Japan Highways Had 1,100 Cave-Ins, Cavities over Decade; Over 40% Triggered by Damage to Underground Pipes

The Yomiuri Shimbun

16:19 JST, July 27, 2025

More than 1,100 cases of cave-ins and underground cavities were confirmed on national highways across the country between fiscal 2015 and fiscal 2024, according to a recent analysis by The Yomiuri Shimbun.

Of this number, more than 40% were caused by surrounding soil and sand flowing into damaged underground pipes, as with the cave-in that occurred earlier this year in Yashio, Saitama Prefecture, according to the analysis.

It was also found that about 20% of the incidents were caused by construction defects, such as insufficient ground compaction. Furthermore, nearly half of the cases occurred within a one-kilometer radius of a similar incident. An expert has stressed the need for the government to strengthen the maintenance and management of national highways.

In the Jan. 28 incident in Yashio, part of the surface of a prefectural road suddenly collapsed at an intersection, and a truck traveling along the road fell into a sinkhole. It is believed to have been caused by damage to a pipe installed in 1983, which allowed surrounding soil and sand to flow into it, creating an underground void.

The sinkhole subsequently expanded to 40 meters wide and 15 meters deep. The truck driver died in the incident, and sewer use was temporarily restricted in 12 municipalities within the prefecture.

To conduct its analysis, The Yomiuri Shimbun requested the disclosure of relevant information from eight regional development bureaus and the Hokkaido Regional Development Bureau under the Land, Infrastructure, Transport and Tourism Ministry, as well as the Cabinet Office’s Okinawa General Bureau. Overall, these bodies manage 24,000 kilometers of national highways.

The Yomiuri then collected documents reporting cave-ins and cavities that were detected in the 10 years from fiscal year 2015 to analyze factors including the dates and times, locations, scale, causes and repair methods. Incidents for which reports were not compiled were also included in the analysis if they had been made public.

At least 1,157 incidents — 730 cave-ins and 427 cavities — had been reported. The prefectures with the highest number were Kochi with 78 cases; Ishikawa 63; Tottori 62; Chiba 59 and Shimane 55.

A total of 509 cases, or 44%, were caused by surrounding soil and sand flowing into corroded or damaged pipes, or deteriorated joints, creating cavities underground. Construction defects totaled 259 cases, or 22%, with most of these resulting from insufficient compaction of the ground during road construction or pipe laying.

In addition, 276 cases, or 24%, were attributed to such factors as decayed tree roots, the collapse of nearby slopes, earthquakes and typhoons.

There were 521 cases in which a cave-in or cavity occurred within a one-kilometer radius of another such incident on the same highway, more than 40% of the total. At the Omano intersection on National Highway Route 4 in Koshigaya, Saitama Prefecture, three cave-ins occurred between 2022 and 2024 due to corrosion and damage to the same drainage pipe.

“At present, the government hasn’t yet been able to fully grasp the precise picture of road sinkholes and cavities,” a senior ministry official said in response to the analysis results. “We will expedite efforts to investigate and proceed with establishing a system to quickly grasp the underground situation.”

“Many national highways are designated as emergency transportation routes in the event of a disaster. If the risk of sinkholes is left unaddressed, road cave-ins could be a frequent occurrence during an earthquake, delaying rescue and relief efforts,” said Reiko Kuwano, a professor of geotechnical engineering at the University of Tokyo.

“The government needs to work with entities in charge of underground infrastructure to strengthen the maintenance and management system for national highways,” Kuwano said.