## 4 Conclusions  
In this study, two different soil improvement methods--injection and mixing--were applied to three different soil types (S1: fine sand, S2: medium sand, S3: coarse sand) with varying particle sizes, and the effects of these methods on the geotechnical properties of the soils were investigated. To evaluate the impact of the improvement methods on geotechnical behavior, various tests were conducted on the treated soil specimens, including hydraulic conductivity, UCS, and calcite content measurements. Additionally, SEM analyses were performed to examine the morphological characteristics of the microbial precipitates. Furthermore, EDS was used to determine the chemical composition of the formed minerals. The key findings obtained from the experimental studies are summarized below:  
* Both methods--injection and mixing--proved effective in enhancing soil strength through the use of SA and MICP additives. While the injection method improved UCS values across all soil types, the combination of SA and MICP yielded successful results only in the S2 soil. In contrast, the mixing method ensured more homogeneous distribution of the additives within the soil matrix, achieving high strength values up to 398 kPa in the S1 soil. The best performance across all soil types was obtained with the combination of 2.5% SA and 15% pellet content. Moreover, the mixing method provided more consistent and effective strength improvements compared to the injection method. These findings indicate that the influence of SA on UCS is more pronounced when applied via the mixing method. * Experimental results demonstrated that both injection and mixing methods have significant potential to enhance soil low hydraulic conductivity. In the injection method, increased SA content and MICP contributed to the clogging of soil pores, thereby reducing hydraulic conductivity. However, factors such as uneven additive distribution, restricted flow in certain zones, and insufficient structural integrity occasionally hindered the achievement of the desired low hydraulic conductivity levels. * The combination of 1-1.5% SA + 10-15% pellet + MICP was found to yield the most effective results in terms of reducing hydraulic conductivity. Nevertheless, due to the variability in soil properties, it is essential to optimize application parameters specifically for each soil type. The success of low hydraulic conductivity improvement depends not only on the quantity of precipitated calcite but also on its spatial distribution within the soil matrix, soil type, and microbe-soil interactions. * The sustained effectiveness of SA and MICP additives requires careful management of parameters such as proper dosage, controlled application conditions, microbial activity level, precipitation time, and ion balance. In this context, optimizing SA + MICP systems according to site-specific conditions is considered a key factor in achieving long-term and efficient low hydraulic conductivity performance. * Overall, the injection method offers advantages in terms of low hydraulic conductivity improvements, while the mixing method delivers higher performance in mechanical strength enhancement. However, the success of both methods depends on various parameters such as additive ratios, microbial activity duration, application conditions, and soil type. Therefore, to ensure sustainable and effective UCS improvement, the selection of the appropriate method should be carefully made based on the soil properties and the intended application purpose. * When both the UCS (unconfined compressive strength) and hydraulic conductivity (k) results are considered together, it is evident that both methods are effective in reducing soil hydraulic conductivity; however, the mixing method provides higher strength values. As the optimal treatment configuration, we recommend applying the SA + MICP combination with 2.5% SA and 15% pellet contents using the mixing method. This combination ensures low hydraulic conductivity while significantly enhancing soil stability and strength.