# Model Test Analysis of Groundwater Level Fluctuations on
## 4 Analysis of Model Test Results
### Effect of Groundwater Level Fluctuations on Cover Deformation  
#### 4.2.1 Analysis of the Influence of Water Level Rise on the Dissolution Cavity  
The case in which the caprock density is 1.4 g/cm\({}^{3}\) and the average water supply rate is 1.2 cm/min was simulated. That is, when the positive pressure change in the dissolution chamber is the same, for different thicknesses of 6 cm, 9 cm, 12 cm, 15 cm, and 18 cm, the impact test of the karst collapse of the caprock in the karst cavity is shown in Figure 6.  
Figure 5: The relationship between rainfall and caprock deformation.  
From Figure 6, it can be observed that there was no vacuum negative pressure in the dissolution chamber before 240 s. This is because the water inlet valve was closed and the pressure in the dissolution chamber was consistent with the external atmospheric pressure during this period. After 240 s, the water inlet valve was opened, and the instrument began to supply water, resulting in the positive pressure in the chamber. When the water supply continued, the groundwater level continued to rise, the space in the chamber continued to decrease, and the positive pressure value continued to increase. After 295 s, the positive pressure of the dissolution chamber reached the maximum of 3542 Pa. When the instrument started to supply water, the positive pressure of the dissolution chamber increased at the fastest rate of 375 Pa/s. With the increase of time, the positive pressure change rate gradually decreased. When the time was greater than 295 s, the positive pressure in the dissolution chamber tended to be stable, and the change rate was 0. This is mainly because with an increase in positive pressure in the cavity, the positive pressure acting on the cap layer gradually increases. When the pressure increases to a certain extent, the particles of the cap layer are connected to the outside pores, so that the gas in the cavity can be discharged. When the volume of water supplied is equal to that of the gas discharged from the cavity, the pressure in the cavity remains unchanged.  
Figure 7 shows the deformation time curve of the caprock. It can be seen that the deformation-time curve of the caprocs with different thicknesses is similar to the positive pressure curve in the cavity, whereas the time required for the stability of caprock deformation is greater than that of the positive pressure curve. That is, when the positive pressure reaches the maximum, it still affects the deformation of the cover layer soil. After a period, the stress state of the cover layer reaches equilibrium again, and the cover layer soil no longer deforms. Before starting the water supply, the covering layer does not deform, the deformation amount is almost 0, and the structure is relatively stable. When the water supply starts, the covering layer begins to deform slowly. In addition, with a continuous water supply, the amount of deformation gradually increases, the final deformation is unchanged, and the structure tends to be stable.  
When the thickness of the covering layer is 6 cm, the covering layer is stable at 376 s, and its deformation is 3.016 mm. When the covering layer thickness is 18 cm, the deformation of the covering layer remains unchanged at 342 s, and its deformation is 0.526 mm. When the caprock density and positive pressure in the cavity are the same, with an increase in caprock thickness, the caprock deformation decreases continuously. Namely, the caprock thickness is inversely proportional to the caprock deformation (Figure 8). In the meantime, when the thickness of the cap layer is larger, the deformation rate in the deformation stage is smaller. When the thickness of the cap layer increases from 6 cm to  
Figure 6: Time-lapse curve of the water supply and positive pressure of the dissolution chamber.  
18 cm, the deformation rate decreases from 0.132 mm/min to 0.0283 mm/min. This result indicates that the larger the cap layer thickness is, the more difficult it is for the cap layer to deform and fail, and the more stable the structure is.