## 5 Conclusions and Suggestions  
Microbial-induced calcite precipitation (MICP) is an emerging technology in recent years, which is widely applied in geotechnical and environmental engineering. This study reviews the microcosmic mechanism of MICP technology including the metabolic reactions of bacteria and changes of soil microstructures. Compared with temperatures and PH values, the nutrient solution concentrations and calcium ion concentrations are the main factors influencing the MICP process, since they are necessary for the survival of bacteria and CaCO\({}_{3}\) formation. MICP technology can improve the water stability of soft soil, since the soil particles will be covered by a layer of white and dense hardened shell of CaCO\({}_{3}\) after treatment, and the water solubility of CaCO\({}_{3}\) is very small. Meanwhile, the permeability of soil samples decreases after the MICP process, given that pores within soil are also filled by CaCO3 crystals. These phenomena result in the improvement of the mechanical  
Figure 10: Dynamic resilient modulus with different CaCO\({}_{3}\) production.  
properties of unsaturated soils. The shear strength of soft soil increases with the MICP curing times. Due to the bio-cementation process of soil particles, the structural strength of the soil sample is improved. Based on the prediction model of resilient modulus, since the fitting parameters increase when the water content decreases, the dynamic resilient modulus of the MICP-treated soil is larger than that of the untreated soil.  
As far as the progress of MICP technology mentioned above, there are still some problems that need to be solved urgently.  
1. The current research focuses on the improvement in mechanical properties of sand soil treated by MICP, but long-term water stability is lacking attention. In addition, the effect of the MICP process on the acid resistance, frost resistance, erosion resistance, and the other durability's of unsaturated soil should be further studied.
2. The diversity and complexities of actual environmental conditions requires the MICP technology to have a corresponding adaptability. Therefore, an environmental adaptability study should be performed for the application of MICP technology.
3. Although MICP is more environmentally friendly than other treatment approaches of soil, there are still some harmful products emitted to the environment, for example, the ammonia produced by urea hydrolysis. Thus, subsequent research should be conducted in a more environmentally friendly direction or to find suitable methods to reduce the impact of the generated waste on the environment.
4. In the existing literature, the methods of using MICP technology mainly include mixing, grouting, and soaking. However, both mixing and soaking are limited by construction conditions in engineering practice. Thus, further research work can be attempted in the field test section to find a better way to apply the MICP technology.  
Conceptualization, J.L.; methodology, W.B.; software, W.B.; validation, Z.L.; formal analysis, Y.Y.; investigation, W.B. and Z.L.; resources, J.L.; data curation, W.B. and Y.Y.; writing--original draft preparation, J.L., W.B., and Z.L.; writing--review and editing, J.L. and Y.Y.; visualization, Z.L.; supervision, J.L.; project administration, J.L.; funding acquisition, Y.Y. and J.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China, grant number 52208426; Special Financial Aid to the Post-Doctorate Research Project of Chongqing, grant number 2021 XM1012; Open Fund of Key Laboratory of Special Environment Road Engineering of Hunan Province (Changsha University of Science and Technology), grant number kfj210501; the Key Research and Development Program of Hunan Province, grant number 2021 SK2050; the Science and Technology Project of the Department of Transportation of Jiangxi Province, grant number 2022H0024.  
Not applicable.  
The authors declare no conflict of interest.