## Model Test Scheme
### Test. Model. Filling  
The fillings are mainly composed of angular graded gravels. Sieve test is carefully performed to obtain the characteristics of particle size distribution. The grading curve is shown in Figure 3. All of the filling particles can pass the 40 mm sieve, indicating that the maximum particle size is smaller than 40 mm. More than 50% particles have a size smaller than 10 mm. Only 6.82% particles could pass the 0.075 mm sieve. Furthermore, nonuniform coefficient of the fillings is 67 and curvature coefficient is 4.67. Therefore, the fillings can be distinguished as fine angular pebbles with some clayey soil. Furthermore, since the top layer of subgrade bed suffers cyclic train load and climate changes directly, its size grading is strictly regulated in Chinese code and should locate in the scope shown by the upper and lower limited grading lines in Figure 3. It is remarkable that the graded gravel adopted in this study have the size grading characteristics that satisfy such a requirement.  
Figure 3: Particle size distribution of model filling.  
Figure 2: Displacement transducers layout.  
After this, standard heavy compaction test is carried out for the fillings to obtain the maximum dry density and optimum moisture content. The compaction work per unit volume applied in the standard heavy compaction test is 2684.9 kJ/m\({}^{3}\). With analysis of the compaction test data, the maximum dry density \(\rho_{d\text{max}}\) is approximately equal to 2.40 g/cm\({}^{3}\), while the optimum moisture content is nearly 5.3%.  
The fillings are prepared to have the optimum moisture content to fill the test model. The model is filled by four layers in a layer-by-layer manner. The thicknesses of the four layers are 20 cm, 20 cm, 25 cm, and 25 cm respectively from bottom to top. Three models with different compaction degrees are constructed as a comparison with each other. The compaction degree \(K\) is defined by the following equation:  
\[K=\frac{\rho_{d}}{\rho_{d\text{max}}} \tag{1}\]  
where \(\rho_{d}\) is dry density of the filling model. The compaction degree of these three models and the corresponding filling mass for each structural layer are listed in Table 1. After each layer is filled into the model, it is compacted by artificially ramming to finish the construction.  
As soon as construction is finished, ground coefficient \(K_{30}\) for each model is precisely measured by load settlement test. It should be noted that ground coefficient \(K_{30}\) refers to the ratio of dead load corresponding to fiducial settlement to fiducial settlement. Generally, fiducial settlement is set as 1.25 mm. \(K_{30}\) is widely adopted in high-speed railway construction in Japan and China. The relationship between settlement and load level is measured for these three models, as shown in Figure 4. According to the test data, ground coefficient \(K_{30}\) can be calculated as 136 MPa/m, 224 MPa/m, and 373 MPa/m for the models with compaction degree of 0.9, 0.95, and 1.0 respectively. It is remarkable that the model with higher compaction degree is inclined to have a larger value of \(K_{30}\)  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Model No.** & **Compaction Degree** & **Layer** & **Thickness (cm)** & **Filling Mass (kg)** \\ \hline Model 1 & 0.9 & Lower two layers & 20 & 222 \\  & & Upper two layers & 25 & 278 \\ Model 2 & 0.95 & Lower two layers & 20 & 235 \\  & & Upper two layers & 25 & 293 \\ Model 3 & 1.0 & Lower two layers & 20 & 247 \\  & & Upper two layers & 25 & 309 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Compaction degree for each model and filling mass for each layer.  
Figure 4: The relationship between settlement and dead load level in \(K_{30}\) test.