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How to Go About Burying 16,000-Ton Tunnel Segments Under a River
Throughout the summer, six large barge-like vessels floated out of Baltimore's harbor. These weren't boats, they were giant concrete tubes, destined for a tunnel in Southern Virginia that will expand a busy subterranean highway that connects two parts of the state. This week, those tunnel segments will start to be installed under the Elizabeth River and engineering firm Skanska has released detailed information about how exactly they are burying those 16,000-ton tunnel segments below a muddy riverbed. The Elizabeth River Tunnels is a transportation project launched in 2011 to double the capacity of the existing Midtown Tunnel, which connects the cities of Norfolk and Portsmouth, Virginia by car. It's one of the largest infrastructure projects currently happening in the U.S. But it's also in an extremely sensitive ecological region—the Elizabeth is the same river that's part of a larger restoration program to clean and repair the watershed that I've written about before. The project needs to have as light a touch on the surrounding environment as possible. Since the tunnel segments were cast in Baltimore, the first step for the journey was to transport those segments almost 220 miles away. Obviously the 350-foot-long concrete tubes were far too heavy (and inefficient) to haul via truck, so engineers used the river's natural transportation power. The segments were closed off on one side so they could float like barges, heading out the Chesapeake Bay and back up the Elizabeth River, where a trench awaited the segments' arrival. The rest of the action goes on underwater, so Skanska created an infographic to illustrate the process. First, 40,000 tons of aggregate and sand are dropped into the trench, where they're graded down to a tolerance of one inch by a kind of sand-plow device suspended from above. Then, the actions used to make the [...]