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Sinkholes are common worldwide and often times occur when rock underneath the Earth is dissolved by acidic water.
Sinkholes tend to form where limestone dominates the subsurface. The soft susceptible rock over time weathers and forms caverns underneath the surface of the Earth, which eventually may collapse.
In case of Hakata’s sinkhole, excavation underground for the subway tunnel, combined with erosion from water likely caused the collapse. One expert speculated that the sinkhole was caused by multiple factors in addition to the construction work, such as high levels of ground water, heavy rainfalls in recent years, and already aging piping systems in the sewerage network in Hakata.
The incident prompted a swift response as construction workers immediately began to fill the hole that spanned the five-lane street. Japanese workers toiled continuously for a week, dumping huge amount of wet cement and sand into the gaping hole and fixing electricity, gas and water lines that had stopped following the accident.
The street reopened at 5 am, November 15. The giant sinkhole was repaired in just 7 days. The newly retrofitted streets are even 30 times stronger than the former one. Many on social media expressed amazement at the quick recovery.
In the past few years in Japan, there have been cases in which underground construction work has caused roads to cave in. In October 2014, about three meters of road in the city of Fukuoka caved in several meters where construction to move sewage was underway. Last December, a sidewalk located in front of a building construction site in the city of Nagoya (the third-largest city) caved in about 5 meters. The site was about 300 meters from Nagoya Station. Also in Nagoya in June, several roads and part of a park sunk-in near a site where construction was underway to build a tunnel to store rainwater. (Floro Mercene)