## 4 Summary and Conclusions  
Based on the fact that embankment materials in South Korea contain low plastic fines, kaolinite was selected as a fine material. Sand represents granular materials of the embankment to simplify the testing matrix. Sand-kaolinite mixtures were tested by changing the fines content. The basic material properties gradually vary as fines content increased. Maximum dry density tended to decrease according to fines content; however, it slightly increased at fines content of 15% and 20%. One-dimensional compression tests (i.e., oedometer tests) provided details on settlement behavior in terms of fines content. To categorize behavior of sand-kaolinite mixture, the transition fines content (TFC), defined as an intergranular void ratio in which the maximum void ratio of the host material (i.e., sand) is equal to, was adopted. SEM images were taken and analyzed to investigate the microscale grain network of the sand-kaolinite mixtures. The findings of this study can be summarized as follows:  
1. An increase in fines content caused an increase in both void ratio and compression index. However, settlement-related properties such as \(e\), \(C_{c}\), and \(C_{c<s}\) decreased or showed the smallest values. Maximum dry density of 15% and 20% of fines content was also the greatest. 2. Presence of fine materials at a certain amount, 15% and 20% of kaolinite in this study, played a role in helping ensure better compaction. Therefore, appropriate fines content of embankment materials resulted in less deformation. 3. With the mixture of 10% KC, the smooth surface of a sand particle under kaolinite particles can be seen in the SEM images. However, in the mixture with up to 25% kaolinite content, the kaolinite particles were found to take over the sand grains. Moreover, the kaolinite particles were overlapped, creating large flat surfaces with the fines content higher than 30%, and inducing the claylike behavior of mixtures. 4. Transition fines content of sand-kaolinite mixture was about 21% to 26%. Different types of behavior were identified: a transition zone (21% \(\leq f_{c}\leq\) 26%), which was  
Figure 12: Overall behavioral of the mixture. a behavioral change point of the mixture; sand-like behavior (\(f_{c}<15\%\)), which was when the mixture behaved like sand; clay-like behavior (\(f_{c}\geq 30\%\)), which was when the mixture behaved like clay, and an optimum fines content (\(15\%\leq f_{c}\leq 20\%\)) which induced a low compression index. Based on oedometer tests and compaction tests, \(15\%\) to \(20\%\) of fines content of sand-kaolinite mixture is the most appropriate fines content for inducing the least settlement. Transition fines content, critical fines content, optimum fines content (mentioned above) were similar to those of \(e\), \(C_{c}\), and maximum dry density. Therefore, the best fines content or transition fines content can be approximately predicted by following these approaches: (1) simple compaction test with method D, (2) TFC by laboratory testing, and (3) calculation of the critical fines content. However, this approach should be verified through more laboratory and field testing with actual embankment materials used in South Korea. Consolidation settlement and long-term settlement should also be investigated according to fines content. Based on this research, the design criteria on materials, especially fines content as embankment materials, can be suggested to control residual settlement. Conceptualization, Y.-T.C. ; Data curation, Y.-T.C. ; Funding acquisition, S.-H.L. ; Investigation, P.K. and Y.-T.C. ; Methodology, C.-Y.Y. ; Project administration, S.-H.L. ; Supervision, S.-H.L. and C.-Y.Y. ; Validation, C.-Y.Y. ; Writing--original draft, P.K. ; Writing--review & editing, Y.-T.C. All authors have read and agreed to the published version of the manuscript.