# Strength Characteristics and Slope Stability Analysis of Expansive Soil with Filled Fissures
## 2 Morphology and Filling Characteristics of Fissues in Expansive Soil  
Based on the geological survey of the Nanyang section of the South-to-North Water Transfer Project, the most typical feature of the expansive soil was that the fissures were extremely developed, and the fissures contained a large amount of filler. The fissures of Nanyang expansive soil were basically filled fissures (Figure 2). Expansive soil fissures were widely distributed. According to the causes of fissures, combined with the distribution and the shape, they could be divided into primary fissures (Figure 2a) and secondary fissures (Figure 2b). The primary fissures were formed due to the uneven shrinkage stress in the soil. Since the shrinkage stress was proportional to the amount of water loss, the stress gradually decreased from the surface to the deep part of the soil. The primary fissures were often wide at the top and narrow at the bottom. They were basically filled with secondary clay and the fissure surface was smooth. The secondary clay was the clay deposited from the original site by wind and water to the distant location. It was gray-green clay in this area, which was deposited in the primary fissures under the action of a series of natural forces, forming the filling clay. Secondary fissures refer to tensile or shear fissures, which were formed due to changes in the external and internal stress conditions of the soil. Shear fissures and tensile fissures were widely distributed and dense. These fissures were irregular, short, and narrow. There was no obvious directionality, the fissure surface was relatively rough, and it was often filled with gray-green clay. The filling clay contained more clay minerals, such  
Figure 2: Filled fissures in the expansive soil. (**a**) Primary filled fissures; (**b**) secondary filled fissures. as montmorillonite and illite, and had a more dense structure, higher content of fine particles, lower permeability and higher plasticity. The expansion of the filling was stronger than that of the original expansive soil, and the engineering properties were extremely poor. The secondary fissure gradually developed into the long-large fissure under the influence of atmospheric camping forces and other factors, and its influence on the slope gradually approached that of the original fissure. The groundwater activity in the expansive soil area was frequent, and most of the fissures in the soil were filled with gray-green clay, and the rest of the fissures were filled with calcium and iron-manganese. The number of unfilled fissures was very small. During the deposition process, the soil crept repeatedly under the action of expansion. Because the creeping effect had a sorting and rounding effect on the soil particles, the fissure surface was often very smooth and had a waxy luster, which was the natural weak surface in the soil. In addition, due to these special deformation characteristics, such fissures were often active areas of water movement, and soils often appeared gray-green after leaching. The filling was formed by the ion exchange or mineral deposition of the clay minerals such as montmorillonite and illite in the expansive soil during the migration of groundwater through the fissures. The content of clay particles and hydrophilic minerals in the filler was extremely high, and the filling gray-green clay was very delicate, and the moisture content was large. The investigation showed that the filled fissures of weak expansive soil accounted for 64.3% to 83.9% of the total fissures. The filled fissures of gray-green clay in the medium expansive soil area had vertical zoning characteristics, and the filled fissures within 6 m depth account for about 80% of the total fissures. However, the development of filled fissures in strong expansive soils was more significant, accounting for more than 90% of the total fissures. The filling was irregularly distributed in the soil in a net shape. The filling thickness was generally 2 to 5 cm.