)
Final lining: Cast-in-place concrete with waterproof membrane 10–45ft below surface level
Vertical Alignment: 0.5–5.6% uphill gradient
Interviewed: Jake Coibion
Atkinson, Tunnel Manager
Chad Frederick
Sound Transit, Construction Manager