JAES Learning

The recent sinkhole tragedy in Bangkok, a faulty pipe rapidly eroding the soil beneath the ground. What happens is that the cavity becomes so huge and eventually… Why didn't the soil collapse when the cavity was small? This would have resulted in a minor accident. In fact, small holes do collapse. However, when the shape is curved, they don't. Arch-shaped structures can support much higher loads. If one fine morning, the doors of your house suddenly won't close, you should be worried. The formation of sinkholes gives very few external warning signs. What occurs is that the top shell of the sinkhole gets slightly compressed. This lowers the foundation a little bit, creating an offset of a few millimeters between the door and the floor. Of course, we humans can't notice a floor shifting a few millimeters, but doors can. Sinkholes almost exclusively form in areas where the rock beneath the topsoil is soluble, for example, limestone, dolomite, and 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—a Karst landscape—is common in Florida. The overburden soil can now fall into this cave, and slowly an arch-shaped cavity gets formed. As we saw in the beginning, the arch shape can carry a good amount of load. So this subsurface erosion leaves almost no signs on the surface, sitting idle like a ticking time bomb. Now, the final episode: the trigger that converts this cavern into a sinkhole. Period of intense rain can rapidly add a huge amount of weight to the soil bridge over the cavern. The flowing water can also aggressively wash away supporting sediment. Eventually, a deadly sinkhole is formed. Even drought can be a trigger for sinkhole formation. During a drought, the water table can drop significantly. The water in a cavern actually provides buoyant support to the roof. When that water is gone, the roof loses that support and can be more prone to collapse. Pumping large amounts of water from wells also causes the same thing. It is very common for sinkholes to continue getting bigger after the initial collapse. The initial event is often just the first and most dramatic step in a longer process of stabilization. The growth can happen in two ways: through further rapid collapses or through slower, gradual erosion. The sinkhole we have discussed so far is known as a cover-collapse type sinkhole—the most dangerous one. On the other hand, cover-subsidence type sinkholes are the less dramatic way the overburden can fail. It happens when the overburden is different, primarily sandy and non-cohesive. 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. All these discussions lead to a very basic question: 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.