## 3 Results and Discussion  
### Effect of Variation in Fly Ash-Stabilized loess on Afterberg Limits  
Afterberg limits, namely, liquid limit, plastic limit, and plasticity index, are used to study the activity and the frost susceptibility of fine grained soil. The plastic limit is the limit between the semi-solid and plastic state, and the liquid limit separates the plastic state from liquid state [63]. The plasticity index is the difference between the liquid and plastic limits [64]. Table 5 shows the effects of the addition of various FA ratios on the liquid limit, plastic limit, and plasticity index of the loess.  
In comparison with the untreated loess, the plasticity index of loess increased significantly by 21.90%-34.79%, whereas the liquid limit of loess only increased slightly by 8.01%-11.01% at the FA ratios from 10% to 20%. The plasticity index of FA-treated soils increased mainly due to the decrease in plastic limit [65]. The plastic limit of loess first slightly decreased by 4.15%-9.83% in soil exposed to FA from 10% to 20% compared with those of the untreated loess (Table 5). This initial increment might be due to the addition of FA of fine particles, thereby resulting in the soil becoming similar to a fine material and increased water holding capacity of loess.  
However, the liquid limit showed a decreasing trend with the increase in FA ratio. A decrease in liquid limit (5.24%) was observed as FA ratio increased to 30% compared with FA ratio increased at 20% (Table 5). The reduction in liquid limit was attributed to that FA created an excellent bonding between the loess and FA particles because the very fine particles of the FA were slightly smaller than the particle size of loess [66]. This condition resulted in variations in grain and pore size distributions [67]. Specifically, the soil particles of natural loess were affected by the reduction in the specific surface area of the FA particles, thereby decreasing the fine particle surface adsorption of water [68]. The increment in plastic limit was also observed, which increased by 17.96% in soil exposed to 30% FA compared with soil exposed to 20% FA (Table 5). The above-mentioned results indicated that the reduction in liquid limit with the increment in plastic limit reduced the plasticity index of the loess with FA piles.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**FA (\%)** & **Liquid Limit (\%)** & **Plastic Limit (\%)** & **Plasticity Index** \\ \hline
0 & 35.69 & 19.02 & 16.67 \\
10 & 38.55 & 18.23 & 20.32 \\
20 & 39.62 & 17.15 & 22.47 \\
30 & 37.54 & 20.23 & 17.31 \\ \hline \hline \end{tabular}
\end{table}
Table 5: The effects of the addition of various FA ratios.  
A decrease in plasticity index (22.96%) was observed as FA ratio increased to 30% compared with FA ratio increased at 20% (Table 5). This result agreed with the previous conclusions that the increase in percentage of FA decreased the plasticity index and liquid limit of the treated soils [46; 66; 69; 70; 71; 72]. The decrease in plasticity index of specimens might be due to cementitious reactions from the FA fraction, which dissociated CaO and formed cementitious and pozzolanic gels [73] and, thus, produced a cement-stabilizing effect that reduced loess plasticity [74]. Thus, the addition of high percentages of FA would result in improved workability of soils. Interestingly, an opposite trend was observed in the previous studies that showed a constant increase or no difference with any ratio of FA [52; 72].  
On the contrary, the liquid limit, plastic limit, and plasticity index were insignificantly changed for all stabilized samples compared with those of the untreated loess (Table 5). This result was consistent with that of a recent study [52]. Thus, the effect of FA treatment on the plasticity index of Shaanxi soil was very small. The reason was that the low plasticity of Shaanxi soil (16.67; Table 1) led to that FA slightly influenced on its plasticity index [52].