The water-holding and strength characteristics of unsaturated expansive soil and modified soil in a high-fill canal embankment along the central line of the South-to-North Water Diversion Project were investigated using a pressure plate apparatus and a GDS unsaturated triaxial test system. The soil-water characteristic curves (SWCCs) of expansive soil and modified soil were obtained by curve-fitting the results of water-holding characteristic tests, thereby revealing the distinctions in water-holding characteristics of the two soil types. The laws governing the effects of matrix suction on the stress-strain relationships and shear strength of the two soil types were explored through unsaturated triaxial drainage shear tests. According to the test results: (1) The moisture content and void ratio of each soil type decreased gradually with the increase in matrix suction, although the void ratio of modified soil decreased at a slower rate than that of expansive soil. (2) Matrix suction induced a transition from strain hardening to strain softening; (3) The shear strength of both soils increases with the matrix suction and confining pressure, with the increment of expansive soil greater than that of modified soil. Notably, the influence of confining pressure became progressively more significant with increasing matrix suction for both soils; (4) The cohesion and internal friction angle of expansive soil and modified soil increases with the matrix suction, with 200 kPa as the critical point of increasing rate; (5) The expansive soil differs from modified soil in cohesion and internal friction angle under different matrix suctions, with matrix suction of 400 kPa as the critical point. (6) The matrix suction thresholds of 200 kPa and 400 kPa can serve as references for engineering design and construction, as well as seepage prevention and slope reinforcement. This study provides technical parameters and theoretical support for the design, construction, and long-term stability of embankments on the expansive soil in the South-to-North Water Transfer Project site.

The persistent trend of rising temperatures and shifting weather patterns caused by climate change has prompted significant concern around the world. This research aims to evaluate the instability of slopes in Almaty, Kazakhstan, under various rainfall patterns, groundwater tables, and slope geometries by incorporating the principles of unsaturated soil mechanics. However, there have been a limited number of studies incorporating the principle of unsaturated soil mechanics with constant rainfall patterns in Central Asia, particularly in Kazakhstan, on the impact of rainfall-causing landslides. Hence, in this research, GeoStudio software (SEEP/W and SLOPE/W) was used to simulate the factor of safety (FoS) and pore water pressure for the investigated slopes under different rainfall patterns. Results from Hyprop and statistical method show that the saturated volumetric water content is 0.502, whereas the residual one is 0.147 and for the permeability function the conductivity coefficient started to sharply decrease at the suction value of 2 kPa when the air-entry value was 24 kPa. Findings from numerical analysis show the change in FoS for the slope of 10 m height and 27-degree slope angle was 6%, 7%, 7%, and 8% for cyclic, delayed, advanced, and normal distributions, respectively. For the slope with 20 m height and the same 27-degree angle, the change in FoS was 8%, 10%, 8%, and 11% for the cyclic, delayed, advanced, and normal distributions, respectively. These same patterns were shown in slopes with 35-degree and 45-degree angles, having the same 10 m and 20 m heights. Comparatively, this shows that slopes under cyclic rainfall patterns (240 mm of rain within 12 days) are less prone to failure compared to slopes under continuous, delayed, or regularly distributed rainfall patterns. Moreover, an increase in slope height and angle also affect the FoS negatively. It should be noted that the results obtained are only applicable to clayey-loam soil.