### Experiment System and Its Accuracy  
Subjected to water buoyancy, a non-contact video measurement system including CCD camera, video gauge, computer, and other accessories was employed to real-time record the movement trajectory of the granary under different working conditions. According to Video Gauge User Manual Version 5.20, the vertical displacement is positive downward and negative upward, and horizontal displacement is positive to right and negative to left. To study the interaction between the underground granary and sand soil under buoyancy, this paper presents a non-contact experiment system. The process is as follows. Step 1: CREO software is used to establish model granary which is made by 3D printing. Step 2: A generalized geological model tank is made according to imaging plane visualization requirement. Step 3: The pressurized water supply system is made considering groundwater effect. Step 4: The video gauge test method (Video GaugeTM) is selected for granary displacement of, and particle image velocimetry (PIV) technology for, soil deformation. Step 5: Accuracy analysis. The non-contact experiment system is shown in Figure 2. The length, width, and height of the model tank are 800 mm, 300 mm, and 500 mm, respectively. The glass tank is made of five pieces of high-strength and high-transparency reinforced glass adhesive with wall thickness of 10 mm. The accuracy of the non-contact measurement system is the key to obtain reliable experiment data under different working conditions. The displacement of the model granary was set as 40 mm. When the displacement of the granary reached the designed height, the water supply pressure and equipment collection were stopped immediately. To control the backfill thickness of each layer of sand sample, a soft ruler was laid on the side wall of the model tank and marked at each 5 cm. To prevent the sample sand from \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Flexural** & **Bending** & **Tensile** & **Tensile** & **Poisson’s** & **Density** \\
**Modulus** & **Strength** & **Modulus** & **Strength** & **Ratio** & **(g/cm\({}^{3}\))** \\
**(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **Ratio** & **(g/cm\({}^{3}\))** \\ \hline
2521 & 73 & 2488 & 48 & 0.40 & 1.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The parameters of the photosensitive resin. Figure 2: Non-contact experiment system. blocking the outlet, the first layer was filled with gravel with particle size of 5-8 mm and height of 10 cm, and a permeable geotextile was laid above the gravel. Since the images collected during the experiment were gray, to ensure the experiment accuracy, red sands, green sands, and blue sands were backfilled at 20 cm, 25 cm, and 30 cm from the bottom of the model tank, respectively. The groundwater level was based on the bottom of the model tank, and the height of the water level plane read in the model tank relative to the bottom of the model tank. Six target points were arranged equidistantly in the middle of the granary to monitor the displacement trajectory of the granary, which is shown in Figure 3.  
The displacement-time curves of the target point in the horizontal and vertical directions are measured by the non-contact video measurement system, as shown in Figure 4. In the horizontal direction, the displacement of the target point is between 0 mm and 2 mm as shown in Figure 4a. When the water supply pressure is stopped, the vertical displacement of each target point fluctuates around 4.05 cm, as shown in Figure 5.  
Figure 3: The upward movement of model granary: (**a**) before upward movement; (**b**) 4 cm upward movement. In summary, when the water supply pressure was stopped, the vertical displacement measured by the non-contact experiment system was 4.05 cm, and the actual displacement was 4.00 cm. The relative error was 1.25%, which indicates that the proposed experiment system satiates the accuracy requirement [15,16].