Bertha, the world’s largest-diameter tunnel-boring machine, sits at a low point in its 9,000-ft bore under downtown Seattle. But, this time, the much-maligned machine finds itself at a low point it wants.
这台57.5英尺的机器位于春天大街下方约120英尺的位置，已到达计划的维护站3,088英尺，最终将为西雅图提供无聊的隧道，以取代阿拉斯加老化的高架桥。After a two-year-plus delay that required raising Bertha’s 4,000-ton face to the surface for repairs, getting to this point under Spring Street—the lowest grade the machine will reach in moving north to its exit portal, near the Space Needle—has elevated the mood of the project team, says Chris Dixon, Seattle Tunnel Partners project manager.
“We are very, very happy to be tunneling and making good progress,” Dixon says. “I know there was a lot of concern [regarding the repairs and finishing the job]. But I always had confidence” that we would succeed.
伯莎（Bertha）于3月和4月29日到达避风港3，以进行计划的维护站。它的平均每天五月的隧道环和六月的每天七个戒指。6月23日，它暂停了大约六个星期的时间，以检查并有可能替换Cutterhead工具，这是与Ballard Marine Construction一起进行的高压维护的一部分。
At the last stop, Seattle Tunnel Partners replaced 11 cutting tools after investigating and cleaning the eight cutterhead spokes. “It would be nice to find we do not have to replace any,” Dixon says. “The clay we have been going through isn’t an abrasive material, so we don’t anticipate excessive wear on the cutting tools.” The machine did have to bust through a relatively thick zone of concrete-type material when leaving the ground that was reinforced around Safe Haven 3.
Dixon expects Bertha to stop again for maintenance in about 2,000 ft and, after that, make a final stop another 2,000 ft. It will move up a 1.6% grade for 3,000 ft, a 3.6% grade for 2,000 ft and a 4% grade for the final 1,300 ft. During this portion of the job, the ground becomes more alluvial.
“We’re pleased with both the progress of the machine and Seattle Tunnel Partners’ ability to control the ground,” says Brian Nielsen, Washington State Dept. of Transportation deputy program administrator.
