## 2 Field Testing
### Analysis of Soil Pressure Amplitude on the Pipe Top and Dynamic Strain Amplitude of the Pipeline  
The variation of dynamic earth pressure amplitude at the pipe top at various compaction times is shown in Figure 4. The figure shows that with increasing filling depth, the amplitude of earth pressure on the pipe roof decreased, indicating that the influence of dynamic load on the pipeline decreases with an increase in the buried depth of the pipeline. After the ninth compaction, the soil pressure on the pipe roof in Condition 3 was close to that in Condition 2, and the increase of the buried depth of the pipeline had little contribution to the decrease of the top pressure of the pipeline. Therefore, a 1.8 m burial depth can be taken as the optimal depth for the project. The amplitude of dynamic earth pressure on the pipe top is affected by compaction times. Meanwhile, the vertical acceleration of the vibratory roller in Condition 2 presented the same trend as the pressure on the pipe top, and its amplitude fluctuated with the compaction times, as shown in Figure 5. At the first compaction, the filling material was relatively loose, and the vibration energy of the vibratory roller was mainly dissipated in the form of plastic deformation energy storage of the filling material. At the second compaction, the filling material became dense, and the plastic energy storage was reduced, so the acceleration amplitude showed a rebound phenomenon. However, at the third compaction, the soil was over-compacted, and the filling material was re-compacted to absorb energy, so the acceleration amplitude decreased. The same trend between acceleration and pipe top pressure indicates that the pipe top pressure increases with the increase of backfill compaction degree. It is noted that at the ninth compaction, the vibration acceleration amplitude reached the maximum value of 5.178 g, indicating that the filling material had reached another better compaction state. The minimum vibration acceleration amplitude measured at the tenth compaction also confirmed this. Due to the large distance between the pipe top and the ground, in Condition 3, the pipe top soil pressure showed the same change trend between several compactions and was not as sensitive to the compaction  
Figure 4: Amplitudes of pipe roof soil pressure. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Working Condition** & **Test Content** & **Mode of Vibration** & **Compaction Times** & **Buried Depth of Pipeline** \\ \hline Condition 1 & Pipeline strain, earth pressure, & Weak vibration mode & 10 & 1.5 m \\ Condition 2 & \
\begin{tabular}{c} Pipeline strain, earth pressure, \\ acceleration of the vibratory roller \\ \end{tabular} & Strong vibration mode & 12 & 1.8 m \\ Condition 3 &
\begin{tabular}{c} Pipeline strain, earth pressure \\ \end{tabular} & Strong vibration mode & 10 & 2.1 m \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Field vibration compaction test conditions. times as the other two conditions. The maximum and minimum values of the pipe top pressure amplitude in Condition 1 were 34.56 kPa, which was much larger than that in Condition 2 and Condition 3. This indicates that when the pipe is shallowly buried, the pipe top pressure is significantly affected by the compaction degree. The maximum and minimum values of the amplitude of the pipe top pressure in Condition 1 differed by 34.56 kPa, which was much larger than that in Condition 2 and Condition 3. This indicates that when the pipeline is shallowly buried, the pipe top pressure is significantly affected by the compaction. Figure 6 shows the amplitude values of soil pressure at different burial depths under Condition 2. It can be seen from the figure that the variation trend of the amplitude value of the #4 soil pressure cell closest to the ground is opposite to that of the other three soil pressure boxes, which is due to the fact that the surface filling soil is compacted before the bottom filling material.