What are sinkholes and how do they form?
Why don't sinkholes collapse when they are small? 
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The recent sinkhole tragedy in Bangkok was caused due to a faulty pipe rapidly eroding the soil beneath the ground. The cavity became so huge and eventually collapsed.
The big question is, why didn't the soil collapse when the cavity was small? The collapse of small sinkhole shown in image below. This would have resulted in a minor accident, but this never happens. Sinkhole always collapse when they become huge. We will see the reason for this and more details of the Bangkok Tragedy by the end of this article.
An experiment to prove Sinkhole's true nature
Let's conduct an experiment to test the true nature of sinkholes. We used small and arch-shaped hole structure for the experiment. The small hole structure collapsed under a heavy weight as shown below.
However, when the structure is curved, they don't. Arch-shaped structures can carry much higher loads. Soil erosion generally produces cavities with an arch shape. The cavity keeps getting bigger until huge sinkholes swallow everything.
Sinkhole Warning signs
If one fine morning your doors suddenly won't close, you should be worried. Sinkholes give few external warnings; the surface layer has simply collapsed slightly.
This lowers the foundation a little bit, creating an offset of a few millimeters between the door and the floor. We humans can't notice a floor shifting a few millimeters, but doors can.
How do sinkholes form?
Sinkholes exclusively form mainly where the bedrock beneath the soil is soluble, such as limestone, dolomite, or gypsum. Water is the active agent that creates the underground void. This water can come from rain or from broken pipes. Over the years, the soil can get eroded chemically and physically. Eventually, a cavern is formed. This kind of soil condition is a Karst landscape. Overburden soil can collapse into the void, forming an arch-shaped cavity that supports significant load and leaves little surface evidence; eventually the cavern becomes a sinkhole.
Cover-collapse sinkhole
An intense rain rapidly adds a huge amount of weight to the soil bridge over the cavern. The flowing water also aggressively wash away supporting sediment and creates a deadly sinkhole. Drought can also trigger sinkhole formation. During a drought, the water table drops significantly. The water in a cavern actually provides buoyant support to the roof. When that water is gone, the roof loses that support and is prone to collapse. Pumping large amounts of water from wells also causes sinkholes.
Sinkholes often expand after the initial collapse. The initial event is usually just the first, most dramatic step in a longer stabilization process. Growth can occur either by further rapid collapses or by slower, gradual erosion. The sinkhole we discussed here is a cover-collapse type—the most dangerous kind.
Cover-subsidence sinkhole
The cover-subsidence sinkholes occur when the sandy and non-cohesive overburden fails. In this case a soil arch never forms. As soil is piped away into the bedrock, the sand particles simply slump and flow gradually into the void, like sand in an hourglass. This results in a slow, gentle sinking or subsidence at the surface, forming a shallow, bowl-shaped depression over time. This process is far less dangerous because it's gradual and visible.
How can sinkholes be prevented?
Is it possible to identify a sinkhole and cure it before it collapses? If you are struggling to close your doors, test the soil below your house with ground-penetrating radar. It works by sending small pulses of high-frequency electromagnetic waves into the ground and then recording the reflections of those waves to create an image of the subsurface.
The most direct evidence is finding an air-filled void underground. On a GPR scan, these often appear as bright, high-contrast 'hyperbolic' reflections, looking like a 'U'. If it is a small void, you can inject gravel and fill it. If the cavity is big, it’s better to vacate the location.
In sinkhole-prone areas of Texas, it is a standard part of due diligence for large commercial or public projects to investigate and perform ground stabilization before construction begins. If the investigation confirms the presence of dangerous voids, the engineer will recommend a stabilization plan. The most common and effective method is grouting. Grouting obviously fills any underground cavities it encounters. Grouting pushes the loose soil together, increasing its density and strength.
Florida : The Sinkhole Alley
The state of Florida undisputedly experiences the highest frequency of sinkholes in the United States. A central portion of the state is nicknamed Sinkhole Alley. The area encompassing Hernando, Hillsborough, and Pasco counties in the Tampa Bay region, is the epicenter of sinkhole activity.
The entire Florida peninsula is built upon a thick platform of porous limestone. This carbonate rock is easily dissolved by naturally acidic rainwater that seeps underground. Over millennia, this process has created a vast network of underground caverns and voids.
Winter Park and Land O' Lakes sinkhole
Winter Park sinkhole might be the most famous sinkhole in Florida's history. The Winter Park sinkhole opened up in May 1981 in the Central Florida city. In a single day, it grew to an astonishing 110 metre wide and 23 metre deep. Before the sinkhole stabilized it swallowed a three-bedroom house and five Porsche cars.
In July 2017, a sinkhole in Land O' Lakes, Pasco County, rapidly expanded to become the largest in the county in recent decades. It eventually reached a width of about 80 metre and a depth of 15 metre. The sinkhole destroyed two homes.
The world's largest sinkhole : China - Xiaozhai Tiankeng sinkhole
Do you know where the world's largest sinkhole is located, it’s in China - Xiaozhai Tiankeng sinkhole. It plunges to a staggering 662 meters at its deepest point. It measures 626 meters by 537 meters across. Its immense size has allowed a complete, unique ecosystem to flourish within its depths, including an underground river and nearly 1,300 species of plants.
What caused the Bangkok sinkhole?
Who can forget the recent sinkhole incident in Bangkok? The collapse occurred at the construction site of the future Vajira Hospital underground station. A high-pressure underground water pipe broke. The gushing water from the broken main flooded the void, which rapidly and catastrophically eroded the surrounding soil, washing much more of it into the tunnel. This created a massive cavern that could no longer support the road above. The road surface then caved in, creating a massive sinkhole 50 meters deep.
Authorities also noted that recent heavy rains may have softened the soil, making it more susceptible to collapse. Remember, Bangkok does not have a Karst landscape like Florida. The Bangkok sinkhole was formed purely due to human activities.
The curious case of sinkhole in Texas?
It is interesting to note that sinkholes in Texas are almost manmade. Texas does have significant karst regions. A significant number of Texas's large sinkholes, especially in West Texas are formed decades of oil and gas extraction. Injecting water to force out petroleum can dissolve underground salt beds, creating massive cavities that eventually collapse.
I hope this article has clarified what sinkholes are, how they form, and how to prevent them—thank you for reading.
