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Underground structures are regarded as most efficient for transport because they leave city areas for other public needs. The metro is a very complicated transport system, including track ways in interstation (running), connector and service tunnels; aboveground lobbies, with ticket offices and automatic fare collection gates or turnstiles. It has escalators linking the underground platforms with lobbies, staff offices and E& M plant rooms for electrical and mechanical equipment including power-driven stations and driving machinery chambers, sanitary engineering and the station’s ventilation plant rooms together with any ancillary accommodation. Lifts are used for the disabled, trolleys and cleaning equipment.

Track construction of the metro and traditional railways differs. The rails in the metro rest on concrete sleepers to keep air free from dust. Were the track embedded in slag, gravel, sand, hard earth or even broken stone, the trains would be followed by dust clouds. The sleepers, of the overland railway track, rest upon a bed of crushed rock or gravel, which is called ballast. In the metro, the sleepers are shorter (0.9 m.) than those of the railway track (2.7 m). A drainage gutter separates them (fig. 21.1). The contact rail is laid along the track throughout the line and carries a high voltage current of 825 volts. It is attached to the brackets and transmits direct current to the train electromotor through the current collector (fig. 21.1).

The main part of any underground railway system is an interstation tunnel. It may be a one-way tunnel with single or double tracks or a four-lane tunnel. The New York Subway has two tracks and two high-speed tracks called “expressway”.

The lining in running tunnels is of a circular shape. As a rule, it is erected of reinforced concrete segments or cast iron liners, i.e. is assembled from separate units. Exits switch the trains from one track to another (fig. 21.2).

The metro is a high-performance passenger transport system designed to carry millions of passengers a year. Needless to say, the stations represent one of its main parts, as they are collection points for passengers. Station planning must balance the conflicting demands of passenger movement, economy of operations and the requirements for electrical and mechanical plant rooms and provide a safe environment for the staff. Taken together, the functional and aesthetic requirements of stations form a challenge to their planners and designers. Attractively designed underground structures represent a continuation of the aboveground architecture. The underground stations are fully air-conditioned with temperature and humidity maintained at required levels as the entrance lobbies and subways are densely packed with retail outlets. Usually passengers do not notice whether they are moving above or below ground because the structural layout itself creates the atmosphere of “a city within a city”.

An atypical underground station may have up to several ground level entrances and exits. They should allow for straight and short passageways which link entrances to the fare paid zone, which is behind the line of automatic fare collection gates (AFC) or turnstiles. Their number as well as sizing of platforms, escalators and subways, which are station elements, is dependent on the number of people using the station. Space planning should also take into account the large streams of people moving in these areas. The width of the underpasses should range between 2 and 6 m, the height cannot be less than 2.5 m. The routes must be unobstructed and well signed. Information and route maps must be installed throughout the station. The signs must display messages in English and in the native language.

It has become common to provide escalators for upward and downward movement, and thus allow passengers to change levels within the station. The carrying capacity of the escalators produced in Russia is up to 65 m. An escalation system involves an inclined tunnel for three moving stairs, a power-drive station to move the stairs, and a chamber (fig. 21.3) for a circulating belt tensioner. As a rule, the diameter of the inclined tunnel is 8.1 m. Sometimes there are stairs between the two moving lanes, which can be used in case the escalator breaks down. Stair and escalator widths must be adequate for emergency evacuation. Today’s escalators provide ever-increasing levels of reliability and comfort. The design of metro escalators reflects both technical and aesthetic requirements. They have roughened handrails, non-slip treads and the driving machinery uses a planetary gear instead of a worm and wheel type gear. Modern escalator systems are based on reliable technology that incorporates the latest developments, and on “intelligent” electronic monitoring and control. If malfunctions occur, automatic control units help to find and remove them quickly.

To determine the optimum capacity of an underground station it is necessary to consider the number of passengers circulating and the number of passengers the station can deal with effectively. The quantity index of any station is measured using these figures: the number of passengers for a station is equal to 20-30% of total train capacity; for stations located near terminals, theatres, etc. it must be equal to 50%, and for the stations next to large stadiums the quantity index is equal to 100% of overall train capacity. The metro is ideally suited to serving venues like a sports stadium or concert arena, where occasional events will generate a very large passenger surge over a relatively short period.

Platforms are very important parts of the station as passengers gather and enter the trains, leave their cars and change routes (fig. 21.4a and b) there. Two factors determine the overall configuration of stations. The first is the train length, which fixes a clear platform length. The second factor is the choice between side or island platforms.

When twin-bored tunnels are proposed, island platforms are preferred. It offers narrower station construction. Single rows of columns in the centre of the platform, or one row on each side of it, are normal structural solutions. A column-free platform also offers many advantages, including an enhanced feeling of spaciousness.

Architecture and interior design should give priority to passenger needs. Seating should be situated so that it does not form an obstruction, but can be within easy reach of all passengers. Floor surfaces should be firm, easy to clean and non-slip. Artificial lightning is used to enhance the interior of the station. Platform edges must have strips of high contrasting colour (hazard tape) to warn passengers and prevent access to the tracks. Some platforms are equipped with automatic platform screen doors, which allow a full load of passengers on and off the cars within 90 seconds. These doors allow passengers to enter the cars only when the doors of each train match the barrier doors. Many stations on the Jubilee Line and the Canary Wharf Station, in London, are equipped with glass protective barriers, and contain bright coloured banding to make them more visible.

Three-vault structures are employed for deep-laying stations (fig. 21.5b). The central vault is used as a platform and the side vaults are used for track ways. Vaulted stations can be erected with columns, which serve as the principle-bearing element. The result is a perfect harmony between eye-appeal and technology. As a rule, the floors of shallow-laying stations are made of flat panels (fig. 21.5a).

The structural system of such stations is a reinforced concrete structure with a structural frame of slabs, beams, crossbars, columns and walling blocks as well as floor slabs and foundations on rock. All station platforms; escalators, corridors and tunnels are under closed circuit television surveillance.

Exercises:

Discuss the following questions using terminology from the Word list: