TBM Finishes Drilling under Beacon Hill in SeattleA Mitsubishi 600-tonne, 91 m-long, 6.4 m-diameter tunnel boring machine has punched on 5th March, 2008 the second tunnel through Seattle's Beacon Hill for the Sound Transit light rail line with 13 transit stations between Westlake Center in downtown Seattle and Sea-Tac airport. Construction workers and transit employees cheered as the TBM broke through and operators crawled across the drill bit to shake hands with Mayor Greg Nickels. The TBM, nicknamed the "Emerald Mole", emerged within five millimetres of its target. The first tunnel under Beacon Hill was completed last May. Click here and here. View a video of the breakthrough at www.youtube.com/watch?v=ndpeszMmLl4 Obayashi launched the TBM on its second trip through Beacon Hill in July 2007. The TBM turns about 2½ half revolutions per minute. It travelled 1,310 metres through the hill and an estimated 46,000 cubic metres of dirt were hauled out from the tunnel. The deepest point of the tunnel is 50 metres underground. In its path the machine installed more than 800 precast concrete segments forming the lining of the tunnel. The TBM has encountered some rough spots, like water and various maintenance issues.The project forms part of the 25.7 km Central Link light rail project from Westlake Center to South 154th Street in Tukwila. Construction is under way on all five segments of the initial segment: underground downtown, Sodo, Beacon Hill, Rainier Valley and Tukwila. The airport connection will add 3.2 kilometres. The complete line is on schedule to open in late 2009 and is estimated to carry about 45,000 people a day. Read E-News Weekly 35/2007, 21/2006 & 30/2003. Click us/42. The University link to the north (dotted on the map) will have three TBMs in total for the project. Two of the TBMs will start at the University site and simultaneously bore the two tunnels south to Capitol Hill. A third machine will start at the Capitol Hill station site and bore one tunnel south to the end of the initial segment construction. That machine will then be disassembled, moved to Capitol Hill and reassembled to bore the second tunnel (just as was done with the Beacon Hill TBM). Read E-News Weekly 50/2006, 24/2004 & 13/2003. Visit www.soundtransit.org 11/08.