A tunneling breakthrough near the stadiums

Seattle Tunnel Partners is preparing for Bertha to break through the southern wall of the SR 99 tunnel access pit, but another, lesser known tunneling breakthrough has already occurred approximately 2,000 feet south of the pit. Earlier this month, crews demolished the temporary wall separating two sections of cut-and-cover tunnel under construction to the west of the stadiums. A cut-and-cover tunnel is just what it sounds like – dig a trench, then build a floor, walls and a roof. This construction method is being used at each of the tunnel portals to connect the bored section of the tunnel to SR 99 lanes at the surface. 
The cut-and-cover section of the tunnel at the north end will be 450 feet long, extending from the north end of the bored tunnel at Sixth Avenue North and Thomas Street to the tunnel portal at Harrison Street. The cut-and-cover tunnel at the south end is a little more complicated. Instead of a single portal for both directions of traffic, there are two portals and they are more than 900 feet apart. The southbound section of the tunnel is 300 feet long. Traffic for this section will come to the surface roughly two blocks west of the north end of CenturyLink Field.  By contrast, the northbound section of the tunnel is more than 1,200 feet long, and the tunnel portal is located about 300 feet north of South Royal Brougham Way. Building two separate portals at the south end of the tunnel was part of STP’s design to reduce the footprint of the ramps and roadways near the stadiums.  
Crews demolish a wall at the south cut-and-cover tunnel.
This graphic shows the bored and cut-and-cover sections of the SR 99 tunnel.