Abstract: The phenomena of “dry shrinkage and wet expansion” and “frost heave and thaw settlement” in expansive soils in seasonally frozen regions have caused numerous engineering problems. This study focuses on the strength degradation and slope instability in expansive soil water channels of the Northern Xinjiang water supply project. Using drying-wetting and freezing- thawing cycles as experimental conditions, the research includes moisture content monitoring at various depths to analyze soil moisture variation patterns during different stages. Additionally, laboratory experiments are conducted to study the effects of these cycles on non-uniform deformation, strength degradation, and microstructure damage in expansive soils. The results reveal that: 1) Under drying-wetting and freezing-thawing conditions, expansive soils at certain depths of the channel foundation exhibit significant moisture content fluctuations. The most significant variations occur during the freeze-thaw phase, establishing a “phase change dynamic zone” within the expansive soil. 2) Drying-wetting and freezing-thawing cycles cause significant microstructural damage in expansive soils, marked by continuous crack development and expansion with increasing cycle frequency. The soil experiences persistent “dry shrinkage and wet expansion” and “frost heave and thaw settlement” effects. In the early stages of drying-wetting and freezing-thawing action, expansive deformation significantly contributes to total deformation. However, after a certain number of cycles, both volumetric and expansive soil deformation gradually stabilize. 3) Expansive soils exhibit varying degrees of degradation in shear strength and strength parameters. Cohesion degrades more significantly, following an exponential decrease, while the internal friction angle experiences a less pronounced reduction. In the early stages of dry-wet and freeze-thaw cycles, cohesion degradation accounts for 41.2% to 48.6% of the total degradation rate. The significant decrease in soil cohesion leads to shallow landslides in expansive soil slopes of channel foundations, highlighting the crucial role of cohesion in slope instability.

Key words: expansive soil drainage channel, dry-wet and freeze-thaw cycles, moisture content, strength attenuation, crack extension