An experimental study of the relationship between water content and strength of unsaturated expansive soil on canal slope

The soil layer in the canal first section of the Middle Route of the South-to-North Water Transfer Project (MR-SNWTP) is mainly composed of expansive soil, and changes in water content of the expansive soil affect the stability of the slope. In order to make a reasonable assessment of the stability of expansive soil canal slopes, it is necessary to test and analyze the relevant physical and mechanical properties of the expansive soil on canal slopes at different water content. In this paper, titration-direct shear tests are carried out on the expansive soil collected from the canal first section of MR-SNWTP to obtain the relationship between the shear strength parameters and water content of the expansive soil. The test results show that the shear strength decreases significantly with increasing water content, and the decay process starts fast and then slows down. The lower the initial water content of the sample is, the slower the cohesion decreases, and the faster the angle of the internal friction decreases, and the shear strengths of the sample with different initial water content all show an inflection point when the water content increases to about 30%, when the decay rate decreases and becomes stable. Hydraulic conductivity tests based on soil-water characteristic curve method and instantaneous profile method are designed to obtain the hydraulic conductivity-water content curve of unsaturated expansive soil. The test results show that the lower the water content is, the smaller the hydraulic conductivity is and the faster the change rate of hydraulic conductivity is. Conversely, the slower the change rate of hydraulic conductivity is, and the more stable the soil water content tends to be. The research results of this paper can be used for quantitative analysis of the relationship between the stability of the expansive soil canal slopes and the groundwater level of the slopes. In practical applications, they can also be used to effectively obtain the distribution characteristics of water content and shear strength in the unsaturated soil of the canal slope, and to obtain the time variation laws of water content and shear strength, and a more accurate stability evaluation model is established based on spatial dynamic shear strength analysis of the expansive soil of the canal slopes.