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How does a tunnel boring machine work?

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How are new subway tunnels constructed underneath a busy city like Barcelona, Spain? This promotional animation demonstrates how a tunnel boring machine—specifically a Herrenknecht TBM with an Earth Pressure Balance Shield—excavated a 3.6 kilometer (2.24 mile) long line extension between Barcelona’s Gornal and Zona Universitària metro stations:

In order to avoid any damage to the sensitive buildings, subsidence, heave and vibrations must be avoided at all cost. In addition, the impact on traffic above ground must remain as small as possible. The S-442 is a classic EPB machine with an outer diameter of 12.06 meters and a total length of 95 meters. With its rotating cutting wheel, the tunneling machine breaks the material from the tunnel phase. The material is then transferred to the belt conveyer system in the rear of the shield via a screw conveyer while the hydraulic cylinders press the machine forward continuously.

The reinforced concrete segments, known as lining segments, are installed under the protection of the shield’s skin. When the ring building has been completed, the machine can push itself against the new tunnel ring and drill further into the soil.

The video above was posted by San Francisco’s Central Subway Project YouTube channel in 2011.

Next, more incredible engineering projects: A monster-sized bridge building machine in action, annotated time lapse assembly of a wind turbine, and The Ingenious Design of the Aluminum Beverage Can.

Thanks, @akent.

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