Abstract: Expansive soil is featured with expansibility, multi-crack and over consolidation. When exposed to natural conditions, it vulnerable to the drying-wetting cycle effect of rain and evaporation, thus the shear strength of expansive soil will decreases with time, resulting in expansive soil slope instability. A series of ring shear tests is conducted on weak expansive soil sampled from Jinmen, Hubei province under the condition of drying-wetting cycles to investigate the variation of peak strength and residual strength. Results show that the shear strength of the sample is related to the normal stress, both its peak strength and residual strength increase with the increase of normal stress. Meanwhile, the greater the normal stress is, the smaller the shear displacement is when the sample reaches the residual strength. As the drying-wetting cycle increases, the peak strength of the expansive soil obviously attenuates, while the residual strength of expansive soils varies slightly, but it is not obvious, which can be approximately considered stable as stable. The peak cohesion and residual cohesion of expansive soils after three dry-wet cycles are almost the same. The difference between the peak internal friction angle and residual internal friction angle is always 2°, which is largely unaffected by the number of drying-wetting cycles.

Key words: expansive soil, wetting-drying cycle, ring shear test, residual strength, cohesion