Break on through...to the other side!

After many months of drilling (or boring), the Beacon Hill tunnel segment of Sound Transit's Central Link project is nearly complete. This morning, the giant Emerald Mole machine broke through the east face of Beacon Hill for the second time, spewing foam and clay as it emerged from the ground:



Here's a quick overview of the Emerald Mole, courtesy of Sound Transit:

• At full size, the machine is approximately the length of a football field. It includes "trailing gear" such as supply tanks, electrical support, exhaust fans, and a conveyor belt.
• The weight of the Emerald Mole and attached training gear is approximately six hundred and forty two tons - more than a fully loaded Boeing 747-8.
• A 21 foot in diameter cutterhead is positioned at the front of machine (that's the spoked wheel in the picture above).
• Equipped with various cutting tools, the cutterhead turns around att the rate of 0.1 to 2.5 revolutions per minute. Excavated material goes through openings in the face of the machine.
• The spoils are brought into the machine by a corkscrew-like screw conveyor located behind the cutterhead. The spoils are then taken out the back to be temporarily stored on site and then loaded into dump trucks.
• Foam is usually added to condition the soil cuttings into a paste in order for it to pass through the coneyor for removal. Water, bentonite or polymers may also be used, depending on the soil type, groundwater, and other factors. The conditioning agents are biodegradable.
• The machine is propelled and steered with sixteen hydraulic jacks that are located around the perimeter of the machine.
• The operator steers the machine using sophisticated positioning technology that is accurate to within an inch.
• The machine was manufactured by Mitsubishi Heavy Industries in Kobe, Japan, and arrived in Seattle by ship. It took twenty five truck loads to deliver the Emerald Mole in pieces to the construction site.

Commenting on the accomplishment, Sound Transit Board Chair Greg Nickels said:

Link light rail in Seattle truly is seeing the light at the end of the tunnel, or tunnels in this case. Later this summer we will be ready to pull Link light rail trains through the hill to test the trains on 10 miles of the finished tracks just south of the east portal as we prepare for the first passengers in 2009. This is a major milestone the thousands of men and women who have been a part of light rail construction from downtown to the airport.

Central Link is now more than 85% complete and on track to open for business as scheduled next year. Sound Transit says that six million plus human hours have gone into light rail construction since the project broke ground.