Every nook and cranny: First responders trying to rescue the driver of a truck that fell into a sinkhole on a street in Yashio, northeast of Tokyo. — AP
The space between the two sinkholes in a prefectural road in Yashio, Saitama prefecture, collapsed Jan 30 morning and created one larger hole, estimated to be more than 20m in diameter.
This occurred just after 2.30am local time on Jan 30. The driver of a truck that fell in the hole, a man in his 70s, still had not been rescued, but firefighters and others were preparing to renew their efforts.
The sinkholes are believed to be related to a broken pipe under the road.
The prefectural government through the respective local governments has called on 1.2 million residents of 12 cities and towns in the eastern part of the prefecture, including a part of Saitama City, to refrain from doing laundry and taking baths, which create domestic wastewater. The prefectural government has been sucking up the sewage and discharging it into another sewer pipe.
To prevent the sewage from overflowing, it also began on Jan 29 night emergency discharge from a pumping station in Kasukabe, Saitama prefecture, to the Niigata River, which connects to the Naka River, after adding chlorine. No impact was expected on drinking water.
The damaged sewer pipe was 4.75m in diameter and made of concrete. It had been in use since 1983. The service life of concrete is usually 50 years. Visual inspections are conducted once every five years. The most recent one, in fiscal year 2021, determined that “no immediate repairs (were) required” for the pipe that may have caused this incident.
The prefecture believes that hydrogen sulfide was the cause of the pipe break. Sewage flowing through sewer pipes contains organic matter such as excrement and kitchen waste.
“When stagnation occurs in sewer pipes, if there is no oxygen, bacterial activity produces hydrogen sulfide from organic matter,” said Dr Hiroaki Morita, a professor at Nihon University’s Civil Engineering Department, who is an expert on sewage systems.
Hydrogen sulfide changes into sulfuric acid when it comes into contact with air in the pipes and can corrode the concrete.
Road subsidence frequently occurs throughout Japan.
According to the Road Bureau of the Land, Infrastructure, Transport and Tourism Ministry, about 10,000 cases of road subsidence, for which the presence of underground cavities is one cause, occur on national and municipal roads nationwide every year.
In fiscal year 2022, there were 10,548 such cases. About 40% of these were caused by broken gutters and storm drains. In urban areas, sewer line damage was the most common cause at 30%. — The Japan News/ANN
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