## 3 Experimental Results and Analysis  
### Unconfined Compressive Strength Tests  
After the sampling preparation procedure described in Section 2.2, we prepared multiple cylindrical specimens from the cured soil sample, each with a diameter of 50 mm and a height of 100 mm, for unconfined compressive strength (UCS) tests. We saturated the specimens using a saturator for a duration of 24 h.  
This study utilizes the Geotechnical Digital Systems (GDS) apparatus (GDS Instruments, Hook, UK), which primarily consists of a confining pressure chamber, seepage tubes, and a pressure-volume controller. These components enable real-time readings of seepage rates, confining pressure, and head changes at the top and bottom of the sample during the seepage process. The GDS reaction frame loading system uses a motor for uniform loading and can measure displacement and reaction force values through the displacement and force sensors located at the top of the reaction frame. During the UCS  
Figure 1: Surface bacterial colony of soil maintained for 7, 14, and 28 d.  
test, the loading rate is set to a strain rate of 1% per minute, corresponding to an actual loading rate of 1 mm/min. Loading is stopped when the strain reaches 20%. Both treated and untreated soils exhibit strain-hardening properties without a strength peak; therefore, the stress value at 15% strain was selected as the unconfined compressive strength of the soil. The experimental results are shown in Figure 2.  
As the treatment duration increased, the unconfined compressive strength of the soil notably improved. The optimal curing age required for the MICP-treated soft clay to achieve maximum strength and durability was 14 days, the time within which the peak value is reached. After this point, the strength increase plateaued. The unconfined compressive strength increased by 17 kPa, with a growth rate of approximately 23%, suggesting that further curing does not substantially enhance the strength.  
The strength development of MICP-treated soft clay can be categorized into three stages. In the early stages (the initial days), the strength increase is minimal because the generation of calcium carbonate is not yet complete, resulting in minimal cementation and a slow strength increase. During the intermediate period (up to 14 days), the strength increase is rapid, as microbial activity leads to the complete generation of calcium carbonate, forming comprehensive carbonate cementation around the microbes, significantly enhancing the strength. In the later stages (beyond 14 days, as shown in Figure 2), the strength increase plateaus as microbial activity gradually declines and the cementation process stabilizes, meaning additional curing does not substantially improve the mechanical properties. The results in [24] indicate that traditional long-term reinforcement methods, such as cement-based reinforcement, exhibit relatively uniform and continuous strength growth across different ages. In contrast, the strength increase induced by the MICP reinforcement method stops relatively early. This is why some studies suggest that to ensure the effects of MICP are long term, periodic treatments are necessary [25].