## 4 Influence of MICP on Properties of Unsaturated Soil
### Resilient Modulus of MICP Unsaturated Soil  
The MR of soil describes the resilient deformation characteristics of soil subgrades under dynamic traffic loads [119]. In the MICP process, CaCO\({}_{3}\) precipitation bio-cemented the soil particles and filled the pores between soil particles, leading to the increase in coarse particle content and the decrease in fine particle content in soil. Therefore, with the decrease in fine particle content, the dynamic resilient modulus of improved soil increases. In order to achieve biological stability, [PERSON] et al. [120] adopted the microbial-induced calcite precipitation (MICP) method. The result shows that the elastic behavior of the treated fine-grained soil was similar to that of coarse grained soil, and the MR of treated soil evidently increased. [PERSON] et al. [121] found that the sensitivity of the MR of the stabilized fine sand to confining pressure decreases when the content of CaCO\({}_{3}\) increases. The increase rate of the MR of the saturated stabilized soil is greater than that of the unsaturated stabilized soil. The reason for this structure is due to the improvement of MICP technology. The generated CaCO\({}_{3}\) crystals will block and bio-cement the soil particle pores, and the pore volume will decrease with the increase in CaCO\({}_{3}\) content. Soil particles can contact closely through CaCO\({}_{3}\) crystals, and the MR increases accordingly. Figure 10 shows the relationship between dynamic MR and CaCO\({}_{3}\) output. It can be seen from the figure that the output of CaCO\({}_{3}\) is closely related to the dynamic MR. The minimum value of dynamic MR increases by about 50% at most, and the maximum value increases by about 66.8%.  
Figure 9: Curves of shear strength for (**a**) comparison between MICP-treated and untreated expansive soils; (**b**) different MICP-treated days.  
After microbial improvement, it can be found that there is evident CaCO\({}_{3}\) precipitation between soil particles. CaCO\({}_{3}\) deposits on the surface of soil particles, forming bio-cement points and connecting soil particles. These bio-cements can be bio-cemented into larger aggregates, which enhances the connection ability between soil particles and improves the stability of soil particles. Therefore, the structural strength of the soil sample is improved, and the physical and mechanical properties of the soil sample are improved.  
The comment model for predicting MR is shown in Equation (19) [122]. The dynamic resilient modulus of subgrade soil is affected by factors such as soil type, soil physical properties, stress state, and humidity state.  
\[M_{R}=k_{1}Pa\bigg{(}\frac{\theta}{P_{a}}\bigg{)}^{k_{2}}\bigg{(}\frac{T_{ \text{oct}}}{P_{a}}+1\bigg{)}^{k_{3}} \tag{19}\]  
where \(M_{R}\) is the dynamic resilient modulus (MPa); \(Pa\) is the atmospheric pressure, and the reference value is 101.3 kPa; \(\tau_{\text{oct}}\) is octahedral shear stress; and \(k_{1}\), \(k_{2}\), and \(k_{3}\) are all model parameters, which meets \(k_{1}\geq 0\), \(k_{2}\geq 0\), \(k_{3}\leq 0\).  
Combined with the research on the mechanical properties of MICP improved soil [123], the influence of soil type, water content, thousand density, and plasticity index on the dynamic resilient modulus of improved soil was analyzed. Previous studies found that the model parameters (\(k_{1}\), \(k_{2}\), \(k_{3}\)) decreased with the increase in water content. With the decrease in plasticity index, model parameter \(k_{1}\) decreases and \(k_{2}\) and \(k_{3}\) increase. Similarly, \(k_{3}\leq 0\), and the influence of the plasticity index on \(k_{1}\) and \(k_{3}\) is different from that on \(k_{2}\)[123]. Therefore, after improving by MICP method, the hydrophilicity of soil is weakened, and the variation range of water content is reduced. The plasticity index of soil is significantly reduced after improvement by the MICP method. Evidently, compared with the unmodified soil, the dynamic resilient modulus of the improved soil increases.