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Seattle’s tunnel project puts construction under pressure POSTED: Monday, June 25, 2012 at 03:06 PM PT BY: Lee Fehrenbacher Tags: Alaskan Way Viaduct, Ballard Diving and Salvage
The laws of physics are a bit different 200 feet beneath the surface of the earth. “When you get to that point, you can’t breathe normal air,” said Joe Olsen, project manager for Ballard Diving & Hyperbaric Services out of Vancouver, Wash. “The concentration of oxygen is too high and your body can’t handle that much oxygen pressure.”
Two workers for Ballard Diving & Hyperbaric Services exit the air locks inside a tunnel boring machine after decompressing on a tunnel project in Portland. A similar system requiring pressurized chambers will be used in the machine that will dig the tunnel beneath the Alaskan Way Viaduct in Seattle. (Photo courtesy of Ballard Diving and Salvage)
At 200 feet deep, the pressure of the soil is equivalent to that of water at 165 feet deep or five times the earth’s atmosphere. At that depth, divers need to replace nitrogen with helium because the former starts to act like a narcotic, inducing a drunk-like effect.
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It’s also the approximate maximum depth of the massive two-mile, 57.5-foot diameter tunnel construction crews are preparing to dig through the ground beneath Seattle to replace the Alaskan Way Viaduct – a project pushing the boundaries of technology while grazing the edge of extreme environments. Facebook social plugin
Right now, a Japanese company called Hitachi Zosen Corp. is building what will be the largest tunnel boring machine in the world for the project. When it arrives in March of next year, it will come in 40 separate 900-ton pieces and will be something of a modern marvel. Ballard Diving will be performing the maintenance – a task in which 45 minutes of work time can require three hours of decompression in a cramped hyperbaric chamber. “That’s the tax you have to pay for going in and having fun,” Olsen said.
The construction process The tunnel boring machine uses an earth-pressure balance system, which means that as the machine digs forward it exerts the same amount of pressure as the surrounding soil. “If you look at it from the front it looks like a big bicycle wheel with a bunch of spokes and then a bunch of cutters mounted on the face of the shield,” said Chris Dixon, project manager for Seattle Tunnel Partners, which is overseeing the project. As the face of the machine spins and scrapes the earth, soil is treated with a conditioner and shoved through openings into a chamber, where it turns into a toothpaste-like consistency. A screw conveyer pushes the dirt out of the chamber and onto a conveyer belt that carries the
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Seattle’s tunnel project puts construction under pressure | Daily Journal of...
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soil out of the tunnel. Dirt is removed at the same rate that the machine advances. At full speed the machine can advance 39 feet per day. Dixon said they will be transporting approximately 1.23 million cubic yards of soil to disposal sites. While that’s happening, a robotic arm will place 6.5-foot-wide, precast concrete segments in a ring around the exposed surface of the tunnel and grout them in place. As each ring is finished, the boring machine pushes forward and begins the next segment. Some 9,720 feet, and 16 months later, the machine will emerge at the north end of the city. Seattle Tunnel Partners is a joint venture between Tutor Perini Corp., a heavy civil construction company out of Sylmar, Calif., and Dragados USA, a large tunnel-boring contractor from Spain. Dragados owns 55 percent of the $1.35 billion contract, and Tutor Perini owns 45 percent. Dixon said that while the tunnel boring aspect of the job gets a lot of attention, the lion’s share of the work (about two thirds) will come in the way of excavation to prepare for the machine, the construction of the two-level concrete highway that will run through the tunnel, and the extension of utilities throughout. Crews performing that work will follow the boring machine as it proceeds, where progress is largely dependent on the soil. “There’s an old saying that good ground makes good miners,” Dixon said. “So it all really depends on the geologic conditions we encounter as we’re tunneling. The machine can digest boulders up to three feet in diameter.” But those boulders – called glacial erratics – can be as big as a house, and Dave Sowers, engineering manager for the Washington State Department of Transportation, said they expect to encounter at least a few of those. That’s because 10,000 years ago, and for tens of thousands of years before that, the entire Puget Sound area was covered in 5,000 feet of solid ice. “It crushed the ground and caused it to densify into this very hard condition, which is good if you’re building a tunnel, good if you’re building a highway, or if you’re building a sky scraper,” Sowers said, of the structurally stable soil conditions. It’s the same soil conditions that allowed crews to dig the Great Northern Tunnel for the Burlington Northern Railroad in 1904 with pick axes and shovels. The tunnel runs beneath Seattle and is still in use today. Obviously, technology has advanced. But when a glacier drops a rock the size of a building in the path of a tunnel boring machine it still poses a problem. In those situations, Seattle Tunnel Partners will need to pull out special cutting tools to break the rocks into three-foot chunks. If the tools wear down or break, they’ll send in Ballard Diving.
The maintenance “If you’re claustrophobic, it’s not the job for you,” Olsen said. While the nearly 60-foot diameter boring machine is downright cavernous by Ballard Diving’s standards, not many people would feel at ease in a diving helmet, 200 feet below the surface, in a five-foot-wide chamber filled with soil. Hitachi Zosen is actually designing the tunnel boring machine so that much of the maintenance work can be done at atmospheric pressure. However, approximately 10 of the 20 planned maintenance stops, or “interventions,” will require hard-hat divers. Crews of five divers each will operate on swing shifts, with each team working for an hour and then decompressing in a tiny metal chamber for two to three hours. During decompression, workers can read magazines– but not newspapers due to their highly flammable nature in oxygen rich environments – or get a bite to eat. “They’ll have their lunch break on air breaks,” Olsen said. An hour of work to three hours of decompression isn’t very productive so if something goes wrong in the digging process that requires more man-power, Ballard Diving is preparing for longer exposures. The company is currently building a 16-person pressurized habitat on the surface that will allow workers to live at pressure for up to a month. In that situation, divers will shuttle between the habitat and the tunnel boring machine via portable hyperbaric chambers. Decompression can be hard on the body and Olsen said staying pressurized helps divers avoid decompression sickness, or the bends – a painful and potentially deadly condition where gas bubbles in the blood rapidly expand as pressure decreases. Compression work is a necessary part of tunneling, and Olsen said he doesn’t anticipate month-long saturation diving being necessary.
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Seattle’s tunnel project puts construction under pressure | Daily Journal of...
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“But we wanted to make sure they had every tool in the tool kit available should something go drastically sideways,” he said. As for the project at large – which is under way and slated to finish by the end of 2015 – Sowers said all eyes will be on Seattle. “The whole world is looking at this project,” he said. “Everyone wants to learn a few things and that comes from getting under way and seeing how our machine which we’ve designed can handle the ground.” More »
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COMMENTS Gerard says: The statement in the second paragraph is not correct. 200 feet of air = 165 feet of water? I’ve been almost 200 feet down in one of NYC’s water tunnels and we didn’t need any decompression. Death Valley is over 200 feet below sea level. As far as I know, you can drive in and drive out. No mandatory decompression chambers at the park entrances! Posted on 06/26/12 at Tuesday, June 26, 2012 Chrisbap says: The paragraph is perhaps poorly worded. I think he means that the pressure of 200′ of soil is equivalent to 165′ of water. Something still seems a little funny about that conversion, but it seems a lot more reasonable than 200′ of air. As you correctly point out, being 200′ below sea level in the open air is not that big of a deal. Posted on 06/26/12 at Tuesday, June 26, 2012 lee.fehrenbacher says: Sorry for the confusion. I was referring to 200 feet of soil pressure, which according to WSDOT is equivalent to the pressure at 165 feet under water in the anticipated soil conditions. The conversion changes in different soil compositions. A soil composed of mostly clay, which is very impervious to water, will have a lower pressure than soil composed of sand. I’ve made a clarification in the story. Posted on 06/26/12 at Tuesday, June 26, 2012
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