Thursday, January 19, 2006

Digging Beacon Hill

Link Light Rail has two tunneled sections: One is the existing Downtown Seattle Transit Tunnel, currently being retrofitted, and the other are double tunnels under Beacon Hill. Beacon Hill Station will be over 165 feet below ground, between Mount Baker and SODO stations. Expected to have nearly a million boardings a year in 2020, it will provide commercial and residential growth for Beacon Hill, and will result in better air quality and better traffic as more people choose it over private vehicles.

While the Rainier Valley segment of the light rail could have been bypassed on the way to the airport, this area of Seattle has been lacking development dollars for decades, and fixed guideway transit brings developers to station locations. For this alignment, in order to both access the operations and maintenance base nearby the Port and to serve the stadia, and to avoid putting surface level tracks on Rainier Boulevard, a tunnel under Beacon Hill was necessary.

The station will begin at ground level at Beacon and McClellan, and extend downward about 180 feet. To begin, the walls of the shaft were cut out and built in place in 12 foot wide sections. As each section is sunk, the hole made is filled with a high density liquid called "slurry" that supports the surrounding earth, preventing a collapse in the 180 foot deep holes. The slurry is then displaced by concrete from the bottom up, and this is allowed to harden before moving on to the next section. The walls of the station shaft were completed before any dirt was removed from inside of them!

This dirt is mined out from the surface, with workers lowered by crane into the main shaft as the depth increases. The crane also removes earth from the excavated station with what's called a muck bucket, filled by earthmovers at the bottom of the excavation. A second shaft with emergency stairs and ventilation will also be sunk between the platforms just to the east of the main station. Air will largely be circulated by the movement of the trains themselves, but I've been told that this system helps maintain more constant air pressure for passengers at the station.

Tunneling was originally to begin in October of last year, but was delayed due to sandy soil near the station site. Apparently, this has little impact on the tunneling itself, but excavation of the station platforms had to wait for what's called "jet grouting" to take place. Jet grouting involves drilling a hole straight down from the surface, then filling it with a concrete-like substance in order to stabilize the soil for excavation. This was included as a provision in the contract with Obayashi, and it saved Sound Transit and the taxpayers a considerable amount of money to have it available.

The tunnel boring machine (TBM), now known as the "Emerald Mole", will start at the west portal under I-5, near the new Operations and Maintenance facility, and bore the two tunnels one at a time. After the first tunnel, it will be brought back around to the west portal to begin again, in order to keep all the earth coming from one source and traveling on one route. As it moves, the TBM actually builds the tunnel inside its skin from these large precast concrete sections, then pushes forward off of them to drill out the next section of earth. A concrete grout fills the slim section between the tunnel liner segments and the outside of the bored tunnel.

Yesterday morning marked the official launch of the Emerald Mole, with elected officials such as Senator Patty Murray, Seattle Mayor Greg Nickels, and King County Council Chair Larry Phillips, with representatives from the offices of several others. Much of the Sound Transit staff also came out, as well as contractor personnel such as Paul Zick from Obayashi. They had cakes from Obayashi featuring Sound Transit colors and logos, as well as coffee provided by Tully's, who are headquartered next door.

Murray and Nickels each gave short speeches, and gave recognition to a number of individuals who came out for the ceremony. The VIPs went up to break bottles of champagne and sake against the TBM, and everyone got a chance to sign the side and ask questions about the machine and the process - I learned a lot about the equipment and the process.

Congratulations to Sound Transit for a crucial milestone on the path to completing Link!