Expansive soil is a kind of clay soil that expands rapidly after being immersed in water and shrinks significantly after losing water. The expansive soil slopes exposed to the atmosphere are in the process of continuous dry and wet cycles and are extremely unstable under the conditions of rainfall. Based on saturated-unsaturated seepage theory, the unsaturated seepage process and humidification process of the expansive soil slope under rainfall were simulated, and a related FORTRAN language program was developed. The effects of matrix suction changes, seepage softening, and moistening expansion during the seepage process were considered. The effects of strength attenuation, seepage softening, and moistening expansion on the overall stability of the expansive soil slope were analyzed. The results show that under the action of rainfall, the failure mode of the expansive soil slope after multiple dry-wet cycles is shallow collapse, and the failure surface is located in the weathered area, which is quite different from the traction landslide of the homogeneous expansive soil slope. After considering the moistening expansion and softening effects, the maximum displacement of the slope increased by an order of magnitude, and the safety factor decreased significantly. The significant increase in the horizontal stress of the lower half slope is the main reason for the landslide. The research results well explain the special tractive and shallow properties of typical expansive soil landslides.

The soft interlayer in the foundation is one of the most important factors that induce landslide in the expansive soil-embankment slope. The existence of large soft layers significantly reduces the overall stability of the slope, and they have the tendency to further develop into a sliding surface. To investigate the influence of the soft interlayer on expansive soil slopes, taking the high-fill embankment slope with a soft interlayer in the Baoshan area of Yunnan Province as an example, the causes and strength effects of the soft interlayer are analyzed via landslide investigation, laboratory test, and numerical simulation, and the stability and instability mode of the slope are studied. Results indicate the following: ① The soft interlayer is located at the bottom of the weathering layer within the depth of the atmosphere. Its strength is less than that of the expansive soil on both sides. ② The overall stability of the slope and the shape of the sliding surface are controlled by weak interlayers, and its strength greatly impacts the damage scope of the slope. When the strength is high, it appears as a collapse of the fill area, and when the strength is extremely low, it appears as the overall sliding of the slope. Using the residual strength as the calculation parameter of the weak layer reflects well the actual stability and failure mode of the slope. ③ Results of soil layer parameter sensitivity analysis indicate that the internal friction angle of the soft interlayers has the most significant effect on slope stability. Using the lightweight embankment fill can effectively improve the stability factor of embankments.

With the rapid development of road engineering today, a large number of high-grade highways need to pass through expansive soil distribution areas. At present, the research on expansive soil slope mainly focuses on the newly excavated cutting slope. However, according to engineering experience, a landslide of fill embankment on expansive soil foundation is also very common. The expansive soil layer is heterogeneous. There are many weak intercalations or large fissures under the ground, which are generally parallel to the trend, with low strength and high permeability. After rainfall, the strength of the weak interlayer and large fissures will be further reduced after moisture absorption, and the sliding surface is easily formed under the load of filler, which is the main factor inducing embankment landslide. On the basis of landslide investigation and a laboratory test, a FORTRAN calculation program is developed in this paper, which can comprehensively consider the special moisture absorption and softening characteristics of expansive soil. Taking a high fill embankment slope with a soft interlayer in the Baoshan area of Yunnan Province as an example, the stability and instability characteristics of the fill slope on the expansive soil foundation are analyzed, and the influence of moisture absorption and softening on the expansive soil slope is emphatically discussed. Finally, this paper puts forward the reinforcement method of the high fill embankment slope on the soft expansive soil foundation, which is proven to have a good reinforcement effect through calculation analysis and field practice. For expansive soil foundation with weak interlayer, it is better to directly reinforce the weak layer through rigid piles.

No abstract