**1. Introduction**

Expansive soil is special catastrophic clay composed of strong hydrophilic minerals such as montmorillonite, illite, and kaolinite, which usually exhibit significant overconsolidation, multiple fissures, and swell-shrink characteristics. Expansive soils are distributed in a wide range in China and exist to varying degrees in more than 20 provinces such as Guangxi, Yunnan, and Sichuan. After expansive soil is exposed to water, it is highly susceptible to swelling deformation, softening and strength decay, and dry shrinkage cracks when water is lost. This repeated expansion and contraction deformation will lead to crack, subgrade uplift and slope sliding which will cause major engineering disasters [1–3]. Therefore, any construction work in expansive soil area must be treated and improved to eliminate potential engineering hazards. Traditional methods widely used in engineering for improving expansive soils are usually divided into physical and chemical improvement methods [4,5]. These methods of improving expansive soil could reduce expansive soil's swell-shrink characteristic effectively and also improve its strength. However, the traditional methods of improvement are not only labor-intensive and time-consuming, but

**Citation:** Yu, X.; Xiao, H.; Li, Z.; Qian, J.; Luo, S.; Su, H. Experimental Study on Microstructure of Unsaturated Expansive Soil Improved by MICP Method. *Appl. Sci.* **2022**, *12*, 342. https://doi.org/10.3390/ app12010342

Academic Editor: Bing Bai

Received: 7 December 2021 Accepted: 23 December 2021 Published: 30 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

also pollute the surrounding environment, and may even have negative impacts on the long-term ecosystem. Therefore, the academic and engineering circles urgently need a new, sustainable, and environmentally friendly alternative method to improve the poor engineering properties of expansive soil.

With the interpenetration of modern biotechnology and engineering science, biogeotechnical engineering has become a new and hot research field. In recent decades, the Microbially Induced Carbonate Precipitation (MICP) method was used by scholars at home and abroad to repair fractures, seepage control, sand consolidation and soft soil improvement with good results [6–9]. In terms of applying the MICP method to improve the engineering characteristics of soft soils, scholars from home and abroad have mainly researched in improving sandy soils and made great progress [10–14]. In recent years, a few scholars have used the MICP method to improve expansive soil and other soft clays, which has made gratifying research progress. Bai studied the coupled migration of ions in soil [15]. Liu demonstrated that the MICP method can effectively improve the strength of loess through unconfined compressive strength test, calcium carbonate content test and scanning electron microscope test, and found the optimal reaction conditions and cementation liquid content for the improvement [16]. Safdar and Gowthaman studied the MICP method for improving peat soils. It was found that *Bacillus licheniformis* was more effective in improving peat soils, and that scallop powder was effective in increasing the activity of microorganisms and promoting the cementation of peat soil particles [17,18]. Some scholars investigated the swelling properties, strength properties, and microstructure of MICP modified expansive soils through a series of tests such as one-dimensional swelling tests, tensile tests, consolidation tests and compression tests. The MICP method of improved expansive soils can control the swelling and shrinking characteristics of the soil and improve the strength of the soil, and the calcite produced by the reaction can also provide a linkage to the soil particles [19–21]. Expansive soil in nature is usually unsaturated. Based on the theory of unsaturated soil mechanics, scholars carried out research on the engineering properties of unsaturated soil [22–26]. Soil-water Characteristic Curves (SWCC) obtained from unsaturated soil tests can be used to study the hydrophilicity, swell-shrinkage and strength characteristics of expansive soil improved by the MICP method.

In this paper, a series of experimental studies were carried out on expansive soil improved by the MICP method using the filter paper method. The SWCC of the expansive soil was measured after the improvement of different solutions with different cementation liquid content. The relationship between the moisture content of the expansive soil and the matric suction was obtained. Following comparison of four empirical models, Fredlund & Xing model was chosen to analyze the SWCC of the expansive soil after improvement by different schemes. The air entry value and water stability of the expansive soil were obtained.

The particle shape and pore space changes in the improved expansive soil were gotten by Scanning Electron Microscope (SEM) tests. From the perspective of microstructural changes in soil samples, the mechanism of the effect of different cementation liquid content on the water stability and water sensitivity of improved expansive soil during the MICP process was revealed.

## **2. SWCC Test Materials and Methods**
