*2.3. Test Scheme and Procedure*

Four sets of tests were set up to investigate the effect of cementation liquid content on the SWCC of expansive soil during MICP. In addition to one group of unimproved expansive soil samples, three other groups of expansive soil samples were successively improved with cementation liquid and bacterial liquid. The improved soil samples were dehumidified and absorbed after 14 days of curing and weighed regularly to make sure they reached the moisture content required for the experimental design. The maximum dry density *ρdm* of the expansive soil is taken as the dry density of the specimen and the degree of compaction is 90%. Calculated the mass of loose soil required for the sample and made it into a ring-knife sample of size 61.8 × 20 mm. Based on relevant studies by Jiang [19] and Li [20] on the microbiological improvement of expansive soil, the admixture of cementation liquid and bacterial liquid in the experimental scheme was designed [19]. In the three groups of improved expansive soil samples, the content of cementation liquid was 100, 125, and 150 mL respectively, and the content of bacterial liquid was 50 mL. The test scheme for the MICP method of improving expansive soil is shown in Table 3.

**Table 3.** Test scheme of improved expansive soil by MICP method.


Dried filter paper ("Double Circle" brand, No. 203) is placed between two soil samples and three layers of filter paper are placed in each set of samples. The top and bottom layers are qualitative filter paper with a diameter of 60 mm and the middle layer is quantitative filter paper with a diameter of 50 mm. Use electrical insulation tape to seal the contact surface of the two ring knives, then wrap the test sample in plastic film and then wrap the plastic film tightly with electrical insulation tape. The wrapped samples are placed in the basin, sealed twice using plastic film and placed in a 20 ◦C thermostat. After 7 days, moisture equilibrium is reached between the filter paper and the expansive soil, at which point the matric suction of the filter paper and the expansive soil can be considered equal. The middle layer of the filter paper is removed with forceps and the mass of the wet filter paper is quickly weighed and recorded.

The test needs to be conducted under three assumptions. Firstly, it was assumed that *Bacillus pasteurii* with consistent absorbance would have the same activity and produce the same urease activity. Secondly, it is assumed that the expansive soils are well mixed during the treatment of the MICP method. Thirdly, it is assumed that the resting specimen is completely sealed during the filter paper method of testing. To ensure the reliability of

the test results and to minimize errors arising during the test, a control group was designed in the experimental study. Measure the matric suction, as shown in Figure 2.

**Figure 2.** Measurement for matrix suction: (**a**) sealed storage; (**b**) dried filter paper.

According to the study of the calibration curve of the "double circle" brand filter paper by Bai Fuqing [29] and Zhang Hua [30], the matric suction of the soil sample is determined by using the calibration formula. The calibration result of matrix suction, as shown in Equation (1).

$$\begin{cases} \lg(\mathcal{S}\_{\mathfrak{M}}) = -0.0767\theta\_{\mathfrak{k}} + 5.493 & \theta\_{\mathfrak{k}} \le 47\%\\ \lg(\mathcal{S}\_{\mathfrak{m}}) = -0.0120\theta\_{\mathfrak{k}} + 2.470 & \theta\_{\mathfrak{k}} > 47\% \end{cases} \tag{1}$$

where *Sm* represents the matrix suction, and *θ<sup>e</sup>* represents the moisture content of the filter paper after equilibrium.
