Saitama Sinkhole: Are Road Collapses on the Rise?

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A 74-year-old truck driver fell into a sinkhole that appeared at a prefectural road intersection in Yashio City, Saitama Prefecture on January 28. Initially 10 meters (33 ft) wide and 5 meters deep, the hole merged with another opening nearby. By January 30, the massive crater measured 40 meters wide and 15 meters deep.

Rescue efforts, ongoing as of January 31, are being hindered by additional cave-ins. Large amounts of water from a drainage pipe and soil continue to flow into the hole. The surrounding area remains highly unstable, with a risk of further collapse.

The recent sinkhole is believed to have been caused by an underground sewer pipe located about 10 meters below the surface. In addition, a drainage pipe about 2 meters underground channels rainwater and runoff into the area.

Sinkhole Statistics 

Road collapses have been occurring more frequently across Japan, with over 10% linked to sewer system issues. Local governments are struggling to replace aging sewer pipes, but progress has been slow due to high costs. 

In response to the recent incident, Japan's infrastructure ministry has requested an emergency inspection of large sewer pipes connected to major sewage treatment facilities, similar to those involved in the Saitama road collapse. The request was made to seven prefectures, including Osaka, Hyogo, and Nara.

In the Saitama collapse, hydrogen sulfide from human waste and other substances in the sewer pipes likely reacted with air to form sulfuric acid. The acid gradually corroded the concrete pipes, causing structural damage. 

Soil then began flowing into the broken sections. However, a routine inspection conducted in fiscal year 2021 determined that immediate repairs were not necessary.

According to the infrastructure ministry, approximately 10,000 sinkholes occurred nationwide in fiscal year 2022, with around 2,700 cases in urban areas. Sewer-related collapses tend to be more common in cities.

Case Study: 2009 Osaka Sinkhole

Japan's sewer system expanded rapidly during the country's economic boom. The standard service life of concrete sewer pipes is 50 years. Older urban areas that implemented sewer systems earlier are now experiencing severe deterioration.

In February 2009, a sinkhole appeared at an intersection on Chuo-Odori Avenue in Chuo Ward, Osaka. The collapse created a hole about 1 meter (3.3 ft) in diameter and 1.3 meters deep. Investigations revealed that a sewer pipe installed in 1954 had been damaged, allowing water to leak and erode the surrounding soil. The pipe had not undergone regular inspections since its installation.

As of the end of fiscal year 2022, Japan had a total of approximately 490,000 kilometers of sewer pipes. Currently, only about 7% have exceeded their service life, but this proportion is expected to surge to 40% in the next 20 years.

Sewer Maintenance Challenges

Maintaining and managing sewer pipes is extremely costly. With a declining population, revenue from sewer usage fees is not expected to increase, making it difficult to replace aging pipes quickly.

In Osaka, the city owns the sewer pipes but outsources maintenance and management to a private company, with the costs fully funded by the municipality. The city also replaces approximately 55 kilometers of its 5,000-kilometer sewer network each year. 

A city official stated, "To prevent waste from accumulating inside sewer pipes, we adjust pipe gradients during replacements. We also use cameras for inspections rather than relying solely on service life estimates to mitigate risks."

To improve inspection accuracy, some municipalities have recently begun using artificial intelligence to assess pipe deterioration.

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Author: Fumiya Suzuki, with additional data from The Sankei Shimbun

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