Built in the 1950s, the Alaskan Way Viaduct was damaged in the 2001 Nisqually Earthquake. While it was strengthened and safe for daily use, the structure remained seismically vulnerable.
The viaduct’s role in moving people into and through downtown Seattle was replaced by the new SR 99 tunnel and a new Alaskan Way surface street . Sigma Engineering Solutions provided Demolition Engineering Services to FERMA Corporation for the removal of the 4-way, 2 story Alaskan Way Viaduct, in Seattle, Washington. The demolition involved sequential removal of viaduct and the various components including 6.5” thick concrete deck, supported concrete beams and bents . The original mid-century structure consisted of 2 ramps at each end of the viaduct and 2 intermediate ramps exciting the viaduct at Seneca Street and Columbia Street and a pedestrian walkway underneath a bridge near Marion St. At the north end of the viaduct the supporting structure was made of steel where it passed over the BNSF Railway. Sigma worked with FERMA, Kiewit, and Washington DOT to come up with a safe and efficient plan for removal of the large concrete structure from a busy waterfront with as little disruption as possible to nearby people, property and businesses while keeping Alaskan Way open.