# Analysis of Vertical Earth Pressure Acting on Box Culverts through Centrifuge Model Test
## 3 Centrifuge Model Tests
### Ground Composition
#### 3.3.2 Sand Pluviation  
In this study, we targeted a degree of compaction of 95%, and according to the compaction test results shown in Figure 4b, the model ground has a dry unit weight of 1.66 t/m\({}^{3}\). Traditionally, the model ground is created using the compaction method and pluviation methods that use a sieve or funnel. In the compaction method, a compaction plate \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Particle Size (mm)** & 0.85–0.60 & 0.60–0.25 & 0.25–0.10 & 0.10–0.075 \\ \hline
**Ratio (\%)** & 40 & 25 & 25 & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Particle size and ratio of sand for well-graded sand. Figure 4: Results of laboratory test conducted using mixed well-graded sand: (**a**) particle size distribution obtained from sieve test and (**b**) compaction test to determine maximum dry unit weight and optimal water content of well-graded sand. is placed on the ground, and an impact is applied on top of it by using a rubber hammer. The compaction energy directly compact the ground. If the impact load applied during compaction is directly transmitted to the embedded sensors, the sensors may malfunction. To prevent this scenario, the ground should be compacted at a certain distance from the location where the sensor is installed; then, the gap between the ground and the sensors was filled with dried sand. However, in this case, it is highly likely that the ground will not be formed homogeneously, and the earth pressure condition may change owing to unexpected deformation of the ground. In the pluviation method using a sieve, soil particles are allowed to fall freely after they pass through a sieve of a certain size. These soil particles are deposited on the chamber (or specimen) at a uniform speed, and the ground is formed with a homogeneous density. This is the most commonly used method for creating a model ground, and the density of the ground can be controlled by varying the drop height and sieve size. However, when the ground is created using well-graded soil, the possibility of material separation exists because the falling speed of soil composed of large particles is greater than that of soil with small particles. In other words, large particles may fall to the ground first, and therefore, it may not be possible to obtain a homogeneous ground composition. Therefore, in this study, we adopted the funnel-based pluviation method, as shown in Figure 5a. Unlike the sieve-based pluviation method, in the funnel-based pluviation method soil is dropped through a steel pipe having a certain diameter rather than an open space, and the location of the ground composition is determined by the funnel location. The principle of this method is similar to that of the Tremi tube used for pouring concrete on cast-in-place piles. Because the sand falls through a steel pipe instead of an open space, the possibility of material separation is small, and the possibility of sensor breakage or malfunction is minimized. The model ground is created from left to right (blue solid line) and then vertically (red solid line), as shown in Figure 5b. Since density differences occur depending on the diameter of the sieve attached to the lower part of the soft pipe and the length of the steel pipe, the steel pipe length and sieve diameter corresponding to the target dry unit weight were selected based on the results of multiple tests (steel pipe length = 2.0 m, sieve diameter = 4 mm). This method requires a skilled operator because density differences may occur depending on the operator's skill. Therefore, a process to verify the homogeneity of the ground is required, and to this end, we performed a small cone penetration test. As the homogeneity of the ground increases, the cone penetration resistance (\(q_{c}\)) increases linearly with the penetration depth. By contrast, in the case of non-homogeneous ground, scattered data are obtained.