A
manhole cover is a removable plate forming the lid over the opening of a
manholeA manhole is an opening used to gain access to sewers or other underground structures, usually for maintenance.Manhole may also refer to:* Manhole , a metal band from Los Angeles* The Manhole, a computer game...
, to prevent anyone from falling in and to keep unauthorized persons out.
Since the era of ancient Rome,
sewerA sanitary sewer is a separate underground carriage system specifically for transporting sewage from houses and commercial buildings to treatment or disposal. Sanitary sewers serving industrial areas also carry industrial wastewater...
grates made from stone have been used to keep people from falling into the sewage and to catch anything large that might otherwise fall in.
Manhole covers usually weigh more than 50 kg, partly because the weight keeps them in place when traffic passes over them, and partly because they are often made out of
cast ironCast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
, sometimes with infills of
concreteConcrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
. This makes them inexpensive, strong, and heavy. A manhole cover sits on metal base, with a smaller inset rim which fits the cover. The base and cover are sometimes called "castings," because they are made by a casting process.
They usually feature "pick holes," in which a hook handle is inserted to lift them. Pick holes can be concealed for a more watertight lid, or can allow light to shine through. A manhole pick or hook is typically used to lift them, though other tools can be used as well, including electromagnets.
Manhole covers are generally made using
sand castingSand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the mold material.It is relatively cheap and sufficiently refractory even for steel foundry use. A suitable bonding agent is mixed or occurs with the sand...
techniques.
IndiaIndia , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
has become a major player in the industry because of cheaper, lower-grade iron and lax labor and safety laws.
Although the covers are too large to be
collectibleA collectable or collectible is any object regarded as being of value or interest to a collector . There are numerous types of collectables and terms to denote those types. An antique is a collectable that is old...
, their ubiquity and the many patterns and descriptions printed on them has led some people to collect pictures of covers from around the world.
Despite their weight and cumbersome nature,
manhole covers are sometimes stolenManhole cover theft is the phenomenon of manhole covers being stolen, usually for resale as scrap. Long considered to be a childish prank or simple vandalism, this type of theft is often expensive to municipalities, and dangerous to their residents. In recent years, the theft of manholes for...
, usually for resale as scrap, particularly when metal prices rise.
Circular shape
The question of why manhole covers are typically round was made famous by
MicrosoftMicrosoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
when they began asking it as a job-interview question. Originally meant as a psychological assessment of how one approaches a question with more than one correct answer, the problem has produced a number of alternate explanations, from the pragmatic ("Manhole covers are round because manholes are round.") to the philosophical.
Reasons for the shape include:
- A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the hole. (A Reuleaux triangle
A Reuleaux triangle is, apart from the trivial case of the circle, the simplest and best known Reuleaux polygon, a curve of constant width. The separation of two parallel lines tangent to the curve is independent of their orientation...
or other curve of constant widthIn geometry, a curve of constant width is a convex planar shape whose width, defined as the perpendicular distance between two distinct parallel lines each intersecting its boundary in a single point, is the same regardless of the direction of those two parallel lines.More generally, any compact...
would also serve this purpose, but round covers are much easier to manufacture. The existence of a "lip" holding up the lid means that the underlying hole is smaller than the cover, so that other shapes might suffice.)
- Round tubes are the strongest and most material-efficient shape against the compression of the earth around them, and so it is natural that the cover of a round tube assume a circular shape.
- Similarly, it is easier to dig a circular hole and thus the cover is also circular.
- The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using a lathe.
- Circular covers do not need to be rotated to align them when covering a circular manhole.
- Human beings have a roughly circular cross-section.
- A round manhole cover can be more easily moved by being rolled.
- A round manhole cover is cheapest to manufacture related to other shapes because it requires the least amount of metal to cover an opening wide enough for a person to get through.
- Most manhole covers are made by a few large companies. A different shape would have to be custom made.
Other manhole shapes can be found, usually squares or rectangles.
Nashua, New Hampshire-Climate:-Demographics:As of the census of 2010, there were 86,494 people, 35,044 households, and 21,876 families residing in the city. The population density was 2,719.9 people per square mile . There were 37,168 housing units at an average density of 1,202.8 per square mile...
may be unique in the U.S. for having triangular manhole covers that point in the direction of the underlying flow. The city is phasing out the triangles, which were made by a local
foundryA foundry is a factory that produces metal castings. Metals are cast into shapes by melting them into a liquid, pouring the metal in a mold, and removing the mold material or casting after the metal has solidified as it cools. The most common metals processed are aluminum and cast iron...
, because they are not large enough to meet modern safety standards and a manufacturer for larger triangles cannot be found. Some manhole covers in
Hamilton, BermudaHamilton is the capital of the British Overseas Territory of Bermuda. It is the territory's financial centre and a major port and tourist destination.-Geography:...
are triangular, and hinged. Some triangular water-main covers also exist in San Francisco.
Manhole covers and race cars
Because of their aerodynamic design, modern
racing carAuto racing is a motorsport involving the racing of cars for competition. It is one of the world's most watched televised sports.-The beginning of racing:...
s create enough
vacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
to lift a manhole cover off the ground. During races on city streets, manhole covers must therefore be welded down to prevent injury. In
1990The 1990 World Sportscar Championship season was the 38th season of FIA World Sportscar Championship racing. It featured the 1990 FIA World Sports-Prototype Championship for Drivers and the 1990 FIA World Sports-Prototype Championship for Teams, both of which were contested over a series for cars...
, during the
Group CGroup C was a category of motorsport, introduced by the FIA in 1982 for sports car racing, along with Group A for touring cars and Group B for GTs....
World Sportscar ChampionshipThe World Sportscar Championship was the world series run for sports car racing by the FIA from 1953 to 1992.The championship evolved from a small collection of the most important sportscar, endurance and road racing events in Europe and North America with dozens of gentleman drivers at the grid,...
raceThe 1990 480 km of Montreal was the eighth round of the 1990 World Sportscar Championship season, taking place at Circuit Gilles Villeneuve, Canada. It took place on September 23, 1990.The race was ended just after half distance due to a heavy accident...
at
Circuit Gilles VilleneuveThe Circuit Gilles Villeneuve is a motor racing circuit, venue for the Formula One Canadian Grand Prix, NASCAR Canadian Tire Series, NASCAR Nationwide Series and Grand-Am Rolex Sports Car Series....
, located in a public park in Montréal, Quebec, a Brun Motorsport
Porsche 962The Porsche 962 was a sports-prototype racing car built by Porsche as a replacement for the 956 and designed mainly to comply with IMSA's GTP regulations, although it would later compete in the European Group C formula as the 956 had...
struck a manhole cover that was lifted by the
ground effectGround effect is term applied to a series of aerodynamic effects used in car design, which has been exploited to create downforce, particularly in racing cars. This has been the successor to the earlier dominant aerodynamic theory of streamlining...
of the car he was following, a Courage C24 Porsche, causing the Porsche to catch fire, and with the safety issues, the race ended shortly afterwards.
Propelled into space
According to
urban legendAn urban legend, urban myth, urban tale, or contemporary legend, is a form of modern folklore consisting of stories that may or may not have been believed by their tellers to be true...
, a manhole cover was accidentally launched from its shaft during an underground nuclear test in the 1950s, at great enough speed to achieve
escape velocityIn physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
. The myth is based on a real incident during the
Operation PlumbbobOperation Plumbbob was a series of nuclear tests conducted between May 28 and October 7, 1957, at the Nevada Test Site, following Operation Redwing, and preceding Operation Hardtack I...
nuclear tests, where a 900kg steel plate cap was blasted off the test shaft at an unknown velocity, and appears as a blur on a single frame of film of the test; it was never recovered. A calculation before the event gave a predicted speed of six times Earth
escape velocityIn physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
, but the calculation is unlikely to have been accurate and they did not believe that it would leave the Earth in reality. After the event, Dr. Robert R. Brownlee described the best estimate of the cover's speed from the photographic evidence as "going like a bat out of hell!!"
Other uses
A robotics research paper in 2011 suggested that robots could examine the shape of specific manhole covers and
use it to calculate their position, as a double-check on GPS data.
See also
- Manhole cover theft
Manhole cover theft is the phenomenon of manhole covers being stolen, usually for resale as scrap. Long considered to be a childish prank or simple vandalism, this type of theft is often expensive to municipalities, and dangerous to their residents. In recent years, the theft of manholes for...
- List of New York City manhole cover abbreviations
- Public utility
A public utility is an organization that maintains the infrastructure for a public service . Public utilities are subject to forms of public control and regulation ranging from local community-based groups to state-wide government monopolies...
- SPQR
SPQR is an initialism from a Latin phrase, Senatus Populusque Romanus , referring to the government of the ancient Roman Republic, and used as an official emblem of the modern day comune of Rome...
- Storm drain
A storm drain, storm sewer , stormwater drain or drainage well system or simply a drain or drain system is designed to drain excess rain and ground water from paved streets, parking lots, sidewalks, and roofs. Storm drains vary in design from small residential dry wells to large municipal systems...
- Coal hole
A coal hole is a hatch in the pavement above an underground coal bunker. They are sometimes found outside houses that existed during the period when coal was widely used for domestic heating from the early 19th century to the middle 20th century...
Further reading
- Melnick, Mimi (1994). Manhole Covers. Cambridge, Mass.: MIT Press. ISBN 0-262-13302-4.
- Raymond, Douglas (2007). Transylvanian Street Metal. http://www.blurb.com/bookstore/detail/77885
- Stuart, Diana (2003). Designs Underfoot: The Art of Manhole Covers in New York City. Sharon, Conn.: Design Books. ISBN 1-58574-639-8.
External links
- Rampant manhole theft in China, The Age
The Age is a daily broadsheet newspaper, which has been published in Melbourne, Australia since 1854. Owned and published by Fairfax Media, The Age primarily serves Victoria, but is also available for purchase in Tasmania, the Australian Capital Territory and border regions of South Australia and...
, Feb 8, 2006
- Manhole Covers in Space – exhaustive article by Debbie Moorhouse
- The Pascal-B nuclear test
- Manhole Lids Wiki
- Report by Siemens
Siemens may refer toSiemens, a German family name carried by generations of telecommunications industrialists, including:* Werner von Siemens , inventor, founder of Siemens AG...
for the State of MassachusettsThe Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
concerning manhole cover explosions
- World Sewer Cover Web site
- Great manhole covers of the world