Development characteristic and formation mechanism analysis of collapse sinkholes in Wugaishan town, Chenzhou city, Hunan province, China

The cover collapse sinkholes occurred and concentrated in Wugaishan town, Chen zhou city since 1996. The results are combined with results of site investigation, geophysical prospecting and in situ groundwater monitoring data, allowing the development characteristics and formation mechanism of surficial collapse incidents to be summarized. Collapse sinkholes are significantly active in recent years and mostly develop in the rainy season ranging from April to June and generally show a zonal distribution along the topography of study area from SW to NE. 92.31 % of total collapse events occurred in the thickness of overburden material ranged from 0 to 15 m, which indicated that overlying material less than 15 m was easier to collapse. The results show that collapse sinkholes have strong relationship with characteristic of overburden material, which sharply decrease in internal physical and mechanical property of bottom layer. Furthermore, substantial cavities formed within bedrock are the best transport channels and storage spaces for the unconsolidated material, especially under the condition of dynamic undulation of groundwater level. The formation mechanism of collapse sinkhole is divided into three types: infiltration erosion, coupling air implosion with vacuum cavitation and saturation erosion. Each formation mechanism is related to changes of groundwater level. When groundwater level rose above the soil-bedrock interface, saturated subsoil were easier to disintegrate into small particles and migrate downward as the vertical seepage of groundwater. The hydraulic gradient increased and became the predominant factor for the development of soil cavity as groundwater level dropped below the soil-bedrock interface. Moreover, when groundwater level sharply surged up at the relative sealed environment, the upward erosion roof of cavity would be more likely to collapse by the entrapped air blasting.

The Münsterdorf sinkhole cluster: Void origin and mechanical failure

<p>We describe and explore a group of collapse sinkholes located on a<br>sports field in the village of Münsterdorf close to Hamburg in northern Germany.<br>The collapse sinkholes develop since 2004 with a rate of one per year, with<br>sizes between 2-3 m in diameter and 3-5 meter depth, and are aligned to<br>a narrow east-west trending region..<br>In 20 m depth, cretaceous chalk is present in the area, topped by peri-glacial<br>and glacial deposits.<br>We summarize hydrological, geodetical, and geophysical data collected<br>and then discuss mechanical concepts for the occurence of the collapse sinkholes,<br>starting with simple analytical solutions and then expanding to distinct-element modelling.<br>We conclude that dewatering of the area might be an important aspect for the collapse sinkhole<br>developement.</p>

Methodological Considerations in Cover-Collapse Sinkhole Analyses: A Case Study of Southeastern China’s Guangzhou City

Cover-collapse sinkholes can present significant hazards to human habitation and communal facilities in soil-covered karst regions. Therefore, for human security and land-use planning in sinkhole-prone areas, appropriate approaches are required prior to construction in order to understand the cover-collapse sinkhole genesis and its likely evolution. The study seeks to contribute to performing an integrated analysis of karst hazards in mantle karst regions where karst evidence can be masked, with the ultimate goal of developing a methodological framework utilizing different techniques and approaches. A small area located in Guangzhou City of southeastern China’s Guangdong Province was analyzed. The detailed typology, morphometry, and chronology inventory of 49 cover-collapse sinkholes in the study area were analyzed using various surface investigation methods, such as field surveys, aerial photography, and photogrammetry. The Quaternary deposits and indicators of the active underground karst features in the aforementioned mantle karst region were geotechnically characterized using drilling and geophysical techniques. These techniques included ground penetrating radar (GPR), electrical resistivity imaging (ERI), natural source audio frequency magnetotellurics (NSAMT), and micro-tremors. During this study’s investigations, three karst fissure zones covered by Quaternary soil were observed using multiple techniques. In addition, it was found that the groundwater dynamic monitoring data confirmed that the sinkholes in the study area were closely related to changes in groundwater levels. Therefore, the efforts which have been made to investigate and monitor the sinkhole development will be required to continue into the immediate future.

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.