Abstract: The engineering background of this article is the subgrade construction of a highway in Hefei, where the soil category is expansive soil. To get closer to the real environment of the soil mass and clarify the change rule of the shear strength of the soil mass after the improvement of weathered sand, the matrix suction, initial dry density and confining pressure are taken as the control parameters to conduct triaxial shear test on the improved soil, and analyze the shear strength parameters, and study the change rule. The test results show that: ① the stress-strain curve of the improved unsaturated expansive soil presents a hyperbolic hardening model, and the tangent slope of the stress-strain curve, that is, the growth rate of the linear elastic modulus, is negatively correlated with the confining pressure; ② The effect of initial dry density on shear strength is less than that of the confining pressure change; ③ The improvement effect of weathered sand on expansive soil mainly comes from the improvement of its internal friction angle; The matrix suction has an effect on the apparent cohesion of the structure, but has little effect on the internal friction angle; Under this specific engineering background, the formula of double stress shear strength of unsaturated soil is simplified to facilitate practical application; ④ The linear regression of the failure strength of improved soil under the joint action of three parameters, namely initial dry density, matrix suction and net confining pressure, has a good effect, providing technical parameter reference and stability basis for subgrade slope engineering improvement in expansive soil area.

Key words: Unsaturated Expansive Soil, Weathered Sand Improvement: Matrix Suction: StressStrain Relationship, Strength Parameter