Rapid Transit Technology - Tunnel Construction

Tunnel Construction

The construction of an underground metro is an expensive project, often carried out over a number of years. There are several different methods of building underground lines.

In one common method, known as cut-and-cover (used in the first New York City subway line), the city streets are excavated and a tunnel structure strong enough to support the road above is built at the trench, which is then filled in and the roadway rebuilt. This method (used for most of the underground parts of the São Paulo and Guadalajara subways, for example) often involves extensive relocation of the utilities commonly buried not far below city streets – particularly power and telephone wiring, water and gas mains, and sewers. This relocation must be done carefully, as according to documentaries from the National Geographic Society, one of the causes of the April 22, 1992 explosions in Guadalajara was a misrelocated water pipeline. The structures are typically made of concrete, perhaps with structural columns of steel; in the oldest systems, brick and cast iron were used. Cut-and-cover construction can take so long that it is often necessary to build a temporary roadbed while construction is going on underneath in order to avoid closing main streets for long periods of time; in Toronto, a temporary surface on Yonge Street supported cars and streetcar tracks for several years while the Yonge subway was built.

Some American cities, like Newark, Cincinnati and Rochester, were initially built around canals. When the railways replaced canals, they were able to bury a subway in the disused canal's trench, without rerouting other utilities, or acquiring a right of way piecemeal.

Another usual way is to start with a vertical shaft and then dig the tunnels horizontally from there, often with a tunnelling shield, thus avoiding almost any disturbance to existing streets, buildings, and utilities. But problems with ground water are more likely, and tunnelling through native bedrock may require blasting. (The first city to extensively use deep tunneling was London, where a thick sedimentary layer of clay largely avoids both problems.) The confined space in the tunnel also limits the machinery that can be used, but specialised tunnel-boring machines are now available to overcome this challenge. One disadvantage with this, however, is that the cost of tunnelling is much higher than building systems cut-and-cover, at-grade or elevated. Early tunnelling machines could not make tunnels large enough for conventional railway equipment, necessitating special low, round trains, such as are still used by most of the London Underground, which cannot install air conditioning on most of its lines because the amount of empty space between the trains and tunnel walls is so small.

The deepest metro system in the world was built in St. Petersburg, Russia. In this city, built in the marshland, stable soil starts more than 50 metres (160 ft) deep. Above that level the soil mostly consists of water-bearing finely dispersed sand. Because of this, only three stations out of nearly 60 are built near the ground level and three more above the ground. Some stations and tunnels lie as deep as 100–120 metres (330–390 ft) below the surface. However, the location of the world's deepest station is not as clear. Usually, the vertical distance between the ground level and the rail is used to represent the depth. Among the possible candidates are:

• Deepest stations in Saint Petersburg Metro, Russia:
  • Admiralteyskaya (The Admiralty, 86 metres (282 ft),launched 2011, probably the best candidate)
  • Komendantsky Prospekt (The Commandant Avenue, 78 metres (256 ft), launched 2005)
  • Chernyshevskaya (Chernyshevsky, 70 metres (230 ft), launched 1958)
  • Ploshad Lenina (Lenin Square, 72 metres (236 ft), launched 1958)
• Arsenalna station in Kiev Metro, Ukraince (102 metres (335 ft), launched 1960, built under a hill)
• Park Pobedy station in Moscow Metro (~80 metres (260 ft), launched 2005, built under a hill)
• Puhung station in Pyongyang, North Korea (the Pyongyang metro doubles as a nuclear shelter)
• Washington Park station on Metropolitan Area Express in Portland, Oregon (built under a hill), 260 feet (80 m)

One advantage of deep tunnels is that they can dip in a basin-like profile between stations, without incurring significant extra costs due to having to dig deeper. This technique, also referred to as putting stations "on humps", allows gravity to assist the trains as they accelerate from one station and brake at the next. It was used as early as 1890 on parts of the City and South London Railway, and has been used many times since.

The proposed West Island extension to the Island Line of the MTR of Hong Kong will have stations over 100 metres (330 ft) below the ground level, to serve passengers on the Mid-levels. According to the latest proposal some of the entrances/exits will be equipped with high-speed lifts, instead of the conventional way to use escalators.