Photo/Illutration Two sinkholes are combined into one at the intersection in Yashio, Saitama Prefecture, on Jan. 30. (Koichiro Yoshida)

YASHIO, Saitama Prefecture—The rescue operation for a truck driver trapped for days inside a sinkhole here will likely be further prolonged because of the constant danger to firefighters in the unpredictable environment.

A cracked underground pipe for rainwater is believed to be leaking water into the hole, which is now 40 meters across at its widest point.

The rainwater is apparently pushing sand and dirt into a ruptured sewer pipe further underground that is believed to have been the original cause of the sinkhole.

The rainwater is also loosening the dirt walls of the expanding sinkhole, raising the risk of additional cave-ins at the intersection of a prefectural road.

Firefighters plan to use sandbags to stem the leak of the rainwater.

On Jan. 30, firefighters had lost sight of the truck cabin because it was buried under dirt and debris.

That day, they started to build a ramp to connect the road with the inside of the sinkhole. They said they hoped to finish the ramp by the end of Jan. 31.

The ramp will be used to install heavy machinery within the sinkhole to remove the dirt and debris.

When the sinkhole developed on the morning of Jan. 28, the 2-ton truck and its driver in his 70s fell in.

During the rescue operation, another sinkhole formed on the road on Jan. 29.

The road later collapsed further, connecting the two sinkholes into one larger one.

DANGER POINTS IN PIPES

According to Saitama Prefecture, the sewage pipe, measuring 4.75 meters in diameter, was built in 1983 and curves about 10 meters deep beneath the intersection.

“At pipe curves, the flow of sewage water slows down and hydrogen sulfide can develop more easily,” said Hiroyuki Kato, former senior technical officer at the land ministry and now an associate professor at the University of Tokyo.

“And hydrogen sulfide leads to corrosion,” he said.

According to the ministry’s National Institute for Land and Infrastructure Management, urine and detergents contained in sewage water produce hydrogen sulfide.

When the sewage water falls steeply or flows over steps, the hydrogen sulfide is released into the air inside the sewage pipe, the institute said. It is then oxidized by germs attached to pipe walls and becomes liquid sulfuric acid, which can corrode the pipe and lead to ruptures.

Among the various types of pipes, concrete-made ones are the most vulnerable to corrosion from sulfuric acid, the research institute said.

“The sinkhole site was one of the areas where we needed to be cautious,” Kato said. “However, it is difficult to check more than once every five years, which is stipulated under law, amid financial and labor shortages.”