## 3 Results and Discussion
### Unconfined Compressive Strength
#### 3.3.1 Effect of Variation in Fly Ash-Stabilized Loess on Stress  
Figure 1 provides information on the UCS stress under submergence and non-submergence conditions at different FA ratios and curing times. With the addition of FA from 0% to 30%, the effect of FA on the UCS stress was significantly great, especially for the FA ratio higher than 20% under the submergence condition. The UCS stress increased by 17.32%-38.27% and 2.07%-38.80% at 0 and 14 days of curing time compared with that of the untreated loess (Figure 1). The impact of the FA ratio on the results was much greater during the curing time of 28 days, and this condition increased UCS by 5.19%-64.56% compared with that of the untreated loess (Figure 1). The aforementioned results indicated that the FA ratio altered the intrinsic soil property. The hydration process and pozzolanic reactions significantly increased the soil strength by producing C-S-H and C-A-H gels through the increase in the amount of CaO in the mixture as the FA ratio increased [52; 76; 80]. The remarkable increment in the UCS stress under submergence conditions might be due to that the UCS stress depended on the chemical compatibilities of FA with plasticity soil with the addition of water [51]. The reduction in the plasticity index with the increase in the FA ratio (Section 3.1) implied an increase in the soil strength because of the inversely proportional relationship between soil strength and plasticity index [69]. This result was consistent with that of other researchers [46; 51; 69; 81]. On the contrary, UCS stress did not change under non-submergence condition. The UCS stress first increased by 2.51% with 10% FA and then slightly decreased with 20% and 30% FA (maximum of 5.84%) compared with that of the untreated loess at zero days of curing time with non-submergence samples (Figure 1). This initial increasing trend indicated that FA-treated loess exhibited a more brittle behavior than the untreated loess. However, the decrease in UCS stress at high FA ratio (20% and 30%) might be due to the excessive amount of FA added, which was probably related to high compaction, water content, and low compaction characteristics; as a result, the specimens failed to stand the amount of stress applied [46; 66]. This change trend was opposite at 14 days of curing time, during which UCS stress first decreased by 3.85%-20.03% with 10% and 20% FA and then slightly increased by 4.55% with 30% FA compared with that of the untreated loess (Figure 1). Specifically, the UCS stress only decreased (3.74%-1.04%) at 28 days of curing time  
Figure 1: The influence of the variation in FA-stabilized loess (FASL) specimen on stress at different curing times under non-submergence and submergence conditions: (**a**) zero days; (**b**) 14 days; (**c**) 28 days; and (**d**) stress increasing. compared with that of the untreated loess (Figure 1). These results suggested that the addition of FA under non-submergence condition slightly affected the UCS stress for the loess. This phenomenon might be due to the uneven distribution of FA without water in the loess-FA mixtures [82]. Therefore, the submergence condition had a greater contribution to the UCS stress due to the addition of FA than the non-submergence condition. Surprisingly, a variation in UCS stress was observed at different FA ratios with the change in curing time (Figure 1). For example, under non-submergence condition, the highest UCS stresses of specimens at 0, 14, and 28 days of curing time were observed with 10%, 30%, and 0% FA, respectively. By contrast, the lowest UCS stresses of specimens at 0, 14 and 28 days of curing time were obtained with 30%, 20%, and 30% FA, respectively. A similar trend was also observed for submergence condition. In particular, the highest UCS stresses of specimens at 0, 14, and 28 days of curing time were observed with 10%, 30%, and 30% FA, respectively. On the contrary, the lowest UCS stresses of specimens at 0, 14, and 28 days of curing time were obtained with 20%, 0%, and 0% FA, respectively.