# Model Test Analysis of Groundwater Level Fluctuations on
## 4 Analysis of Model Test Results
### Effect of Groundwater Level Fluctuations on Cover Deformation
#### 4.2.2 Analysis of the Results of the Dissolved Cavity Water Level Drop Test  
In the simulation test, the density of the cap layer is 1.4 g/cm\({}^{3}\) and the drainage rate is 1.5 cm/min. That is, with the same change of vacuum negative pressure in the dissolution chamber, different thicknesses of 6 cm, 9 cm, 12 cm, 15 cm, and 18 cm are simulated via experiments to study the effect of caprock on karst collapse. The variations of air pressure in the cavity are shown in Figure 9.  
Figure 8: Settlement curve for each cover layer during water level rise.  
Figure 7: Rising water table cover settlement.  
It can be seen from Figure 9 that before 240 s, there is no vacuum negative pressure in the dissolution chamber in which the pressure is equal to the external atmospheric pressure. At 240 s, the dissolution chamber drainage switch is turned on to start drainage. The continuous process makes the space in the dissolution chamber increase continuously, and the vacuum negative pressure also keeps increasing. When the test goes on for 310 s, the vacuum negative pressure reaches the maximum of 2321 Pa. After drainage, the vacuum negative pressure increases rapidly during 240-270 s. After 270 s, the vacuum negative pressure increases slowly with the increase of time. When the time is greater than 310 s, the negative pressure in the dissolution chamber tends to be stable. Due to the pressure difference between the top and bottom of the cap layer, the vacuum suction force acting on the cap layer gradually increases with the vacuum negative pressure. The external air enters the karst cavity through the connected pores. When the volume of the discharged water is equal to that of the gas entering from the outside, the negative pressure in the karst cavity remains constant.  
Figure 10 shows the deformation-time curve of the caprock. Before drainage, the cover layer soil is relatively stable and hardly deforms, and the deformation amount is approximately 0; after the drainage starts, the cover layer soil begins to deform slightly. With the increase in drainage time, the cumulative deformation of the covered soil also increases. As the test continues, the cover soil no longer deforms when other conditions remain unchanged, and finally tends to be stable. When the thickness of the cover layer is 6 cm, the deformation rate in the curve deformation stage is the largest. As the thickness of the cover layer increases, the deformation rate of each curve becomes smaller and smaller, indicating that the deformation and displacement of the cover layer are inversely proportional to its thickness. That is, the thicker the cover layer, the more difficult it is to deform and break. It can be seen from Figure 11 that under the same negative pressure of the cavity, the larger the thickness of the overburden, the smaller the deformation, and the more stable the overburden soil is. the shear strength increases accordingly, and the ability of the cover layer to resist deformation and damage also increases. The deformations of the cover may have a nonlinear relationship with its thickness. With the increase of cover thickness, the change rate of soil deformation gradually slows down.  
Figure 9: Time-lapse curve of the drainage and cavity pressure.