Analysis on Precipitation-Induced Subsidence of Covered Karst Soil Caves Regarding Spatial Shape

In order to reveal the precipitation mechanism of covered Karst caves, the influence of the shape and size of the cave body and the internal law under the limit equilibrium, a common ellipsoid soil cave in straight collapse is investigated and its mechanical model of precipitation-induced subsidence is constructed. The calculation formula of the cavity negative pressure for the soil cave is deduced according to Boyle-Maliot law, so as to obtain the expression of the stability coefficient for the soil cavecollapse, and the feasibility of the calculation formula is verified by comparison. Further, the internal relations among the physical and mechanical parameters of soil mass, precipitation parameters, the spatial shape and size of soil hole and the overburden soil thickness under the limit equilibrium are obtained. Utilizing a calculation example, the influence of groundwater precipitation parameters and the shape and size parameters of the soil cave, and the internal law analysis under the limit equilibrium state are carried out. It is pointed out that when the initial water level is higher than the cave top, the stability coefficient of soil cave collapse and groundwater drawdown show a “Z”-shaped change, and it is very easy to cause soil cave collapse the moment the falling stable water level falls over the vault. When the initial water level is in the cave body, they show a negative correlation with changes steep in the front and slow in the back, and the higher the initial water level in the cave, the greater the decline; when the initial water level is lower than the cave bottom, the effect of drawdown is very small. The influence of the ratio of the long and short half axes of the ellipsoid on the stability coefficient conforms to the pattern of the increasing function. The greater the eccentricity of the cross section, the more stable it is, while the circular sphere is the most unfavorable. There is a linear relationship between the arch height and the stability coefficient. The arching effect is significant when the arch height increases, and the soil hole is more stable. Under the limit equilibrium, when the initial water level is fixed, the drawdown is positively correlated with the thickness of the overburden layer, showing a trend of slow change before and steep change after; however, when the thickness of the overburden layer is fixed, the drawdown is negatively correlated with the initial water level. The greater the horizontal section eccentricity of the soil cave or the higher the arch height, the deeper the groundwater required to reach the limit equilibrium, which is characterized by the changes gentle front and steep back.