International Journal of Advances in Science Engineering and Technology, ISSN: 2321-9009, Vol-5, Iss-2, Spl. Issue-2 Jun.-2017 http://iraj.in DISCUSSION ON THE SINKHOLE FORMING MECHANISM
1YUSHIK HAN, 2HAE-WON HWANG
1,2Geotechnical Engineering Research Institute of Korean Geotechnical Society, South Korea E-mail: [email protected], [email protected]
Abstract- The definition, classification, and occurrence mechanism of sinkholes are investigated in detail through various case studies. Furthermore, the mechanism by which sinkholes occur in South Korea is discussed
Keywords- South Korea, Sinkholes, Mechanism, Definition, Classification
I. INTRODUCTION about by natural causes and those brought about by artificial causes. Sinkholes have occurred in various places around the The occurrence of sinkholes by natural causes is as world and concerns about public safety have been follows: acidification of surface water occurs when raised in recent years. Particularly, a sinkhole may surface water absorbs carbon dioxide or participates occur due to a variety of conditions when developing in the metabolic activities of plants and such acidified underground spaces since the development of surface water flows through bed rocks and weakens underground spaces is essential for urban the structural integrity of the rocks. In particular, development. If the cause of sinkholes can be when bed rocks are composed of soft minerals such identified accurately, the occurrence of sinkholes can as limestone or carbonate rock, weakening of the be prevented in advance through human intervention. structural integrity becomes more severe. Some of the Thus, the present study investigated the mechanism typical types of naturally-caused sinkholes are ground of sinkhole formation in detail through various case collapse, ground subsidence, and ground dissolution. studies. Artificially sinkholes are caused by cavity expansion due to loosening ground and collapse during the II. DEFINITION AND CLASSIFICATION OF underground construction of structures such as SINKHOLES tunnels, underground cavity expansion due to defective compaction of backfill materials during A sinkhole refers to a phenomenon whereby the construction of structures, and erosion due to water ground surface sinks to create a cylinder or funnel leakage in urban water supply and drainage pipes shape in the ground after the ground or underground followed by expansion of an underground cavity and cavity has collapsed. The cause of sinkholes due to underground water disturbance. The classification of the collapse of the ground or underground cavity can sinkholes according to this causal concept is shown in be categorized into two types: sinkholes brought Fig. 1.
Fig. 1 Classification of sinkholes according to causal factors
2.1. Naturally caused sinkholes under the ground, sinkholes do not occur without Naturally caused sinkholes occur in limestone areas changes in the underground water. Since atmospheric where dissolution happens easily. Limestones are pressure increases by a magnitude of 1 for every 2.5 dissolved due to the flowing of underground water for m of underground depth, pressures of 10 and 100 long periods of time and a large cavity is formed in atmospheres are present at 25m and 250m depth in the underground eventually when such dissolution the rock formation. Such forces are supported by continues for a long time. Even if cavities are formed underground water. However, when the level of
Discussion on The Sinkhole Forming Mechanism
42 International Journal of Advances in Science Engineering and Technology, ISSN: 2321-9009, Vol-5, Iss-2, Spl. Issue-2 Jun.-2017 http://iraj.in underground water is reduced in some instances due 2.2. Artificially caused sinkholes to various reasons such as an increase in underground A sinkhole that occurs in urban areas is presumably water usage, a cavity which was filled with due to abnormal or defective underground water underground water becomes empty and water networks. As the level of underground water pressure disappears. Then, the cavity cannot sustain decreases as a result of rapid increase in usage of the increased pressure and collapses to create a underground water and changes in underground water sinkhole in the ground surface. A naturally caused flow, soil pressure supported by underground water is sinkhole can be classified into three types. transferred so that the ground collapses to form a A cover-collapse sinkhole occurs when upper sinkhole. If a large amount of underground water is sediment is composed of soft materials such as clay. pumped due to the large scale development of potable An underground cavity is created under the bedrock water sources, underground excavation, or subway so that some of the upper sediment falls into the construction, for example, ground which is far away empty space created by the cavity thereby weakening from such activities can also sink. This is because the topsoil gradually (Fig. 2(a)). The weakening underground water flows from higher to lower places upper sediment begins sinking and finally a sudden (water head difference) in a manner similar to surface collapse occurs. Although the occurrence of a large water, and erosion occurs through the movements of size sinkhole can take a long period of time, the underground water from higher to lower places if collapse occurs suddenly so the damage caused can water is pumped from a place where the underground be serious. water level is lower. Continuous erosion forms a A cover-subsistence sinkhole occurs in the form of cavity and ultimately a sinkhole develops. A sinkhole small to mid-size holes. Many sinkholes of this type can also be created when the flow of surface water are generated in sandy soil where no viscosity is changes. When moisture content is increased due to present in the surface layer and it occurs mainly at a changes in surface water flow in the soil ground place where underground water is close to the where moisture content is low, a pore water pressure ground. The subsidence occurs gradually rather than is increased thereby reducing the effective stress to suddenly so sinking occurs gradually as well (Fig. collapse the ground after excessive loads are exerted. 2(b)). In addition, moisture content can be increased in the In a dissolution sinkhole, the process of subsidence is ground when water from the water supply and not visible in many cases and dissolution in the drainage pipes leaks in cities or when reservoirs are shallow surface layer progresses rapidly due to developed for industrial use in a sandy surface layer changes in the surface water flow thereby causing or when erosion occurs to form a cavity, all of which erosion that ultimately extends to the bedrock. Since can lead to the development of a sinkhole later. An subsidence, in terms of shape, mainly takes place in artificially caused sinkhole can be classified into the shape of a cone rather than a funnel, it is difficult three types. to visually verify the tendency to subside (Fig. 2(c)). If a tunnel is excavated in a place where the cover value is small and structures are positioned nearby, stress conditions in nearby ground can change and surface subsidence and deformation of adjacent structures can occur due to the occurrence of loosening of the ground around the tunnel (Fig. 3).
(a)Cover-collapse sinkhole (Modified from USGS)
(b) Cover-subsidence sinkhole (Modified from USGS)
Fig. 3 Behavior of nearby ground due to tunnel excavation
As upper soil is introduced to defective segments of aging conduits, a space can be formed in the lower part of the pavement. The pavement can maintain its stiffness to some extent so it can bear the subsidence
(c) Dissolution sinkhole(Modified from Walters, 1978) within a range due to internal forces until a sudden Fig. 2 Naturally caused sinkholes collapse takes place when a limit has been crossed. Fig. 4 shows a schematic shape of sinking ground,
Discussion on The Sinkhole Forming Mechanism
43 International Journal of Advances in Science Engineering and Technology, ISSN: 2321-9009, Vol-5, Iss-2, Spl. Issue-2 Jun.-2017 http://iraj.in which is extended up to the ground surface as the Sinkholes that have occurred in South Korea have upper soils are introduced to the damaged conduits. been created mainly due to artificial causes rather Since the sink ground occurs up to the upper surface than natural causes. In particular, the most frequent of the buried conduits, the depth varies according to occurrence of sinkholes is due to aging conduits the locations of buried pipes. For example, a followed by sinkholes caused by underground water relatively shallow depth is formed in urban areas. disturbance. The sinking of the ground occurs due to damage to aging conduits that have been distributed along urban areas for more than 30 years.
Fig. 4 Schematic diagram of sinkhole occurrence due to aging conduits(Modified from Ottawa Citizen, 2012)
A load applied to an aquifer where ground water is distributed is shared by both the skeleton of the Fig. 5 Sinkholes due to underground water disturbance aquifer and the ground water. A change in volume in the skeleton does not occur in circumstances where ACKNOWLEDGMENTS ground water inflow and outflow are equivalent. However, once the ground water level is reduced due This research was supported by the Korea Agency for to the excessive collection of ground water, the load Infrastructure Technology Advancement under the applied to the ground water is transferred to the Ministry of Land, Infrastructure and Transport of the skeleton so that the skeleton contracts due to the Korean government. (Project Number: 15SCIP- increase in load. Then, the load applied to the B108153-01) skeleton increases so that the skeleton contracts further. As a result, particles are re-arranged due to REFERENCES plastic deformation and pore water flows to the outside so that the storage capacity in the aquifer is [1] Ottawa Citizen(2012) . [Online]. Available : reduced gradually and the ground subsides www.ottawacitizen.com permanently (Fig. 5).. [2] U. S. Geological Survey (USGS) (2014), Sinkholes. [Online]. Available : DISCUSSION https://water.usgs.gov/edu/sinkholes.html [3] Walters, R.F., 1978, land subsidence in central Kansas related to salt dissolution: Kansas Geological Survey Bulletin 214, 82 p.
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