## 3 Results and Discussions
### Displacement of Underground Granary  
Figure 6 shows the displacement-time curves of the target points in the horizontal and vertical directions. When the soils around the granary are loose sands, the displacement trajectory of the granary is roughly divided into four stages named as the stability stage (before 152 s), floating stage (152 s to 170 s), inclined stage (170 s to 175 s), and unstable failure stage (after 175 s), respectively. Before 152 s and 158 s, the granary does not move in vertical and horizontal directions, respectively. The reason is that the soil around the granary is not fully saturated, and the water buoyancy at the bottom of the granary is less than or equal to the sum of the gravity of the granary and the friction between the granary and the soil around the granary. After 152 s, with the increase of water supply pressure time, the vertical displacement of the granary increases nonlinearly. The gap between soil particles is filled with water, resulting in the soil reaching a fully saturated state. With the increase of pore water pressure, the friction between particles decreases, resulting in increasing the granary displacement. Therefore, 152 s is the time for the granary to float, and the corresponding water level is 32.5 cm, as shown in Figure 7a. In the horizontal direction, after 158 s, the granary begins to shake around, and after 170 s, the granary tilts toward left. After 175 s, the horizontal displacement of the granary suddenly changes which indicates that the granary experiences instability failure, and the corresponding vertical displacement is about 45.0 mm. When the soils around the granary are the medium dense sands, the displacement trajectory of the granary is roughly divided into four stages, named as the stability stage (before 242 s), floating stage (242 s to 262 s), inclined stage (262 s to 278 s), and instability stage (after 278 s). The floating time of the granary is 242 s, and the corresponding floating water level is 34 cm, as shown in Figure 7b. In the floating stage, compared with the loose state, the time of resisting water buoyancy of the granary increases, and the displacement of the granary decreases significantly. With the increase of degree of compaction of the soil around the granary, the weight of soil in unit volume increases, and the mutual friction between particles increases, which increases the friction between the soil and granary wall. After 278 s, the horizontal displacement changes suddenly, which indicates that the granary experiences instability failure. The corresponding vertical displacement is about 37 mm, which is reduced by 17% compared with that of the loose sands around the granary. When the soils around the granary are dense sands, with the increase of water supply pressure time, the vertical displacement trajectory of the granary shows a slow upward trend, and the horizontal displacement is small. The displacement trajectory of the granary is roughly divided into three stages, named as the relative stability stage (before 510 s), floating stage (510 s to 520 s), and inclined stage (520 s to 580 s). The maximum displacement of the granary in the horizontal direction is about 12 mm, and the granary does not experience unstable failure. The floating time point of the granary is 510 s, and the corresponding floating water level is 36 cm, as shown in Figure 7c. In the floating stage, compared with the loose sands and medium dense sands, the compactness of the soil around the granary is greatly improved. The pores between the soil particles are compressed and the mutual embedded force is enhanced, which cause the lateral friction resistance on the granary wall to increase, resulting in the unit volume of soil limiting further displacement of the granary. From 520 s to 580 s, the granary experiences slight fluctuation. After 580 s, the granary begins to tilt until the water level of the model tank reaches the water level at the end of the experiment, resulting in unstable failure.