Study on Ground Collapse of Covered Karst Soil Caves by Sudden Drop of Groundwater

In this study, the ellipsoidal soil cave with vertical collapses in the covering karst area is studied. Based on certain assumptions, the mechanical model of karst collapse caused by groundwater drop was established. Then, based on the negative pressure calculation formula of soil cave cavity according to Boyle–Mariotte’s law, the expression of the stability coefficient of the soil cave was proposed. Subsequently, the feasibility of the theoretical formula was verified. The calculation example analyzed the relationship of groundwater parameters and overburden thickness. The results show that when the initial groundwater level is higher than the top of cave, the law between the stability coefficient of soil cave and groundwater drawdown shows the jumping horizontal broken line. Thus, soil cave tends to collapse when the falling groundwater level drops over the vault; when the initial groundwater level ranges from the bottom to the top of the cave body, the stability coefficient and groundwater drawdown show a negative correlation law, the curve is steep at the early stage and then becomes gentle at the latter stage, and the higher the initial groundwater level in the cave is, the greater stability coefficient of soil cave reduces; when the initial groundwater level is lower than the bottom of the cave, the effect of drawdown is limited. In addition, for the small drawdown or low initial groundwater level, the stability coefficient of soil cave first decreases and then increases with the increases in thickness of overburden, and the thinner the overburden is, the greater the drawdown rate is; when the drawdown or the initial groundwater level is higher, the stability coefficient of soil cave positively relates to the thickness of the overburden layer.

Land Degeneration due to Water Infiltration and Sub-Erosion: A Case Study of Soil Slope Failure at the National Geological Park of Qian-an Mud Forest, China

Sustainable development of the natural landscape has received an increasing attention worldwide. Identifying the causes of land degradation is the primary condition for adopting appropriate methods to preserve degraded landscapes. The National Geological Park of Qian-an mud forest in China is facing widespread land degradation, which not only threatens landscape development but also endangers many households and farmlands. Using the park as a research object, we identified the types of slope failure and the factors that contribute to their occurrence. During June 2017, a detailed field survey conducted in a representative area of the studied region found two main types of slope failure: soil cave piping and vertical collapse. Physicochemical properties of the soil samples were measured in the laboratory. Results show that soil slope failure is controlled by three factors: (1) the typical geological structure of the mud forest area represented by an upper layer of thick loess sub-sandy soil and the near-vertical slope morphology; (2) particular soil properties, especially soil dispersibility; and (3) special climate conditions with distinct wet and dry seasons.