Roughly around 5:15 AM on the 8th of November, 2016, the unsuspecting people residing in the vicinity of the JR Hakata Station (a highly popular commercial district located in Fukuoka City, Japan) awoke to a deafening uproar. By 9:45 AM, a large-scale evacuation was being undertaken by the city officials and police in order to ensure the safety of the public present in the Hakata Station locality. So what exactly was the root of this massive commotion?
The source turned out to be a gigantic sinkhole which measured roughly 30 meters in length, 27 meters in width, and 15 meters in depth. Let us try to put that numerical measurement into perspective. A four-story building is generally a tad bit shorter than 15 meters in height. As you may have observed from the photo above, the width of 27 meters is more than enough to completely swallow the five-lane roadway. Lastly, the length of 30 meters can be equated to 5 meters more than half the length of an Olympic-sized swimming pool.
Now that we have a rough (and a bit disturbing) imagery of what the sinkhole looks like, let us gain some knowledge as to what exactly are sinkholes, what causes them, and how they can be fixed.
Merriam-Webster defines a sinkhole as “a low area or hole in the ground that is formed especially when soil and rocks are removed by flowing water.” Not especially helpful, is it? Can’t blame Merriam-Webster since they are not specialized in that field, so let us learn the definition from a source that is.
The United States Geological Survey, or USGS, defines a sinkhole as “an area of ground that has no natural external surface drainage–when it rains, all of the water stays inside the sinkhole and typically drains into the subsurface.” Oh dear, looks like it is getting more and more complicated.
Allow me to try to mash up the two definitions and make, hopefully, an easier-to-understand definition. In a nutshell, a sinkhole, as the name suggests, is the sinking of a piece of land thus forming a hole! Sounds pretty simple, right?
(Proceed with extreme caution or skip this section altogether if the concepts of civil engineering are too boring for you.)
You would have already probably formed some opinion of the cause by reading the two official definitions of a sinkhole.
Basically, different soils are made up of different components (sandy soil, clayey soil, alluvial soil, etc.) which in turn results in different strengths of soils. Sinkholes are common in areas wherein the rock below the land surface comprises of carbonate rocks, limestone, or salt beds, etc. The reason being that these types of rocks have the potential to be naturally dissolved and dissipated by groundwater. It is this dissolving of rocks that ends up being the catalyst for the formation of sinkholes.
Probably the most dangerous fact about a sinkhole is that it has the potential to still swallow up more land mass and in an instant! The rationale for this expansion is that even after the creation of the first sinkhole, sometimes the land still retains a bit of its shape and strength. However, when the spaces down under become just too massive to contain, an instant land collapse happens.
By now, you would be trying to formulate a guess as to the cause of the sinkhole in Hakata. Well, to tell the truth, the actual reason is yet to be determined by experts. But there are a few speculations that the incident could be connected to the construction in a nearby area related to the Nanakuma Subway Line. The concerned area was undergoing a tunnel construction project for the subway line’s expansion and this might have unknowingly been the cause of the sinkhole.
Now, the burning question of all the commuters in the region probably is, “Can they fix the roadway so we can go back to work already?” Uhm, sure, they can fix it up. But it is highly doubted that they can do it in the next few days.
For now, let us try to figure out how they will fix the huge hole. The savior would probably be a technique called grouting, which in layman’s term is like an injection of cement (yes, there are other materials but cement is the most general one) given to the weak and sick ground to rejuvenate its lost strength and make it healthy again.
Thankfully, this sinkhole was born in the early wee hours and could not have any human beings for breakfast (though it did have a bite of a huge chunk of the road) and hence, there were no reported injuries. While no casualties were reported, there was an enormous power shortage in the area with over a hundred households affected.
Feel free to Google up the various techniques that grouting encompasses or the different kinds of soils and hypothesize your own theory for the Hakata sinkhole. Since you have already read this article, it can perhaps qualify you for an undergraduate degree in the field of civil engineering. Kudos, mate!