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.

The study of the compression characteristics of loess in seasonal regions involves analyzing the mechanical properties and mesoscale damage evolution of intact loess subjected to dry-wet freeze-thaw cycles. This study meticulously examines the evolution of the stress-strain curve at the macroscale and the pore structure at the mesoscale of loess by consolidation and drainage triaxial shear tests, as well as nuclear magnetic resonance (NMR), under varying numbers of dry-wet freeze-thaw cycles. Then, utilizing the Duncan-Chang model (D-C), the damage model for intact loess is derived based on the principles of equivalent strain and Weibull distribution, with testing to verify its applicability. The results indicate that the stress-strain curve of undisturbed loess exhibits significant strain softening during the initial stage of the freeze-thaw dry-wet cycle. As the number of cycles increases, the degree of strain softening weakens and gradually exhibits a strain-hardening morphology; the volume strain also changes from dilatancy to shear contraction. According to the internal pore test data analysis, the undisturbed loess contributes two components to shear strength: cementation and friction during the shear process. The cementation component of the aggregate is destroyed after stress application, resulting in a gradual enlargement of the pore area, evidenced by the change from tiny pores into larger- and medium-sized pores. After 10 cycles, the internal pore area of the sample expands by nearly 35%, indicating that the localized damage caused by the dry-wet freeze-thaw cycle controls the macroscopic mechanical properties. Finally, a damage constitutive model is developed based on the experimental phenomena and mechanism analysis, and the model's validity is verified by comparing the experimental data with theoretical predictions.

A water conveyance open channel project in the northern Xinjiang region crosses a large area of collapsible loess. The mechanical properties of the collapsible loess have undergone severe degradation after years of exposure to rainfall, evaporation, and seasonal temperature fluctuations, making it highly susceptible to engineering phenomena such as channel foundation collapse and slope failure. To delve into the deterioration mechanism, direct shear, compression, and microscopic scanning tests were conducted on the collapsible loess under dry-wet & freeze-thaw cycles. The deterioration patterns of shear strength and compression properties, as well as their damage mechanisms, were analyzed at both macro and microscopic scales. The results of the study indicate (1) Straight shear test: with increasing the number of dry-wet-freeze-thaw cycles, the peak shear strength exhibits a three-stage trend: rapid decrease, decelerated rate of decrease, and eventual stabilization. The cohesion decreased exponentially, with the largest reduction occurring during the first cycle, and stabilizing after 5 cycles, reaching a degradation degree of 44.55%. The change in internal friction angle, which varied within 2.1 degrees, was less affected by the wet-dry-freeze-thaw cycles, with a maximum degradation of 7.04%. (2) Compression test: the compression curve can be divided into two stages of elastic deformation and elastic-plastic deformation according to the consolidation yield stress sigma(k), and sigma(k) shifts forward as the cycle times increase. The compression coefficient and compression index decreased exponentially or in a power function form with increasing cycle times, indicating reduced overall compressibility of the soil body. (3) Microstructure: through scanning electron microscope (SEM) analysis, under cycling, the number of large pores decreased while the number of medium and small pores increased, with the arrangement tending towards disorder. Large particles gradually transformed into medium and small particles, and their morphology tended to become rounded. Correlation analysis indicates that pore size and its angle are the main factors influencing shear strength. Pearson's correlation coefficient reveals that particle morphology and pore size have the greatest influence on compression indices.