Photo/Illutration Police officers limit access to a road in Chofu, western Tokyo, where a cave-in occurred in October 2020. (Provided by reader)

Shoddy tunneling work is being blamed for a series of cave-ins in busy Chofu city, western Tokyo.

An experts’ panel set up by East Nippon Expressway Co. explained in a report released Feb. 12 that the company had been tunneling deep underground for its Tokyo outer ring road when a number of cave-ins occurred and huge cavities formed.

According to transport ministry officials, this is the first such instance in connection with a law that took effect in 2001 related to the usage of underground areas at great depths.

The report found that heavy bore machinery encountered difficulties tunneling into the bedrock. Chemicals were poured into the site to facilitate the work. But that led to the machinery excavating an excessive amount of dirt and creating cavities in the area above the machinery that formed the uppermost section of the tunnel.

The study also concluded that the surface where the cave-ins occurred was not solid but artificially created. Other special conditions included a layer of sand and gravel that made the area unstable.

These circumstances triggered the cave-ins and cavities, the report concluded.

East Nippon Expressway pledged to compile steps to prevent a recurrence before deciding when to resume construction work.

At a Feb. 12 news conference, Kazuyoshi Akaba, the transport minister, also said the latest findings would likely affect work on the Linear Chuo Shinkansen project being undertaken by Central Japan Railway Co. because tunnels along that line are also at depths similar to the road work under Chofu.

After the cave-ins, East Nippon Expressway conducted boring tests in the vicinity but no other cavities were found. Repair work will be carried out for areas deemed to have been weakened.

Shinya Inazumi, a professor of geotechnical engineering at Shibaura Institute of Technology, said working at depths of 40 meters or more required a different mindset from traditional civil engineering technology.

“In terms of civil engineering, there are no guarantees of safety when working at depths of 40 meters,” he said. “Steps need to be reviewed to ensure that companies are not negligent in the measures they will have to implement.”

(This article was written by Takaoki Yamamoto, Ari Hirayama and Yuta Ichijo.)

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