2.2.1. Bacteria and Culture Media

The microorganism used to improve the expansive soil was *Bacillus pasteurii*, purchased from the Chinese General Microbial Strain Collection (CGMCC) under the number ATCC11859. This strain is a nonpathogenic, Gram-positive bacteria, which has no adverse effects on humans or the biological environment, a good ability to secrete urease. and is present in natural soil [27]. The *Bacillus pasteurii* used for the test is shown in Figure 1. The culture medium consisted of 20 g/L urea, 20 g/L agar, 15 g/L casein peptone, 5 g/L soy peptone and 5 g/L sodium chloride. *Bacillus pasteurii* was inoculated in a slant medium and incubated at 30 ◦C for 24 h and stored at 4 ◦C. The culture was inserted into an agar-free medium and incubated for 36–48 h in a constant temperature shaking incubator at a temperature of 30 ◦C and a speed of 150 r/min. The absorbance *OD*<sup>600</sup> value of the bacterial liquid at a wavelength of 600 nm was measured using a spectrophotometer. When the *OD*600≈1.6, the bacterial solution is used to improve the expansive soil.

**Figure 1.** *Bacillus pasteurii* used in testing.

## 2.2.2. Preparation of Cementation Liquid

The cementation liquid is a source of calcium for the MICP process and provides the raw material and hydrolytic environment for the metabolic processes of the microorganisms. Based on the chemical reaction equation for the MICP process, it can be calculated that for every molecule of calcium carbonate produced, 1 molecule of calcium chloride and 1 molecule of urea are required [28]. Therefore, the ratio of calcium chloride to urea in the cementation liquid was determined to be 1:1 and the concentration of the cementation

liquid was 1 mol/L. Each 100 mL of cementation liquid contained 21.91 g of *CaCl*2·6*H*2*O* and contained 6.01 g of urea.

## 2.2.3. Preparation of Soil Samples

The soil samples used in the test were divided into four groups, one of which was an unimproved expansive soil as a control group and the other three groups were samples that had been improved using the MICP method. Due to the high liquid limit and low permeability of expansive soil, conventional grouting and soaking methods cannot be used to improve expansive soil by the MICP method. In the tests, the expansive soil was amended by spraying and then mixing. The unimproved expansive soil was mixed with the cementation liquid and the bacterial fluid, respectively, and then kept in a constant temperature and humidity biochemistry cultivation cabinet at 30 ◦C for 14 days. During the curing process, holes need to be poked in the plastic film to provide the aerobic environment for bacterial metabolism to survive.
