Sinkholes Sinkholes are depressions or holes on the ground that resulted for the collapse of the surface layer of the soil. Sinkholes occur in areas where the soil foundation is made of soft minerals and rocks such as limestone, salt beds, or any acidic rocks. The rocks can be weakened further by frequent submersion in waters, such as when there is repeated rainfall or an unmanaged flood. The rocks dissolve in water and gradually weaken, curving and forming spaces and underground passages for water. When the spaces underground become bigger and the layer of bedrock above it can no longer support the weight above it, a sudden collapse of the land surface dramatically takes places. The depth of sinkholes varies from a few meters to even kilometers deep. Sinkhole Development As rain falls, it absorbs carbon dioxide, making percolating water slightly acidic. As this water migrates downward, the carbonate bedrock is slowly dissolved, creating bedrock voids. Over time, the underground void becomes larger as the soil and rock from above fall into the cavity and are washed away. The void continues to grow and stope upward toward the surface. Eventually, the structural integrity of the overlying material is breached and a sinkhole forms. Aerial view of a deep sinkhole that opened up in Itogon, Benguet, in the aftermath of Typhoon ‘Lando’. The collapse of the ground can also be triggered by any type of land movement that cause vibrations such as earthquakes, volcanic activities, and even human development activities such as construction and industrial activities. Most of the new sinkhole recently discovered are linked to land-use practices, especially groundwater pumping, ground excavation, and land development. When the land is altered, the natural water drainage is oftentimes changed as well due to levelling and carving of land. It should be noted that sinkholes are natural formations that serve as an underground basin or a reservoir of water. The topmost layer of the surface layer of the ground that covers the hole or cavity is maintained by the pressure of the groundwater. Hence, this top layer should not be built on by anything heavy or should not even be modified or touched at all. In urban development, however, ground modification cannot be prevented. This is one reason that sinkholes are formed earlier than usual or that disasters take place due to damage or destruction of structures built on areas prone to sinkholes. Types of Sinkholes According to geologists, there are three main types of sinkholes—solution, coversubsidence, and cover-collapse sinkholes. However, we also have artificial sinkholes, caused by human activities. Cover Collapse Sinkholes Cover-collapse sinkholes may develop abruptly (over a period of hours) and thus cause catastrophic damages. They occur where the covering sediments contain a significant amount of clay. Over time, surface drainage, erosion, and deposition of sinkhole into a shallower bowl-shaped depression. 166 Cover Subsidence Sinkholes Cover-subsidence sinkholes tend to develop gradually where the covering sediments are permeable and contain sand. In areas where cover material is thicker or sediments contain more clay, cover-subsidence sinkholes are relatively uncommon and therefore may not be seen frequently. They are smaller and thus may go undetected for long periods. Solution Sinkholes Solution sinkholes occur in areas where limestone is exposed at land surface or also is covered by thin layers of soil and permeable sand. Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface. Aggressive dissolution also occurs where flow is focused in pre-existing openings in the rock, such as along joints, fractures, and bedding planes, and in the zone of water-table fluctuation where groundwater is in contact with the atmosphere. Solution sinkholes are generally small in size and also slow to develop. Artificial Sinkholes As stated earlier in this article, humans also can create sinkholes. Artificial sinkholes are actually quite common, especially in urban areas. That’s why you are advised to conduct a thorough investigation before you begin to construct a building. These types of sinkholes can be caused by various human activities, including groundwater pumping and construction activities. However, the most common activities that result in artificial sinkholes are mining, drilling, considerable changes in weight, as well as a tremendous increase in water flow, such as a construction of an artificial pond. So, how does all these cause sinkholes? What Causes a Sinkhole? Sinkholes are all about water. Water dissolved minerals in the rock, leaving residue and open spaces within the rock. (This is called "weathering".) Water washes away the soil and residue from the voids in the rock. Lowering of groundwater levels can cause a loss of support for the soft material in the rock spaces that can lead to collapse. Changing groundwater gradients (due to removing or introducing water to the system) can cause loose material to flush out quicker from the voids and the surface to collapse in response. Any change to the hydrologic system (putting more water in or taking it out) causes the system to become at least temporarily unstable and can lead to sinkholes. Sinkholes can result from seasonal changes in the groundwater table, freeze and thaw of the ground, and extremes in precipitation (drought vs heavy rain). Karst landscapes develop naturally through the weathering process so a sinkhole can be considered a natural occurrence. But, human influence causes sinkholes to occur where they might not naturally have happened. Or, they may occur sooner or more abruptly than under natural conditions. Typical activities that can lead to sinkholes are: Decline of water levels - drought, groundwater pumping (wells, quarries, mines) Disturbance of the soil - digging through soil layers, soil removal, drilling Point-source of water - leaking water/sewer pipes, injection of water Concentration of water flow - storm water drains, swales, etc. Water impoundments - basins, ponds, dams Heavy loads on the surface - structures, equipment Sometimes several factors combine to cause a sinkhole. A sinkhole left open to take more water may continue to grow and can contribute to the appearance of more sinkholes nearby. https://www.usgs.gov/special-topic/water-science-school/science/sinkholes?qtscience_center_objects=0#qt-science_center_objects https://www.dep.pa.gov/Citizens/My-Water/Sinkholes/Pages/What-causes-asinkhole.aspx#:~:
