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Why are manhole covers/manhole covers round? – Enlightenment

Why are manhole covers/manhole covers round? - Enlightenment ;rung

Round and smooth, there are no dents if you bump into it. At least not like when a corner stands in the way. The manhole cover, or in technical terms the manhole cover, closes the upper outlets of underground sewage systems and is circular. It is usually made of cast ironor of a strong steel plate. Holes are always drilled in the manhole cover to let escaping gases out or to compensate for the different air pressure. Numbers or the name of the foundry that manufactured the cover are incorporated for identification.

There are also square manhole covers

Why not make the manhole cover square? In fact, there are also square end caps where the sewer exit is in the form of a square. However, this is more likely to happen in commercial systems and not in the public sector. The round manhole cover has the advantage that it can absorb more load on the same area than a comparable square cover, since the tapering edges are missing here. This makes it possible for a heavy truck to drive over the round manhole cover on a busy street without breaking in.

It won't fall in

The round shape has another advantage: Nobody can sink the lid into the manhole. Because the curve has the same diameter at every point, you can tilt it as you like, the lid always stays up. It is different with a four-edged lid, which would fit perfectly into the diagonal of the shaft. In the worst case, such a cover could fall on the sewer worker's head.

Cover with profile

On closer inspection, the round lids have another special feature: they are not simply smooth on their surface, but have a structure made up of various struts. On the one hand, they are there to ensure that nobody can slip over the smooth cover, which can happen quickly when the iron is wet, and on the other hand, the profile provides additional stability. Seen in this way, a simple manhole cover is an ingenious construction. So that the heavy lid can be lifted and cleaned by a specialist, special holes have been incorporated into which a lifting device fits.

Water-swallowing gullet

The term gully is borrowed from English, the word “gullet” means gorge, this word comes from the old French “Goule”, which means something like throat. This name suggests that a manhole cover represents something like the tip of the iceberg: the visible upper entry into a long and widely branched sewer system. This runs along under the town, the village and the commercial halls and towers. It keeps our streets, houses and sidewalks from flooding when it rains and keeps water out of the city and into rivers and streams.

The World Below the Gully

Not only is the rainwater runoff from the roofs and streets integrated into the sewage system, our toilets, showers and bathrooms, the waste water from the washing machines and dishwashers also feed the so-called gray water into the sewage system. Of course, this dirty water must not simply be discharged into the rivers, it is first given a stay in the sewage treatment plant. Bacteria do a great job there: They separate the components into sewage sludge and clean water. The mud is applied to the fields and used as fertiliser, the clean water is returned to the rivers.

Round rolls

There is another, simple explanation for why the manhole cover is round. In earlier times, cranes and means of transport were still rare. People had to make do with other means. What's round can be rolled. This is how the ancient Egyptians used rollers to move their heavy blocks of stone from the desert to the building sites of the pyramids. A round manhole cover can also be rolled up. It was not necessary to first harness a draft horse on the construction site to bring the heavy cover from the beginning of the street to its place of use.

No support without socket

However, the manhole cover alone is only half the truth. He would sink into the earth in no time. Equally important is the steel insert ring that sits underneath and crowns the canal exit. It must always be tight and clean, otherwise the lid sits crooked and becomes a tripping hazard for passers-by. But don't worry, the sewers have to be serviced regularly and the sewer cleaning staff will clean everything and properly put the manhole cover back on top.

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