## 1 Introduction  
Unsaturated soil is a type of soil consisting mainly of solid particles, a liquid matrix, and pore air. Due to the shrinkage interface between liquid and air, the matrix suction becomes an important structural stress within saturated soil [1]. The mechanical properties of unsaturated soil closely depend on the humidity characteristics [2; 3]. The dependence of mechanical behaviors on matric suction or moisture involved in many engineering problems has been reported on in studies of foundation engineering, subgrade engineering, and slope engineering [4; 5; 6; 7; 8]. For example, after the soft soil humidifying, the stiffness of the soil subgrades decreased evidently, resulting in the settlement and collapse of the pavement structure. Thus, unsaturated soils should be improved in practice by physical and chemical methods. The existing soil treatments mainly include dynamic compaction methods and the cement grouting method, but these methods all have certain shortcomings [9; 10]. [PERSON] et al. [11] strengthened the collapsible dam foundation by means of the dynamic compaction method, and found that its availability of treatment is limited by construction and geological conditions. The grouting method is often used for soft foundation treatment,but it uses a large amount of cement and its production would increase the quantity of CO\({}_{2}\) emissions [12]. More and more attention has been placed on reducing CO\({}_{2}\) emissions in the life cycle of engineering practices, with the increasing awareness of people concerned about the environment [13]. These green and low-carbon practices require a novel technology to decouple the dependence of the soil's treatment by cement. Therefore, the microbial-induced calcite precipitation (MICP) technology has become one of the interest points in geotechnical and geological engineering fields in recent years, since it is an environmentally friendly, noiseless, and low-cost approach for improving the engineering properties of unsaturated soil [14; 15]. At present, the cement process of MICP could mainly be classified into four types of bio-mineralization reactions [16], including the urea hydrolysis type [17], ferric reduction type [18], sulfate reduction type [19] and denitrification type [20]. Urea hydrolysis is the most efficient and advantageous way to conduct MICP technology, since it has the advantages of simple operation and is easily controllable [21]. In addition, it can quickly produce calcium carbonate precipitates and has a high microbial survival rate without a special nutrient solution [22]. Studies have been conducted to investigate the improvement of soil mechanical behaviors by urea hydrolysis through macroscopic and microcosmic experiments [23; 24]. [PERSON] et al. [25] applied the MICP grouting method on expansive soil columns and found that the swelling potential and hydrophilicity of expansive soil decreased after the treatment of MICP. [PERSON] et al. [26] demonstrated that the penetration grouting approach of MICP-solidified fluid can improve the stability of soil slopes, since the generated CaCO\({}_{3}\) can fill the volume of the micro-pore structures within soil by 9.9%. [PERSON] and [PERSON] [27] found that the compressive strength of MICP-treated soil was 1.45 to 2.26 times of that of the untreated soil by laboratory tests. Some researchers have reported the effectiveness of MICP treatment on soil mechanical properties, while less considerations and comparisons have been made on the changes of the soil hydraulic properties induced by MICP treatment [28; 29; 30]. Specially, it is important to investigate the moisture dependence on the stiffness and deformation of unsaturated soil in subgrade construction. In this paper, the reaction mechanism of microbial-induced calcite precipitation and its influence on the physical and mechanical properties of unsaturated soil are reviewed.