Issue link: https://read.dmtmag.com/i/279349
I DA U N I V E R S A L M a rc h -A p r i l 2 0 1 4 23 Designed as a win-win. Bertha is designed to eas- ily handle rocks under 3 feet in size, because the machine is large enough to simply consume those chunks. Larger boulders could be crushed by the front of the machine as it moves forward, but workers also have the option of break- ing apart problematic rocks. In December 2013, the machine's progress was halted by an unexpected impediment. Crews worked extensively over weeks to try to identify the mystery blockage. ey examined Bertha's excava- tion chamber just behind its cutter-head; drilled wells to relieve water pressure around the machine, and drilled 17 exploratory holes near the front of the machine to fi nd the obstruction. Finally, enough soil had been removed, and the water pressure had dropped enough, that crews could inspect the top 15 feet of the excavation chamber. at's when workers saw a pipe protruding through one of the many openings in the cutterhead. e machine's cutting blades encountered an 8 inch (200 mm) diameter, 119 foot (36m) long steel pipe, one of several castings le over from previous drilling done in 2002. e pipes' locations were known to the Washington Director of Transportation and were supposed to have been removed. e machine grinds through rock, dirt and timber, but not through thick metal. At that point, Bertha had tunneled 1,019 feet (311 m) or 11% of the total 9,270 feet (2,830 m) length of the tunnel and sits 60 feet below ground. A er much work, she bore 4 more feet in January, until the warning light in the control room signaled overheat- ing near the cutterhead. e workers found busted seals around several cutterheads, as well as around the machine's main bearing. To repair the seals, the contractor digging the High- way 99 tunnel will have two options. One is to dig a sha in front of the idled boring machine, which sits 60 feet beneath the ground in Pioneer Square, amid groundwater- inundated earth. e other is to access the bearing through the back of the tunneling rig. e front-end option seems most likely, and it has been done before. A machine digging a railroad tunnel in Sarnia, Ontario, had some of the same symptoms as Bertha. e Canadian engineers dug a vault in front of their machine, the Excalibore, li ed the cut- terhead away and fi xed a dam- aged seal. It took seven months to complete the job. Based on the contract terms, Hitachi Zosen, the company that manufactured the machine, owns it for the fi rst 1,300 feet of digging. A er that point, Seattle Tunnel Partners, a part- nership between Dragados-USA and Tutor Perini, will take over ownership of the tunneling rig. Hitachi Zosen representatives are working with Seattle Tun- nel Partners to diagnose and fi x Bertha's mechani- cal problems. In February 2014, WSDOT stated that it would be several months before the tunneling could be resumed. ● A surveyor sets up his equipment inside the SR 99 tunnel beneath Seattle. Tunnel- boring machine Big Bertha has completed 1,000 feet of the tunnel.