## 2 Field Testing
### Instrument Layout and Test Plan  
A trapezoidal ditch was excavated for pipeline burial in the original subgrade with a slope of 1:0.33. The compactness of backfill soil followed regulatory standards [24], as shown in Figure 1. The circumference of the pipe was filled with stone chips and stone powder, and 50 cm of stone powder was used as fill above the pipe, followed by graded gravel and soil-rock mixture. The thickness of each layer was 30 to 40 cm after compaction. The design elevation of the pavement was 46.02 m, the design elevation of the pipe bottom was 42.71 m, and the buried depth of the pipe was 3.31 m, as shown in Figure 2.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type of Pipeline** & **Inside Nominal Diameter** & **Elastic Modulus** & **Poisson’s Ratio** & **Circumferential Stiffness** & **Density** \\ \hline Double-wall corrugated pipe & 800 mm & 800 MPa & 0.4 & 10 kN/m\({}^{2}\) & 950 kg/m\({}^{3}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic parameters of HDPE pipe used in testing.  
Figure 1: Trench soil backfill requirements.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mode of Vibration** & **Vibration Frequency** & **Vibration Amplitude** & \
\begin{tabular}{c} **Excitation Force** \\ **Amplitude** \\ \end{tabular} &
\begin{tabular}{c} **Static Pressure of the** \\ **Roller** \\ \end{tabular} \\ \hline Weak vibration mode & 33 Hz & 0.93 mm & 290 kN & \\ Strong vibration mode & 28 Hz & 1.86 mm & 374 kN & 110 kN \\ \hline \hline \end{tabular}
\end{table}
Table 2: Basic parameters of vibratory roller used in testing.  
Four strain gauges were arranged at the top and sidewall of the pipe. The strain gauge arrangement is shown in Figure 3. A total of five earth pressure cells were arranged. The #1 cell was arranged at the top of the pipeline, and the #2 earth pressure cell was placed 80 cm above the #1 cell. The #3 cell was placed in the soil-rock mixture layer, 30 cm away from the #2 cell. The #4 cell was placed at 40 cm above #3, and the #5 cell was installed after laying two layers of an earth-rock mixture, 30 cm away from the #4 cell. The detailed layout is shown in Figure 2.  
To analyze the mechanical properties of buried pipelines under different buried depths and dynamic load frequencies, vibration compaction tests with different frequencies were carried out in three stages of backfill, which are shown in Table 3. The vibration frequency under the strong vibration mode was 33 Hz, and under the weak vibration mode was 28 Hz. Taking Condition 1 as an example, the roller advanced at a speed of 2.63 km/s from 15 m away from the measuring point in the weak vibration mode and stopped advancing and vibrating 3 m away from the measuring point. When the signal was stable, the roller reversed and returned to the starting point. To fully compact the filler, the process was repeated ten times. The soil dynamic stress amplitude and pipeline dynamic strain amplitude represent the most dangerous state of the pipeline, and, thus, the test focused on monitoring the dynamic pressure and pipeline strain amplitude under different test conditions.  
Figure 3: Layout of pipeline strain measuring points.  
Figure 2: Layout of earth pressure cells in the filling soil.