# Engineering Properties and Environmental Impact of Soil Mixing with Steel Slag Applied in Subgrade
## 3 Results and Discussion
### Determination of Optimum Proportion (SL/S) between SL and Intact Soil
#### 3.1.3 CBR Test  
The CBR values of the test soils (S1 and S2) mixed with SL after 96 h-immersion are shown in Figure 8. From Figure 8, it can be seen that the CBR values of the test soil specimens were 15%-60%. According to the requirements of the Chinese specifications on subgrade design, the roadbed on expressways and Class I roads is greater than 8%, that on Class II roads is 6%, and that on Class III and IV roads is 5%. The CBR values of the test soil specimens (S1 and S2) before improvement were 5% and 22%, respectively, which have a certain bearing capacity, but the water stability was poor. After adding SL, the CBR values of the test soil improved significantly, and the water absorption gradually reduced, which ensures the strength and stability of the test soil used as roadbed for high-grade roads, and the embankment and subgrade could also meet requirements. A higher CBR value means a higher bearing capacity. Therefore, soil mixed at a proportion of 50% SL was the best choice.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Sample Type**} & \multicolumn{1}{c}{**SL Content (\%)**} & \multicolumn{1}{c}{**Liquid Limit Water**} & \multicolumn{1}{c}{**Plastic Limit Water**} & \multicolumn{1}{c}{**Plasticity Index**} \\  & & **Content (\%)** & **Content (\%)** & **Plasticity Index** \\ \hline \multirow{4}{*}{S1} & 0 & 43.7 & 31.5 & 12.2 \\  & 30 & 38.1 & 26.5 & 11.6 \\  & 50 & 36.0 & 25.2 & 10.8 \\  & 70 & 25.2 & 19.1 & 6.1 \\ \hline \multirow{4}{*}{S2} & 0 & 59.6 & 27.7 & 31.9 \\  & 30 & 43.7 & 26.7 & 17.0 \\ \cline{1-1}  & 50 & 34.4 & 24.9 & 9.4 \\ \cline{1-1}  & 70 & 29.3 & 20.0 & 9.3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Limit water content of soil mixed with different SL contents.  
Figure 7: The plastic zones of soil specimens in this study.  
The CBR of the test soil mainly came from materials and structures, and there was little hydration of active substances. With the addition of SL, the CBR of the improved soil SSL first increased and then decreased due to its structural composition. The addition of SL makes the soil and SL form a reliable and stable structure. However, with excessive SL, this balance is broken, leading to a decrease in strength and the \"inflection point\", as shown in Figure 8a. In addition, it was observed that with the increase of compactness, the CBR value also increased, and the water absorption and expansibility decreased, making the subgrade structure more stable. From Figure 8b, it can be seen that the swelling capacity decreased with increasing steel slag content.  
To summarize, the compaction characteristics, Afterberg limit value and CBR values showed that the improved soil mixed with SL (i.e., SSL) performed well, especially the SSL with 50% SL which had the highest CBR strength, and can solve the potential stability problems caused by the high liquid limit of intact soil. Considering the economic and compaction characteristics, the improved soil mixed with 50% SL also had the characteristics of a high utilization rate of SL, which was selected for the following study.