Multiscale study on the microstructural evolution and macromechanical deterioration of expansive soil under dry–wet cycles

To explore the influence law of dry–wet cycles on the microstructure of expansive soil and the deterioration effect of macroscopic shear strength, the correlation between the change in the soil internal structure and the deterioration of the mechanical properties is analysed. The expansive soil in the test section of the slope support project in Hanzhong city, Shaanxi Province, China, is selected for sample preparation, three groups of different dry–wet cyclic water content variation paths are defined. The volume damage rate of the soil sample caused by dry–wet cycles is tested, and the microscopic evolution law of soil sample structure was analyzed. On the basis of the conclusion of microscopic analysis, the deterioration effect of macro shear strength of soil samples is further elaborated. The results show that expansive soil is rich in hydrophilic minerals such as illite and montmorillonite. The larger the amplitude of the dry–wet cycle, the more significant the volume change is. With the alternating dry–wet cycle treatment, the microscopic analysis shows that the water migration channels gradually become larger until a new balance is reached. The T2 spectra of the NMR test also show that the overall internal structure develops from stable to unstable. With dry–wet cycles, the unstable change in the soil internal structure leads to the attenuation of the macroscopic shear strength. These micro- and macroscopic research results show that the deterioration effect of drying and wetting on expansive soil cannot be ignored.