# Role of Bimodal Water Retention Curve on the Unsaturated Shear Strength
## 5 Discussion  
The saturated volumetric water content of 70S30K is lower than that of 50S50K, as shown in Table 3. It can happen because 70S30K has a lower void ratio, which indicates less amount of void to be filled with water at saturation. The experimental results \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Zone**} & \multirow{2}{*}{**Metric Suction (kPa)**} & **70S30K** & **50S50K** \\ \cline{3-4}  & & \(\phi^{\textbf{b}}\) (°) & \(\phi^{\textbf{b}}\) (°) \\ \hline
1 & \textless{}AEV_{1}\) & 35 & 28 \\
2 & \textless{}AEV_{1}\textless{}AEV_{2}\) & 20 & 17 \\
3 & \textless{}AEV_{2}\) & \textless{}11 & \textless{}10 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: \(\phi^{\textbf{b}}\) angle for different matric sutions. Figure 13: [PERSON] et al. [44] and the proposed equation in this study for predicting shear strength of 50S50K. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **70S30K** & **50S50K** \\ \hline \(AEV\) (kPa) & 15 & 25 \\ \(l_{p}\) (\%) & 12.74 & 18.28 \\ \(c^{\prime}\) (kPa) & 8 & 12 \\ \(\phi^{\prime}\) (°) & 35 & 28 \\ \(n\) & 2 & 1.59 \\ \(\sigma\textless{}\textless{}u_{a}\) (kPa) & 0 & 0 \\ \(y\) & 0.987 & 1.141 \\ \(b\) & 0.889 & 0.908 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Shear strength prediction parameters based on [PERSON] et al. [44] Equation. from this study are in agreement with the previous studies from [PERSON] et al. [12], and [PERSON] et al. [17; 23], who stated that bimodal grain size distribution may lead to bimodal WRC if the soil is compacted in dry optimum with certain percentages of fine and coarse-grained particles. There are two main constituent particles (i.e., medium-sized sand and silt-sized kaolin) for both specimens that can be correlated to two AEVs of the respective specimens. The AEVs and inflection points of the first and second sub-curves of WRC for 70S30K are lower than those of 50S50K (Table 3), indicating a smaller water storage of 70S30K as compared to that of 50S50K. This can occur since 70S30K has larger particle sizes and lower liquid and plastic limits than those of 50S50K. Additionally, it indicates that 70S30K has larger sizes of dominant macropore and micropore than those for 50S50K. [PERSON]'s equation describes that pore sizes of soil are inversely proportional to the ability of soil to retain water at high matric suction [27]. Furthermore, the lower saturated volumetric water content of 70S30K shows that total amount or frequency of pores inside 70S30K are less than those inside 50S50K. At a fully saturated state, water is equivalent to all available pores inside the specimen by assuming water cannot penetrate into solid particles. Based on Table 3, standard deviation of first and second sub-curves of WRC correspond to the width of the dominant macropore and micropore sizes in PSD, respectively. In this research, lower standard deviations are observed in WRC of 50S50K, which has a higher number of fine-grained particles. Therefore, the greater percentages of fine contents, the more unlikely the appearance of two dominant pore sizes in the PSD. Mohr-Coulomb failure envelopes of both specimens (Figure 10) demonstrate that typically 70S30K has a lower effective cohesion intercept than that of 50S50K. One of the possible reasons is that the inter-particle bonding inside 70S30K is weaker than that of 50S50K since 70S30K has a smaller proportion of fine-grained particles. Figures 12 and 13 illustrate that equation from [PERSON] et al. [44] tends to significantly overestimate the measured shear strength of both specimens especially at high matric suctions or beyond \(\text{AEV}_{2}\). The possible reason is that equation from [PERSON] et al. [44] incorporates parameters from the [PERSON] and [PERSON] [43] equation which cannot fit the experimental data of the unsaturated shear strength with bimodal WRC data well. Therefore, the equation from [PERSON] et al. [44] is not applicable to predict the shear strength of soil with bimodal WRC.