Mesh-free methods, such as the Smooth Particle Hydrodynamics (SPH) method, have recently been successfully developed to model the entire wetting-induced slope collapse process, such as rainfall-induced landslides, from the onset to complete failure. However, the latest SPH developments still lack an advanced unsaturated constitutive model capable of capturing complex soil behaviour responses to wetting. This limitation reduces their ability to provide detailed insights into the failure processes and to correctly capture the complex behaviours of unsaturated soils. This paper addresses this research gap by incorporating an advanced unsaturated constitutive model for clay and sand (CASM-X) into a recently proposed fully coupled seepage flow-deformation SPH framework to simulate a field-scale wetting-induced slope collapse test. The CASM-X model is based on the unified critical state constitutive model for clay and sand (CASM) and incorporates a void-dependent water retention curve and a modified suction-dependent compression index law, enabling the accurate prediction various unsaturated soil behaviours. The integration of the proposed CASM-X model in the fully coupled flow deformation SPH framework enables the successful prediction of a field-scale wetting-induced slope collapse test, providing insights into slope failure mechanisms from initiation to post-failure responses.

A tensor-type capillary stress, instead of a scalar suction, has been proposed to serve as a stress-like state variable to capture the effects of capillarity in the mechanics of unsaturated granular soils. However, the influence of water content on the evolution of capillary stress in such soils remains insufficiently understood. This study performs numerical simulations of unsaturated granular soils in the pendular regime using the Discrete Element Method (DEM) involving a volume-controlled capillary bridge model. In these simulations, water content is maintained constant by redistributing the water from ruptured capillary bridges to adjacent ones. The evolution of capillary stress with varying water contents during triaxial and biaxial loading conditions is systematically examined. The DEM simulation results show that, under both loading conditions, the mean component of the capillary stress generally decreases, while its deviatoricity gradually develops. These changes are observed to become less significant as the initial degree of saturation increases. At low saturation levels, capillary bridges between non-contacting particle pairs rupture due to soil deformations, and the water from these ruptured bridges redistributes to existing contacts. This redistribution leads to an anisotropic distribution of pore water aligned with the contact network. At higher saturation levels, non-contacting capillary bridges persist due to their ability to sustain large relative displacements between particles, allowing the spatial distribution of pore fluids to remain less constrained by the solid contact network. Additionally, at higher water contents, relative sliding and particle rearrangement are the primary factors influencing the directional distribution of capillary bridges.

The entrance of permafrost tunnels in cold regions is particularly vulnerable to frost damage caused by complex thermal-hydro-mechanical (THM) interactions in unsaturated frozen soils. The effects of temperaturedependent volumetric strain variations across different stratum materials on heat and moisture transport are often neglected in existing THM coupling models. In this study, a novel THM coupled model for unsaturated frozen soil integrating volumetric strain correction is proposed, which addresses bidirectional interactions between thermal-hydraulic processes and mechanical responses. The model was validated through laboratory experiments and subsequently applied to the analysis of the Yuximolegai Tunnel. The results indicate that distinct layered ice-water distribution patterns are formed in shallow permafrost under freeze-thaw cycles, driven by bidirectional freezing and water migration. Critical mechanical responses were observed, including a shift in maximum principal stress from the invert (1.40 MPa, frozen state) to the crown (5.76 MPa, thawed state), and periodic lining displacements (crown > invert > sidewalls). Frost damage risks are further quantified by the spatial-temporal zoning of ice-water content-sensitive regions. These findings advance unsaturated frozen soil modeling and provide theoretical guidance for frost-resistant tunnel design in cold regions.

This study evaluates dykes stability of bauxite residue storage facility using limit equilibrium (LEM) and finite element methods (FEM), considering diverse construction phases. In LEM, steady state seepage is simulated using piezometric line while factor of safety (FOS) is determined by Morgenstern-Price method using SLOPE/W. In FEM, actual loading rates and time dependent seepage is modelled by coupled stress-pore water pressure analysis in SIGMA/W and dyke stability is assessed by stress analysis in SLOPE/W, referencing SIGMA/W analysis as a baseline model. Both the analysis incorporated suction and volumetric water content functions to determine FOS. FEM predicted pore pressures are validated against in-situ piezometer data. The results highlight that coupled hydro-mechanical analysis offers accurate stability assessment by integrating stress-strain behaviour, pore pressure changes, seepage paths, and dyke displacements with time. It is found that inclusion of unsaturated parameters in Mohr-Coulomb model improved the reliability in FOS predictions.

This study addresses the cracking issue of airport foundations in marine and coastal regions by proposing an unsaturated reinforcement method based on Microbially Induced Calcium Carbonate Precipitation (MICP) combined with coconut fibers. Composite sand columns incorporating coconut fiber and bioslurry were prepared, and the effects of fiber length and content on the mechanical properties of MICP-treated sand columns were investigated. Experimental results revealed that the addition of short fibers (1-5 mm) significantly improved the unconfined compressive strength and ductility of the MICP-treated sand columns. As the bioslurry content decreases in the sand columns, the enhancement effect of short fibers on the unconfined compressive strength becomes more pronounced, with fiber addition improving compressive strength by up to 98 %. However, the inclusion of medium fibers (5-10 mm) and long fibers (10-15 mm) negatively affected the mechanical properties of the sand columns. Microstructural analysis further confirmed the synergistic reinforcement effect of short fibers and calcium carbonate precipitation. Short fibers acted as bridges, forming additional contact points between sand particles, which facilitated calcium carbonate precipitation at critical contact points, thereby enhancing the overall stability and strength of the sand columns. This characteristic was more pronounced under unsaturated conditions. This study provides a feasible technical solution for the effective reinforcement of airport foundations and demonstrates potential in unsaturated reinforcement and improving the ductility of sandy soil foundations.

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.

This paper presents an experimental investigation into the interaction mechanism between aqueous foam and unsaturated granite residual soil during conditioning. Contact filter paper tests and undrained shear tests were used to analyze foam's effects on soil water retention and shear behavior, while surface tension tests, capillary rise tests, and microscopic observations examined the role of soil particles in foam stability. The findings demonstrate that foam-conditioned granite residual soils exhibit three distinct saturation- dependent phases (soil-only, transition, and soil-foam mixture) governed by foam's gas-liquid biphasic nature, with foam injection effectively reducing matric suction in unsaturated conditions. Increasing foam injection ratio reduces shear stress while enhancing pore water pressure, with vertical displacement transitioning from contractive to expansive behavior at low shearing rate. Effective cohesion stress varies with gravimetric water content via a rational function, while other effective cohesion stress and friction angles with respect to foam injection ratio, shearing rate, and gravimetric water content obey exponential relationships. The probability distribution function, cumulative distribution function, and decay pattern of bubbles in foam-only systems and soil-foam mixtures all exhibit exponential relationships with elapsed time. Furthermore, a new water-meniscus interaction model was established to characterize rupture and stabilization mechanisms of foam in unsaturated granite residual soils, with particular emphasis on capillary-dominated behavior. Saturation-dependent particle contact modes were identified for foam-conditioned unsaturated granite residual soils, offering valuable guidance for enhancing soil conditioning protocols in earth pressure balance shield tunneling operations.

Rainfall-induced debris slides are a major geological hazard in the Himalayan region, where slopes often comprise heterogeneous debris-a complex mixture of rock and soil. The complex nature makes traditional soil or rock testing methods inadequate for assessing such debris's engineering behaviour and failure mechanisms. Alternatively, reduced-scale flume experiments may aid in understanding the failure process of debris slopes. Here, we present findings from reduced-scale laboratory flume experiments performed under varying slope angles (ranging from shallow to steep), initial volumetric water contents (ranging from dry to wet), and rainfall intensities (ranging from light to heavy) using debris materials with a median grain size (D50) 20.7 mm sampled from a rainfall-induced debris slide site in the Himalayas. Hydrological variables, including volumetric water content and matric suction, were monitored using sensors, while slope displacement was tracked indirectly, and rainfall was monitored using rain gauges. The entire failure process was captured via video recording, and index and shear strength tests were performed to characterize the debris material. Our results reveal that the failure of debris slopes is not driven by sudden increases in pore water pressure but by the loss of unsaturated shear strength due to reduced matric suction and a decreased frictional strength from reduced particle contact between grains during rainfall. We also find that the saturation of debris slope by rainfall was quick irrespective of the slope angles and initial moisture contents, revealing the proneness of debris slopes to rainfall-induced failures. These findings provide critical insights into the stability of debris materials and have important implications for improving risk assessment and mitigation strategies for rainfall-induced debris slides in the Himalayas and similar regions worldwide.

The majority of existing effective stress-based constitutive models approach thermal effects through the temperature dependency of surface tension and its effects on the soil-water retention curve (SWRC) and effective stress. Experimental tests and theoretical studies, however, suggest that the temperature effect on surface tension alone is not sufficient to properly explain thermal-induced changes in the effective stress and SWRC. This study focuses on the temperature-dependent elastoplastic behavior of low plasticity unsaturated soils by developing a set of constitutive-level relations that incorporate temperature-dependent SWRC and effective stress models. These models account for the effect of temperature on the enthalpy, contact angle, and surface tension. The application of the presented constitutive relations was demonstrated and validated for low plasticity soils, specifically incorporating temperature effects into the hardening modulus, specific volume change, yield stress of the modified Cam-Clay model, and stress-strain relationships. The proposed relationships are incorporated in any effective stress-based constitutive model for modeling temperature dependency of elastoplastic response in low plasticity unsaturated soils. Employing these relationships can enhance the numerical simulation of low plasticity unsaturated soils under thermo-mechanical or other coupled processes involving temperature-dependent conditions.