KOBE - Across Japan, road collapses have been occurring with increasing frequency, often blamed on aging infrastructure. In one recent incident in Ashioshi, Saitama Prefecture, a truck was swallowed by a gaping sinkhole.
Sewer systems are indispensable to daily life, yet many are deteriorating. In Kobe's Nishi Ward, construction was underway in a quiet suburban neighborhood to restore an old sewer pipe without removing it. The target was a 30-meter section of reinforced concrete sewer pipe, about 30 cm in diameter, that had been in use for around 40 years.
To begin, a robot-mounted camera small enough to navigate the narrow pipe was lowered through a manhole to inspect the interior. Although wastewater had flowed through the lower portion for four decades, and corrosive gases from the water and air passed through the top, the pipe showed no major damage and remained relatively clean.
The Ministry of Land, Infrastructure, Transport and Tourism defines the standard service life of sewer pipes as 50 years. Kobe City, which rapidly expanded its sewer network during Japan's postwar economic boom, now manages a system extending roughly 4,200 kilometers. About 30% of this infrastructure is already over 50 years old; within 10 years, this figure is expected to reach 60%, and 80% within 20 years.
To deal with the growing need for repairs, the city has adopted trenchless rehabilitation methods. One such method, called Cured-in-Place Pipe (CIPP), involves inserting a flexible lining made of glass fiber into the old pipe, sealing both ends, and curing it with high-temperature steam. The liner hardens inside the existing pipe, creating a new pipe within the old one. This process takes only half a day, much faster than traditional methods that require excavation and pipe replacement.
The cost of rehabilitating a small-diameter pipe is about 200,000 yen per meter. For the 30-meter stretch being repaired, the total cost is estimated at around 5 to 6 million yen. The new pipe, visible as a green lining inside the old one, is expected to last more than 50 years. With its improved flexibility, it is also more resistant to earthquakes.
According to Kobe City, multiple sewer repair projects are underway almost daily. The city aims to restore around 30 kilometers annually, but at that pace, it would take roughly 140 years to upgrade the entire network. Labor shortages make it difficult to accelerate progress. Although some young workers are joining, many leave the job early, leaving the workforce stretched thin.
The Ministry’s 50-year guideline is being questioned. In a recent road collapse in Yoshii, Saitama Prefecture, the failed sewer pipe had been installed in 1983, only 42 years ago. Jun Mishina, a technical advisor who has spent over 50 years researching concrete corrosion, warns that some pipes can deteriorate faster, especially under the influence of hydrogen sulfide gas generated from sewage.
Mishina explained that many older pipes are concrete and prone to weakening from corrosive gases and wastewater. "They may look fine on the surface, but crumble like tofu when touched," he said. Depending on the concentration of gases, concrete can erode by up to 8mm a year. He emphasized that not all pipes will last 50 years, and that maintenance frequency must increase to avoid further accidents.
According to his data, areas where water converges—such as pipe junctions or elevation changes—are especially prone to gas generation and concrete decay. These vulnerable locations make up about 10% of all sewer pipes. One such site exists in Kobe, near Higashi Yuenchi Park, where wastewater from the city gathers before heading to a treatment plant.
To investigate, Kobe City staff first ventilate the manhole to reduce hydrogen sulfide levels before descending with a small camera. About 10 meters below, a large junction chamber is inspected visually. No abnormalities were found on this occasion, although the smell of household wastewater was evident. Further down, where a 3-meter-diameter sewer pipe transports water from higher elevations, the flow and condition are carefully checked, especially since water collisions at such points heighten gas production and risk of deterioration.
Despite the rain on the day of inspection, which increased water flow, no major cracks or damage were discovered. Normally, crews descend further to inspect deeper sections. Each year, Kobe conducts manual inspections on about 6% (roughly 230 kilometers) of its total network. Inspectors say they still rely heavily on human eyes because AI cannot yet accurately assess pipes coated with residue from waste.
Moreover, even using AI requires preliminary video footage—meaning human labor is still essential. As a result, AI adoption in this field remains limited.
Day after day, sewer maintenance crews work behind the scenes to uphold urban safety. Even though the work is out of sight and often stigmatized as dirty, those involved hope the public will recognize the importance of their contribution.
One worker said, "We’re often looked at with disdain because of the nature of the job, but I hope people remember that there are those of us working hard in places you never see."
Source: ABCTVnews
