## 2 Materials and Methods
### Test Method  
#### 2.2.1 Response Surface Experimental Design  
To simplify the engineering grouting process and reduce reinforcement costs, it is essential to identify the controllable factors that may affect the grouting effectiveness in a bacterial native environment. A comparison of literature [20; 21; 22; 23; 24; 25] and analysis of past grouting experiences indicate that the bacteria\(-\)centeration ratio, cementation solution concentration, and grouting amount are potential controllable factors influencing the final grouting outcome. An excessively high bacteria\(-\)centeration ratio can lead to wastage of bacterial liquid and accelerate the MICP reaction rate, resulting in a substantial accumulation of calcium carbonate at the ends of the sample, which hinders subsequent grout infiltration. Conversely, a low bacteria\(-\)centeration ratio results in a reduced reaction rate, diminishing the efficiency of MICP reinforcement and wasting raw materials. A higher cementation solution concentration can expedite the formation of calcium carbonate, leading to uneven cementation. Furthermore, excessively high concentrations may be toxic to bacterial activity, adversely affecting the MICP reinforcement effect. In contrast, a low cementation solution concentration similarly reduces reinforcement efficiency. An excessive grouting amount can cause significant disturbance to the sample, compromising its uniformity, while an insufficient grouting amount may fail to provide adequate hydraulic head, hindering grout penetration into the sample. Based on the above analysis, this study considers the bacteria\(-\)centeration ratio, cementation solution concentration, and grouting amount as the subjects of investigation, denoted as A, B, and C, respectively. The compressive strength of the uranium tailings solidified by injection after 3 and 7 times is selected as the response variable.  
This study employs the CCD (Central Composite Design) method in Design-Expert 13 software to create a 3-factor, 5-level experimental scheme, comprising a total of 20 experimental  
Figure 1: Grading curve of tailings.  
Figure 2: Macro\(-\)micro morphological images of uranium tailings; (**a**) tailings particles, (**b**) SEM image of tailings particles under 100 \(\upmu\)m, and (**c**) SEM image of the surface of tailings particles under 10 \(\upmu\)m.  
runs, which includes 14 factorial points and 6 center point repetitions. This design minimizes the number of experiments while allowing for the assessment of the effects of each experimental factor and their interactions on the response variable. Based on preliminary experimental parameters from the research group [15], and to enhance the operability of the experiments, the design sacrifices rotational properties while ensuring orthogonality, setting the distance from the axial points to the center point at 1. The experimental factors and their levels are detailed in Table 1.  
The sand-filling pipe used in the tailing body grouting test was a transparent plexiglass tube that could be detached in half along the axis. The inner diameter of the sand-filling pipe was 39 mm, and its height was 80 mm, as shown in Figure 3. The inner wall of the detachable two-petal transparent plexiglass tube was coated with petroleum jelly and then assembled. The outer wall of the tube was bonded with water-sealing tape. The prefabricated disassembly joint was glued, and stainless-steel restraint hoops were sleeved on the upper and lower sides of the tube. This prevents the deformation of the sand-filling pipe during the preparation of the tailing columns and grouting. A drainage hole was placed on the lower side. A strain-controlled unconfined pressure gauge with a test loading rate of 1 mm/min was used as the test instrument. The criterion for judging the failure of the specimen was whether the tailing sample had oblique shear during the test; if so, the test is stopped.