A manhole cover or maintenance hole cover is a removable plate forming the lid over the opening of a manhole, an opening large enough for a person to pass through that is used as an access point for an underground vault or pipe. It is designed to prevent anyone or anything from falling in, and to keep out unauthorized persons and material.
Manhole covers date back at least to the era of ancient Rome, which had sewer grates made from stone.
Description
Manhole covers are often made out of cast iron, concrete or a combination of the two. This makes them inexpensive, strong, and heavy, usually weighing more than . The weight helps to keep them in place when traffic passes over them, and makes it difficult for unauthorized people without suitable tools to remove them.
Manhole covers may also be made from glass-reinforced plastic or other composite material (especially in Europe, or where cover theft is of concern). Because of law restricting acceptable manual handling weights, Europe has seen a move towards lighter weight composite manhole cover materials, which also have the benefits of greater slip resistance and electrical insulating properties.
A manhole cover sits on a metal base, with a smaller inset rim which fits the cover. The base and cover are sometimes called "castings", because they are usually made by a casting process, typically sand-casting techniques.
The covers usually feature "pick holes", into which a hook handle tool 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.
Although the covers are too large to be easily collectible, their ubiquity and the many patterns and descriptions printed on them has led some people to collect pictures of covers from around the world. According to Remo Camerota, the author of a book on the subject titled Drainspotting, 95% of Japanese municipalities have their own cover design, often with colorful inlaid paint.
Despite their weight and cumbersome nature, manhole covers are sometimes stolen, usually for resale as scrap, particularly when metal prices rise.
Manhole covers may be marked to indicate what sort of services exist under them and what group maintains those services, such as the local water system, telecommunications firm, subway or train services, or natural gas firm.
The name "manhole" is sometimes changed to make it gender neutral. The city of Berkeley, California, for example, changed the official term to "maintenance hole" in 2019.
Shape
Circular
The question of why manhole covers are typically round (in some countries) was made famous by Microsoft 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 alternative explanations, from the tautological ("Manhole covers are round because manholes are round") to the philosophical.
Reasons for the shape might include: A round manhole cover cannot fall through its circular opening, whereas a square manhole cover might fall in if it were inserted diagonally in the hole. 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. (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.) Round tubes are the strongest and most material-efficient shape against the compression of the earth around them. A round manhole cover of a given diameter has a smaller surface area than a square cover of the same width, thus less material is needed to cast the manhole cover, meaning lower cost. 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 with the manhole. A round manhole cover can be more easily moved by being rolled. A round manhole cover can be easily locked in place with a quarter turn (as is done in countries like France), which makes them hard to open without a special tool. Lockable covers do not have to be made as heavy to resist being dislodged. A round flat metal casting will typically warp in its center (concave or convex) leaving the rim flat and evenly supported, whereas a casting with multiple sides will typically warp at its corners which allows the grate to rock as traffic crosses it.