###### Abstract  
In South Korea, Honam High-Speed Railway has a relatively large residual settlement issue and high fines content has been pointed out as one of the causes. Design guidelines regulate not to use soils containing fines content higher than 25%. However, there is no background information on the effect of fines content on settlement. Therefore, this paper aims to investigate compressional behavior according to fines content using sand and kaolinite. Oedometer test results showed that the compression index is lowest with fines content of 15% to 20% at which the mixture produced maximum density. The optimum fines content for inducing low settlement would be 15% to 20% for the sand-kaolinite mixture. Transition fines content (TFC), which shows sand-like to claylike behavior, was observed to have between 21% and 26% of fines content. Critical fines content (fcrit) where a minimum void ratio occurs was estimated as 21.67%. These behavioral changes appear when fines content is greater than the optimum fines content. SEM also shows that the kaolinite particles were overlapped, creating flat surfaces with a fines content higher than 30%, and showing clay-like behavior. Based on the analysis results, engineers can simply identify the behavior of embankment materials to ensure optimum fines content and consequently minimize long-term settlement potential. compaction; oedometer; intergranular void ratio; transition fines content; compression index; optimum fines content +
Footnote †: journal: _Sciences_  
soils. The behavior of sand-silt mixtures has been investigated by researchers for low, intermediate, and high contents of fine particles [3; 4; 5]. [PERSON] et al. [6] carried out a laboratory study to observe the effect of fines (kaolinite, crushed silica fines, and 70-140 silica sand) and gradation on the collapse behavior of loosely compacted soil mixtures. The experimental results showed that an increase in fines content has a noticeable effect on the undrained condition at large strains, while the gradation variation of the sand seems to have a minor effect on the undrained condition. [PERSON] et al. [7] investigated the mechanism of liquefaction for the artificial mixture of sand with various clay contents by means of a ring shear apparatus. They could divide liquefiable and non-liquefiable clayey soils according to plasticity index. Moreover, the increase of clay content (bentonite clay) increased the liquefaction resistance. [PERSON] and [PERSON] [8] conducted oedometer tests on kaolin-sand mixtures to investigate compression behavior based on transition fines content (TFC). The results showed that when the fines content was below the TFC, the mixture behaves like sand-silt mixtures while, in the range exceeding the TFC, silt controls the compression behavior. [PERSON] et al. [9] carried out undrained monotonic triaxial tests on sand with non-plastic silt. The outcomes indicated that undrained shear strength can be linked to the fines content, intergranular void ratio, and saturated hydraulic conductivity. [PERSON] et al. [10] studied the effects of low-plastic silt content on geotechnical properties based on the static triaxial, cyclic triaxial, and resonant column tests. Their results indicated that an increase in silt content caused an increase in cohesion and a decrease in the internal friction angle, cyclic stress ratio, and maximum shear modulus. [PERSON] et al. [11] performed drained and undrained triaxial compression tests with the specimens of sands with low plastic fines content. The results showed that an increasing fines content gradually causes a high compressibility of sand-silt mixture. Judging by the aforementioned studies, most of the research has been concerned with the overall effect of fines content on simple artificial mixtures. In South Korea, embankment materials consist of low-plastic fines. In this sense, sand and kaolinite have been selected to simulate embankment materials in a simple and systematic way.