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Downtown Fukuoka sinkhole highlights risks of urban infrastructure damage

The exposed foundations of a building near the sinkhole in Hakata Ward, Fukuoka, are seen in this photo taken from a Mainichi Shimbun helicopter on Nov. 8, 2016. (Mainichi)

FUKUOKA -- A huge sinkhole that opened up on a major thoroughfare here on Nov. 8 highlights the risks of infrastructure damage in big cities.

The cave-in in front of JR Hakata Station in Hakata Ward, Fukuoka, also caused a power blackout at railway stations and other facilities, and seriously affected communications networks. The municipal government is poised to fill in the sinkhole up to the level of groundwater that has flowed into the pit, or about 3 meters below street level, by the evening of Nov. 9, officials said.

The Fukuoka Municipal Government attributes the cave-in to tunnel boring at the site for an extension of the Fukuoka municipal subway system's Nanakuma Line. Road cave-ins caused by aging sewerage pipes and other factors have occurred across the country. It is essential to take preventive measures as underground redevelopment is going on in big cities.

"There is a possibility that soil containing groundwater flowed into the (subway expansion) tunnel through cracks and decayed sections of bedrock, causing the cave-in," Hidetaka Sumi, manager of the construction department of the city's Transportation Bureau, told a Nov. 8 news conference.

Workers were expanding the 9-meter-wide, 5-meter-high tunnel upward when part of the bedrock above the tunnel collapsed at around 4:25 a.m. on Nov. 8. The workers sprayed concrete to solidify the bedrock, but failed to stop the collapse.

Water started to leak inside the tunnel, forcing the workers to flee the construction site. The cave-in occurred about 15 minutes later.

The bedrock was formed after shale, a kind of clay, became solid. Boring and other surveys showed that the bedrock was solid enough to dig a tunnel. Sumi described the quality of the bedrock as "relatively good."

In the work, the so-called NATM method was employed. In this method, a smaller tunnel is dug, then widened and the interior surface is solidified with concrete as the excavation progresses by the meter. This method is suitable for boring through solid bedrock, and makes it relatively easy to change the shape of the tunnel as it nears a station.

Taisei Corp., which leads the joint venture responsible for the work around the scene, says it is still investigating the cause of the cave-in.

Hajime Imanishi, professor at Tohoku Institute of Technology who inspected a road cave-in during construction on the same line in 2000, pointed to difficulties in digging underground tunnels in Fukuoka.

"There's a lot of groundwater in Fukuoka, and the water table is close to the surface. If mixed with water, earth and sand tend to flow, making it difficult to conduct (tunnel digging) work," he explained.

Noriyuki Yasufuku, professor at Kyushu University and a member of a technological panel on the Nanakuma Line project, said, "We had in-depth debate on the construction method and concluded that it would be all right. However, the strata may not be thick enough to block water."

The cave-in seriously affected local residents' livelihoods. The incident caused a power outage at stations and a glitch in the Bank of Fukuoka's computer system, temporarily making it impossible to deposit or withdraw money at all its branches. Since some of the sewage pipes were ruptured, the municipal government is urging local residents and businesses to try to discharge as little waste water as possible. The foundations for buildings near the sinkhole are also exposed, forcing those staying nearby to evacuate.

Fukuoka University professor Satoshi Murakami underscored the need to confirm the safety of nearby buildings while pointing out that the structures are unlikely to collapse soon.

"The buildings will unlikely collapse soon if their foundation piles were driven into the bedrock. However the buildings' structures should be examined to ensure safety before using them again," he said.

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