A tunnel beneath Seattle: the megaproject to replace the Alaskan Way Viaduct

The program to replace the Alaskan Way Viaduct with a single-bore tunnel under Seattle's waterfront is a lesson in outliving a worst case scenario. Designed over a decade in a highly inclusive democratic setting, the program's ambition is measured in its time to fruition, realized risk, and scale of public improvement. Budgeted at $3.1 billion, this constellation of 30 projects revolves around one $1.35 billion contract to use one of the world's largest tunnel boring machines. With just a three percent contingency, managers were stunned when, just 10 percent into its route, the machine ground to a halt. Though still litigating the cost of repairs, Washington State's compartmentalization of risk has kept the tunnel cost to a 10 percent increase, with the program at a 7 percent overrun for $3.35 billion with a three year schedule increase, while public waterfront improvements have accelerated an already sizeable boom in property values.