This study investigates the simultaneous influence of particle shape and initial suction on the hydromechanical behavior of unsaturated sandy soils. Anisotropic loading-unloading tests at constant water content conditions were conducted on three sands with distinct shapes (Firoozkooh-most angular, Babolsar-Subangular, and Mesr-roundest) using a direct shear apparatus. Particle shapes were quantified in terms of sphericity, roundness, and regularity using the results of scanning electron microscopy (SEM) tests. In addition, a coupled hydromechanical model based on elasto-viscoplasticity was developed and validated against the experimental results first. The model was then employed to conduct a parametric study (compressibility, pore water pressure, and permeability) with an emphasis on the role of particle morphology and shape. The findings revealed rounder particles (higher regularity) experienced higher volumetric strain (epsilon v) under lower suction but less epsilon vwith increasing suction compared to angular sands. Moreover, the rate of permeability reduction during loading in Mesr sand was 1.5 times and 2.4 times higher than that of Babolsar and Firoozkooh sands at near-saturation condition. However, this amount decreased with increasing suction. Pore water pressure (PWP) generation was highest in the most angular sand due to its retention characteristics. The relationship between void ratio and PWP was independent of loading cycles and exhibited a linear dependence. Particle shape significantly impacted this relationship, with rounder sands showing a higher rate of void ratio change per unit change in PWP.

To investigate the unloading mechanical properties of deeply buried silty soil in dam foundation cover layers, a series of consolidated drained triaxial compression tests along multi-stage loading-unloading path were performed on both undisturbed samples (including horizontally and vertically oriented samples) and remolded samples. The test results demonstrate that: (1) the vertically oriented soil samples exhibit strain softening under low confining pressures (100, 200, and 400 kPa), transitioning to strain hardening at high confining pressures (800 and 1600 kPa). In contrast, the horizontally oriented specimens consistently exhibit strain softening across all confining pressures, whereas the remolded samples display strain hardening under all confining conditions; (2) the strength of vertically oriented soil specimens is significantly higher than that of horizontally oriented specimens, ranging from 1.18 to 1.43 times greater. Remolded samples, however, remolded samples are slightly weaker than horizontally oriented specimens under low confining pressures (100, 200, and 400 kPa), while at high confining pressures (800 and 1600 kPa), their strength approaches that of horizontally oriented specimens; (3) the deeply buried silty soil also exhibits pronounced unloading-induced volume contraction characteristics, which increase with the initial axial strain at the beginning of unloading and diminish as confining pressure increases; (4) the unloading modulus is obviously higher than the initial loading modulus, with the ratio of the unloading modulus to the initial loading modulus ranging from 1.4 to 3.6. This ratio increases with increasing confining pressure but decreases with increasing axial strain at the onset of unloading.

To address loading and unloading issues in civil and hydraulic engineering projects that employ coarse-grained soil as fill material under plane strain conditions during construction and operation, cyclic loading-unloading large-scale plane strain tests were conducted on two types of coarse-grained soils. The effects of coarse-grained soil properties on shear behavior and various modulus relationships were analyzed. The research results showed that coarse-grained soils with better particle roundness exhibit significant shear dilation deformation; it was also found that low parent rock strength can lead to strain softening, and an increase in confining pressure suppresses shear dilation deformation. During the cyclic loading-unloading process, the initial unloading modulus (E-iu) > unloading-reloading modulus (E-ur) > initial reloading modulus (E-ir) > initial tangent modulus (E-i), with the unloading modulus considerably greater than the others. In finite element simulations and model calculations, it is essential to select appropriate modulus parameters based on the stress conditions of the soil to ensure calculation accuracy. In this work, an elastoplastic and nonlinear elastic theory was used to establish a cyclic loading-unloading constitutive model. By comparing the values obtained using this model with experimental measurements, it was found that the model can reasonably predict stress-strain variations during cyclic loading-unloading of coarse-grained soils under plane strain conditions.