Subway engineering Article by: Moh, ZaChieh Moh & Associates, Inc., Oriental Technopolises Tower, Taipei, Taiwan. Hwang, Richard N. Moh & Associates, Inc., Oriental Technopolises Tower, Taipei, Taiwan. Last updated: 2014 DOI: https://doiorg.ezproxy2.library.drexel.edu/10.1036/10978542.664750 (https://doiorg.ezproxy2.library.drexel.edu/10.1036/1097 8542.664750)
Feasibility study Planning Design Construction Operation
Links to Primary Literature Additional Readings
The branch of transportation engineering that deals with feasibility study, planning, design, construction, and operation of subway (underground railway) systems. In addition to providing rapid and comfortable service, subways consume less energy per passenger carried in comparison with other modes of transportation such as automobiles and buses. They have been adopted in many cities as a primary mode of transportation to reduce traffic congestion and air pollution.
The first subway opened in London in 1863 using steam locomotives fueled by coal. In 1896, the first subway on the European continent was placed in service in Budapest. In the United States, the first subway line was completed in Boston in 1895, and since then subways have been constructed in cities such as New York, Chicago, Washington, Baltimore, Houston, San Francisco, Los Angeles, Philadelphia, Pittsburgh, and Atlanta. Subways are found in numerous cities worldwide, including Tokyo, Singapore, Hong Kong, Seoul, Taipei, Shanghai, Beijing, Cairo, Toronto, Sydney, Moscow, Paris, Brasilia, Santiago, and Caracas.
Modern subway systems are electrified, with trains powered by direct current that cruise at speeds up to 80 km (50 mi) per hour. Some trains are fully automated, with drivers on board only for handling unexpected incidents. Cars are air conditioned for passengers comfort, and most of the underground stations are airconditioned as well. It is becoming popular to install doors along the trackside edges of platforms in airconditioned stations to protect passengers as well as conserve energy. See also: Air conditioning (/content/airconditioning/017000); Electric distribution systems (/content/electricdistributionsystems/215800); Railroad control systems (/content/railroadcontrol systems/571900)
Feasibility study Subways are designed for short trips with frequent stops, compared to aboveground, intercity railways. Many factors considered in the planning process of subway systems are quite similar to those for railway systems. Subway system planning starts with a corridor study, which includes a forecast of ridership and revenues, an estimation of construction and
operational costs, and a projection of the potential benefits from land development. The feasibility of constructing a subway system is often governed by economical, social, and political factors.
It is important to integrate a subway system with other modes of transportation to maximize its effectiveness. In mass transportation, air transport and railway systems are used for longdistance travel, subways for intermediatedistance travel within individual metropolitan areas, and buses as feeders to the others. All these systems should form a network to provide maximum convenience for travelers. In many cities, subway and bus systems have already been combined so that they are managed by the same agency. In some cities, unified farecollection systems have been adopted for the two systems so that riders do not have to purchase separate tickets for different segments of the same trip. See also: Railway engineering (/content/railwayengineering/572000); Transportation engineering (/content/transportation engineering/706700)
Planning All subway systems have three major types of structures: stations, tunnels, and depots. The most important task in planning a new subway system or a new subway line is to locate stations and depots and to determine the track alignment. Subway lines are normally located within the rightofway of public roads and as far away as possible from private properties and sites of importance. Protection of historical sites and monuments has often been a reason for the realigning routes. Because stations and entrances are usually located in densely populated areas, land acquisition is often a major problem. One solution is to integrate entrances into nearby developments such as parks, department stores, and public buildings, which lessens the visual impact of the entrances and reduces their impediment to pedestrian flow. Since underground structures are more costly and take longer to construct, subway systems in suburbs and the countryside are usually laid aboveground and, if necessary, elevated.
The environmental impact due to construction is a major concern in terms of disturbance to traffic, vibration, and noise. In addition, dumping sites are usually difficult to find in urban areas, making dumping of construction waste a problem. Offshore dumping is an option, but the effects of such an action must be carefully assessed and minimized.
Design Design of the permanent works includes structural and architectural elements and electrical and mechanical facilities. There are two types of structures: stations and tunnels. For stations, space optimization and passenger flow are important. Figure 1 shows the CKS Memorial Hall Station of the Taipei (Taiwan) Rapid Transit Systems and the major elements in a typical station, such as rails, platform, staircases, and escalators. For handicapped passengers, provisions should be made for the movement of wheelchairs in elevators and at fare gates, and special tiles should be available to guide the blind to platforms. In Singapore, selected stations have been designed as shelters, with structures strengthened for civil defense purposes.
In both stations and tunnels, ventilation is essential for the comfort of the passengers and for removing smoke during a fire. Sufficient staircases are required for passengers to escape from the station platform to a point of safety in case of a fire. The plastic curtains shown at the top of Fig. 1 are required by the fire code to prevent smoke from spreading. See also: Ventilation (/content/ventilation/729900)
The electrical and mechanical facilities include the rolling stock (trains), signaling, communication, power supply, automated fare collection, and environmental control (airconditioning) systems. In addition, there are depot, station, and tunnel service facilities. For stations designed also as civil defense shelters, extra air, water, and power supplies are provided.
Architectural design of station entrances is important in terms of function and esthetics. Landscaping and art exhibitions can reduce the impact of the entrances on the environment. Parking lots and bus bays should be properly located to avoid traffic congestion.
Corrosion has caused problems to structures in some subways; therefore, corrosionresistant coatings may be required. See also: Paint and coatings (/content/paintandcoatings/482300)
To minimize noise and vibration from running trains, floating slabs can be used under rails or building foundations in sections of routes crossing densely populated areas and in commercial districts where vibration and secondary airborne noise inside buildings are unacceptable. Floating slabs contain vibrationabsorbing materials, such as rubber pads, sandwiched between concrete slabs.
Ground conditions are important for both design and construction of underground structures and should be thoroughly investigated. Ground water has been responsible for many major incidents and should not be overlooked. Construction of subway systems usually is complicated by problems involving traffic, utilities, and buildings. These problems are usually responsible for delays and accidents. The conditions of nearby buildings should be inspected and recorded, and the construction impact on these buildings should be estimated. Excessive ground movements resulting from inadequate
Fig. 1 Taipei Rapid Transit Systems’ CKS Memorial Hall station.
retaining systems during excavations have been responsible for most of the catastrophic incidents involving collapse of buildings, explosions due to leaking gas pipes, flooding of pits due to dislocated water mains, and so on. Programs should be prepared to monitor ground movements to ensure the safety of the buildings that most likely will be affected.
With the rapid advancement of computer technology, geographic information systems (GIS) have been widely adopted in the construction industry. Such systems can be used to compile information relevant to the geology and ground conditions, utilities, and buildings. It is desirable to establish a data center to store all the electronic information and to facilitate sharing of data via the Internet. See also: Database management system (/content/databasemanagementsystem/180980); Geographic information systems (/content/geographicinformationsystems/757430); Internet (/content/internet/757467)
Construction Underground stations are normally constructed by using an opencut method. For open cuts in soft ground, the sides of the pits are normally retained by wall members and braced using struts. The pits are fitted with decks for maintaining traffic at the surface. For new lines that pass under existing lines, it is not possible to have open cuts. In such cases, stations have to be constructed using mining methods (underground excavation).
Tunnels linking stations can be constructed by either the opencut method or by underground excavation. On stable ground, underground excavation can be done by using light machines or tunnelboring machines with minimum support. In soft ground, it has become popular to bore tunnels using shield tunneling machines and then line the tunnels with reinforced concrete segments. Figure 2 shows a shield machine which just completed a section of tunnel and entered the station which was still under construction. For tunnels running across waters, submerged tubes are frequently used, such as between Oakland and San Francisco (Bay Area Rapid Transit System) and across the Hong Kong harbor (Hong Kong Mass Transit Railway).
Wherever waterbearing strata exist, construction should proceed with great care. This is particularly true when openings are made on structures at great depth. If leakage occurs, the soils surrounding the water path may quickly liquefy due to the great hydraulic gradient, and the flow may become uncontrollable in a short time. Grouting is usually carried out to cut off water path, although its effectiveness is difficult to ascertain. Ground freezing has been adopted in many cases, either as a precautionary measure or as a remedial measure in rehabilitation, and has been proved effective in dealing with groundwater problems. Freezing is carried out by circulating coolant through looped pipes inserted in drilled holes for the purpose of blocking the water path by solidifying a large volume of wet soil. See also: Grout (/content/grout/301600); Soil mechanics (/content/soilmechanics/631900)
Fig. 2 Shield tunneling machine.
Ground settlement must be minimized to protect structures in zones affected by construction. Ground settlement is closely monitored using computerized instrumentation. Warning systems have been adopted in the construction of many new subway lines that enable contractors, designers, and construction managers to take preventive actions in time and effectively eliminate potential dangers, such as collapse of retaining walls, flooding of pits, and building damage.
Operation Many modern subway systems are fully automated and require only a minimal staff. Train movements are monitored and regulated by computers in a control center. Therefore, engineering is limited to the function and maintenance of the electrical and mechanical facilities. The electrical and mechanical devices requiring constant care include the rolling stock, signaling, communication and broadcasting systems, power supply, elevators and escalators, automated fare collection, and environmental control systems. Also included are depot facilities, and station and tunnel service facilities.
ZaChieh Moh Richard N. Hwang
Links to Primary Literature C. Wang and Z. Su, Feasibility study on step method excavation of tunnels with large cross section and small interval, Procedia Environ. Sci., 12:550–554, 2012 DOI: https://doiorg.ezproxy2.library.drexel.edu/10.1016/j.proenv.2012.01.316 (https://doiorg.ezproxy2.library.drexel.edu/10.1016/j.proenv.2012.01.316)
Additional Readings J. P. Copsey et al., Proc. Inst. Civ. Eng., Part 1, 86:667–707, 1989
T. W. Hulme, P. H. J. Chapman, and P. S. Foo, Singapore Mass Rapid Transit System: Planning and implementation, Proc. Inst. Civ. Eng., Part 1, 86:627–665, 1989
J. R. Lambrechts, R. N. Hwang, and A. Urzua, Big Digs Around the World, Geol. Spec. Publ., no. 86, ASCE, 1998
S. Y. Chang, S. K. A. Bahar, and J. Zhao (eds.), Advances in Civil Engineering and Building Materials, CRC Press, Boca Raton, FL, 2013