## 2 Materials and Methods
### Test Equipment and Design  
Figure 5 illustrates transparent soil model test equipment composed of a loading system, CCD industrial camera, laser, model box, optical platform, and computer. The loading system was a microcomputer control electron universal testing machine WDW-100D, the measuring force range was 0-100 kN, and the displacement measurement accuracy was 0.01 mm. Additionally, the accuracy of force measurement was 0.03 F.S., the resolution of the CCD industrial camera was 1280 \(\times\) 960, and the maximum frame rate was 40 fps. The laser model was YN-532-3W, the wavelength was 532 nm, the maximum power was 5 w, and the working distance was 1 m. The length, width, and height of the model box were 30 cm, 20 cm, and 15 cm, respectively, and the box was bonded by acrylic plates with a thickness of 15 mm. \begin{table}
\begin{tabular}{c c c c} \hline
**Model Type** & **Cement Content** & **t/h** & **s/d** \\ \hline SN-8 & 8\% & 0.5 & 1 \\ SP-8 & 8\% & 1 & 1 \\ MN-8 & 8\% & 0.5 & 2 \\ MP-8 & 8\% & 1 & 2 \\ LN-8 & 8\% & 0.5 & 3 \\ LP-8 & 8\% & 1 & 3 \\ SN-15 & 15\% & 0.5 & 1 \\ SP-15 & 15\% & 1 & 1 \\ MN-15 & 15\% & 0.5 & 2 \\ MP-15 & 15\% & 1 & 2 \\ LN-15 & 15\% & 0.5 & 3 \\ LP-15 & 15\% & 1 & 3 \\ \hline \end{tabular}
\end{table} 
Table 1: Test scheme. Figure 4: Schematic diagram of hard crust and grid structure. The gridded hard crust model studied in this experiment was different from the previous study of piles or other small components. In the past, the model box could be directly placed on a professional optical test platform, and the laser was irradiated from the side to form a speckle field for testing. However, the laser could not penetrate the transparent soil at the interval of the grid structure in this test owing to there being no gap between the side of the model and the bottom of the model box. Thus, an optical test table was designed for testing. The optical test table consisted of welded steel plates, and the test table was provided with a square hole with a width of 20 cm in the vertical direction facing the load plate. In this way, the fan-shaped laser surface generated by the laser could pass through the model box from bottom to top. Before the test, the transparent soil was put into the model box, and the model box with transparent soil was put into a vacuum bucket for vacuum degassing for 2-3 h. This step aimed to discharge bubbles caused by disturbance during sample loading. After the air pumping was finished, the model was embedded into the transparent soil of the model box according to the calibrated position. Subsequently, the transparent soil was added to the positions on both sides of the model and put into the vacuum bucket again for air pumping for 2-3 h. After standing for 12 h, the model box was placed at the center position of the desktop of the optical platform aligned with the gap opening of the desktop. A laser emitter was placed at the bottom of the model box to make the laser spot, and the position of the laser was adjusted to ensure that the sheet laser emitted by the laser faced the middle of the transparent model box. Besides, the industrial camera was installed on the tripod and fixed in front of the model box. It was focused manually, and the position of the camera was constantly adjusted until a clear speckle surface could be photographed. The image acquisition software was debugged and operated to ensure that the pictures could be collected normally and continuously. The photo storage format was BMP. Normal and continuous data acquisition was performed by data acquisition software. Before loading, three load plates with lengths of 10.5 cm, 15 cm, and 19.5 cm were prepared following models of different sizes. Each load plate was 20 cm in width and 2 cm in thickness, and threaded holes were drilled in the middle to connect with the loading equipment. The concentrated load applied by the universal testing machine was transformed into a uniform load through the load plate.