The unified effective stress equation based on suction stress, a widely accepted method for calculating effective stress in unsaturated soils, provides a closed-form solution that enables the characterization of soils in both saturated and unsaturated states. The effect of desaturation on the water content of natural and treated soils was studied with respect to unconfined compressive strength (UCS) and indirect tensile strength (ITS). The soil's moisture-dependent behavior was characterized by the van Genuchten (Soil Sci Soc Am J 44:892-898, 1980. https://doi.org/10.2136/sssaj1980.03615995004400050002x) and Lu et al. (Water Resour Res, 2010. https://doi.org/10.1029/2009wr008646) models and implemented using the equation. Suction tests were conducted using the dew point and filter paper methods, alongside UCS and ITS tests, on silty clay soil and microsilica-treated soil with microsilica contents of 5%, 10%, and 15%. The equation was validated by comparing mean total stress (p) and mean effective stress (p ') to deviatoric stress (q) and analyzing the friction angle at different suction levels. It proved applicable to both natural and treated soils, with valid moisture content ranges of 4-17.5% and 6-20%, respectively. This study experimentally confirms the equation's effectiveness in characterizing the hydro-mechanical behavior of soils under varying moisture conditions.

Slow-moving landslides are typically characterised by pre-existing shear zones composed of thick, clay-rich, and mechanically weak soil layers that exhibit heightened sensitivity to changes in moisture content and hydrological conditions. These zones, often governed by variations in suction and degree of saturation, play a critical role in the stability and long-term behaviour of slow-moving landslides. In this study, we investigate the influence of the degree of saturation on the mechanical properties of shear-zone soils from a reactivated slow-moving landslide in the Three Gorges Reservoir area, China. A series of laboratory experiments, including consolidation, reversal direct shear, and ring-shear tests, were conducted on reconstituted shear-zone soil samples at varying degrees of saturation. The test results indicate that increasing the degree of saturation has a marked impact on the compressibility of the soils, with saturated samples exhibiting greater compressibility and unsaturated samples demonstrating reduced compressibility. Both shear tests indicate that higher saturation leads to a reduction in peak and residual shear strength, likely due to elevated pore water pressures and a decrease in inter-particle bonding forces. These insights emphasise the need to account for varying degrees of saturation when analysing the mechanical behaviour of slow-moving landslides, contributing to an improved understanding of their deformation patterns and failure mechanisms.

Foundation designs typically rely on traditional soil mechanics principles, which assume the soil is either completely saturated or entirely dry. However, the impact of soil suction associated with the alternate wetting and drying conditions in the unsaturated zone (i.e. soil suction) is generally overlooked in traditional design approaches. This may lead to ground heave or differential settlement contributing to extreme distress to various infrastructures built in unsaturated expansive soils. Shallow foundations are usually built above the groundwater table, leaving much of the soil beneath them unsaturated. As a result, soil suction greatly affects the bearing capacity and settlement behaviour. Further, deep foundations extend through the active layer of unsaturated expansive soil until reaching the bedrock or rest on a high-quality soil-bearing stratum. The volume-changing behaviour of the unsaturated expansive soil typically moves upward along the pile, creating additional positive friction that can potentially uplift lightly loaded structures. This paper presents a review of foundation behaviour in unsaturated expansive soils. Particularly, this review focuses on the influence of matric suction on soil-volume expansion which contributes to the ground heave, soil-structure interface shear strength properties, bearing capacity, and load-settlement behaviour of foundations.

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.

In India, particularly within its Northeastern territories, landslides triggered by rainfall following dry periods are a major concern, consistently causing extensive damage to both life and infrastructure. This study focuses on mitigating their impact through preemptive measures, with an emphasis on analyzing slope stability to determine critical intervention points. The investigation includes experimental tests on soil samples to assess key parameters, such as soil matric suction and unconfined compressive strength, alongside an analysis of slope failures during the 2017 monsoon in Mizoram's Lunglei district. Employing Soil-Water Characteristic Curves (SWCC) derived from ASTM D5298-10 standards and a microwave drying technique for preparing soil samples, the research evaluates the condition of the slopes before and after monsoonal rains. This study utilizes a blend of numerical modeling and empirical laboratory investigations to explore the factors contributing to slope instability. The findings underscore the necessity of advanced landslide warning systems, suggesting that a deeper understanding of rainfall-induced slope failures could significantly enhance disaster preparedness and reduce potential damages.

The major problematic soils in semi-arid regions include expansive soils and collapsible soils. These two types of soils cause problems and are hazardous for buildings when moisture is introduced following a dry or semi-dry season. In order to assess the risk and damage likely to occur, a protocol of investigation needs to be considered by geotechnical engineers to quantify and assess the possible heave or collapse that may occur. The characterization and prediction of unsaturated soil behavior in semi-arid areas can now be enabled following the advancement of unsaturated soil mechanics. Heave is associated with the wetting of expansive soils, while excessive settlement or the sudden loss of support may occur when water is introduced to collapsible soils. This work calls for more than one parameter for the assessment of problematic soils to avoid misleading predictions based on a single test. This study presents an investigation of two sets of soil samples obtained from semi-arid areas in Saudi Arabia known for their collapsible or expansive nature. Tests under controlled suction and variable effective stress were conducted. The air entry values, inflection points, and residual points were established and compared for the two problematic soils. A series of oedometer tests was conducted for typical soils, and settlement and collapse were measured and assessed. The swell potential for the tested clays varied from 4% to 22%. It is possible to integrate the data from the soil-water characteristic curve (SWCC) and compressibility tests with any project specification and applied stresses to produce reliable recommendations for the construction and protection of structures in hazardous soils.