Sunday, December 20, 2009

Engineering Contest Sees Tunnels in Urban Future


An innovative deep-bore tunneling operation devised for Sound Transit in Seattle is one of five projects in competition for the Outstanding Civil Engineering Award of 2009, a contest conducted by the American Society of Civil Engineers. One of the Seattle project's competitors is another tunnel in California. All in all, tunnel technology is being revolutionized these days, with extensive implications for urban design as well as transportation.

All five nominated 2009 civil engineering projects are impressive and tend to renew one's confidence that technology can provide breakthroughs in human life comparable to the great feats of the past century. The successful Sound Transit project is also significant for the next deep-bore program in Seattle, a tunnel under the downtown to replace the Alaska Way Viaduct, a 60 year old elevated freeway alongside the harbor. Eventually, the waterfront tunnel project may offer a national model for cities that wish to recover surface land in high density urban areas for multiple uses--land now used for the single purpose of motor traffic. Tunnel technologies like those in Seattle also could help remove the reputation for waste acquired by the "Big Dig" project in Boston.

Here is what Erik Sofge of Popular Mechanics says about the already completed Sound Transit tunnel:

"Whether or not you're a believer in the universal benefits of public transit, this project deserves respect. To build a passenger rail station in the Beacon Hill area, south of downtown Seattle, contractors had to create the largest and deepest soft-ground sequential excavation method (SEM) tunnels in North America. SEM refers to the practice of digging a tunnel in sections, supporting each segment as you go. The pair of mile-long tunnels—part of a 14-mile light-rail project—were nearly twice the depth and diameter of previous such projects, running under a 352-feet-high hill. When initial test shafts found a surprisingly large amount of fine sand, engineers quickly rearranged the design and path of the tunnels, pioneering new construction techniques that should benefit future SEM projects in soft soils. The final result is inherently unassuming—the Beacon Hill station is 160 feet underground, accessible in 20 seconds by elevator—so the 642-ton, 330-feet-long earth-pressure-balancing tunnel-boring machine that dug the tunnels will have to stand testament to this nimble and literally ground-breaking project."

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