## 3 Results
### Analysis of Interactive Factors
#### 3.3.2 Analysis of Interactive Factors on Internal Friction Angle  
The interactive impact of two factors on internal friction angle is shown in the contour maps in Figure 4. As illustrated by Figure 4a, the internal friction angle decreased with the increase in clay content and moisture content when the soil density was 2.3 g cm\({}^{-3}\). Similar to cohesion, the maximum internal friction angle was obtained when clay content was within 36-40% (the coded value within 1-1.682) and moisture content within 30-34% (the coded value within \(-\)1.682-1). However, the contour map suggested that the moisture content has a greater impact on the internal friction angle than clay content, which was opposite to their effects on cohesion, as discussed in Section 3.3.1.  
When the moisture content was kept constant at the value of 40%, the internal friction angle increased with both the clay content and density, as shown in Figure 4b. The  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Clay/\%**} & \multirow{2}{*}{**Moisture/\%**} & \multirow{2}{*}{**Density/**} & \multirow{2}{*}{**Cohesion/kPa**} & \multirow{2}{*}{**Friction Angle/\({}^{o}\)**} \\ \cline{4-7}  & & & & **g\(\cdot\)cm\({}^{-3}\)** & & & **MV** & **PV** & **MV** & **PV** \\ \hline
1 & 22.3 & 35 & 2.1 & 11.3 & 11.9 & 22.6 & 22.2 \\
2 & 22.8 & 40 & 2.6 & 17.5 & 11.1 & 32.6 & 26.7 \\
3 & 23.5 & 40 & 2.0 & 15.2 & 9.9 & 23.9 & 19.3 \\
4 & 26.1 & 40 & 2.0 & 10.7 & 10.0 & 20 & 26.2 \\
5 & 30.8 & 35 & 2.3 & 21.6 & 17.3 & 30.2 & 39.1 \\
6 & 30.8 & 40 & 2.6 & 28.3 & 15.2 & 10.3 & 9.3 \\
7 & 36.0 & 35 & 2.3 & 25.3 & 28.0 & 30.2 & 30.7 \\
8 & 36.0 & 40 & 2.6 & 21.3 & 25.9 & 14.3 & 21.2 \\ \hline Average & & & & 18.9 & 16.2 & 20.0 & 24.3 \\ PBIAS & & & & 15\% & & \(-\)6\% & \\ RMSE & & & & 2.05 & & 1.94 & \\ RRMSE & & & & 11\% & & 8\% & \\ \hline \hline \end{tabular}  
* MV: measured value; PV: predicted value; PBIAS: percent of bias; RMSE: root mean square error; RRMSE: relative root mean square error.  
\end{table}
Table 5: The measured and predicted values of the shear parameters model.  
maximum internal friction angle was reached when clay content was between 36 and 40% (coded value 1-1.682) and density was between 2.6 and 2.8 g\(\cdot\)cm\({}^{-3}\) (coded value 1-1.682). Meanwhile, as compared to clay content, the wet bulk density had a greater impact on internal friction angle.  
Figure 4c implies that the internal friction angle decreased with the increase in moisture content and density when the clay content was 31%. The maximum internal friction angle was obtained when the moisture content and density were within 30-34% (coded value \(-\)1.682\(\sim\)\(-\)1) and 2.6-2.8 g\(\cdot\)cm\({}^{-3}\) (coded value 1-1.682), respectively. Moreover, the variation rates of the internal friction angle in different directions suggest that the soil moisture content has a greater impact on the internal friction angle than density.  
Table 4 shows that the contribution rates of individual factors to the internal friction angle from high to low were \(X_{1}\), \(X_{2}\), and \(X_{3}\) in turn, while the impact of two interactive factors on the internal friction angle from high to low was ordered \(X_{1}X_{2}\), \(X_{2}X_{3}\) and \(X_{1}X_{3}\). However, all three combinations of the interactive impact on the internal friction angle were non-significant (\(p>0.1\)).