General News

Another Collapse in Saitama Pref. Road Connects Two Existing Sinkholes; Cause Apparently Was Sewer Pipe Corroded by Hydrogen Sulfide

The Yomiuri Shimbun

17:48 JST, January 30, 2025

YASHIO, Saitama — the space between the two sinkholes that had already opened in a prefectural road in Yashio, Saitama Prefecture, collapsed early on Thursday morning and expanded into an even larger hole.

The collapse occurred just after 2:30 a.m. on Thursday. It is estimated to be more than 20 meters in diameter. As of Thursday morning, 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 due to a broken pipe under the road.

The prefectural government has called on residents of 12 cities and towns in the eastern part of the prefecture, including a part of Saitama City, through their respective local governments, to refrain from washing clothes and taking baths, which create domestic wastewater. About 1.2 million people are subject to the notice.

The prefectural government has been sucking up the sewage and discharging it into another sewer pipe. To keep the sewage from overflowing, they also began emergency discharge from a pumping station in Kasukabe, Saitama Prefecture, to the Niigata River, which connects to the Naka River, late at night on Wednesday, after adding chlorine. No impact was expected on drinking water.

The damaged sewer pipe was 4.75 meters 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, and the most recent one, in fiscal 2021, determined that “no immediate repairs [were] required” for the pipe that may have caused this incident.

Then why did the pipe break? The prefecture believes that hydrogen sulfide was the cause.

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 Hiroaki Morita, a professor at Nihon University’s Civil Engineering Department, who is an expert on sewage systems. Morita also said that hydrogen sulfide changes into sulfuric acid when it comes into contact with air in the pipes and corrodes 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 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%.