Empirical Model for Predicting Shear Strength of Chengdu Expansive Soil Under Dry–Wet Cycles Considering Water Content and Dry Density

To investigate the variation in shear strength of expansive soil under dry–wet cycles, laboratory direct shear tests were conducted on remolded soil from a foundation pit in the Chengdu area. The tests were performed under controlled drying and wetting paths, with systematic variations in water content (w), number of dry–wet cycles (N), and dry density (ρ). The characteristics and evolution of shear strength under these conditions were analyzed. Using a nonlinear multiple surface fitting method, empirical relationships were established between the soil’s shear strength parameters (cohesion c and internal friction angle φ) and the variables w and N. Furthermore, equations describing the attenuation of these parameters with respect to ρ and N were derived. Based on the experimental data and within the framework of the Mohr–Coulomb strength theory, a practical predictive model was developed for the shear strength of expansive soil under the coupled effects of dry–wet cycles, water content, and dry density. Verification results demonstrate that the model predictions are in good agreement with experimental measurements. The proposed model provides a practical reference for estimating the shear strength of similar expansive soils in the Chengdu area under cyclic drying and wetting conditions.