Seattle’s New SR 99 Tunnel

Seattle has seen many new development and construction projects over the past several years, with none quite so well known as the State Route (SR) 99 Tunnel, which was created to replace the Alaskan Way Viaduct. With is opening in February of 2019, the completed tunnel hosted its first commuters and capped off a years-long planning and construction process that attracted attention from many residents across the city and from people across the US curious about the large and complicated project. The new, four lane underground tunnel required the work of the city’s top construction outfits, from engineers to Seattle commercial concrete contractors, and will change the look and dynamic of the waterfront district above, as well as Seattle’s downtown as a whole.

The need for a new route

The Alaskan Way Viaduct had traversed the the edge of Seattle along the waterfront since 1953, sending 110,000 cars a day north and south, high above its namesake surface street. While some city residents enjoyed the view of Elliot Bay from the upper deck of the double-decker freeway, others believed the structure was an eyesore that cut the city off from its bustling waterfront. The decision to find a more desirable replacement for the viaduct, however, came as a result of its questionable structural integrity, revealed by the Nisqually earthquake of 2001. After Nisqually’s tremors subsided, inspections of the viaduct determined that the structure was susceptible to total collapse if it were to endure another earthquake of the same or greater magnitude. After city officials and residents explored several options for a replacement, it was decided to widen the surface street Alaskan Way for local traffic and construct a deep-bore tunnel to carry two miles of SR 99 traffic as a bypass route under the city.

Cutting-edge technology

The enormous scale and complexity of the tunnel project required cutting-edge design and technology. One of the most notable elements of the construction process was the tunnel-boring machine (TBM), dubbed Bertha by Seattleites, that at the time of construction was the largest-diameter machine of its kind. The 57 foot wide TBM was designed specifically for the project and featured a cutterhead that funneled the excavated soil into an internal removal system that transported the soil on rail cars out and behind the machine, while an erector arm placed curved, precast concrete segments to create the exterior walls of the tunnel. Once Bertha had completed its work, the two decks, one for northbound traffic and the other for southbound traffic, were constructed from precast concrete slabs that underwent post-tensioning to increase the concrete’s strength and ability to withstand lateral forces.

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