Soil suction and the degree of saturation influence the resilient modulus, which should be considered when designing road and railway foundations. To reasonably account for these factors, a suction stress model incorporating both suction and degree of saturation was established for application in transportation design. In this study, the suction stress was considered by categorizing water into bulk and meniscus water and applying cyclic loading to the samples. The soil water characteristic curve (SWCC) was generated using discrete element method (DEM) analysis. Additionally, the resilient modulus was observed to vary with both deviator stress and the degree of saturation. The trends observed in this study are consistent with the results of laboratory tests conducted by other researchers. These findings demonstrated that the water division and cyclic loading algorithm effectively represented the unsaturated soil state. Furthermore, DEM analysis revealed that the suction stress was influenced the resilient modulus, with the resilient modulus increasing as suction stress increased.

A method directly using rainfall records to predict a slope's potential instability is devised. The method consists of three sequential steps: identifying the critical suction stress (pore water pressure when soil is saturated) profile of a given slope, developing a rainfall intensity-duration threshold curve for the slope, and using rainfall record to determine if the threshold is reached (failure occurs) or not (no failure). It innovatively uses a slope's strength parameters and slope angle to develop the critical suction stress (tensile) or compressive pore water pressure profile where, at each depth within the slope, the effective stress reaches the failure state. Hydromechanical numerical modeling is then conducted under various rainfall intensities to identify their corresponding duration for slope failure, thus, the rainfall intensity-duration threshold curve of the slope. Two previously well-documented and studied rainfall-induced slope instability cases; one near the town of Edmonds, Washington State, and the other near the village of R & uuml;dlingen in northern Switzerland are used to validate the method. Excellent predictions of the slope failure depth and timing are demonstrated, indicating the effectiveness of the proposed method. Because the suction stress-based rainfall intensity-duration curve is characteristic of a given slope and it can be determined a priori, the method provides a practical way to conduct real-time rainfall monitoring and predict instability for a specific slope, and a pathway to forecast instability of natural slopes in a region.

Air-fall pyroclastic soil deposits usually display a loose fabric composed of alternating layers of ashes and pumices. Such deposits, when lying on steep slopes, represent a major geohazard due to the occurrence of landslides. This is the case of the carbonate massifs in Campania (southern Italy), a wide landslide-prone area of approximately 400 km2 covered with pyroclastic soils. In such cohesionless deposits, the additional shear strength provided by soil suction in unsaturated conditions is important for ensuring slope stability and can be jeopardized by soil wetting during rainwater infiltration. This paper provides a comprehensive view of the hydraulic and shear strength characteristics of different layers of pyroclastic deposits at different sites in Campania, revealing a broad view of their similarities and differences. To that end, some datasets from previous studies and novel data are gathered, linking the index properties, the hydraulic behavior of the soils and the contribution of suction to the shear strength of the studied materials. Two types of ashes at different positions within the stratigraphic sequence are identified: ashes interbedded between pumice layers, where landslide failure surfaces usually occur, and altered ashes in contact with the bedrock, which affects water leakage from the overlying soil profile. The former show quite uniform characteristics, and this allowed testing some predictive models for the assessment of the unsaturated shear strength of pyroclastic ashes in the absence of direct measurements. In contrast, the latter may exhibit significantly different behaviors, with great variability in hydraulic and mechanical properties.

Granite residual soil is a widely encountered clayey soil with unique microscopic soil structures. Its soil structures mainly stem from the presence of interparticle cementation by iron oxide. An attempt is made herein to experimentally quantify the impact of interparticle iron oxide cementation on soil's mechanical properties, particularly in the high suction range. The amount of interparticle cementation is artificially generated by mixing soil samples with varying mass fractions of iron hydroxide colloid. The mechanical behavior of these soil samples in the full suction range is measured via the drying cake test. Preliminary experimental results demonstrate that interparticle iron oxide cementation can significantly decrease soil shrinkage (by up to 52%), and substantially increase soil elastic modulus (by up to 1.83 times) and negative suction stress change (by up to 0.82 times).

Under the combined effect of rainfall and water level fluctuation, the slope of the reservoir bank is prone to collapse. Ideal elastic-plastic Mohr-Coulomb criterion is used to analyze the stability of reservoir slope, which is difficult to characterize the complex mechanical characteristics of slope soils, such as over consolidation dissipation under wet-dry cycles. It is difficult to analyze the stability of bank slopes under the action of dry and wet cycles to reflect the complex mechanical properties such as overconsolidation and dissipation of slope soils. In this study, focusing on weakly overconsolidated unsaturated red clay at the reservoir bank slope of the Xingan shipping-hydropower junction project in Jiangxi Province, unsaturated direct shear tests were conducted and an overconsolidated unified hardening (UH) model for red clay was constructed. The UH model incorporates the mathematical-physical relationship between suction stress and matric suction using the arctangent function. Subsequently, based on the UH model, an application program of FLAC3D was developed in C++. The SEEP/W module of GeoStudio software was employed to compute the unsaturated seepage field during the rainfall infiltration, and an interface program was created to import FLAC3D data for stability calculations of the reservoir slope. Comparisons between the horizontal displacements obtained from the improved UH model and the classical unsaturated elastic-plastic model revealed significantly larger displacements in the former, suggesting that the improved UH model can provide reasonable predictions of the overconsolidated unsaturated red clay for reservoir slope stability. This research offers valuable insights for similar projects involving analyses of reservoir bank slope.

Rainwater infiltration is one of the main triggering factors in slope failure. Therefore, exploring the unsaturated slope behavior is essential. However, studies generally ignored the impact of soil -water characteristic curve (SWCC) hysteresis caused by wet -dry cycles in engineering practice. SWCC measured in the drying process is commonly used to estimate slope behavior in the wet -dry cycle. Three soils of Toyoura sand, Hiroshima decomposed granite soil (Masado soil), and DL clay will be taken as examples to examine the infinite slope stability under the effect of SWCC hysteresis. Firstly, this research examines soils' SWCC and suction stress characteristic curves (SSCC). Then, the factor of safety (FOS) changes are further analyzed when suction stress is considered the confining pressure. The results indicate that FOS for soils with small cohesion and air -entry value is greatly affected by SWCC hysteresis. As the depth between the selected slip surface and slope surface increases, the disparity between FOSs calculated through wetting FOS and drying FOS will decrease sharply. Therefore, for shallow slope stability analysis, only using the SWCC measured during the drying process to evaluate the entire wet -dry cycle might lead to underestimating slope failure potentiality.

Suction stress, the part of effective stress induced by soil-water interaction, is the source for the intrinsic cohesion of fine-grained soil slurries. Here, the previous unified effective stress equation is generalized to extend the suction stress variation from the liquid state to the oven-dry state, yielding an augmented closed-form equation. This equation includes a new term, named slurry adsorptive suction stress, to incorporate the adsorptive mechanism of soil slurries at the liquid state. This adsorption mechanism involves the interparticle van der Waals attraction, face-to-edge attraction, and electrical double-layer repulsion when soils are in the liquid state. The proposed equation is validated with a wide array of 12 fine-grained soils' shrinkage curves, modulus, and suction stress data measured by the drying cake test. It is demonstrated that the proposed equation can excellently capture the experimental data across all saturations. Furthermore, the practical implications of the proposed model are illustrated via its relevance to rheological properties of soil slurries and correlations with both liquid limit and plastic limit. (c) 2024 American Society of Civil Engineers.