Traffic has reopened on a street in downtown Fukuoka a week after a section of it suddenly collapsed in a giant sinkhole, knocking out gas and electricity, cutting off communications cables and halting online banking services, and prompting an evacuation advisory for the occupants of nearby buildings. It was only due to sheer luck that nobody was killed or injured in the incident that took place early in the morning of Nov. 8. Had the sinkhole appeared later in the day when traffic was heavy on the busy street that leads to JR Hakata Station, it could have caused many casualties.
Authorities closed the road to traffic around 5 a.m. when workers at the site where construction was underway to extend the local subway line noticed signs of trouble underground and evacuated the site. Two separate sinkholes started to grow and joined each other about two hours later, forming a gaping cavern that swallowed traffic light poles and sidewalks and eventually swelled in size to 30 meters long, 27 meters wide and 15 meters deep in less than three hours.
Now that the collapsed section has been repaired and the road reopened to traffic, the Fukuoka Municipal Government must identify the cause by mobilizing experts to scrutinize what happened and why, including whether there were any defects in the construction design and plan.
Although on a smaller scale, numerous sinkhole incidents are happening across the country, especially in big cities where underground development for public transportation and commercial purposes is taking place. Once the cause of the Fukuoka sinkhole is known, other local authorities and construction firms should closely study the case and examine whether adequate precautions are being taken to prevent similar disasters in their underground construction projects.
The Fukuoka tunnel was being dug through a layer of bedrock that was below a section of clay that blocked the flow of water. Above the clay was a layer of sand containing water. The project’s design required that at least 2 meters of the bedrock layer remain over the tunnel. According to the digging method, each time the front end of the tunnel advanced 1 meter, concrete had to be sprayed over the tunnel wall to reinforce the structure. But after a recent boring test found that the bedrock layer inclined downward on the left side, the construction design was changed so that the height of the tunnel would be 90 cm lower than originally planned.
There is a possibility that earth and sand containing water gushed into the tunnel. It must be determined whether the tunneling was taking place strictly in accordance with the newly altered design, and whether the city and the joint venture in charge of the construction correctly understood the conditions of the geological layers, including possible weathering of the bedrock and the way in which the layers ran — straight or wavy. Wavy layers increase the risk of an accident.
It should be noted that the area around Hakata Station was originally swampy, which makes it likely that geological layers there contain more water. Two road cave-ins, in 2000 and 2014, occurred at construction sites along the same subway line, although the sizes of the sinkholes were smaller. One wonders whether the earlier incidents had impressed upon local authorities the need to take maximum precautions in underground construction projects. The 2014 cave-in occurred only 400 meters from where last week’s incident took place.
Tunnel construction is a common cause of cave-ins and underground flooding, including an incident in 2005 at a subway construction site in Nagoya. In more recent years, flooding at tunnel construction sites delayed the opening of the Shinagawa route of the Chuo Kanjo Expressway in Tokyo and that of the Kyoto-Jukan Expressway. Old, decaying underground sewer pipes are also to blame for many cave-ins, with the infrastructure ministry confirming that around 3,300 decay-related incidents took place nationwide in fiscal 2014.
Local authorities must strive to ensure cave-ins do not take place when they plan and carry out the construction of projects such as subways and subterranean shopping centers. They must also make greater efforts to identify irregularities in existing facilities and repair them. Many of the nation’s underground sewer pipes laid in the 1960s and ’70s are decaying and becoming susceptible to rupture. Adequate local funding, manpower and expertise need to be secured to address the problem.
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