Cover-collapse Sinkholes Research Articles

Formation process of cover collapse sinkholes related to groundwater level decline in karst areas

AbstractThe decline in groundwater level is a key factor contributing to cover collapse in karst areas. In this study, the model tests and numerical simulations are conducted to reveal the breeding process and formation mechanism of cover collapse sinkholes caused by the decline of groundwater level in karst area. Firstly, the model tests confirm that the decline of groundwater level generates negative pressure at the lower edge of overlying soil. The negative pressure experiences four distinct phases during the groundwater drawdown process: rapid rise, slow decline, rapid decline, and gradual dissipation. The maximum negative pressure is influenced by the particle size distribution of the overlying soil. Then, the numerical simulations are carried out to investigate the change process of negative pressure caused by the loss of fillers in karst pipe. The simulated results indicate that the rate of groundwater decline and the thickness and initial void ratio of the overlying soil can affect the maximum negative pressure. As groundwater level drops, a negative pressure zone forms underground, causing tensile failure in the surrounding soil and creating an arched soil hole, which weakens the support for the overlying soil. This phenomenon can also lead to the collapse of the overlying soil under its self-weight. Groundwater table decline in karst areas can result in both internal and surface collapses. When the overlying soil is thin, internal and surface collapses occur simultaneously. In contrast, for thick overlying soil, internal collapse happens first, followed by a layer-by-layer collapse, ultimately forming sinkholes. Finally, the breeding process and formation mechanism of the Yujiawan Reservoir sinkholes are discussed. Geological conditions and groundwater level decline significantly affect internal collapse in karst areas, requiring careful consideration from on-site engineers.

Investigating the drivers of urban cover-collapse sinkholes in shanghai: analyzing dominant factors and proposing mitigation strategies

Urban cover-collapse sinkholes pose a significant global challenge due to their destructive impacts. Previous studies have identified groundwater fluctuations, subsurface soil conditions, pipeline leakage, precipitation, and subterranean construction activities as key contributors to these phenomena. However, unique geological settings across different urban environments lead to variations in the primary factors influencing sinkhole formation. This study focuses on Shanghai, a city notable for its extensive urbanization and rich historical context, to explore the dynamics of sinkholes within urbanized areas worldwide. We employ spatial analysis and statistical methods to examine data on sinkholes recorded in the past two decades in Shanghai, correlating these events with the city’s shallow sand layer, ground elevation, and proximity to surface water. Our goal is to identify the dominant factors governing sinkhole occurrence in Shanghai and to lay the groundwork for their effective scientific management and prevention. Key findings indicate that most sinkholes in the area are associated with a thin shallow sand layer, low to moderate ground elevations, and the absence of nearby rivers. Additionally, many sinkholes correlate with subterranean voids within the confined aquifer beneath the cohesive soil layer. The lack of historical river channels, obscured by urban development, also indirectly contributes to sinkhole formation. We recommend enhancing urban river management and drainage systems to mitigate potential damage from water accumulation.

Geological and Hydrochemical Processes Driving Karst Development in Southeastern Riyadh, Central Saudi Arabia

This study investigates the processes leading to karst development in the southeastern part of Riyadh city extending up to Al Kharj. Numerous solution features such as sinkholes, collapsed dolines, and solution caverns are common in the area. The role of water in the development of the karst features was investigated using an integrated geological and hydrochemical approach. Geological investigations included the petrographic analysis of rock samples collected from zones of intense karstification with special emphasis on mineral dissolution. The study showed that the Sulaiy Formation is commonly fractured, brecciated, foliated, and contains numerous cavities, vugs, and openings. These features have formed by mineral dissolution by circulating groundwater, which has removed anhydrite beds from the underlying Arab–Hith sequence. Karstification likely started from the tectonically weak zones when there was more groundwater recharge. Studies show that during the early to mid-Holocene period, the climate in the Arabian Peninsula was humid, promoting groundwater recharge and subsequent mineral dissolution, though the process of karstification must have started much earlier. Hydrochemical findings reveal that mineral dissolution (halite and calcium sulfate) is the main process affecting groundwater chemistry. The Piper plot revealed two main hydrochemical facies: the (Ca2+ + Mg2+)–(Cl+ SO42−) Type (Type A) and the (Na+ + K+)–(SO42− + Cl) Type (Type B). Most of the samples belong to Type B, typical of groundwater facies affected by dissolution of halite and anhydrite mineral. The absence of the (Ca2+ + Mg2+)–(CO32− + HCO3−) type of groundwater facies indicates a lack of recent groundwater recharge and the removal of carbonate minerals from the system through precipitation, as evidenced by the saturation indices. Plots of the major ionic pairs (cations vs. anions) in groundwater indicate strong halite and gypsum/anhydrite dissolution. Of the three carbonate minerals, calcite has the highest average saturation index followed by aragonite and dolomite. This suggests significant past rock–water interaction leading to carbonate dissolution. Presently, any additional calcium or carbonate ions introduced into the water lead to calcite precipitation. The study indicates that the process of karst development may not be active today. Currently, groundwater chemistry is mainly influenced by rock–water interaction leading to gypsum/anhydrite dissolution, which has resulted in a high concentration of Na+, Ca2+, Cl− and SO42− ions in groundwater. The dissolution of gypsum and halite from the Hith Formation weakens the structural integrity of the overlying Sulaiy Formation, creating large underground cavities. These cavities increase the risk of roof collapse, leading to cover-collapse sinkholes as the roof becomes too thin to support the weight above.

Cover collapse sinkhole formation is delayed in time and uncorrelated to distance from quarry in a long-term study of a karst basin

Collapse sinkholes are problematic for the hazards they bring to property, infrastructure, and human life but the prediction of their formation in time and space remains elusive. We make use of long-term data gathered in a small, highly monitored, hydrologically constrained basin; Primrose Creek, Bucks County, Pennsylvania, USA, to examine the relation between groundwater extraction, precipitation, terrain position, and cover collapse sinkhole development. The selected site is unique as a natural laboratory to explore induced sinkhole processes in that it is a clearly demarcated topographic basin with geological structural and lithologic boundaries; the stressor is extreme, has been plainly identified, and effectively captures all upstream runoff and groundwater; the groundwater withdrawals and water-level elevations have been closely monitored over a period of 20 years; and the general record of land conditions goes back more than a century. We evaluated the history of a carbonate-rock quarry, its development and expansion over many decades, as well as associated precipitation and ground-water level data, and cover collapse sinkhole occurrence within the basin. Interception of discrete high conductivity zones in the Paleozoic carbonate rocks by quarry expansion was an important factor in propagating collapse occurrence. Collapses occurred mainly, though not exclusively, along drainageways and at lithologic boundaries. They formed in time-based clusters with delay of up to several years. Surprisingly, distance of collapse formation from the pumping area was not directly correlated with time elapsed; i.e., collapses did not begin to form close to the quarry, and then later form farther away. Monitoring wells were unsuccessful in delineating the pumping zone of influence although some documented water table decline and response to weather and anthropogenic events. This comprehensive study shows that in order to successfully monitor and control impacts from quarry dewatering in Paleozoic carbonate rocks it is critical to 1) develop a detailed conceptual model of groundwater pathways, 2) try to avoid excavating into high permeability zones, 3) clearly document karst features when exposed, and 4) carefully site a suitable number of monitoring wells and collect data at short intervals.

Investigation of Subsurface and Geological Structures Contributing to Collapse Sinkholes in Covered Karst Terrain, Northeast Thailand

This study focuses on covered karst terrain situated in Phu Pha Man District, Khon Kaen Province, Northeast Thailand, where records of collapse sinkholes are limited. Here, we investigate the subsurface characteristics and potential causes of sinkhole formation within this area using geophysical methods, hydrogeological techniques, and precipitation analysis. We collected field data by measuring groundwater levels, and conducting electrical resistivity tomography (ERT) surveys. We identified eight cover-collapse sinkholes of various shapes and sizes. Analysis of the groundwater flow indicated that the predominant flow direction runs from north to southeast. Examination of rainfall data showed a progressive increase in total rainfall on a yearly basis, with a significant precipitation event preceding the initial occurrence of sinkholes. The ERT results revealed the presence of highly resistive bedrock, water-saturated layers, and potential cavities. Notably, the tomograms indicated variations in resistivity values, suggesting the presence of irregular surfaces of limestone bedrock and weathered zones as characteristics of karst settings. Intense precipitation is a possible dominant trigger for the formation of the sinkholes. This study contributes to understanding sinkhole formation in karst environments and provides key information for hazard mitigation, not only in the Phu Pha Man District but also in areas with similar geological settings.

The application of distributed optical fiber sensors (BOTDA) to sinkhole monitoring. Review and the case of a damaging sinkhole in the Ebro Valley evaporite karst (NE Spain)

Distributed optical fiber sensors (DOFS) have been postulated as a suitable technique for long-range monitoring of sinkhole-related subsidence, and possibly for the anticipation of catastrophic collapse (early-warning systems). The strain data published in previous works refer to artificial experiments considering real and virtual cover collapse sinkholes characterized by rapid subsidence and sharp lateral deformation gradients. The influence of the subsidence mechanism (sagging, collapse, suffosion) on the capability of DOFS to satisfactorily detect active subsidence is discussed. Sagging sinkholes with poorly-defined lateral edges, low lateral deformation gradients and slow subsidence are identified as the most challenging scenario. The performance of BOTDA optical fiber for monitoring such type of sagging sinkholes is evaluated in the active Alcalá sinkhole, which affects a flood-control dike creating a high-risk and -uncertainty scenario. This sinkhole shows active subsidence in sections tens of meters long with maximum subsidence rates ranging between 5 and 35 mm/yr. The comparison of vertical displacement data measured by high-precision leveling and the strain recorded by two types of fiber optic cables shows good spatial and temporal correlation. The subsidence sections are captured in the strain profiles by: (1) troughs of negative strain (contraction) in the area affected by subsidence, with the maximum strain associated with the point of most rapid settlement; and (2) lateral ridges of positive values (extension) in the marginal zones. A subsidence acceleration phase associated with a flood is also captured by substantial increments in the strain values. In this challenging scenario, despite the reasonably good spatial and temporal correlation between the displacement and strain data, the unambiguous identification of the active subsidence area with the fiber optic data alone might be difficult. Better results could be obtained improving the monitoring system (e.g., tighter cable-ground coupling) and testing other types of sinkholes with more localized deformation zones and higher subsidence rates.

Modelling Cover-Collapse Sinkholes That Appeared after the M6.2 Petrinja Earthquake in Croatia Using Electrical Resistivity Tomography Data

After the destructive earthquake in Petrinja measuring M6.2 occurred on 29 December 2020, which was followed by a series of foreshocks and aftershocks in the area of the rural settlements in Mečenčani and Borojevići, cover-collapse sinkholes suddenly appeared. The investigated area is located 20 km southeast of the epicentre. Some months later, more than 90 cover-collapse sinkholes appeared, jeopardising local infrastructure and residential buildings. The sinkholes appear in the area of covered karst, where there are clastic deposits 2–10 m thick on the fractured and weathered limestone bedrock. There are two geological models located in the investigated area: GM-1, where the base consists of clastic strata covering comprising Lithothamnium limestone, which in turn leads to the formation of underground cavities and cover-collapse sinkholes, and the GM-2, where the base comprises clay deposits without any cover-collapse sinkholes. These models can be effectively distinguished due to tomographic resistivity models; hence, numerous measurements were undertaken using two-dimensional electrical tomography in several phases. An estimate of the threat to infrastructural facilities was conducted, and the boundaries of the geological models were precisely determined according to which underground cavities and cover-collapse sinkholes did not develop. Tomographic measurements were also conducted over the largest cover-collapse sinkhole measuring 25 m in diameter and helped to more precisely define the entire hydrogeological model and the mechanisms involved in the formation of cover-collapse sinkholes.

COMPARISON OF THREE APPROACHES TO CALCULATING DIAMETER OF A DROPOUT SINKHOLE

The possibility of sinkhole size prediction in the areas, where soluble rocks are covered by impermeable clay layer is considered. Such sinkholes are named by English-speaking authors as “dropout sinkholes” or “cover collapse sinkholes”, because the process of their formation includes collapse (dropping) of clay covering soluble rocks into a karst cavity. Triggers of this effect are: (1) widening of a cavity below the clay due to dissolution of a soluble rock by groundwater; (2) lowering of hydraulic head in confined karst aquifer, primarily caused by groundwater pumping, which results in decreased support of the overlying clay layer; (3) a dynamic load on the clays, which can have either natural or man-made origin. Fresh dropout sinkholes generally have configuration similar to vertical right circular cylinder, but sometimes their shape can be dome-like with very unstable ground “cantilevers”. The appearance of these forms under foundations can lead to deformation and even to destruction of buildings, structures, and infrastructure facilities. Two concepts exist, which can be used as basis for creating models of soil mechanics to calculate diameter of a dropout sinkhole. The first one is based on a viewpoint that clay collapse into a cavity after a trigger action is displayed immediately at the ground surface as a collapse sink. According to the second concept, the preliminary subsurface collapse of clay occurs as fall of ground block resembling a circular paraboloid. As a result, a dome-like cavity appears in the clay layer and above-lying clay fall into it to form a collapse sink. There are no prediction methods based on the two above-mentioned conceptions. The developed approaches to prediction of a dropout sinkhole diameter are adequately comparable, as they use the same input calculation parameters. The prediction results based on these approaches have been compared to an actual diameter of fresh dropout sinkhole. This verification have allowed us to infer that most adequate is the approach based on concept of initial subsurface clay’s collapse preceding the final collapse sink formation.

Geotechnical reconnaissance of an extensive cover-collapse sinkhole phenomena of 2020–2021 Petrinja earthquake sequence (Central Croatia)

This article presents geotechnical reconnaissance data that characterize the formation of 122 new and 49 historical cover-collapse sinkholes within 1.13 km2 area in 12 months following the MW 6.4 earthquake that occurred on 29 December 2020, in Petrinja, Croatia. Data include a geological background, seismic sequence information, sinkhole geometric characteristics, rainfall data, and results of detailed geotechnical subsurface investigation. The sinkhole geometrical features were collected using aerial and satellite imagery, terrestrial lidar, and manual measurements. Soil properties and groundwater levels were obtained from four geotechnical boreholes, accompanied by in situ geotechnical characterization and standard penetration tests (SPTs). Soil parameters were obtained from consolidated undrained conventional triaxial compression, oedometer, soil water retention, and index tests performed on 31 soil samples. Clayey cover, 4–10 m thick, with sporadic gravel lenses overlying cavernous, intensely karstified carbonate rocks, characterizes the sinkhole area. Clays are mostly overconsolidated, with varying degrees of saturation ranging from very small to fully saturated. Seasonal and climate-induced variations in the groundwater table interact with artesian/subartesian karst aquifer, thus affecting the suction and the shear strength. Soil water retention curves (SWRCs) indicate that desaturation is possible for deeper groundwater tables, thus further affecting the effective stress, shear strength, and interparticle tensile forces. Finally, the observed vertical walls that accompanied sinkholes opening can occur in the overconsolidated cohesive cover clay layer with varying degree of saturation. The presented data provide essential geomechanical information necessary to understand the associated sinkhole failure mechanism. This article will help future investigators to perform detailed analyses and provide a background for complementing future sinkhole precursor research. Geotechnical, geological, seismic, and precipitation data generally indicate that the formation of cover-collapse sinkholes in the study area is a consequence of a specific local geological setting but is significantly expedited by earthquake-induced dynamic loading and complemented by multiple hydro-mechanical factors.

Probabilistic spatial susceptibility modeling of carbonate karst sinkhole

Sinkhole is one of major geohazards in karst area where soluble carbonate bedrock is underlain by cover soils (e.g., sandy or clayey soils) and active groundwater circulation is present. Groundwater recharge easily erode the overburden soils away into cavities in the bedrock, and ultimately structural collapse in ground. The induced ground collapse/subsidence may pose a great threat to human safety, environment, and buildings and infrastructure. The goal of this study is to develop a method/tool to spatially assess the possibility of sinkhole occurrence. The paper presents two main taks: (1) development of a probabilistic spatial prediction model of sinkhole susceptibility and (2) a geographical information system (GIS)-based regional-scale sinkhole susceptibility map. The study area is the east central Florida (ECF) region that has been experiencing more abrupt and larger cover collapse sinkholes. The research methodology employs two statistical methods for the spatial prediction model, frequency ratio (FR) and logistic regression (LR), accounting for six criteria that are soil erosion, mechanical stability, distance, geology, human activity, and climate condition. The final section of factors includes head difference, soil permeability, thicknesses of aquifer systems (overburden, surficial/intermediate aquifer), distance to karst features, surficial geology, lithology, land use/land cover, and rainfall. Once the models were constructed, both models were validated and compared by using receiver operating characteristics (ROC) analysis. The results indicate that the LR model better delineates high susceptibility areas of sinkhole and hydrological factors (recharge and head difference) are more accurately reflected on the model of the LR.

Formation mechanism analysis of cover collapse sinkholes in Wugaishan Town, Chenzhou City, Hunan province, China

Formation mechanism analysis of cover collapse sinkholes in Wugaishan Town, Chenzhou City, Hunan province, China

A multidisciplinary approach in cover-collapse sinkhole analyses in the mantle karst from Guangzhou City (SE China)

Mantled karst is characterized by solution processes in underground conditions, where karst evidence can be masked. In mantled karst environments, cover-collapse sinkhole can be considered one of the most hazardous elements. This study seeks to develop a methodological framework utilizing different techniques and approaches to understand cover-collapse sinkhole genesis and its likely evolution. The study area is located in the Conghua district of Guangzhou city, Guangdong Province, in the south-eastern region of China. A mapping procedure was introduced to combine data from aerial photographs and intensive field investigations. In addition, data interpretations from borehole drilling activities and different geophysical approaches were performed to reconstruct the Quaternary deposit features, rock head morphology, and karst features. During these investigations, the detailed typology, morphometry, and chronology inventory of 49 cover-collapse sinkholes were analysed, and three karst fissure zones covered by Quaternary soil were observed. In the study, the hydrogeological data suggested that karst aquifer pumping triggered the drop in groundwater levels. Cover-collapse sinkholes might be ascribed to erosion of the soil layers due to the groundwater level decline.

Effects of declining water levels on water-air interactions in cover collapse sinkhole

Water-air interactions caused by water level changes over a karst cavity can lead to sinkhole collapse. Declining water levels in a two-layered cover (clayey upper layer and sandy lower layer) above a karst cavity will lead to changes in the air pressure inside both the cover layers and the cavity. The change in air pressure can cause changes in the rate of water level decline and induce subsidence and collapse. In this study, a mathematical formula was derived and used to better understand the water-air interaction mechanism. Air velocity, vacuum variation in the cover, cumulative air intake, and elapsed time in the water level decline process are considered. A physical model was used to conduct laboratory simulations of the water-air interaction process and to determine the relationship between collapse and the water-air interaction. The main findings of this work are as follows: (1) Water level declines cause changes in air pressure both inside the cover and the cavity, and, conversely, the velocity of the water level decline was influenced by the variable air pressure; water-air interaction is obviously occurring. (2) With a low permeability cover layer, the declining water level mainly generates vacuum suction, but also erosion of the underlying high permeability sandy layer; vacuum suction and erosion can lead to subsidence or collapse of the cover. (3) Physical model simulations are useful for representing the phenomena of water-air interactions in karst cavities, and provide a practical way to understand the mechanism of collapse in karst terrain.

The impact of groundwater drawdown and vacuum pressure on sinkhole development. Physical laboratory models

A considerable proportion of the damaging sinkholes worldwide correspond to human-induced subsidence events related to groundwater withdrawal and the associated water-table decline (e.g. aquifer overexploitation, dewatering for mining). Buoyancy loss in pre-existing cavity roofs is generally claimed to be the main underlying physical mechanism. It has been also postulated that rapid water-table drawdowns may create a vacuum effect in the subsurface and contribute to enhance sinkhole activity in karstic terrains with a low effective porosity cover. Our laboratory physical model explores the role played by vacuum pressure induced water-table drops with different magnitudes and rates on sinkhole development, simulating an invariable mantled karst comprising cavernous bedrock and a low-permeability cover. The multiple tests performed include real-time monitoring of the water level drawdown (magnitude, duration, rate), the negative air pressures in the bedrock cavity and the cover, and several features of the subsidence phenomena (deformation style, size, magnitude, rate). The main findings derived from the test results include: (1) Vacuum pressure may trigger the development of cover collapse sinkholes in areas with low-permeability covers. (2) Different water-table decline patterns (magnitude, duration, rate) may result in different subsidence styles or rheological behaviours: sagging versus collapse. (3) Ground fissuring, frequently related to extension at the margin of sagging depressions, may cancel or significantly diminish the vacuum effect. (4) An overall direct relationship between the water-table decline rate and the subsidence rate. Some possible strategies are proposed to ameliorate the adverse effect of the negative air pressure on sinkhole hazard, which most probably has a local impact restricted by the concurrence of rapid water drawdowns and low-permeability covers.

Detection of Cover Collapse Doline and Other Epikarst Features by Multiple Geophysical Techniques, Case Study of Tarimba Cave, Brazil

Reliable characterization of the karst system is essential for risk assessment where many associated hazards (e.g., cover-collapse dolines and groundwater pollution) can affect natural and built environments, threatening public safety. The use of multiple geophysical approaches may offer an improved way to investigate such cover-collapse sinkholes and aid in geohazard risk assessments. In this paper, covered karst, which has two types of shallow caves (vadose and fluvial) located in Tarimba (Goias, Brazil), was investigated using various geophysical methods to evaluate their efficiency in the delineation of the geometry of sediments filled sinkhole. The methods used for the investigation were Electrical Resistivity Tomography (ERT), Seismic Refraction Survey (SRS), Seismic Refraction Tomography (SRT) and the Very Low Frequency Electromagnetic (VLF-EM) method. The study developed several (2D) sections of the measured physical properties, including P-wave velocity and electrical resistivity, as well as the induced current (because of local bodies). For the analysis and processing of the data obtained from these methods, the following approaches were adopted: ERT inversion using a least-square scheme, Karous-Hjelt filter for VLF-EM data and time-distance curves and Vp cross-sections for the SRS. The refraction data analysis showed three-layered stratigraphy topsoil, claystone and carbonate bedrock, respectively. The findings obtained from ERT (three-layered stratigraphy and sediment-filled doline), as well as VLF-EM (fractured or filled caves as a positive anomaly), were found to be consistent with the actual field conditions. However, the SRS and SRT methods did not show the collapsed material and reached the limited the depth because of shorter profile lengths. The study provides a reasonable basis for the development of an integrated geophysical approach for site characterization of karst systems, particularly the perched tank and collapse doline.

A reactivated cave system induces rapidly developing cover-collapse sinkholes in Tasmania, Australia

The reasons of rapid development of cover-collapse sinkholes near Railton, Tasmania were investigated after sinkholes first appeared in a mature pine plantation in 2011. Sinkhole monitoring, dye tracing, chemical analyses of stream water and field observations indicated that large sinkholes in Quaternary alluvium overlying Ordovician limestone resulted from deepening of a limestone quarry south of the plantation. Intersection of a cave system during quarrying lowered the local water table, initiated streamsinks, and caused streams that previously flowed north towards the Mersey River to flow south to the quarry, via streamsinks and reactivated subsurface channels. Concurrently, the thickness of the vadose zone in thick Quaternary sediments overlying the limestone increased, hydraulic support for the sediments decreased, and numerous cover-collapse sinkholes formed. Chemical data including isotope analyses confirmed that the local aquifer is mainly recharged after high winter rainfall during high stream flow, when a proportion of the water in surface streams also flows directly northwards to the Mersey River. During low (summer) flow most water in surface streams flows into streamsinks and south into the quarry via the cave system. It is then pumped to a surface stream running north through the area in question, returns into the cave system via streamsinks and sinkholes, and finally returns to the quarry. The study demonstrates that water-table lowering by quarrying in limestone is not confined to cones of depression but may extend for kilometres if relict subsurface stream channels are intersected. In this area, the rapid development of cover-collapse sinkholes presents risks to infrastructure (a railway line, an electricity transmission line, and roads); to stream biodiversity because of altered water flow duration and water quality; to the safety of forest contractors, particularly those using heavy machines; and to the future use of presently productive land for economic return.

Identification and quantitative analysis of sinkhole contributing factors in Florida's Karst

Sinkholes are one of the major geohazards in karst areas, causing loss of life and significant economic loss as well. Sinkhole forming mechanism varies depending on the geological environment, and Florida's sinkholes are formed through the process of interaction between groundwater flow and cover soils. The significance of this paper is (1) to identify key sinkhole contributing factors and (2) to quantify the impact of those factors on sinkhole formation. Particularly, this study accounted for geomechanical factors that contribute to sinkhole raveling (due to erosion) and mechanical stability of overburden soils. Identification of key sinkhole contributing factors will allow developing a probabilistic sinkhole prediction model. The study area was focused on East Central Florida, where cover-collapse sinkholes often occur. Uniform-distribution transformation was employed for the quantitative analysis on each factor, and then case studies with specific examples are then presented to validate the results of the quantitative analysis. In conclusion, the identified key factors are hydraulic head difference, soil permeability, thicknesses of aquifer systems, and proximity to karst features (e.g., existing sinkhole). The critical permeability (kcrit) showing a rapid increase of sinkhole occurrence is about 20 cm/h. Thickness and characteristics of cover soils seem a controlling factor of sinkhole types, and the overburden thickness around 40 to 45 m tends to form the collapse type. A multivariate regression analysis indicates that the thickness of the surficial aquifer system is the most significant factor, and then the head difference and the proximity to karst features follow in order.

Hydraulic fracturing effect on punching-induced cover-collapse sinkholes: a case study in Guangzhou, China

Punching-induced cover-collapse sinkholes occur rapidly, and multiple collapses occur simultaneously compared with those caused by other common factors, such as pumping, rainstorms and mining. Erosion cannot explain the formation of this type of collapse entirely. We studied the hydraulic fracturing effect from geological analysis, field groundwater monitoring, laboratory testing, and pressure calculation. Four stages can be established for the development of cover-collapse sinkhole based on a case study. Hydraulic fracturing and the suction effect cause sinkhole opening with the percussion bit falling and pulling repeatedly when the pile foundation is being built. The behavior can be likened figuratively to a piston action. The pressure of the calculated hydraulic fracturing effect is 6.05 × 104 Pa, which is equivalent to a 5.93-m-high water column and is much bigger than the critical pressure of 0.7 m at which hydraulic fracturing occurs in laboratory testing. The significance of this study from a practical perspective is to provide a preventative, early warning method for sinkhole opening by groundwater monitoring during pile foundation construction.

Cover collapse sinkhole over a deep buried carbonate bedrock: The case study of Fossa San Vito (Sarno - Southern Italy)

Sinkholes are widely found in Italy in both carbonate slopes and alluvial plain settings. Since they may occur catastrophically, without showing premonitory signs and resulting in severe economic losses and casualties, they constitute a significant risk in many areas, and even pose a threat to human life. In this paper, we carried out a detailed multidisciplinary study of the Fossa San Vito sinkhole (southern Italy) that is a particular type of cover collapse sinkhole. The latter is a sub-circular, closed depression, with a diameter of ~200 m and with inner walls up to 25 m high, located in the piedmont area placed along the inner border of the Sarno River alluvial plain. The study entailed morphological and stratigraphic analysis by means of field surveys, including nine trenches, four boreholes, and the reinterpretation of stratigraphical data from the literature. In addition, hydrogeological and soil gas analysis were carried out to investigate the possible role of deep-water uprising. The study attempted to identify the genetic mechanism and include the reconstruction of a detailed subsurface stratigraphic model, showing that the infilling of the depression is about 45 m thick and consists of well-laminated, silty-clay lacustrine sediments interbedded with tephra layers. 14C dating of fossil wood found at the base of the lacustrine deposits constrains the age of the collapse at 5483 cal. years B.P. The collapse of the topographic surface involved a volume of about 400,000 cubic metres and originated at depth >80 m.

Modeling cover-collapse sinkhole based on the theory of shells

The shape and height of a natural balanced soil arch are two of the critical factors for the development of a sinkhole. The exposure of the natural balanced soil arch can be described as the initiation of the surface-collapse phase of the cover-collapse sinkhole in karst terrain. In this paper, by simplifying the natural balanced soil arch as a thin shell in a limit equilibrium state, a theoretical model is developed using the nonmoment theory of rotary shells with the shape and height of the natural balanced soil arch derived based on the Protodyakonov’s theory. First, the developed model is validated using a case study (a cover-collapse sinkhole occurred in Guizhou, China). It demonstrates that the shell theory used in this study can describe the equilibrium state of a natural balanced soil arch reasonably well. After model validation, a series of numerical simulations are then carried out to investigate the critical factors which govern the collapse of a sinkhole. The results show that buried depth serves as a compulsory condition for the formation of the natural balanced soil arch. Furthermore, it shows that a buried depth less than six times of the radius of a cave could result in the formation of a natural balanced soil arch in the cone surface.