In urban road management, circular manhole covers are generally used because they are not easy to tilt and can better protect the safety of pedestrians and vehicles.
In rural areas and cable wells, they are generally square, which can better prevent rainwater and other liquids from entering.
Extended data:
1958, Japan made mandatory standards for key indicators such as thickness, material and bearing capacity of manhole covers. The current industry standard requires that the manhole cover must be cast with high-strength ductile iron, with a thickness greater than 17 cm and a weight of 40 kg.
In China, most manhole covers are not closely attached to the road surface, and tires will collide with manhole covers when vehicles run over them, so manhole covers have become one of the noise sources in cities. In the 1970s, many Japanese people suffered from the noise of manhole covers, and they asked local governments to solve the "noisy manhole covers".
The Japanese government immediately revised the standard of sewer manhole cover, changed the cylindrical manhole cover into a conical manhole cover with a large top and a small bottom, and required that the manhole cover casting must be accurate to millimeter. In this way, the manhole cover and the road surface can be seamlessly attached together, which not only eliminates the noise, but also increases the difficulty of stealing the manhole cover.
References:
Baidu encyclopedia-manhole cover