By Bob Ortblad

SDOT recently issued the “West Seattle High Bridge Long-Term Replacement Planning Compiled Report”.

This report is engineering propaganda touting the replication of the current bridge.

To eliminate a tunnel option, SDOT evaluated a bizarre tunnel with 360- degrees spiral entrances two miles apart. The tunnel is not defined as a bored, excavated, cut-and-cover, or immersed tube tunnel. This technology has been proven time and time again and is growing in popularity around the world.

The SDOT design however shows a tunnel 14,000 feet long passing under a 1,000-foot wide and 42-foot deep channel. An honest engineering design would give a 1,500-foot long immersed tunnel (500 feet in-river) that passes under a 30- foot deep channel.

The United States Geological Survey’s “Seismic Stability of the Duwamish River Delta” locates the Seattle Fault Zone directly below the West Seattle Bridge, and soil will liquefy during an earthquake.

The Washington State Department of Transportation design manual states it is not cost-effective to build a bridge on a deep liquefaction site.

WSDOT also has a YouTube video on why a tunnel is extremely safe in an earthquake. Immersed tube tunnels are almost immune to earthquakes. 

The SDOT report also fails to mention an unsafe steep 6% bridge grade in their plan and the need for light rail. The steepest interstate bridge in the nation is Tampa Bay at 4%. 

SDOT estimates the cost of a new bridge will exceed $1 billion. In 2020, Gothenburg, Sweden completed a six-lane immersed tunnel that crosses a 1,000-foot-wide river for $225 million.

Bob Ortblad MSCE, MBA