Photo/Illutration Two sinkholes become connected to form one massive hole at an intersection in Yashio, Saitama Prefecture, on Jan. 30. (Asahi Shimbun file photo)

Nearly 300 kilometers of aging sewer pipes across Japan pose a serious risk of road collapse, according to a nationwide investigation by the infrastructure ministry.

The findings follow a fatal road cave-in that occurred in Yashio in Saitama Prefecture in January, where a truck driver died after his vehicle plunged into a sinkhole. 

The ministry presented the results on Sept. 17 during a panel meeting of experts tasked with developing measures to prevent similar accidents.

In March, the ministry launched a special priority survey, targeting around 5,000 km of sewer pipes that are at least two meters in diameter and have been in use for more than 30 years.

These pipes were inspected using visual checks and TV cameras, and assessed for corrosion, sagging and structural damage.

Each issue was rated on a scale from A to C.

Pipes receiving an A rating in any category were classified as “urgency level 1,” requiring immediate action within a year.

Those with at least one B rating were designated “urgency level 2,” requiring emergency measures and full remediation within five years.

Of the 5,000 km, 810 km were prioritized for inspection due to factors such as soft ground similar to the Yashio site, elevation differences that increase corrosion risk, or a  history of collapses or confirmed corrosion.

At the Sept. 17 meeting, the ministry reported on the condition of 621 km of these priority areas.

Of these, 297 km in total, or 48 percent, were found to require countermeasures.

Among these, 72 km managed by 71 municipalities were classified as urgency level 1, while the remaining 225 km fell under urgency level 2.

Subsequent investigations of the affected pipes revealed six underground cavities in surrounding areas. Four have already been addressed, and the remaining two are scheduled for immediate corrective measures.

Previously, the ministry had conducted statutory inspections over five years on about 3,500 km of corrosion-prone pipes. Under the old criteria, urgency level 2 applied only if a pipe had one A rating or multiple B ratings. A single B rating did not qualify.

This outdated standard contributed to the misjudgment in Yashio, where a prefectural inspection three years ago rated the pipe’s corrosion as B. At the time, authorities concluded that no immediate action was necessary.

In response, the ministry revised its evaluation criteria for the current survey to better reflect the risks.

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