The present work introduces an analytical framework based on the limit-equilibrium method for the determination of the local factor of safety (FS) and global factor of safety (FSG), and local displacements along the critical slip surface using the Morgenstern-Price (MP) method of slices. This proposed work computes displacements along the critical slip surface in addition to a single FSG. The unsaturated shear strength models, in conjunction with the soil-water characteristic curve (SWCC), are considered. The MP-based equilibrium equations to determine FSG are utilized as an objective function in the metaheuristic search algorithm of particle swarm optimization to determine the critical center, critical radius, and minimum FSG for unsaturated finite slopes. It is recommended to use a particle size of 75 and conduct 50 iterations for optimal results. The effects of SWCC fitting parameters on the critical slip surface, FSG, point FS, and point displacements are also investigated. Two distinct benchmark slope scenarios with and without negative pore water considerations are utilized in the current study. This approach enables a detailed investigation into the influence of various unsaturated soil parameters, such as af (related to the air-entry value), nf (related to the slope of the SWCC), and mf (related to the residual water content), as well as constitutive model parameters including the linear shear modulus (G) and the fitting parameter (rho). The maximum displacement occurs at the slope's top crest. Under benchmark conditions, the first scenario shows a reduction in point displacement by 3.30%, 1.98%, and 10.23% for SWCC-1, SWCC-2, and SWCC-3, respectively. However, in the second scenario with SWCC-3, the critical slip surface's position changes, affecting local displacements. This results in an increase of 32.72% (i.e., from 21.45 to 28.47 mm) in point displacement at the top when comparing SWCC-3 with no SWCC consideration. The current study advocates that the effect of fitting parameters of the SWCC should be used to better understand the local FS and displacement, because the critical slip surface is contingent on the values of the SWCC. Ignoring SWCC parameters can lead to an underestimation of slope displacement, because they significantly influence the critical slip surface position and displacement magnitude. Their inclusion is essential for accurately assessing slope stability and preventing errors in displacement prediction.

The laboratory experiment is an effective tool for the rapid assessment of the unsaturated soil slopes instability induced by extreme weather events. However, traditional experimental methods for unsaturated soils, including the measurement of the soil-water characteristic curve (SWCC), soil hydraulic conductivity function (SHCF), shear strength envelope, etc., are time-consuming. To overcome this limitation, a rapid testing strategy is proposed. In the experimental design, the water saturation level is selected as the control variable instead of the suction level. In the suction measurement, the suction monitoring method is adopted instead of the suction control method, allowing for simultaneous testing of multiple soil samples. The proposed rapid testing strategy is applied to measure the soil hydro-mechanical properties over a wide suction/saturation range. The results demonstrate that: (1) only 3-4 samples and 2-5 days are in need in the measurement of SWCC; (2) 7 days is enough to determine a complete permeability function; (3) only 3 samples and 3-7 days are in need in the measurement of the shear strength envelope; (4) pore size/water distribution measurement technique is fast and recommended as a beneficial supplement to traditional test methods for unsaturated soils. Our findings suggest that by employing these proposed rapid testing methods, the measurement of pivotal properties for unsaturated soils can be accomplished within one week, thus significantly reducing the temporal and financial costs associated with experiments. The findings provide a reliable experimental approach for the rapid risk assessment of geological disasters induced by extreme climatic events.

Previous studies provide ample experimental evidence highlighting the effect of temperature on the volume change response of unsaturated soils. However, analytical efforts to capture the temperature dependency of dilatancy under shear stresses are notably scarce. This paper aims to fill this gap by presenting a thermodynamics-based dilatancy model incorporating the influence of the degree of saturation, temperature, soil type, and suction. The model is derived from the first law of thermodynamics, formulated in terms of stored and dissipative energies. Various sources of energy dissipation, including entropy, water flow, friction, as well as energies associated with volume change and rearrangement of soil grains, are considered. The temperature-dependent model is calibrated, and its accuracy is validated using data from 27 triaxial experiments available in the literature. This data set encompasses tests conducted under different temperatures, suctions, stress states, and initial void ratios. The accuracy of the proposed model is compared to three classic models present in the literature that do not account for suction and temperature. The findings demonstrate that the model adeptly captures the complex stress-dilatancy behavior of unsaturated soils with considerably higher accuracy than alternative models. Further, the proposed model's application to simulate the volume change response is demonstrated for two soils under varying saturation levels. The model can readily be incorporated into constitutive modeling of unsaturated soils under thermo-hydro-mechanical conditions.

Soil-water characteristics, which vary with hydrological events such as rainfall, significantly influence soil strength properties. These properties are crucial determinants of the bearing capacity of foundations. Moreover, shear strength characteristics of soils are inherently spatially variable, and considering them as homogeneous parameters can result in unreliable design. This paper presents a probabilistic study of the two-dimensional bearing capacity of a strip footing on spatially random, unsaturated fine-grained soil using Monte Carlo simulation. The study employs the hydro-mechanical random finite difference method through MATLAB programming along with FLAC2D software. The undrained shear strength under saturated conditions is modelled as random fields using a log-normal distribution. The generated random values are then made depth-dependent by correlating them with matric suction. Initially, matric suction is assumed to be under a hydrostatic condition and decreases linearly with depth to zero at the groundwater level. Afterward, unsaturated soil is subjected to rainfall with different durations, resulting in the non-linear distribution of matric suction and, consequently, the mean value of undrained shear strength in depth. The results showed that rainfall infiltration impacts the strength characteristics of near-surface heterogeneous strata, leading to significant effects on the bearing capacity and failure mechanism of footing.

This study investigated contact distribution and force anisotropy associated with elliptical particles in granular soils within the pendular state of unsaturated soils, employing the discrete element method. The high cost of determining the micromechanical factors through laboratory tests justifies the use of this method. The macromechanical behavior of unsaturated granular soils depends on interparticle contact characteristics and liquid bridge behavior. The findings indicated that as the degree of saturation increased, both the shear strength and the anisotropy of the normal and shear forces initially rose before subsequently declining. Notably, the contact normal anisotropy exhibited minimal variation with changes in saturation. Furthermore, it was observed that as confining pressure increased at a specific eccentricity and degree of saturation, the associated anisotropies exhibited a continuous increase. In this context, as the eccentricity of the particles increased, the peak shear strength and its corresponding anisotropies initially increased and then decreased. Conversely, residual soil strength showed a consistent increase in shear strength and anisotropy with rising eccentricity.

This study investigates the cyclic response of unsaturated soils, focusing on the dynamic properties such as damping characteristics and soil stiffness, under varying matric suction and confining stress conditions during cyclic triaxial loading. Despite challenges in evaluating unsaturated soils compared to saturated ones, cyclic triaxial testing emerges as an efficient method for exploring their cyclic behavior. Through a series of experiments with different loading frequencies, stress levels, and suction conditions, the research reveals that as matric suction increases, stiffness rises while the damping ratio decreases. Additionally, comparisons between isotropic and anisotropic stress conditions show that the shear modulus is higher under anisotropic consolidation due to particle reorientation. The study proposes a semi-empirical equation to address the stress and suction dependency of shear modulus, finding a consistent trend between predicted and measured values. Ultimately, the findings underscore the significance of stress state, suction, cyclic shear strain, number of loading cycles and confining pressure in determining soil shear modulus.

The spatial distributions of hydraulic conductivity and shear strength parameters are influenced by the soil structure, property and mineral composition. However, hydraulic conductivity is not only determined by the intrinsic soil property but also influenced by external factors such as fractures and interlayers. This study investigates the impact of the asynchronism between the spatial distribution of hydraulic conductivity and shear strength parameters on the reliability assessment and failure mechanism of unsaturated soil slopes with different titled stratifications under rainfall conditions. The results indicate that the asynchronism in the rotational angles (alpha) of hydraulic conductivity and shear strength parameters shows the greatest impact on the probability of failure (Pf) of slopes. By contrast, the asynchronism in the scales of fluctuation of hydraulic conductivity and shear strength parameters and employing different autocorrelation functions (ACFs) show minor impact on the Pf. The impact of using different ACFs, alpha, and scales of fluctuation to characterise the spatial variability of hydraulic conductivity on sliding mass and failure modes is minimal.

Currently, our understanding of material-scale deterioration resulting from meteorologically induced variations in pore water pressure and its significant impact on infrastructure slopes is limited. To bridge this knowledge gap, we have developed an extended kinematic hardening constitutive model for unsaturated soils that refines our understanding of weather-driven deterioration mechanisms in heterogeneous clay soils. This model has the capability of predicting the irrecoverable degradation of strength and stiffness that has been shown to occur when soils undergo wetting and drying cycles. The model is equipped with a fully coupled and hysteretic water retention curve and a hysteretic loading-collapse curve and has the capability to predict the irrecoverable degradation of strength and stiffness that occurs during cyclic loading of soils. Here, we employ a fully implicit stress integration technique and give particular emphasis to deriving a consistent tangent operator, which includes the linearisation of the retention curve. The proposed algorithm is evaluated for efficiency and performance by simulating various stress and strain-driven triaxial paths, and the accuracy of the integration technique is evaluated through the use of convergence curves.

In this research, the thermomechanical formulation proposed by Ziegler and formalized by Houslby and Puzrin to build up hyperplastic constitutive models is applied to the case of unsaturated materials. The mechanical model is based on two main equations: the free energy and the dissipation functions. The former represents the elastic behavior while the latter accounts for the plastic behavior of the material. The dissipation function can be split into two parts: one represents the flow rule and the other the yield surface. The shape of the yield surface can be modified by a single parameter while the plastic flow is of the non-associated type and can also be modified with a single parameter. The yield surface rotates at the origin depending on the anisotropy of the material. The volumetric behavior of the soil is related to the distance between its current state and the normally consolidated, the critical state, and the unloading-reloading lines. The model considers the phenomenon of suction hardening and employs Bishops equation to determine the effective stress on unsaturated materials. The mechanical model is coupled to a porous-solid model that can simulate the soil-water retention curves during wetting-drying cycles and accounts for the hydro-mechanical coupling phenomenon. In that sense, this procedure does not require pre-establishing the shape of the yield surface or the flow rule. The resulting model is a three-dimensional hyperplastic coupled model that requires few parameters. Comparisons between experimental and numerical results show that the proposed model can simulate the behavior of soils with fair precision.

This article studies the undrained behavior of filtered copper tailings in unsaturated conditions under monotonic and cyclic loading at controlled matric suction. Two dry densities with the same water content were used. The material behavior was studied in terms of the increase of saturation, the evolution of suction, and volumetric strain during the transition from unsaturated to saturated conditions. The results show that, during the shearing, suction decreases and the degree of saturation increases, regardless of the type of load applied (cyclic or monotonic). This effect is related to the volume reduction of the air phase during the transition to a fully saturated condition. In terms of undrained shear strength, the evolution of the material is studied in terms of the phase transformation line, comparing its location in saturated and unsaturated cases. Regarding the cyclic strength ratio, the unsaturated condition shows a higher value than the saturated case by about 26% and 58% for the high and low densities, respectively. However, when the volumetric strain is higher than 3% or the double amplitude axial strain exceeds 2%, a cyclic strain localization takes place, leading to an extension failure over the sample.