As Yokohama-shi had international trade port, we started maintenance of the sewer as hygiene measures of foreign settlement early from other cities from 1869. Afterwards, maintenance was late by the Great Kanto Earthquake, World War II, but we start the full-scale sewer maintenance with final sewage disposal plant from 1957, and, as for the sewer diffusion rate of the end of March, 2005, it is 99.7%.
The sewer which we maintained in this way builds enormous networks in undergrounds such as roads, but thinks that it is only manhole on road that citizen's everybody meets eyes directly.
In addition, manhole of the sewer is installed to perform maintenances such as pipe kyo check or cleaning work, and, as for the number, it is to about 510,000 only in the city.
By the way, cast iron cover made which was rich in strength, the durability is adopted nationwide to prevent vibration and the noise by vehicle traffic about manhole lid with change of recent road circumstances including increase of large vehicle.
As there are more hill countries in Yokohama-shi than other cities, there become relatively many immediate slopes in road. Therefore, manhole lid arranges design for the surface as slipper measures, but hopes that we are warned enough as we may skid depending on type of sole including rainy day.
In addition, we examine preventive measures against slips to manhole lid of steep slope to cope with walker measures and barrier-free for future aging society.