Cover-collapse sinkholes pose a major geohazard occurring in mantled karst regions in the south of China in recent years. Cover deposits generally mask the subsurface development and propagation of the deformation to the topographic surface. Human security and land-use planning in sinkhole-prone areas need to be preceded by detailed investigations focused on identification of pre-existing sinkholes, subsurface dissolution, subsidence features, and groundwater condition. Thefore the abnormal, early hidden danger signs (ground, underground and hydrodynamic) of karst collapse are studied. Further, the corresponding identification techniques and methods are presented, such as surface surveying techniques (satellite remote sensing, the drone, etc.), underground detection technology (ground penetrating radar, microgravity, micro tremor, etc.), and monitoring of groundwater conditions. Finally, all kinds of techniques and methods can be combined to form a comprehensive system for identifying hidden dangers of karst collapse. This paper aims at identifying the early hidden dangers of karst collapse geological disasters. This has important theoretical, technical and social significance in order to minimize the impact of disasters on the people and the environment.
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.
Eighty-six sinkholes occurred in a karst area of 2 km2 from 1990 to 2016, resulting in cracked houses and abandoned farmland. The affected area is adjacent to three underground mines with mining depth up to 600 m below ground. Sinkhole characteristics, hydrogeochemical and isotopic data, and groundwater-gas pressure measurements were evaluated to understand the dynamic karst system. An aquiclude layer was identified between the Triassic Nanlinghu Formation and Donggangling Formation, leading to recognition of two karst groundwater subsystems—Dongmaanshan subsystem and Nanlinghu subsystem. The sinkholes occurred in the Dongmaanshan subsystem where Xinhua Copper Mine and a water supply wellfield are located, and the influence distance can reach 1.2 km. A larger groundwater cone of depression induced by dewatering in a pyrite mine and gold mine in the Nanlinghu subsystem caused differential settlement. Because the sinkhole occurrence is sensitive to groundwater extraction in the Dongmaanshan subsystem, it is recommended that underground engineering activities should be avoided in this subsystem in future land use planning unless effective measures are taken to prevent groundwater level decline and high-frequency groundwater-gas pressure monitoring is implemented.
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