Abstract: The strength of expansive soil undergoes significant changes with saturation, and understanding the shear strength of unsaturated expansive soil is crucial for studying engineering problems related to this type of soil. This paper focuses on the expansive soil found along a highway in Nanyang City. In-situ borehole shear tests are conducted on the expansive soil under different saturation levels in its natural state, providing insights into the shear strength characteristics of expansive soil under natural conditions. The results show that as the saturation of expansive soil increases, the shear strength, cohesion, and internal friction angle gradually decrease. To calculate the shear strength of unsaturated expansive soil, two models are proposed in this study. The first model considers normal stress and saturation as variables, while the second model incorporates normal stress and matrix suction as variables. These models are developed based on field test data and the soil-water characteristic curve. The accuracy and applicability of the models are verified using the test data. Compared to commonly used shear strength calculation models for unsaturated soils, the two new models require fewer fitting parameters. They are applicable to calculating the shear strength of unsaturated expansive soils with a wider range of suctions and different normal stresses. Additionally, the results obtained using these models are more accurate.

Key words: unsaturated expansive soil, saturation, borehole shear test, shear strength, matrix suction, calculation model