The subsurface of a major city teems with subsurface utilities: sewers, storm drains, steam tunnels, and utility corridors. Access ways, called manholes are dug down to these subsurface conduits at regular intervals to allow maintenance workers to reach them. Manholes are required so that people can clean, inspect, or repair the subsurface utilities. Manholes can be quite shallow or as deep as 70-stories in the third New York City water supply tunnel. Manhole covers are the round iron plates sunk into streets and sidewalks that keep passers-by from falling into manholes.
Manhole covers must be a minimum of 22 in (56 cm) in diameter, but can be as much as 60 in (1.5 m) in diameter. The average cover weighs between 250 and 300 lb (113-136 kg). It is important for sewer manhole covers to be heavy as sewers can produce methane gas that could push lightweight covers out of the way, letting noxious gases up into the street.
As soon as people began to live in cities the problem of what to do with human waste became an issue. The first cities were built along great rivers that served as open sewers. This was hardly satisfactory due to the periodic plagues that resulted from too much human waste in close contact with people. Roman civil engineers solved this problem with the invention of the underground sewer. The Roman sewers, dug by hand and lined with brick, collected a city's waste and deposited it far downstream. The Romans constructed access ways to these sewers to allow for periodic cleaning. The stone manhole covers that capped these access ways can still be seen in the old Roman City of Jerash in Jordan.
It would be quite a while until modern civilization rose to the level of the Romans. Construction began on the Chicago sewer system in 1856. New York City had only 200 mi (320 km) of sewer line laid by 1870, compared to 6,200 mi (10,000 km) today. The first manholes with covers were probably constructed in the early nineteenth century, not for sewers but for water or town gas pipelines. None of the covers for these manholes are known to survive to the present.
Manhole covers are made out of cast iron. Cast iron means that the iron is melted and then poured, or cast, into a mold. Typical manhole covers are cast using gray cast iron. Ductile cast iron, because of its greater strength, is used for special manhole covers, like those that would be found near airplane terminals. Gray cast iron consists of the element iron and the alloying elements carbon and silicon. The alloying agents, chiefly carbon, give cast iron its strength and durability. Ductile cast iron is produced by adding manganese to the molten iron. The manganese causes the carbon in the iron to form nodules instead of flakes, giving ductile cast iron its greater strength and malleability.
Besides iron, the other raw material required to make manhole covers is green sand, which is sand bound together with clay. The green sand is used to produce the molds into which the molten iron is poured. The sand mixture consists of about 90% silica sand, 4-10% clay, 2-10% organics (e.g., coal), and 2-5% water. The sand is not colored green. Green refers to the fact that it is allowed to remain wet during the casting process.
Every manhole cover, from the simplest to the most ornate, is first modeled in wood or aluminum. The model is used to make the mold into which the molten cast iron will be poured. The designs that have been created for the surface of the manholes are as varied as the skilled artisans who created them. All manhole covers are round because a round object cannot be dropped into a round hole of the same diameter. This is vital since the weight of the manhole could easily kill a worker standing underneath it. Round manhole covers are also easier to move around on the surface as they can be rolled. There are rectangular utility box covers, but they are not installed over manholes.
All castings, including manhole covers, are made in large factories called foundries. Scrap steel comes into the foundry, is melted and alloyed, and leaves as iron casting. Cast iron is everywhere. A typical home in the United States contains around 2,000 lb (900 kg) of iron castings, mostly as pipe and pipe fittings, but also in furnaces and air conditioners. The casting process consists of five steps, pattern making, mold preparation, melting/pouring, and cooling and finishing.
Cast iron is usually made with scrap steel. As the raw materials are not controlled, casting houses must carefully analyze the molten metal before it is used to assure that it contains the proper percentages of iron, carbon, and alloying metals. After casting, the strength and ductility of the cast irons must be tested to assure that the manhole covers made from the iron will perform as designed. Strength and ductility is assessed by casting bar test specimens from the same metal used to cast the manhole covers. The bars are placed into a tensiometer which pulls on their ends until they either break or elongate past their elastic limit—the elastic limit is the point that the bar can be pulled to and still regain its original shape if the tension is released. Ductile cast iron can usually withstand between 2% and 10% elongation before it will break. Gray cast iron is brittle, and hence will break before it elongates significantly. Engineers who design products made with gray cast iron must always bear in mind that because the product is brittle, it will break without providing any warning if it is overloaded. As this could cause a disaster, cast iron components usually have much higher factors of safety than ductile iron components.
Gaseous emissions, such as carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrous oxide, and benzene are produced when the molten iron contacts the green sand. In the past, lead was often used as a binder in some types of molds. Landfilling these used molds created heavy metal pollution problems. Resin bound castings produce volatile organic compounds when they are baked to set them.
Most of the sand in green sand can be recycled in new castings. However, a certain percentage of the sand becomes too fine during the casting operations and must be discarded.
It is unlikely that the production process for manhole covers will change much in the future. Nor is it likely that alternative materials will be used to produce manhole covers as cast iron is extremely economical. The exciting prospect for manhole covers involves computer-aided design and computer-aided manufacturing (CAD-CAM). With CAD-CAM, manhole cover designers can produce intricate patterns that can be cut out of plastic molds by automatic machinery. It will not be necessary for a highly-paid artisan to spend days or weeks creating particularly intricate models for special manhole covers. Once the design is ready, the model can be cut in just a few minutes. A golden age of manhole cover design may be at hand. Rather than dull, utilitarian circles, manhole covers may once again add a touch of artistry to city streets and sidewalks.
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