## 2 Materials and Methods  
### Materials  
In this study, loess was collected from Yangling, Shaanxi Province. The loess sample was excavated at a depth of 3.5 m. The engineering properties of the studied soil on laboratory determination of liquid limit, plasticity index, and particle size characteristics were in accordance with the Specification of Soil Test SL237-1999, the industry standard of P.R. China [60]. This method of soil classification is similar to the American Society for Testing Materials (ASTM). The natural moisture content of Yangling loess, which was obtained using drying method, is 17.42%. Its natural dry density is 1.26 g/cm\({}^{3}\). Its natural void ratio is 1.151, and its natural degree of saturation is 41.9%. The loess was composed of 32.1% sand, 61.4% silt, and 6.5% clay. Its CBR value after being soaked for 4 days is 4.40%, and UCS value after being soaked for four days is 81.51 kPa, which are lower than the allowable value of subgrade filling in China specification. Table 1 shows the physical properties of loess sample used in this investigation.  
Class C FA used in this research was collected from the Xianyang Weihe power plant near Xi'an. The physical property indexes of FA was conducted on the basis of grain size distribution terms SL237-001-1999 [60], which is similar to ASTM D2487, as listed in Table 2. The chemical analysis was done by X-ray fluorescence spectroscopy (Hitachi, Japan), which consists of the excitation of the sample using an X-ray source in accordance with ASTM C618 specification [61]. Table 3 shows the chemical composition of FA. As shown in the table, the FA had CaO content of 21.89% (more than 20%) and could, thus, be considered high calcium FA or Class C FA in accordance with American Association of State Highway Transportation Officials (AASHTO) M 295-11 [50; 62].  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Specific** & **Liquid** & **Plastic** & **Plasticity** & **Natural** & **CBR \({}^{1}\)** & **UCS \({}^{2}\)** & **OMC \({}^{3}\)** & **MDD \({}^{4}\)** \\
**Gravity** & **Limit** & **Limit** & **Index** & **Water** & **Value** & **(kPa)** & **(\%)** & **(g/cm\({}^{3}\))** \\
**G\({}_{\text{S}}\)** & **W\({}_{\text{L}}\) (\%)** & **W\({}_{\text{P}}\) (\%)** & **(I\({}_{\text{P}}\))** & **Content** & **(\%)** & & & \\  & & & & & **(\%)** & & & \\ \hline
2.71 & 35.69 & 19.02 & 16.67 & 17.42 & 4.40 & 81.51 & 18.02 & 1.72 \\ \hline \hline \end{tabular} \({}^{1}\) California bearing ratio, \({}^{2}\) Unconfined compressive strength, \({}^{3}\) Optimum moisture content, and \({}^{4}\) Maximum dry density.  
\end{table}
Table 1: The physical properties of loess sample.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Specific** & \multicolumn{3}{c}{**Constrained Diameter/mm**} & \multirow{2}{*}{**Coefficient of**} & \multirow{2}{*}{**Coefficient of**} & \multicolumn{3}{c}{**Composition of Particles/\%**} \\ \cline{2-2}
**Gravity** & & & & & **Uniformity C\({}_{\text{U}}\)** & & **Curvature C\({}_{\text{C}}\)** & **2-0.075** & **0.075–0.005** & **\textless{}0.005** \\
**Gs** & & & & & & **mm** & **mm** & **mm** \\ \hline
2.07 & 0.055 & 0.19 & 0.005 & 11.00 & 1.31 & 27.0 & 63.0 & 10.0 \\ \hline \hline \end{tabular}
\end{table}
Table 2: The physical property indices of FA.