# Centrifuge Model Investigation of Interaction between Successively Constructed Foundation Pits
## 2 Centrifugal Model Test Design  
A centrifuge model test was performed in the Key Laboratory of Geotechnical and Underground Engineering at the Department of Geotechnical Engineering of Tongji University. The test model was a tlj-150 composite geotechnical centrifuge, and the centrifuge model test was performed using a geotechnical centrifuge, which placed the geotechnical test model in the centrifuge model box according to the design requirements. When the model box rotated at a high speed, the model experienced the effect of gravitational acceleration to compensate for the scaled gravitational loss of geotechnical structures. A centrifugal force equivalent to gravity was applied to make the stress level and deformation of the model soil and the prototype soil the same to provide a reference for theoretical research and numerical methods. In a centrifuge model test, the hypergravity model on a small scale can achieve the stress level of a large-scale prototype in Earth's gravity. Moreover, many long-term phenomena in Earth's gravity can also be reproduced on an observable timescale in centrifuge model tests [30; 31; 32]. In this study, the centrifuge test simulated the successive construction process of adjacent foundation pits. Compared with a single foundation pit, the required model size was larger. Combined with the influence of the boundary effect, the operability of the test process, the stable working conditions of the centrifuge, and the existing test experience of the centrifuge, it was important to properly select the size and similarity range of the model. On the basis of these considerations, the acceleration during the stable operation of the centrifuge model test was 100 G (G is gravitational acceleration), and the corresponding model similarity rate was 1:100. Compared with cohesive soil, sandy soil has better characteristics suitable for centrifuge tests. For example, sandy soil can be used in a high-quality centrifugal simulation if certain preparation methods, including certain compactness, are followed [26]. Therefore, standard sand was used in this model test, and the soil parameters are shown in Table 1. In this model test, the retaining wall and support of the foundation pit were made of aluminum alloy instead of reinforced concrete, and the thickness of the alternative material was determined according to the principle of similar flexural stiffness to the prototype material model [12]. The converted model dimensions are shown in Table 2. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Soil Layer** & **Moisture Content/\(\omega\) (\%)** & \begin{tabular}{c} **Internal Friction** \\ **Angle/\(\varphi\) (\({}^{\circ}\))** \\ \end{tabular} & \
\begin{tabular}{c} **Unit Weight/\(\gamma_{0}\)** \\ **(kN\(\cdot\)m\({}^{-3}\))** \\ \end{tabular} &
\begin{tabular}{c} **Compression** \\ **Modulus/\(E_{8}\) (MPa)** \\ \end{tabular} \\ \hline Standard sand & 4 & 31.0 & 16.1 & 11.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic physical and mechanical parameters of the soil layer. The size of the model box used in this test was 900 mm \(\times\) 700 mm \(\times\) 700 mm (length, width, and height). The original test design was divided into two areas through the middle partition with a certain stiffness. Because some channels were damaged during the test, it was adjusted to select half of the areas for the centrifuge test. The size of the selected area was 900 mm \(\times\) 346 mm \(\times\) 600 mm (length, width, and height). A thick aluminum alloy plate was selected for the diaphragm, both sides of the bottom were reinforced with corbel, and silica gel was used to seal between both ends of the diaphragm and the sidewall of the model box to prevent the outflow of sand. After preparing the soil sample and installing all the components, the total weight of the model box was approximately 1100 kg, and the design capacity was 150 g centrifuge equipment that could operate smoothly under 100 g acceleration.