Tunnel - SR 99 Tunnel

Seattle, WA

The SR99 tunnel under the city of Seattle replaces the Alaskan Way Viaduct structure.  This 9,000 foot long tunnel begins south of downtown and ends at South Lake Union.  The project was bid as a design-build, and Seattle Tunnel Partners was selected as the contractor.

Inside the tunnel is a two level interior structure with northbound traffic running on the lower deck.  The lower deck was converted from cast-in-place to precast construction as a value engineering solution.  The lower deck consists of 1,152 pieces of 14” deep x 8’ wide precast slabs that span 30’ across the width of the tunnel.

Precast slabs were pre-tensioned in the longitudinal direction of the slabs.  After slabs were placed in the tunnel, multiple slabs were post-tensioned in 300’ sections in the longitudinal direction of the tunnel. Once slab installation and post-tensioning was complete a modified concrete overlay was installed onto the slabs.

The precast slab solution provided significant acceleration to the project schedule.


1,152 pcs. – 14” deep x 8’ wide x 30’ long prestressed solid slabs


  • High-performance, low-permeability concrete
  • Full depth, precast concrete deck slabs


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