Photo/Illutration The site of a large-scale road cave-in in Yashio, Saitama Prefecture, on Feb. 10 (Tatsuya Shimada)

Following the road collapse in Yashio, Saitama Prefecture, a nationwide emergency inspection of sewer pipes discovered corrosion and other abnormalities in three locations in the prefecture. 

In addition, six cavities under roads were found in the prefecture as well as elsewhere, the infrastructure ministry reported on Feb. 14. 

The ministry said these underground spaces were not caused by deteriorating sewer pipes, but were in need of urgent repair.

The ministry said that none of these anomalies or cavities are likely to lead to a large-scale sinkhole, as occurred in Yashio on Jan. 28.

The three areas where anomalies were found are in sewer pipes leading to the Shingashigawa river water circulation center in Wako.

The pipes were found to be misaligned and the inner walls of the manholes were corroded.

If left unrepaired, holes could form, and earth and sand could flow into the pipes, creating cavities and triggering a cave-in. 

The Saitama prefectural government has already begun taking countermeasures, the ministry said.

Two cavities were found in Saitama and Nara prefectures, respectively. Two additional underground spaces were found in locations in Tokyo and Kanagawa Prefecture.

All six cavities were in need of emergency repair.

All were shallow, less than 1.5 meters deep, and all have already been filled in, the ministry said.

The latest emergency inspections covered approximately 420 kilometers of large-scale sewer conduits in seven prefectures—Tokyo, Saitama, Chiba, Kanagawa, Osaka, Hyogo and Nara—which are similar to the one in Yashio where the sinkhole occurred.

The inspections also covered roads where these sewer conduits are buried.

The municipalities that manage them visually inspected the underside of about 1,700 manholes in those sewer lines, and also inspected under the surface of the manholes with cameras.

Electromagnetic waves were used to detect empty spaces under roads.

The results were reported to the ministry by Feb. 7.

The ministry also announced that the first meeting of a panel of experts to study measures to prevent a recurrence of the Yashio cave-in will be held on Feb. 21.

“We will thoroughly study and implement the necessary measures to ensure safety and security based on the panel’s discussions,” infrastructure minister Hiromasa Nakano said at a news conference on Feb. 14.

The sewerage law requires that sewer pipes at a high risk of corrosion be inspected at least once every five years.

The scope of inspections is determined by local governments.

According to the ministry, as of September 2024, 3,463 km of the total 490,000 km of sewer pipes are subject to such inspections.

The longest is in Tokyo (229 km), followed by Ishikawa Prefecture (208 km), Aichi Prefecture (182 km), Shizuoka Prefecture (180 km), and Osaka Prefecture and Hokkaido (163 km each).

According to the Saitama prefectural government, the sewer pipe passing through the cave-in site in Yashio was not subject to the requirement.

Nonetheless, the prefectural government said it voluntarily inspected the sewer pipe in fiscal 2021.

The prefectural government also said it has voluntarily inspected all sewer pipes under its jurisdiction once every five years.

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