## 3 Results and Discussion
### Hydraulic Conductivity Results  
In the experimental studies conducted using the injection method, three different soil types (S1, S2, and S3) were improved by applying five different additive ratios--0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5%--of SA, in combination with MICP. The effects of different SA ratios and SA + MICP combinations on the hydraulic conductivity (cm/s) of the S1, S2, and S3 soil specimens improved by the injection method are presented in Figure 7. The results obtained after treatment generally indicate that increasing the SA content significantly reduces hydraulic conductivity. Except for the S3 soil, the hydraulic conductivity values further decreased in the combinations where SA was applied together with MICP. The lowest hydraulic conductivity values were recorded at the highest SA ratios. This suggests that the viscous structure of SA and the calcite precipitation induced by MICP play a complementary role in reducing soil hydraulic conductivity. The obtained data showed that the addition of SA was effective in improving low hydraulic conductivity across all soil groups. As the SA content increased, the gel-like structure narrowed the water flow paths, resulting in a decrease in the hydraulic conductivity coefficient, with this effect becoming particularly evident up to a 1% SA ratio. However, further increases in SA content, in some cases, adversely affected hydraulic conductivity reduction due to the non-uniform distribution of the additive. The addition of MICP to the SA treatment resulted in a further decrease in hydraulic conductivity. The biologically induced calcite precipitates formed by MICP more effectively filled the pores within the soil, significantly restricting water flow. However, in some mixtures, the effect of MICP appeared to be limited, which waslikely due to factors such as insufficient microbial activity, pH imbalance, or inadequate precipitation time. In the experimental studies conducted using the mixing method, three different soil types (S1, S2, and S3) were improved using a combination of SA at 0%, 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% and pellet contents of 5%, 10%, and 15%, along with MICP. Figure 8 illustrates the effects of different SA ratios and 5%, 10%, and 15% pellet contents on the hydraulic conductivity of S1, S2, and S3 soils treated using the mixing method, under both SA and SA + MICP treatments. In general, increasing SA and pellet contents tended to reduce hydraulic conductivity. However, in some cases, the SA + MICP combinations resulted in higher hydraulic conductivity values compared to only SA. This can be interpreted as SA blocking the soil pores more extensively, while MICP, despite inducing mineral precipitation, leaves voids that still allow fluid flow. In the mixtures where only SA was applied to the S1, S2, and S3 soil groups, a general increasing trend in low hydraulic conductivity was observed with increasing SA content. This effect can be attributed to the gel-like structure that SA forms upon contact with water, filling the voids in the soil and narrowing the water flow paths. In the S1 soil group, the lowest hydraulic conductivity  
Figure 7: Hydraulic conductivity results of soil specimens improved by the injection method (S1: fine sand, S2: medium sand, S3: coarse sand). coefficient of 1.9 \(\times\) 10\({}^{-5}\) cm/s was obtained in the mixture containing 1% SA and 10% pellet content. This value indicates that the additive provided an optimal level of low hydraulic conductivity. Although SA also improved low hydraulic conductivity in the S2 and S3 soils, the hydraulic conductivity values obtained were higher compared to those in the S1 soil. In the mixtures where MICP was applied in addition to SA, low hydraulic conductivity became more pronounced. MICP contributed positively to low hydraulic conductivity by producing calcium carbonate through microbial activity, which filled the soil porosity with mineral precipitates.