## 3 Test Results and Analysis
### Time-Frequency Characteristic Analysis
According to the comprehensive analysis of Figure 12, it can be found that the evolution characteristics of the vibration signal can be better described by the Hilbert-Huang transform, and the vibration energy carried by the vibration wave from top to bottom is gradually reduced. During the first pass compaction process, the vibration energy of the wheel varies little from T = 0 s to T = 10.0 s, mainly distributed between the fundamental wave frequency of 20 and  
Figure 9:0 Intrinsic Modal Function (IMF) components and their Fourier spectrums. 30 Hz, and a small amount is distributed between 40 and 50 Hz of the first harmonic wave frequency. The energy carried by each measuring point of the filling layer is concentrated between T = 2 s and T = 3 s when filling materials are loose, and also distributed near the frequency corresponding to the fundamental wave and the first harmonic in the frequency domain. With the increase of compaction times, the density of filling material increases, the vibration energy of the wheel and each measuring point of filling material increases obviously, and the action time increases from 1s to 2 s, but the distribution in the frequency domain does not change much, which shows that the effective rolling time of the vibrating roller increases gradually during the compaction process of the filling material. Based on the Hilbert spectrum, the marginal spectrum of each measurement point is obtained in order to further analyze the distribution of vibration energy on the frequency-amplitude scale. Figure 13 shows the variation of the Hilbert marginal spectrum of each point of wheel and filling materials during the compaction process. It can be found that the vibration energy of each measuring point is mainly concentrated between 10 and 100 Hz, of which the vibration energy of wheel is mainly distributed at the frequency corresponding to the fundamental wave and partially distributed at the frequency corresponding to the first harmonic wave, while the vibration energy has little difference in the distribution of the fundamental wave of the filler and the frequency corresponding to the first harmonic wave. A small amount is distributed in the frequency corresponding to the third harmonic wave. As the number of rolling compaction times increases, the vibration energy carried by the fundamental wave and the first harmonic wave of the wheel increases continuously, similar to the acceleration peak, and tends to be stable after the fifth pass compaction reaches the maximum value. In the filling, the energy carried by the vibration signal began to decrease after the fifth compaction, especially the peak value of the amplitude of the first measuring point, which decreased by about 0.1. It shows that the energy carried by vibration signal gradually transfers from fundamental wave to higher harmonic wave with the increase of filling materials stiffness. When the number of compaction times is increased, the compaction energy increases gradually, but the difference is small with the fifth compaction Figure 9:1 _Cont._  
Figure 9:2 Hilbert energy spectrum of each measuring point in the first pass (left) and the eighth pass (right). Figure 9:3 _Cont._