Sinkhole Application Research Centre

Information about the sinkholes

In the world and in our country, disasters that result in the loss of many lives and properties occur due to natural events and events beyond the control of people such as earthquakes, mass movements, landslides, mud and debris flow, rockfalls, etc., excessive rainfall as a result of snow and glacial melting, floods, avalanches, forest fires, storms, ground subsidence, and sinkholes.

The sinkhole is a word of Turkish origin and is a pipe, chimney, or well-shaped depression with sharp corners in newly formed and more flattened old formations. The sinkhole formations are a kind of karstic landform and the collapse dolines formed by the current karstification processes in the Konya Closed Basin are defined as the “sinkhole” in reference to the Obruk (Sinkhole) Plateau, where it was first formed in the region. 

Karstification and Sinkhole Formation

The word “Karst”, which is derived from the Slavic word “Kras” meaning “rocky mountain without water” or the Italian word “Carso”, describes the typical morphological landforms formed as a result of the dissolution and erosion of limestones on the limestone plateaus in the mountainous area facing the Adriatic Sea in the former Yugoslavia (Sür, 1994). "Karst topography" is a group of landforms such as polje (interior valley), doline (sink), uvala, swallow hole (stream sink), sinkhole, lapie and cave, which develop in natural conditions in regions where there are carbonate, sulfate and chloride rocks suitable for dissolution, and according to their size and the formation processes of the shapes are called “karstification” (Figure 1). Unlike the external factors that shape the earth (such as rivers, glaciers, wind, volcanoes, and coastal geomorphology), karstification develops as a vast, interconnected system both on the surface and underground (Nazik, 2018).

Evaporitic, carbonate, sulphate and chloride soluble rocks generally spread throughout the middle latitudes in the world, and these rocks cover about 15% of the earth's surface (Figure 2). In this zone, in which our country is located, countless karst events and sinkholes that reach tens of kilometers in length and hundreds of meters in depth occur every year (Figure 3).

In our country, the number of sinkhole formations has been increasing rapidly in recent years, especially in Central Anatolia and Southeastern Anatolia regions where Konya and the neighboring provinces of Konya are located. Increasing sinkholes pose a serious danger to primarily residential areas and human life, agricultural areas, pastures, energy investment areas, transportation networks such as roads and railways, oil and natural gas pipelines, electricity, water and other infrastructure investments.

Figure 1. Major landforms developing in a karst terrain (Nazik, 2018)

Figure 2. Distribution of carbonate karst areas in the world (https://www.fos.auckland.ac.nz/our_researchkarst/)

Causes of Sinkhole Formation

There are dozens of factors for karstification and special sinkhole formations. The foremost of these are; geological conditions including lithological, mineralogical, geochemical, structural geological and sedimentological features, level of surface and groundwater, hydrogeological conditions such as pH, EC, temperature and hydrochemical properties of waters, meteorological conditions such as climate, precipitation, temperature and evaporation, altitude, surface forms geographical and geomorphological conditions such as vegetation (Table 1).

Table 1. Main factors for sinkhole formations (Arık et al., 2020a).

The sinkholes are formed by the effects of natural geological, hydrogeological, structural geological and geomorphological conditions. Gutierrez et al. (2014) discussed the causes of sinkhole formation in two main groups as natural and anthropogenic causes and gathered the main factors under 8 main headings, 1) Increased water inflow to the ground, 2) Water table decline, 3) Storage of Water, 4) Erosion and Excavations, 5) Static loads, 6) Dynamic loads, 7) Dissolution of Frozen Soils and 8) Vegetation reduction and erosion (Arık, 2022; Table 2).

Table 2. Natural and anthropogenic causes of sinkhole formation

On the other hand, the type of cover material present in the area where soluble rocks are found might also influence the structure of the potential collapse. The cavities beneath the ground are filled with materials such as fine sand and silt moving down from the cracks and fractures of the rocks, and pits of varying sizes with roughly circular geometry gradually form on the surface in areas where the cover material is composed of cohesionless clastics such as sand and silt (Figures 4, 5 and 6). Since the structures of these sinkholes continuously collapse, the collapses can be seen from the surface. Since sudden collapse is not anticipated in these sinkholes, safety measures can be taken by creating monitoring and early warning systems for the local population (Galloway et al., 1999).

Figure 4. Dissolution mechanism of limestone and dolomites in the karstic system (edited from Galloway et al., 1999)

Figure 6. Sinkhole development in karstic areas with cohesionless cover (from Galloway et al., 1999)

The space at the bottom gradually increases and rapid, sharp-edged collapses take place with the weight of the cohesive soil cover material if there are clay-rich cohesive soil covers. In the development of these types of sinkholes, it is quite difficult to take precautions, as sometimes no signs occur on the surface before collapse (Figures 7, 8 and 9). Therefore, in regions where there is a cohesive cover or ceiling rock with a danger of sinkholes, the subterranean structure should be periodically examined.

Figure 7. The sinkhole that occurred in İnoba Neighborhood, south of Karapınar (Konya), where the cohesive cover material is located.

Figure 8. Development of depression in karst areas with cohesive cover (from Ferrara, 2020).

Figure 9. Sinkhole formations in karst regions with cohesive and non-cohesive cover (edited from Ferrara, 2020, SFWMD, 2021).

Sinkhole Formations in Turkey

In several interior regions of our country, particularly along the Mediterranean coast, carbonate rocks and evaporites suitable for karstification are widespread. Especially in the Mediterranean zone, the Taurus Mountains and the southern regions of the Eastern Black Sea Region, Paleozoic and Mesozoic aged carbonate rocks suitable for dissolution are covered. In the Mediterranean Zone and the Taurus Mountains, carbonate rocks are common in the provinces of Antalya and Mersin, and there are many caves, poljes, dolines and sinkholes, some of which are heavily visited by tourists. An important part of Turkey is covered with carbonate and evaporitic rocks formed in Cenozoic aged shallow lakes. Especially in the Inner Aegean, Central Anatolia and Southeastern Anatolia regions where potholes are most common, such rocks are common and most of the potholes formed in recent years have occurred in these regions (Figure 10).

Figure 10. Distribution of karst areas and sinkholes in Turkey

With climate change and drought in Turkey, and as a result of excessive and uncontrolled groundwater consumption, sinkhole formations have increased in the Konya Closed Basin and neighboring regions. In the Central Taurus Mountains, where karstification has been common since the past, sinkhole formations are widespread in the provinces of Karaman, Aksaray, Afyonkarahisar and Eskişehir, which are neighboring provinces of Konya, apart from the regions with carbonate rocks in the provinces of Antalya and Mersin (Figure 11).

In addition, in recent years, some of the sinkholes that have been encountered in areas with sulfated rocks such as Denizli, Bilecik, Manisa, Sivas, Çankırı and Çorum and individually in Yozgat, Erzurum, Şanlıurfa, Batman and Siirt threaten the settlements (Figure 12). The dangerous sections of the sinkhole must be thoroughly investigated in order to prevent loss of life and property as well as damage to transportation and other infrastructure, including oil-natural gas, energy transmission and communication lines, roads, and railways.

Figure 11. Sinkhole formations in provinces adjacent to Konya

Figure 12. Some potholes formed in Turkey in recent years

Sinkhole Formations in Konya Closed Basin

The Late Miocene-Pliocene-aged İnsuyu formation, which is exposed in extremely extensive sections of the Konya Closed Basin, is composed primarily of marls, although it also contains clastic interlayers in some regions. It also contains limestone, clayey limestone, dolomitic limestone, and clayey limestone. The rocks making up the İnsuyu formation were impacted by nearly NW-SE and NE-SW trending normal faults that formed in the area during the Neotectonic period. According to where the faults were, there were major uplifts and falls in the fractured rocks (Figure 13). The Paleozoic and Mesozoic carbonate rocks in the near neighborhood of Konya have a large number of paleokarstic structures, including uvala, polje, lapia, caves, dolines, and sinkholes.

Most of the current sinkholes are formed within the Upper Miocene – Pliocene aged İnsuyu formation. The İnsuyu formation starts from the Kazımkarabekir-Bozkır line in the south and continues northward to Aksaray and Eskişehir provinces outside the borders of Konya Province and from Çeltik-Yunak-Sarayönü- Kadınhanı-Selçuklu-Meram and Akören districts in the west to Altınekin, Karatay, Çumra, Karapınar, Emirgazi and Ereğli in the east. Studies done in the area (Törk et al., 2019; Ark et al., 2020a; Ark et al., 2020b) indicate that karstification and the risk of sinkholes are still present in significant portions of the İnsuyu formation.

Figure 13. Geology of Konya Closed Basin and distribution areas of sinkholes (Khorrami et al., 2021)

The Hotamış formation, which was formed in the Quaternary-Holocene period in the region, starts with coarse clastics from the coast to the deep and shows a transition to fine sand, silt and clays towards the top. Carbonate rocks, calcareous clastics, sulphate, and chloride evaporitic levels can all be found in the upper parts of the formation. Cavities and mostly sinkholes are formed in the Hotamış formation as a result of the reaction between soluble rocks and water. In some regions, the subsidence of the lower İnsuyu formation also affects the Hotamış formation. In recent years, depending on the decrease in the groundwater level, the formation heights of the sinkholes have also decreased. Therefore, some of the potholes occurring in the lower İnsuyu formation seem to have formed within young lacustrine units within the Hotamış formation.

In the İnsuyu formation, which spreads over a wide area in and around central Meram, Selçuklu and Karatay districts of Konya, Akören, Çumra, Karapınar, Eskil, Altınekin, Sarayönü, Kadınhanı, Cihanbeyli, Tuzlukçu, Kulu and Yunak districts and Aksaray province Eskil and Sultanhanı, Niğde Province Altunhisar District, Karaman Center, Kazımkarabekir and Ayrancı districts within the Konya closed basin, karstic structures such as polje, uvala, doline, sinkhole, and crevices are formed intensively, especially between Karapınar and Tuzgölü (Figure 14).

Although sinkholes have formed in the area between Karapnar and Eskil in the past, their number has significantly increased in recent years (Figure 15, Figure 16). The sinkholes, which were formed at high altitudes due to the higher groundwater level in the past, are now formed in lower areas due to the decrease in the groundwater level. Therefore, sinkholes have started to form in towns and villages where people live, small settlement areas defined as plateaus, agricultural areas, important highways and energy investment areas. As a result of drought and excessive groundwater use, sinkhole formations increased after the 2000s, and as of the end of 2017, 299 sinkholes were detected in the region, while 700 of them were deeper than 1 m and around 1850 with diameters of 50-60 meters from a few meters by the end of 2021, 700 sinkholes deeper than 1 m and shallower than 1 m, varying from a few m to 50-60 m in diameter around 1850, were found in the form of subsidence/sagging. 

Figure 16. Some sinkholes formed around Karapınar (Konya).

Figure 15. Distribution of sinkholes in the Konya Closed Basin (from Arık et al., 2021).

Figure 16. Current sinkhole formations in Yağmapınar (Karapınar-Konya) (Photo: Chris McGratt)

Types of Sinkhole

Ford and Williams (1989) proposed three different mechanisms for how sinkholes form: 1) solution, 2) cover collapse, and 3) cover subsidence (Figure 17).

Figure 17. Subsidence doline (sinkhole) types (from Ford and Williams, 1989; Doğan and Yılmaz 2011)

Potholes (dolins) are divided into five classes by Jennnings (1985) according to their formation types as subsidence, dissolution, settlement, base rock failu,res and formations due to alluvial spring discharges. Waltham et al. (2005), on the other hand, divided the sinkholes into six groups, taking into account some features such as their morphological structure, formation in the basement rock or cover rock, and the type of cover material; 1) solution dolines, 2) collapse dolins, 3) caprock dolins, 4) dropout dolins, 5) suffosion dolins, and 6) buried dolins. Later, a model for the formation of sinkholes in evaporitic and karstic lands was developed and the sinkholes were divided into 7 classes (Gutierrez et al., 2008a and b) and then 8 classes according to the type and morphological structures of the rocks in which they developed (Gutierrez et al., 2014; Gutierrez et al. ., 2016; Nam & Kim, 2017; Muzirafuti et al., 2020, Youssef et al., 2020). Gutierrez et al. (2008, 2014 and 2016) divided the potholes into 8 different types depending on the type of rock they were formed, 1) cover rock, 2) bed rock and 3) cap rock, and according to their morphological structures 1) collapse, 2) sagging and 3) suffosion type sinkholes. and solution potholes (Figure 18).

Figure 18. Types of rocks they are formed, types of sinkhole formation according to their morphological shapes.

Earth fissures

Generally, the cracks observed on the surface in alluvial lands are associated with ground settlements that occur as a result of excessive fall of groundwater. Various mechanisms have been proposed for the formation of fissures. The most widely accepted of these is that as the groundwater level decreases in unconsolidated alluvial basins, less compression and collapse occurs in the thinner alluvium near the edge of the basin than in the deeper, thicker alluvium near the middle of the basin and that the stress due to differential compression causes crack formation in the upper cover. (Figure 19).

Horizontal stresses and the breaking of a rigid plate as a result of resting on relatively solid rock, which is incompressible and less compacted underneath, are other proposed mechanisms, but the formation of new cracks parallel to the existing cracks weakens these mechanisms. Hydrocompaction or subsidence of low-density soils in water-saturated areas and increased soil-moisture tension have also been suggested as possible mechanisms. In addition, pipe erosion, soil rupture during earthquakes, renewed faulting, cave or mine collapse, oxidation of organic soils, and diapirization can be counted. Underground soil erosion along a fissure certainly plays a role in the opening, expansion and subsequent development of fissure grooves (Eaton et al., 1972).

Figure 19. Mechanism of formation of surface crevices (Galloway et al., 1999)

In Arizona, for example, groundwater has been used for irrigation, mining, and municipal utilities since 1900, and in some areas, more than 500 times the amount of water that naturally replenishes aquifer systems has been withdrawn (Schumann and Cripe, 1986). The resulting drop in groundwater levels has led to increased pumping costs in some places with the withdrawal of water from a depth of 200 m, reduced groundwater quality in many places, and uneven permanent compaction of compressible clayey and fine-grained alluvium. Ground crevices caused by irregular or differential compression have damaged buildings, roads and highways, railways, flood control structures and sewer lines. (Galloway et al., 1999).

Numerous surface crevices have formed due to different compression of alluvial material within the Büyük Menderes, Küçük Menderes and Gediz Grabens in Western Anatolia in Turkey. In Konya Closed Basin, on the other hand, as a result of climate change, drought, and excessive and uncontrolled groundwater use, groundwater levels are decreasing at an increasing rate. The locations where the alluvial thickness abruptly changes in the outlying parts of the basin have seen an increase in surface crevice formation as a result of groundwater level declines. The crevices in the basin, on the other hand, are mostly caused by the elevation differences due to the paleotopographic conditions under the alluvial cover material. In the Konya Closed Basin, the formation of surface crevices has increased in the Konya-Aşağı Pınarbaşı, Aslımyayla, Hotamış-İsmil, Karapınar-Sultaniye, Emirgazi, Ereğli-Sazgeçit, Yunak-Çeltik and Akşehir sub-basins (Figure 20).

Figure 20. View of some surface crevices formed in Konya Closed Basin