A few hundred people showed up at Milepost 31 – the information center in Pioneer Square for the SR 99 Tunnel project – on the evening of Sept. 6 to take a guided tour from the Washington State Department of Transportation, and were rewarded with a close up glimpse of the work zone and many facts.

Attendees were led on a free mile-long walk to the west of the work zone where crews are currently digging an 80 foot deep “launch pit” from where the world’s largest tunnel-boring machine will launch in 2013. Towering cranes and massive drilling rigs were hard at work in preparation for the machine’s arrival from Osaka, Japan where it is being built.

The goal of the $3 billion project is to replace the Alaskan Way Viaduct by building a tunnel under downtown Seattle’s waterfront section, connecting the new SR 99, south of downtown, to Aurora Ave. in the north and opening it up for traffic in 2015. According to WSDOT, the advantages will be a safer, “seismically sound replacement for the viaduct” and the freeing up of precious land on Seattle’s waterfront for future development.

Guiding the tour, giving out facts and answering questions were Matt Preedy, WSDOT viaduct project director, construction manager consultant Chris Bambridge and Chris Dixon, project manager for the Seattle Tunnel Partners contractor on the job.

Here are some facts shared during the tour:

- The tunnel boring machine (TBM) will eventually go under the existing viaduct at Yesler Way in its 14-plus month journey, so WSDOT is doing temporary reinforcement from that point north to ensure it remains stable during construction (the reinforcements will do nothing for earthquake safety). - Preedy

- In preparation for the TBM’s arrival, drilling rigs are punching holes in the earth that will eventually be filled with five foot diameter concrete piles on both sides of the tunnel “to protect the viaduct (and port infrastructure) as they do the excavation.” Imagine two vertical walls of concrete, hence the term "Tunnel in a Box" – Bambridge

- The boring begins west of CenturyLink Field and the TBM will cut down at a four degree angle through alluvial fill that came from the Denny Regrade (where there used to be water) until it hits natural ground and starts heading back up.

- As the TBM bores, several monitoring instruments will be put in the ground and on buildings surrounding the project to monitor how the earth and buildings are moving in reaction to the work. - Bambridge

- The TBM itself, the largest ever made, is 57.5 feet in diameter, 326 feet long and weighs in at 6700 tons. The manufacturing is nearly complete and the Japanese manufacturer will use the rest of the year assembling it there for testing, then break it back down for shipment. - Dixon

- Speaking of shipment, the machine will be broken down into 40 pieces and sailed to Terminal 46. Seattle Tunnel Partners originally hoped to have fewer shipments of larger pieces, “but with the capacity of the pier from a structural standpoint and the weight of the pieces … we had to go with a smaller option.” The largest pieces will weigh 900 tons. – Dixon

- Five tons of metal will grind off the TBM’s 900 ton cutter head as it bores. - Dixon

- Tunneling will take 14 months at a minimum as the TBM moves at 6.5 foot increments. At each stop, the crew will install 20 ton concrete rings to form the tunnels shell. A total of 1455 rings, being made in Puyallup, will be installed. It will take 7000 truck trips to get the rings to the tunnel.

In other news, Preedy pointed out that by mid-September the northbound lanes of Highway 99 (from Holgate to King Street) will open. The southbound lane has been carrying both directions of traffic.

As for the remaining viaduct columns and rusty pipes seen jutting into the sky just west of CenturyLink, Preedy said those are the beginning of a railroad bypass bridge at S. Atlantic Street intended to keep freight moving when trains block the intersection at Alaskan Way S. The bridge is expected to be completed in 2013.

Interested in taking the tour yourself? (Information from WSDOT)

For those who did not attend, a new self-guided tour is available. Displays have been installed along the bicycle/pedestrian path to describe construction activities, machinery at work and the area’s history. Access to the path’s north entrance is available at South King Street and Alaskan Way South, a few blocks south of Colman Dock, or the south entrance is at South Atlantic Street and Alaskan Way South, west of Safeco Field. Maps are available at Milepost 31.

Learn more about the Alaskan Way Viaduct Replacement Program atwww.alaskanwayviaduct.org/ and Milepost 31, which is open 11 a.m. to 5 p.m. Tuesdays through Saturdays and is open late during Pioneer Square’s First Thursday Art Walk. Admission to Milepost 31 is free.