# Strength Characteristics and Slope Stability Analysis of Expansive Soil with Filled Fissures
## 1 Introduction  
In expansive soil slope engineering, it has always been known that it will slide every time after cutting [1; 2; 3]. The mid-route of the South-to-North Water Transfer Project in China has a total length of over 1472 km, among which expansive soil areas will cover about 340 km. Expansive soils are mainly Middle Pleistocene alluvial and flood sedimentary clay, Lower Pleistocene flood sedimentary clay, and Neogene hard clay. Strong expansive soil is mainly distributed in the Xingtai-Handan area and Nanyang Basin. The instability of the expansive soil slope will seriously affect engineering construction and safe operation [4; 5; 6; 7]. Fissures are one of the three major characteristics of expansive soil, and they are also the most intuitive and obvious feature in expansive soil engineering. The development of fissures complicates the strength characteristics of expansive soil [8; 9; 10]. The strength of the expansive soil is comprehensively controlled by the distribution, density, tendency, inclination, extension, and filling of the fissures, and the directional arrangement of the fissures causes the anisotropy of the strength of the expansive soil [11; 12; 13]. Due to the significant effect of fissures on the stability of expansive soil slopes, research on fissures in expansive soil has been carried out repeatedly [14; 15; 16; 17]. A digital image acquisition system wasemployed to capture the evolution and propagation of fissures in the soil specimen subjected to desiccation [18]. The effect of the drying environment, i.e., temperature and relative humidity, on the engineering properties and the development of fissures of an expansive soil was investigated [19]. Suction measurements, free shrinkage tests, constrained shrinkage tests, and splitting tensile strength tests (STTs) were undertaken to investigate the shrinkage cracks in expansive soils [20]. The influential factors such as soil matric suction on the formation of surficial fissures and the stability of unsaturated soil slopes were studied [21]. The researchers divided the strength of expansive soil into the soil strength and the fissure surface strength, carried out direct shear and triaxial tests [22; 23; 24], and introduced CT scanning [23], image recognition and other methods [24] to quantitatively analyze the effect of fissures on the strength of expansive soil. These studies obtained some useful conclusions. Expansive soils swell in the presence of the water and shrink in its absence, thereby producing fissures which significantly alter their mechanical and hydraulic performance [18]. Fissures decrease the soil's strength and increase its hydraulic conductivity and compressibility [20]. However, there are still deficiencies. Most studies have not systematically classified and studied the fissure itself. Fissures in different sizes and states in the expansive soil have different effects on the engineering properties of expansive soil. If the fissures contain fillers, the engineering properties of the fillers also have a great influence on the strength of the expansive soil. The shape of the fissures in the expansive soil is random, and the shear plane in the direct shear test is difficult to extend completely along the fissure surface. In addition, when using undisturbed soil to make triaxial samples, the fissures are easily disturbed, and the shape of the fissures is not easy to control. The limit equilibrium method [25; 26] and finite element method [27] are commonly used to analyze the stability of unsaturated soil slope, which are suitable for the analysis of the stability of expansive soil slopes. Numerical analysis using the finite element method was conducted to analyze frequent shallow slope failures on highway embankments of expansive soil in the North Texas region [28]. The finite element method was used to study the effect of rainfall on the factor of safety of fill slopes constructed with expansive Yazoo clay soil in Mississippi [29].