## 4 Conclusions
Although the injection method is effective in reducing hydraulic conductivity through controlled additive delivery, it yields lower UCS values compared to the mixing method. Therefore, for achieving high strength and low hydraulic conductivity simultaneously, the mixing method with an appropriate SA concentration and pellet ratio is considered the optimum solution. * The calcite contents obtained through the injection method indicate that this approach provides relatively controlled and homogeneous calcite formation across all soil types. In contrast, the SA + MICP combination applied via the mixing method resulted in higher calcite accumulation. This broader variation suggests that the mixing method offers a higher yield potential but is also more sensitive to soil properties and application conditions. Therefore, the choice of method should be made based on the desired strength level, soil characteristics, and requirements for uniformity. * The amount of calcite obtained through the mixing method was associated with a greater increase in mechanical strength compared to the injection method. * It strongly demonstrates that, in the improvement of unconfined compressive strength, not only the amount of precipitated calcite but also its spatial distribution within the soil and the soil-microorganism interaction is among the critical determining factors. * Calcite precipitation induced by MICP acts as a complementary mechanism to the gelation effect of SA by filling the remaining voids and enhancing interparticle bonding. The distinct calcium peaks observed in the EDS spectra support the occurrence of this precipitation. Therefore, it is concluded that the gel structure formed by sodium alginate is the primary factor responsible for permeability reduction, while MICP-induced calcite formation provides a secondary sealing contribution. * Injection and mixing methods yielded distinct microstructural outcomes: In fine-grained soils (S1), injection led to uniform film formation and dense calcite bridges, while in coarse soils (S3), both methods showed localized and limited precipitation. SEM and EDS analyses confirmed that homogeneous additive distribution and sufficient CaCO\({}_{3}\) crystallization are critical for effective cementation--highlighting that soil type and method compatibility are key to microstructural improvement. Based on the findings of this study, future research is recommended to focus on optimizing additive ratios according to soil type, developing application techniques that ensure homogeneous distribution, and thoroughly investigating the conditions that enhance microbial activity in the MICP process. Furthermore, the long-term performance of bio-based soil improvement should be evaluated under environmental conditions such as freeze-thaw cycles, wetting-drying, and repeated loading. Exploring the combined use of different biopolymer types (e.g., xanthan, chitosan) with MICP can also expand the method's applicability in field-scale implementations. In this context, the present study provides a valuable foundation in the literature for biopolymer-assisted MICP applications. In addition to the findings of this study, it is evident that not only the amount of calcite but also its spatial distribution plays a critical role in the overall strength. Uneven precipitation may create weak zones within the soil, potentially leading to sudden failure under load. Therefore, developing a Discrete Element Method (DEM)-based modeling approach that links bond strength and distribution to macroscopic behavior could significantly enhance the understanding of this mechanism. It is recommended that future studies incorporate DEM-based analyses. Although the mixing method demonstrated higher unconfined compressive strength under laboratory conditions, its application is generally limited to shallow depths and is difficult to implement over large areas. In contrast, the injection method can be applied in-situ to deeper soil layers using pressurized injection equipment.