Seattle tunnel will go under 158 buildings

Seattle tunnel cross-section

Seattle tunnel cross-section

The tunnel replacing the Alaskan Way Viaduct (SR-99) will pass beneath 158 existing structures requiring an extensive program of vibration and settlement monitoring as well as some remediation. Of the 158 buildings, WSDOT identified 20 as needing some remediation, but the contractor has shortened that list to 7 structures. For those 7 the tolerable settlement is 1/2 inch, for the remaining, it is 1 inch. Remediation will consist of grouting in at least one case, and micropile walls in other cases where the tunnel alignment doesn’t pass directly underneath the structure. The existing viaduct will also get micropiles for stabilization. Utility relocation and geotechnical instrumentation will take place this year with additional work beginning next year. Tunneling won’t begin until mid-2013 on the $1.4 Billion megaproject. [Source: Seattle Times Newspaper. Image: Storify]