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

Abstract

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.