## 2 Materials and Methods
### Experimental Details
#### 2.2.1 Bacterium, Liquid Medium and Cementation Solution  
The bacteria used in the experiment was _Viridiobacillus arenosi_ K64 (GenBank Acceptance No.: KR873397) procured from Ataturk University, Faculty of Science, Department of Biology. It has been confirmed in vitro that bacteria participate in calcite formation [1,2]. The liquid medium was prepared with specific amounts of urea (20 g/L), MnSO\({}_{4}\cdot\)H\({}_{2}\)O (12 mg/L), tryptic soy broth ((TSB) (Laboratorios Conda S.A., Madrid, Spain) was used for the cultivation of microorganisms.) (30 g/L), and NiCl\({}_{2}\cdot\)6H\({}_{2}\)O (24 mg/L) and dissolved in a measured volume of distilled water [3]. All components of the mixture, except for urea, were sterilized using an autoclave at 121 \({}^{\circ}\)C for 45 min. Due to the thermal decomposition of urea during autoclaving, it was sterilized via filtration and subsequently incorporated into the mixture under aseptic conditions. The prepared liquid medium was incubated on a shaker at 120 rpm and a constant temperature of 25 \({}^{\circ}\)C for 48 h. The cementation solution (CS) was prepared using CaCl\({}_{2}\) (Calcium chloride dihydrate (TEKIM, Istanbul, Turkiye)) and urea (TEKIM, Turkiye) with the aid of a magnetic stirrer. The CS was used without sterilization, as maintaining sterile conditions was not feasible in field applications. In this study, the culture medium was prepared under sterile conditions to ensure the effective growth of bacterial cells. However, the cementation solution was not sterilized, as the sterilization of such solutions may present practical challenges, especially when considering real field applications. Moreover, the literature has shown that effective bioementation can be achieved under low-sterility conditions. For example, one study reported that impure bacterial cultures were able to successfully strengthen the sand without the need to isolate  
Figure 2: Visual appearances and SEM images of the soil types S1, S2, and S3 used in the study.  
a pure strain. The researchers also demonstrated that biocementation could be effectively achieved under low-sterility conditions. This finding is particularly important for large-scale applications, as it highlights that bacterial cells can be cultivated on-site, eliminating the need for importing pure cultures from external sources [30]. Based on these findings, the use of a non-sterile cementation solution was considered appropriate for the purpose of this research (Figure 3). The CS consists of 0.5 mol/L urea and 0.5 mol/L CaCl\({}_{2}\). In the literature, it is reported that using equimolar concentrations of urea and CaCl\({}_{2}\) enhances the cementation effect of the MICP process [31]. Several studies have shown that a 1:1 ratio of these two compounds leads to more effective calcite precipitation and improved soil stabilization [32]. Accordingly, the cementation solution used in this study was prepared with 0.50 mol/L of urea and 0.50 mol/L of CaCl\({}_{2}\)[33]. In this study, soil improvement was carried out using both injection and mixing methods by applying 120 mL of culture medium (CM) or 120 mL of SA solution (SS), along with 350 mL of CS. In the specimens treated only with sodium alginate, SA was dissolved in distilled water. However, in the specimens treated with both sodium alginate and MICP, SA was intentionally dissolved in the culture medium to ensure direct interaction with the bacterial solution and to achieve a homogeneous distribution within the same nutrient environment. This approach was adopted to support the simultaneous and effective occurrence of biopolymer gelation and microbially induced calcite precipitation.  
Figure 3: Experimental processes (**a**) Preparation (**b**) Injection (**c**) Mixing phase.