## 3 Results and Discussion
### Vibration Isolation of the Composite Subgrade Structure after Freeze-Thaw Cycles  
The main reason for road diseases in seasonally frozen areas is that the stability and durability of the subgrade material decreases after freeze-thaw cycles, which results in the increase of soil subsidence deformation and uneven sinking of the road. It is necessary to ensure the anti-freeze-thaw performance of the composite subgrade structure in seasonally frozen areas. The acceleration peak value of the composite subgrade structure embedding 5 cm XPS plates after 1, 2, 3, 4, and 5 freeze-thaw  
Figure 8: The isolation coefficient of the composite subgrade structure under different falling heights. cycles were measured. The vibration isolation analyses in the three main axes were conducted as well as the comparative analysis with G1 and G2. The results are demonstrated in Figure 9.  
From Figure 9, the acceleration peak values of the three test groups fluctuated with the increase in the number of freeze-thaw cycles, especially G1 (filled by silty clay). To better describe the fluctuation characteristic of the acceleration peak, statistical analysis was performed to calculate the mean, vibration amplitude range, and standard deviation of G1, G2, and G3. The results are listed in Table 5.  
Among the three test groups, the mean values of G2 in the three directions were all the largest, which means that the vibration isolation of the modified soil was poor and the reason has been explained in Section 3.1. However, the standard deviations and fluctuation ranges of G2 were small, \begin{table}
\begin{tabular}{c c c c c} \hline
**Direction** & **Type** & **Mean (g)** & **Range (g)** & **Standard Deviation (g)** \\ \hline \multirow{4}{*}{X-direction} & G1 & 0.278 & 0.188 & 0.074 \\  & G2 & 0.439 & 0.103 & 0.041 \\  & G3 (5 cm XPS) & 0.179 & 0.185 & 0.070 \\ \multirow{4}{*}{Y-direction} & G1 & 0.399 & 0.109 & 0.044 \\  & G2 & 0.435 & 0.160 & 0.059 \\ \multirow{4}{*}{Z-direction} & G3 (5 cm XPS) & 0.211 & 0.251 & 0.093 \\  & G1 & 2.406 & 0.878 & 0.376 \\ \cline{1-1}  & G2 & 2.566 & 0.370 & 0.135 \\ \cline{1-1}  & G3 (5 cm XPS) & 0.355 & 0.364 & 0.139 \\ \hline \end{tabular}
\end{table} 
Table 5: The static results of the three test groups. Figure 9: Acceleration peak of the three test groups after freeze–thaw cycles. (**a**) Acceleration peak in the X-direction; (**b**) Acceleration peak in the Y-direction; (**c**) Acceleration peak in the Z-direction. which represents that its stability of freeze-thaw resistance was great. Comparing the results of G1 to G2, after five freeze-thaw cycles, the acceleration peak in the Z-direction of G2 was close to that of G1. This means that the vibration isolation performance of G2 was similar to G1 after five freeze-thaw cycles. However, the dynamic response stability of freeze-thaw resistance for G2 was far better than that of G1, and indicates that the modified soil performed better than the silty clay when they underwent the freeze-thaw cycles, which is the reason why the modified soil is selected as a better subgrade filler in road engineering in seasonally frozen areas. The mean and range of G3 in the Z-direction was the smallest. The mean value decreased by 86.2% (2.211 g) when compared with G2. The range and standard deviation were basically consistent with G2, decreasing by 1.6% (0.006 g) and increasing by 2.9% (0.004 g), respectively. These data fully prove the high efficiency and stability of the vibration isolation effect of the composite subgrade structure. From the above results, it can be concluded that the vibration isolation of the composite subgrade structure after several freeze-thaw cycles was still significant, especially in the Z-direction. The isolation coefficients of the composite subgrade structure after the freeze-thaw cycles are described in Figure 10. The isolation coefficients in the X-direction and Y-direction fluctuated drastically, while the isolation coefficient in the Z-direction maintained relatively stable.