## 3 Results and Discussion
### The Analysis on the Vibration Isolation Effect of Test Groups  
To compare the vibration isolation effect of silty clay, the modified soil, and the composite subgrade structure, the relationship curves between the acceleration peak and falling heights of hammer for the three groups were drawn, which are depicted in Figure 5. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Height of the Fall**} & \multicolumn{3}{c}{**5 cm**} & \multicolumn{3}{c}{**10 cm**} & \multicolumn{3}{c}{**15 cm**} \\ \cline{2-10}  & **X-** & **Y-** & **Z-** & **X-** & **Y-** & **Z-** & **X-** & **Y-** & **Z-** \\ \hline G1 & 0.1908 & 0.1606 & 0.9825 & 0.194 & 0.2238 & 1.3739 & 0.233 & 0.2815 & 1.621 \\ G2 & 0.1818 & 0.272 & 1.2744 & 0.203 & 0.2873 & 1.7267 & 0.2815 & 0.2871 & 2.0253 \\ G3 (3 cm XPS) & 0.0848 & 0.1481 & 0.1565 & 0.1396 & 0.2422 & 0.2819 & 0.1879 & 0.321 & 0.4096 \\ G3 (5 cm XPS) & 0.0819 & 0.094 & 0.0998 & 0.1213 & 0.1542 & 0.1737 & 0.1427 & 0.251 & 0.2589 \\ G3 (9 cm XPS) & 0.076 & 0.082 & 0.0767 & 0.088 & 0.1546 & 0.1027 & 0.1335 & 0.203 & 0.1253 \\ \hline \multirow{2}{*}{**Height of the Fall**} & \multicolumn{3}{c}{**20 cm**} & \multicolumn{3}{c}{**25 cm**} & \multicolumn{3}{c}{**25 cm**} \\ \cline{2-10}  & **X-** & **Y-** & **Z-** & **X-** & **Y-** & **Z-** & & \\ \hline G1 & 0.2875 & 0.2964 & 1.8718 & 0.2756 & 0.3539 & 2.1532 & & \\ G2 & 0.336 & 0.3693 & 2.4321 & 0.3903 & 0.4507 & 2.7335 & & \\ G3 (3 cm XPS) & 0.2242 & 0.354 & 0.4894 & 0.2905 & 0.369 & 0.5692 & & \\ G3 (5 cm XPS) & 0.1907 & 0.2752 & 0.2962 & 0.212 & 0.2968 & 0.3584 & & \\ G3 (9 cm XPS) & 0.164 & 0.257 & 0.1647 & 0.176 & 0.2814 & 0.205 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Peak value of acceleration under different test conditions. (Unit: g). Figure 5: Acceleration peak values of the three test groups. (**a**) Acceleration peak in the X-direction; (**b**) Acceleration peak in the Y-direction; (**c**) Acceleration peak in the Z-direction. From Figure 5, the acceleration peak values in the three main axes (X, Y, and Z-direction) for G1, G2, and G3 embedding different thicknesses of XPS plates rose as the falling heights increased. Comparing the data of G1 to G2, the acceleration peak values of G1 were lower than that of G2, especially in the Z-direction. This means that the vibration isolation performance of the modified soil was worse than that of silty clay. The reason for this phenomenon is that fly ash reduces the damping of improved soil [22], while rubber particles increase the damping of modified soil. The strengthening effect of 2% rubber particles is very limited. The 32% content of fly ash dominates the change of damping of modified soil, which leads to the phenomenon that the damping of modified soil is less than that of silty clay and the vibration amplitude of modified soil is larger than that of silty clay. Thus, it is necessary to improve the vibration isolation performance of the modified soil subgrade. Just as shown in Figure 5, the acceleration peak values of G3 were basically lower than that of G1, except for the accelerations of G3 with XPS plates of 3 cm in the X-direction and Y-direction. The results prove the viability of using XPS plates to improve the vibration isolation effect of the modified soil, especially using the thicker XPS plates. This phenomenon is attributed to the composition and structure of foam boards. Both EPS and XPS plates are made of polystyrene foam. The large amount of pores in the foam plate are beneficial to absorb the vibration and fluctuation [16; 23]. The perfect closed-cell honeycomb structure makes the XPS plates have excellent stability, which is an important reason as to why it can be used as a light subgrade material. Furthermore, it was noted that the acceleration peak values in the Z-direction were far higher than that of the X-direction and Y-direction. For road engineering, the vibration generated by traffic loads would propagate downward in the pavement structure.