## 3 Results and Discussion
### Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS) Analysis Results  
SEM and EDS analyses were conducted on soil samples improved by injection and mixing methods (S1, S2, S3; S1-SA, S2-SA, S3-SA; S1-SA + MICP, S2-SA + MICP, S3-SA + MICP). The results are presented in Figures 13 and 14. In the S1 soil specimen, since the particle size is smaller compared to the S2 and S3 soil specimens, the SEM images presented in Figures 13 and 14 were taken at a magnification of \(\times\)1600 for S1, while \(\times\)100 magnification was used for S2 and S3. These analyses were performed to visually and chemically verify the microstructural changes and calcite precipitations induced by the addition of SA and MICP within the soil particles. In this context, the effects of the applied improvement methods on soil performance were explained at the micro scale, and the observed increases in strength and low hydraulic conductivity were scientifically substantiated. In the S1 series, SA applied via the injection method was homogeneously distributed within the microstructure by forming regular film structures on the particle surfaces. A similar observation was reported in a study involving biopolymer and MICP applications, where, in moderately bio-cemented sand specimens, clusters of rhombohedral CaCO\({}_{3}\) crystals were observed on the smooth surfaces and surface grooves of sand particles. These crystals were found to precipitate in significant amounts on the particle surfaces, promoting interparticle bridging and resulting in a notable increase in erosion resistance [45]. In the S2 series, SA applied via the injection method adhered to the particle surfaces and formed fibrous film structures within the pores, effectively reducing hydraulic conductivity. Following the application of SA in combination with MICP, irregular yet effective CaCO\({}_{3}\) precipitations were observed. These structures are believed to have enhanced interparticle bonding, thereby improving microstructural strength. In the S3 series, however, due to the coarse and smooth surfaces of the particles, the SA and MICP additives could not be homogeneously distributed throughout the soil. Even though the injection method was employed, the effectiveness of the treatment remained limited. The resulting calcite precipitates were localized on the surface, failed to form binding bridges between particles, and thus led to low cementation efficiency. Overall, the effectiveness of injection-based soil improvement is considered to be directly dependent on the surface characteristics and grain size of the soil particles. In coarse-grained soils (S3), the influence of the additives remained largely restricted to the surface, and sufficient cementation could not be achieved. In contrast, in fine- and medium-grained soils (S1 and S2), both chemical cementation and physical gel bonding were observed more prominently. EDS analysis indicates that SA adheres physically to the particle surfaces; however, crystallization is chemically confirmed through MICP and cementation additives. In the S1 soil, the SA additive was observed to cover the grain surfaces with fibrous film structures in SEM images, and this adhesion is considered to be chemically supported by the presence of Cl elements detected in EDS analysis. The crystalline calcite precipitations observed in SEM images following the application of the SA + MICP combination were further validated by a notable increase in carbon (C) and calcium (Ca) levels in the EDS results. Additionally, the decrease in the silicon (Si) content suggests that the grain surfaces were partially covered with additive products. The consistent presence of aluminum (Al) across all EDS spectra is thought to originate from environmental contamination, clay phases naturally present in the soil, or as a secondary signal from the sample mounting plate.