Building Tunnels

Tunneling is the most costly and labor-intensive of all engineering enterprises and, in the early days of the railroads, it was also the most dangerous. Working long shifts lit by candlelight in cramped conditions, workers risked serious injury or loss of life as they burrowed underground using only basic tools. From the mid-19th century their picks, hand-drills, and explosives were gradually replaced by tunnel-boring machines (TBMs), the first of which was used after 1862 to dig the Fréjus Rail Tunnel beneath the Alps (see Fréjus Tunnel). Since then, strict safety regulations have reduced risks considerably, and the introduction of computer control has increased the machines’ efficiency.

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Immersed tunnels

Suitable for routes that cross shallow bodies of water, immersed tunnels on the seabed are a cost-effective alternative to boring beneath it. The first tunnel to be built in this way was the Bay Area Rapid Transit (BART) tunnel in San Francisco in the late 1960s. Sections of the tunnel are floated to the tunnel site, sunk into a precut trench on the seabed, and secured with layers of gravel, concrete, and backfill.

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How it works

TBMs consist of several connected systems that are operated in different phases of construction to bore and line the tunnel, lay the rails, and convey waste material away from the site. The cutting wheel can be up to 63ft (19.3m) in diameter, while the TBM apparatus can be as much as 490ft (150m) in length.

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