Bellevue, WA - The SEM excavation of the 36 ft wide, 32 ft high, 1,985ft (605m) long light rail tunnel under downtown Bellevue is well underway. Soon after the bid result in November 2015, Atkinson received change order approval 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 where 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.
The following video link provides more in-depth information.
Interviewed: Jake Coibion, Atkinson Tunnel Manager; Chad Frederick, Sound Transit Construction Manager