U-turn On Seattle's Alaskan Way Bore

摘要

The bored tunnel option to replace the central waterfront section of Seattle's Alaskan Way viaduct and seawall has won backing by state and local political leaders in the latest twist in the saga but their recommendation will need legislative approvals. Backing for the approximately 2.7km long twin tube tunnel came as a surprise as the option had been left on the sidelines in the most recent cost analysis, which late last year produced a shortlist of only two surface alternatives. Construction of the tunnel is to begin in 2011 and the completed link be opened in 2015. The option is the longest construction period (5.5 years for major works) but, critically, it was the least disruptive overall for the city and businesses although its impact is on par with other options at the waterfront. Deep tunnelling would also allow more flexibility in the demolition schedule for the viaduct. In addition, the authorities cited environmental benefits, and improving bored tunnelling technology and economics, as having been important to their preference for the tunnel option. The estimated total cost of the bored tunnel and associated improvements works to improve the streets, waterfront and transit, and also for the cost of demolishing the viaduct on the SR99, is approximately US$4.24bn.