TBM Breaks Through Beacon Hill in SeattleA Mitsubishi Heavy Industries tunnel boring machine broke through the east side of Beacon Hill in the Seattle's south end a little after 8:15 a.m on 8th May, almost a year and a half after it started digging a light rail tunnel. The TBM is equipped with a laser guidance system that dug the 1,337 m-long hole within five millimetres of engineers' plans. It made about 15 metres of progress per day on average, even 18 metres once crews really got going and comfortable with the operation.The freshly dug tunnel is the first of two being built for a 25.7 km Sound Transit line that will stretch from downtown to Seattle-Tacoma International Airport. The USD2.7 billion light rail line from Seattle to the airport is on schedule to open in 2009.Sound Transit crews will finish laying the concrete rings that line the tunnel, then take the machine apart, move it back to the west side of Beacon Hill, reassemble it, test it, then start digging the parallel, southbound tunnel. The machine takes 21 truck loads to haul. Drilling for the second tunnel is planned for June.Sound Transit board members on 10th May approved nearly USD1 million more for geotechnical engineering done by Shannon & Wilson, reallocating USD990,000 into a contingency account to pay for unanticipated work. Shannon & Wilson's additional geotechnical work dealt with unanticipated amounts of water found 48 metres below the surface of the hill. The company helped Sound Transit devise a system to remove the water and stabilize the soil with a form of cement so that construction of the tunnel and the underground station could proceed. In addition to requiring the dewatering and grouting, the water and sand forced Sound Transit to move the Beacon Hill underground station platform 27 metres at greater cost. Visit www.shannonwilson.com Breakthrough of MHI's TBM dubbed the Emerald Mole Other delays, including a shutdown after a fatal construction accident at the site on 7th February, could hold up by two months the opening of the section between downtown Seattle and Tukwila. To prevent that, the agency is trying to get subcontractors to complete work on part of the station ventilation system a few weeks earlier than planned and is pressing other contractors to complete electrical signaling and communications systems early as well. Beacon Hill stationBeacon Hill station consists of working from the top of Beacon Hill downward to build the 48 m-deep Beacon Hill station, using techniques known as slurry wall construction and sequential excavation mining (SEM).Riders will access the Beacon Hill station via high speed elevators that transport them down to the underground platforms. The station is being built on a site located at the intersection of Beacon Avenue South and McClellan Street South. The project's designers grappled with how to dig 48 m-deep shafts, including a 15.2 m-wide main shaft and a 9.1 m-wide ancillary shaft, without their vertical walls becoming unstable during the process. To meet this challenge, the team adopted a slurry wall design. Working in small sections or panels, special equipment including a hydrofraise was used to dig approximately 0.9 m-wide holes to full depth around the shaft perimeters that would be filled with concrete to form vertical walls. To keep these narrow and very deep panels from collapsing inward during the excavation phase, they were filled with thick bentonite slurry - thus the name slurry wall. Once a panel was excavated, reinforcing steel was installed and then concrete was pumped in, displacing the slurry. When all the panels were done, the finished product was a 0.9 m-thick concrete wall. Beacon Hill station's tunnels are being excavated in sections. Temporary structures built in the middle help support the ground during the process and will ultimately be removed Meanwhile, using jet grouting techniques from the surface, crews worked to improve the stability of underground areas where they would later mine. This involved using a tall-mast jet grout drill boom to pump cement grout into the ground at high pressure. This work, to reduce mining risks, targeted key underground areas where geotechnical tests showed particularly crumbly soils. Once the main vertical shaft was excavated, Obayashi began horizontal sequential excavation mining with specialized articulating-boom excavators and tunnel mining drill rigs. The SEM work uses a predetermined, highly disciplined sequence for excavating horizontal passages in sections and temporarily supporting the ground using steel girders and shotcrete. The continual cycle of excavating and reinforcing requires 24-hour work shifts. Visit www.obayashiusa.comSEM techniques have been used extensively in Europe and Asia, and to a lesser extent in North America. The Beacon Hill project is the deepest North American application of the techniques in glacial soils. Read E-News Weekly 21/2006, 30/2003 & 13/2003. Click us/42. Visit www.soundtransit.org 21/07.