Some numerical simulations of drained and undrained triaxial tests on granular materials with different initial densities are carried out with the three-dimensional discrete element method. An in-depth particle-scale analysis is performed quantitatively to illustrate the physical mechanism of the shear mechanical behaviors, with a special attention paid to the characteristics of quasi-steady state and critical state. The simulation results show that the initial density and shear drainage condition both have significant effects on the evolution of stress–strain, coordination number, fabric anisotropy factor, force chains and clusters. The chained grains ratio and the mean length of force chains in the specimens are constantly adjusted to bear and transfer the changing external loads. The transitions between small clusters and large clusters are also continually taking place in varying degrees, correlating to volumetric contraction or dilation. For the loose undrained triaxial specimen presenting quasi-steady state during shearing, the coordination number decreases obviously to nearly 4 and then increases again; the chained grains ratio decreases after a slight increase in the initial loading stage, and then begin to increase again after a period of lower value of around 0.285; the volume ratio of small, submedium and medium clusters all first decreases and then increase gradually, meanwhile volume ratio of large clusters increases sharply to as much as 0.28 and then decreases gradually. The macroscopic critical state of granular materials is a comprehensively external manifestation when the microscopic coordination number and mesoscopic force chains and clusters all evolute to a dynamic equilibrium. At the critical state, the deviator stress, void ratio, coordination number, fabric anisotropy factor, and the volume ratio of small clusters and large clusters all manifest a respectively unique linear relationship with the mean effective stress.

Given the frequent occurrence of expansive soil slope failures during rainy season, in situ borehole shear tests were conducted to investigate how loading-added modes and rainfall conditions affect the shear strength parameters of expansive soils. The results obtained were used to analyze an actual engineering project under various construction practices and depths of rainfall infiltration, with the goal of elucidating the mechanisms behind expansive soil landslides. The test results indicate that rainfall infiltration leads to a significant reduction in cohesive force, while the internal friction angle decreases slightly. Cohesive force increases linearly with greater preload pressures, whereas the internal friction angle decreases linearly as preload pressure increases. Numerical simulations revealed that rainfall-induced sliding occurs at shallow depths along a broken-line sliding surface. For unprotected expansive soil cut slopes, the safety factor experiences a sharp decline as the depth of rainfall infiltration increases. Timely support measures significantly enhance slope stability; however, the supporting load can mitigate the softening effect only up to a certain extent. Additionally, two methodologies for selecting preload pressures in practical engineering applications have been proposed.

The interaction between salt solutions and expansive soils is critical for engineering in chemically aggressive environments. However, the effect of iodide salts on pore water distribution in expansive soils remains poorly understood. This study investigated the transverse relaxation time (T₂) characteristics of pore water in expansive soils under varying KI concentrations (0–20%), moisture content (8.7–26.0%), and dry density (1.26–1.79 g/cm³) using nuclear magnetic resonance (NMR). All T₂ curves exhibited a single peak. Increasing moisture content from 8.7% to 26.0% resulted in increases of approximately 63% in T₂ at peak and 408–439% in peak area. Increasing KI concentration decreased both T₂ at peak by up to 33.3% and peak area by up to 44.0% within the tested range, attributed to diffuse double-layer compression and signal loss. Increasing moisture content broadened the T₂ distribution and linearly increased T₂ at peak and peak area, indicating water gradually occupied larger pore spaces as moisture content rose. T₂ at peak was independent of dry density, while the peak area showed a linear relationship with dry density, consistent with mass balance. The observed systematic linear relationships among T₂ at peak, peak area, and the three experimental variables suggest that NMR is a promising tool for the quantitative assessment of salt solution effects on pore water in expansive soils. These findings provide a theoretical basis for evaluating salt-affected expansive soils in coastal and arid regions.

Expansive soils are prone to strength degradation after rainfall, yet conventional slope stability analyses often rely on laboratory-derived shear strength parameters that may overestimate in-situ conditions. This study evaluates and compares the shear strength of expansive soils using the borehole shear test (BST), laboratory direct shear test (DST), and consolidated undrained test (CUT) under natural and flooding conditions. Results show that shear stress- shear displacement (or axial strain) curves generally exhibit weak hardening characteristics. Significant reductions in cohesive strength occur after flooding. The cohesive force obtained by DST and BST decreases by 18.2 kPa and 13.1 kPa after flooding respectively, while the internal friction angle decreases slightly within 3°. Laboratory tests consistently yield higher strength parameters than BST, while flooding BST values closely align with those measured on actual slip surfaces. Numerical modeling of an unstable cut slope confirms that using flooding BST parameters produces more realistic safety factors. The findings highlight the importance of incorporating in-situ testing under representative moisture conditions for reliable slope stability design in expansive soils.