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A diagram shows where Bertha will tunnel beneath Seattle’s Alaskan Way Viaduct. (Via WSDOT.)

Bertha, the giant tunneling machine making its way along Seattle’s waterfront, will start a delicate phase of its 9,270-foot journey on Friday as it begins to pass beneath the Alaskan Way Viaduct, the roadway it is working to replace.

The 385-foot path from one side of the 63-year-old elevated highway to the other (at the intersection of Yesler Way and Alaskan Way) will force a Viaduct closure of approximately two weeks. Depending on how you look at things, redirecting the roughly 90,000 vehicles that use the road daily into Seattle’s already congested commute could be the scariest element of the whole operation.

Bertha map
(Via WSDOT)

But all eyes will be on the Viaduct and its ability to remain stable as the massive tunneling machine comes closer to it than any structure during the SR 99 project. How close? As detailed in a Washington State Department of Transportation video (below), Bertha will pass 80 feet below the ground surface, but just 15 feet below the foundations that support the Viaduct, as shown in this rendering.

viaduct1
(Via WSDOT)

David Sowers, a deputy administrator with WSDOT, explains in the video what measures have been taken over time to strengthen the Viaduct following its damage during the 2001 Nisqually earthquake. Those measures include giant steel braces on the columns, micropile installation to underpin the foundation, and 142,000 square feet of carbon-fiber wrapping on parts of the structure.

In the FAQ section of a special website set up for the roadway closure, WSDOT doesn’t shy from saying that tunneling under the Alaskan Way Viaduct is “some of the most challenging mining conditions of the tunnel alignment.” A map set up to track Bertha’s movement for the entirety of the project is divided into 10 zones, and the soil conditions in each zone are spelled out.

On Friday, Bertha will begin tunneling through a mix of glacially-compacted soils including sand, gravel, silt and clay. If there is ground movement or if the Viaduct begins to settle, project crews can inject grout from the surface and from the tunnel boring machine to fill voids, according to WSDOT.

In another WSDOT video, released on April 22, a state-of-the-art ground-monitoring system is detailed. This video spells out how the system keeps watch on the ground, buildings, the Viaduct and downtown Seattle streets during tunneling operations. More than 4,000 different monitoring points are used to take measurements and share data with engineers, 24 hours a day, 7 days a week.

Speaking of videos, it’s no coincidence that the most popular upload on WSDOT’s YouTube channel is a 2009 thriller called Alaskan Way Viaduct — Earthquake Simulation. After all, that’s why Bertha is down there, slowly carving out a tunnel to replace the structure that sways and collapses in the scary animated simulation of a 7.0 quake (Nisqually was 6.8). Want to see liquified soil ruin a summer night in Seattle? Watch:

The stability of the viaduct, the movement of traffic while it’s closed and the monitoring of everything related to both will all create different challenges for those involved in the tunnel project. For WSDOT, the focus is on the public and keeping people moving despite the headache of shutting down a major Seattle corridor.

“For most people, getting through a two-week closure of the viaduct will be the most challenging,” said Laura Newborn, communications manager for the Alaskan Way Viaduct Replacement Program. “While we can’t reroute vehicles, we are asking drivers to change their commutes for approximately two weeks.”

Newborn pointed out that Seattleites have faced this challenge before.

“WSDOT had a long closure of the Viaduct in 2011, when we tore down the southern mile of the Viaduct,” Newborn said. “We asked for driver help and many cooperated. We hope drivers will once again respond to our requests to alter their commutes and/or work hours.”

The 99 Closure website points out that while it may feel like Déjà vu, the region has experienced tremendous population growth since 2011 and that means more cars on the road. The site offers a variety of resources and current information for those who are impacted by the closure or just curious about how Bertha is moving along. The machine has not been without its well-documented problems along the way.

Another video provides a quick overview on how to plan around the closure by using the WSDOT site as well as by following the organization on Facebook and Twitter for the latest updates.

Bertha has a Twitter account as well and, as GeekWire has previously reported, many aspects of the Viaduct project have been captured and shared via a flickr account.

Elsewhere on flickr, an account run by Seattle Municipal Archives offers a very different picture of Seattle and its waterfront.

In a photo dated July 3, 1952, a Port of Seattle sign stands in front of a skyline that lacks the glass skyscrapers which fill images of today’s tech-centric city. The newly built Alaskan Way Viaduct runs through the image like an artery, from one end of town to the other.

Seattle Viaduct
(Via Seattle Municipal Archives flickr)
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