## 3 Model Test  
The test model box is made of welded steel plates, the length, width, and depth of the model box are 1500 mm, 500 mm, and 1700 mm respectively. The soil is made of silt, and the particle size composition of silt is obtained by the particle analysis test, which is shown in Table 1. The model pile is made of plexiglass pipes. The diameter of the model pile is 50 mm, the wall thickness is 8 mm, the lengths of the uplift pile and compressive pile are 1200 mm, and the elastic modulus of the model pile is 2.1 GPa. The soil is made by layered filling in box, and each layer is approximately 200 mm thick. The next layer was filled after 2 days rest period with uniform water sprinkling on top. Using burying methods to set up model piles, the model piles are fixed in the design position in advance and simultaneously buried in the soil when each silt layer is filled. The compressive pile is soil-free at the bottom of the pile. Under the pile, bottom is a hollow bamboo cage with a length of 60 mm and a diameter of 50 mm. During the test, the compressive pile can penetrate the bamboo cage to ensure that the pile bottom does not produce a tip resistance of the pile.  
The additional stress in the soil at the side of the pile is measured using micro earth pressure cells, which are buried according to the design requirements during soil produc  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Particle Diameter (mm)** & \(d_{s}\) **< 0.075** & \(0.075\leq d_{s}\) **< 0.15** & \(0.15\leq d_{s}\) **< 0.3** & \(d_{s}\geq 0.3\) \\ \hline Proportion & 76.3\% & 12.3\% & 7.5\% & 3.9\% \\ \hline \hline \end{tabular} Notation: \(d_{s}\) is particle size.  
\end{table}
Table 1: Particle size composition of silt.  
Figure 1: Additional stress caused by uplift piles and compressive piles in soil: (**a**) Uplift pile; (**b**) Compressive pile.  
tion. In the vertical direction, the micro earth pressure cells are set down at 300 mm below ground level and at 300 mm intervals downwards, i.e., 300 mm, 600 mm, 900 mm, and 1200 mm downwards from ground level. In the horizontal direction, earth pressure cells are buried 75 mm (1.5 d, where d is the pile diameter), 150 mm (3.0 d), 225 mm (4.5 d), and 300 mm (6.0 d) away from the axis of the pile. The layout of earth pressure cells is shown in Figure 2, and the embedment of micro earth pressure cells is shown in Figure 3.  
After completion of the soil filling in the model box, the static load test of the pile can be performed after resting for 20 days. At the end of the model test, ring knives were used to collect soil which is at about 200 mm from the pile axis and 500 mm depth for physical and mechanical tests. The physical and mechanical parameters are shown in Table 2.  
The static load test of the model pile adopts the slow maintenance load method [24]. The static load test of uplift piles and compressive piles adopts a servo hydraulic loading instrument, which has a displacement sensor and can automatically collect displacement. The compressive pile test diagram is shown in Figure 4.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Water Content** & **Density** & **Cohesion** & **Internal Friction Angle** & **Compression Modulus** & **Plasticity Index** \\ \(\omega\) (\%) & \(P\) (g/cm\({}^{3}\)) & \(c\) (kPa) & \(\varphi\) (\({}^{\circ}\)) & **Es (MPa)** & \(I_{P}\) \\ \hline
9.8 & 1.61 & 5 & 26 & 8.1 & 6.4 \\ \hline \end{tabular}
\end{table}
Table 2: Physical and mechanical parameters of silt.  
Figure 3: Embedment of micro earth pressure cells.  
Figure 2: Layout of the earth pressure cells.