London's Massive Subway Tunneling Machines Build as They Destroy

The eight sensor-loaded machines boring tunnels under London are accurate to within a few millimeters

Breaking through into a station at London’s Canary Wharf in June 2013
Photograph by Andrew Winning/Reuters

The monstrous machines that dig subways, viaducts, and underground highways don’t look anything like the pointy-nosed tractors imagined by sci-fi prop masters. Tunnel boring machines, as the industry calls them, look more like flat-faced worms. Since 2012 there have been eight at work beneath London, churning through 26 miles of rock and soil to build 10 subway tunnels. The £14.8 billion project ($24.8 billion) will improve tube travel between East and West London and is expected to begin operation in 2018.

The London worms—all named after famous female Brits such as Queen Victoria and programmer Ada Lovelace—were built by the German company Herrenknecht. Each is about the height of a two-story house and nearly as long as London’s 41-story Gherkin skyscraper is tall.

A rock-gnawing worm performs two simultaneous tasks: destroying and building. The cutting face is studded with tungsten carbide teeth that are only slightly less hard but significantly less expensive than diamond. The face does not whir so much as leisurely grind along at one to three turns per minute. It bristles with sensors that report pressure, soil makeup, and position to a crew of 20 people. The 12 working onboard have access to a canteen and bathroom while they keep the beast in motion 24/7. The gnashed rock it produces is dropped onto a conveyor belt and transferred out of the tunnel.

Five-foot-thick slabs of curved concrete are moved from the back of the machine to a rotating hydraulic arm just behind the cutting face. The arm lifts and bolts each piece into place, forming new rings of the subway tunnel. If things go well, the boring machine can advance about 2 feet per hour, leaving behind it fresh new concrete linings, like a snake sloughing off its skin.

Of course, things don’t always go well. In Seattle, a borer named Bertha has been stuck underground since late December. The machine, built by Hitachi Zosen of Osaka, Japan, struck an unmarked pipe and busted a bearing while digging a subterranean highway. Officials say Bertha won’t get grinding again until March 2015, and the delays will cost more than $125 million.

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