*3.2. Effect of Cementation Liquid Content on SWCC of Soil Samples*

The Fredlund & Xing model was chosen to analyze the test results and the parameters of the model obtained after fitting, as shown in Table 5. As a result, the SWCC of the microbially improved expansive soil can be obtained for different cementation liquid content, as shown in Figure 4.


**Table 5.** Fitting parameters of Fredlund & Xing model.

The Fredlund & Xing model assumes that the parameters *α*, *n* and *m* are independent of each other. Where α is the soil parameter associated with the air entry value and can be used to characterize the value of matric suction at the inflection point of the soil-water characteristic curve [35]. As can be seen from Table 5, the air entry values for the expansive soils are 226.948, 155.666, 102.804 and 64.240 kPa from unimproved, A1 and A2 to A3 respectively. The air entry value of the improved expansive soil is significantly lower than that of the unimproved soil, and the air entry value of the improved soil tends to decrease slowly and non-linearly as the content of the cementation liquid increases. The reason for this is that there is a large amount of freedom *Ca*2<sup>+</sup> in the cementation liquid, which displaces with the *Na*<sup>+</sup> in the expansive soil, reducing the thickness of the electric double layer, increasing the gravitational force between the particles and effectively promoting the coalescence of the particles. At the same time, the coagulation of the soil particles results in a significant reduction in the dispersion of the soil, an increase in the agglomerates and an increase in the porous between the soil particles, which causes a reduction in the air entry value. On the other hand, the larger porous produced by the microbially induced carbonate precipitation, adsorbed on the contact surface between soil particles and the particle surface, effectively enhance the interparticle linkage and reduce the air entry value of the expansive soil [36,37]. *n* is positively correlated with the slope at the inflection point of the SWCC, which can be used to characterize the water-holding capacity of the soil. The lower the slope at the SWCC inflection point, the better the water holding capacity of the soil [38].

As can be seen from Table 5, the values of the soil parameter *n* for the improved expansive soils have decreased compared to before the improvement. According to the value of *n* , the unimproved expansive soil has the weakest water-holding capacity. The value of *n* in the improved expansive soil decreases gradually from A1, A2 to A3 as the content of cementation liquid in the expansive soil increases under the condition of controlling the content of bacterial liquid of 50 mL.

**Figure 4.** SWCCs of expansive soils under different cementing liquid content.

Figure 4 shows that the SWCC intersection of the expansive soils is located near the optimum moisture content under the conditions of the different improvement schemes. When low volumetric moisture content, the greater the content of cementation liquid, the greater the matric suction at the same volumetric moisture content. When the volumetric moisture content is higher than the moisture content at the SWCC intersection, the matric suction of the soil sample decreases with increasing the content of cementation liquid. From unimproved expansive soils to A1, A2, and A3, the SWCC of the expansive soils gradually levelled off, which means that the dehydration rate of the microbiological improved expansive soils gradually decreased, indicating that their water stability was gradually enhanced.
