# Study on Deformation Characteristics of Low-Highway Subgrade under Traffic Load
## 2 Dynamic Triaxial Test of Subgrade Soil under Traffic Load
### Testing Scheme  
The testing scheme is listed in Table 2. It can be seen that the tests aim to investigate the deformation mechanism of subgrade soil under traffic load. The test is designed according to the wheel-axle load and subgrade depth, affecting the subgrade deformation.  
The amplitude of traffic load in this test was determined by axle-load spectrum. This test considered the axle-load distribution of three different types of single axle, double axle, and triple axle. According to Bin-Lai Expressway's traffic-volume investigation results, each type was converted into standard axle times, and each type was converted into a vehicle with corresponding weight. It was found that the peak area the of axle-load spectrum occurred in axle loads of 50 kN, 130 kN, and 240 kN, respectively.  
Figure 3 is the schematic diagram of subgrade stress under traffic load. The additional stress of traffic load is related to the single-wheel load and the depth of the subgrade soil layer. The calculation formula of additional subgrade stress caused by wheel-concentrated load is shown in Formula (1). In the consolidation process, the hydraulic conductivity of the alluvial fill will change significantly. The vertical stress of the subgrade under different depths and different axial loads calculated by Formula (1) is shown in Table 3.  
\[\sigma_{Z}=\alpha P_{1}/Z_{n} \tag{1}\]  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Maximum Dry** & **Liquid Limit (\%)** & **Plastic Limit (\%)** & **Plasticity Index** & \
\begin{tabular}{c} **Optimum Water** \\ **Content (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Cohesion (kPa)** \\ **Angle (°)** \\ \end{tabular} \\ \hline
1.76 & 36.96 & 18.20 & 8.70 & 17.03 & 4.59 & 30.78 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic material parameters of subgrade soil.  
Figure 2: Particle size distribution curves.  
where \(\alpha\) is the stress coefficient, the reference value is 0.4775; \(P_{1}\) is the concentrated load converted from the wheel load; \(Z_{n}\) is the buried depth of the action point causing additional stress under the action of the centerpoint of the concentrated load of the road wheel.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Test** & **Borrow** & **Confining** & **Wheel-Axle** & **Dynamic** & **Frequency** \\
**Number** & **Depth (m)** & **Pressure (kPa)** & **Weight (kN)** & **Stress (kPa)** & **(Hz)** \\ \hline I-1 & - & - & 50 & 4.2 & \\ I-2 & 0.5 & 14 & 130 & 11.0 & \\ I-3 & - & - & 240 & 20.2 & \\ \hline II-1 & - & - & 50 & 1.2 & \\ II-2 & 1.5 & 24 & 130 & 3.2 & \\ II-3 & - & - & 240 & 6.0 & \\ \hline III-1 & - & - & 50 & 0.6 & \\ III-2 & 2.5 & 34 & 130 & 1.5 & \\ III-3 & - & - & 250 & 3.0 & \\ III-1 & 0.5 & 14 & - & - & \\ III-2 & 1.5 & 24 & - & - & \\ III-3 & 2.5 & 34 & - & - & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Testing scheme.  
Figure 3: Schematic diagram of wheel action.  
The subgrade height of the low expressway is generally controlled within 5 m. Therefore, the subgrade depths are 0.5 m, 1.5 m, and 2.5 m. The vibration frequency reflects the speed of vehicles on the road. This test refers to the statistical results of traffic-load frequency in the relevant literature, and the vibration frequency of traffic load in this test is set as 1 Hz.