The shear strength of compacted bentonite is crucial for preventing tilting and damage of the waste canisters in deep geological repositories (DGRs). The shear strength evolution along the confined wetting path also needs to be investigated, given the long saturation time of the bentonite buffer. This study conducted direct shear tests on densely compacted Gaomiaozi bentonite after suction control under confined conditions to determine its peak shear strength and strength parameters. Furthermore, the shear strength evolution along the confined wetting path was modeled on the basis of the effective stress principle. The results show that, for a given dry density, the peak shear strength at a given vertical pressure and the strength parameters exhibit an overall decrease along the confined wetting path. Moreover, the peak shear strength of the specimen that underwent confined wetting was considerably lower than that of the as-compacted specimen with the same total suction, indicating that the suction value and microstructure codetermine the peak shear strength of compacted Gaomiaozi bentonite. For this reason, the peak shear strength in the as-compacted state and the dual-porosity water retention curves established along the confined wetting path were used to model the shear strength evolution along the confined wetting path. The substitution equation for the effective stress parameter chi was selected on the basis of the experimental evidence. Finally, the model parameters were calibrated from the shear strength evolution at a given vertical pressure, and they reasonably reproduced the shear strength evolution under other vertical pressures. These findings can be helpful for the design and safe operation of DGRs under extreme geological conditions.

Agricultural soils are often affected by compaction due to machinery loads, which alters pore-size distribution and thus hydraulic properties. Up to date most studies on traffic and its impact on soil functions lack a detailed analysis of the effect on pore-size distribution (PSD). Our study aimed to understand how different machinery types, load levels, and moisture conditions impact the water retention curve (WRC) and PSD at various soil depths and field areas (headland or inner field). Eight field campaigns were conducted between 2016 and 2019 on a variety of sub-fields within one agricultural farm site with a clayey-silty soil. Undisturbed soil samples were collected before and after the harvest of winter wheat, silage maize, and sugar beet, and before and after digestate application. The van Genuchten model was fitted to the laboratory data, and parameters were interpreted to deduce WRC features. Additionally, the pore water pressure head at the pore-size density maximum (PSDmax) was determined and interpreted. The parameter alpha responded to all types of field traffic and decreased with increased load, indicating a shift from coarser to finer pores. The parameter n generally increased due to field traffic, suggesting a narrowed pore-size distribution. The theta s parameter, associated with porosity, decreased in all trials, with the tendency of lowest values occurring after wheeling under moist conditions. Load-induced shifts in the PSDmax towards finer pores were obvious down to 50 cm depth, even with relatively low loads. Our findings indicate that the majority of vehicles utilized in conventional agricultural operations can lead to severe soil compaction.

This study investigated the hydraulic and mechanical behaviors of unsaturated coarse-grained railway embankment fill materials (CREFMs) using a novel unsaturated large-scale triaxial apparatus equipped with the axis translation technique (ATT). Comprehensive soil-water retention and constant-suction triaxial compression tests were conducted to evaluate the effects of initial void ratio, matric suction, and confining pressure on the properties of CREFMs. Key findings reveal a primary suction range of 0-100 kPa characterized by hysteresis, which intensifies with decreasing density. Notably, the air entry value and residual suction are influenced by void ratio, with higher void ratios leading to decreased air entry values and residual suctions, underscoring the critical role of void ratio in hydraulic behavior. Additionally, the critical state line (CSL) in the bi-logarithmic space of void ratio and mean effective stress shifts towards higher void ratios with increasing matric suction, significantly affecting dilatancy and critical states. Furthermore, the study demonstrated that the mobilized friction angle and modulus properties depend on confining pressure and matric suction. A novel modified dilatancy equation was proposed, which enhances the predictability of CREFMs' responses under variable loading, particularly at high stress ratios defined by the deviatoric stress over the mean effective stress. This research advances the understanding of CREFMs' performance, especially under fluctuating environmental conditions that alter suction levels. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Featured Application The findings of this study establish the behavior of sanitary landfill cover materials, such as compacted clay and compacted polyurethane-clay, in unsaturated conditions under several wet-dry cycles, which would aid in predicting the performance of the material under varying environmental conditions. By predicting the unsaturated hydraulic conductivity and understanding the effects of environmental stresses, the findings can aid in the design and implementation of more durable and efficient landfill liners and covers.Abstract Sanitary landfill covers are exposed to varying environmental conditions; hence, the state of the clay layer also changes from saturated to unsaturated. The study aimed to predict the unsaturated hydraulic conductivity of the locally available compacted clay and clay with polyurethane to determine their behavior as they change from wet to dry using matric suction and empirical models proposed through other studies. The specimens underwent three wet-dry cycles wherein the matric suction was determined for several moisture content levels as the specimen dried using the filter paper method or ASTM D5298. The results showed that the factors affecting the soil structure, such as grain size difference between clay and polyurethane-clay, varying initial void ratios, and degradation of the soil structure due to the wet-dry cycles, did not affect the matric suction at the higher suction range; however, these factors had an effect at the lower suction range. The matric suction obtained was then used to establish the best fit water retention curve (WRC) or the relationship between the matric suction and moisture content. The WRC was used to predict the unsaturated hydraulic conductivity and observe the soil-water interaction. The study also observed that the predicted unsaturated hydraulic conductivity decreases as the compacted specimen moves to a drier state.

Water loss in paddy fields occurs through various pathways, and previous studies have primarily focused on water seepage in the field, often overlooking the potential for the field-bund area. In this study, 3 typical paddy fields in the plain river network area of southeastern China were selected to clarify the differences in the soil structure and hydraulic characteristics at different positions within the field-bund area: the field, inner bund, middle bund and outer bund. The interactions between basic soil properties and hydraulic characteristics were also evaluated. The results revealed that the outer bund presented the lowest soil porosity (6.92 %), followed by the field (7.52 %), middle bund (7.77 %), and inner bund (8.09 %). The soil pores in the field presented the smallest mean diameter and fractal dimension and the highest degree of anisotropy. The deep layer of the bund contained more macropores, and the soil pores exhibited greater spatial distribution heterogeneity. The bottom layer in the field and bund presented the lowest average Ks value of only 0.05 mm min(-1), indicating the presence of a plow pan and a notable tendency for lateral seepage. Differences in the soil structure and hydraulic parameters between the field and bund created a driving force for lateral seepage and rendered the field-bund area a hotspot for water loss. For the analysis of the underlying water loss mechanism, the structural equation model represented 65 % of the total variance in the hydraulic parameters. The micropore characteristics had the greatest positive direct effect on the hydraulic parameters, with a standardized path coefficient of 0.39 (p < 0.001). The soil physical properties were not directly related to the hydraulic parameters but exerted an indirect effect through aggregate stability and micropore and macropore characteristics, with a total indirect standardized path coefficient of -0.41.

In this paper, the EC-5 water sensor and the MPS-6 water potential sensor were used to measure water content and suction, respectively, to investigate the evolution of soil-water retention properties of compacted loess samples prepared at different dry densities and subjected to different numbers of wetting-drying cycles. The water retention data were integrated with a detailed microstructural investigation, including morphological analysis (by scanning electron microscopy) and pore size distribution determination (by nuclear magnetic resonance). The microstructural information obtained shed light on the double porosity nature of compacted loess, allowing the identification of the effects of compaction dry density and wetting-drying cycles at both intra- and inter-aggregate levels. The information obtained at the microstructural scale was used to provide a solid physical basis for the development of a simplified version of the water retention model presented in Della Vecchia et al. (Int J Numer Anal Meth Geomech 39: 702-723, 2015). The model, adapted for engineering application to compacted loess, requires only five parameters to capture the water retention properties of samples characterized by different compaction dry densities and subjected to different numbers of wetting-drying cycles. The comparison between numerical simulations and experimental results, both original and from the literature, shows that only one set of parameters is needed to reproduce the effects of dry density variation, while the variation of only one parameter allows the reproduction of the effects of wetting and drying cycles. With respect to the approaches presented in the literature, where ad hoc calibrations are often used to fit density and wetting-drying cycle effects, the model presented here shows a good compromise between simplicity and predictive capabilities, making it suitable for practical engineering applications.

The mechanical behavior and strength characteristics of unsaturated fine-grained soils with dual-porosity are of crucial importance in geotechnical designs. Nanyang fine-grained soils have been selected as typical dual-porosity structure soils to perform experimental tests under a wide range of suction and different initial densities while studying its stress-strain-strength properties constitute the main scope of this study. Axial translation and vapor equilibrium techniques are jointly employed to apply a wide suction range. Our data suggest that soil behavior transits from strain-hardening with shear-induced contraction to strain-softening with shear-induced dilation as suction and density increase. By exploiting a bi-modal soil-water retention curve (SWRC) explicitly separating capillarity and adsorption mechanisms, the shear strength is allowed to be analyzed in the capillary suction stress-shear stress space. The strength envelop exhibits bi-linear characteristics. Building upon these findings, we propose a bi-linear shear strength criterion specifically for dual-porosity fine-grained soils. We utilize the obtained test data to evaluate existing strength criteria based on effective stress and dual stress variables that consider the bi-modal SWRC characteristics. The comparison indicates that the proposed bi-linear shear strength criterion can more reasonably represent the variation of shear strength under a wide range of suction for unsaturated dual-porosity fine-grained soils.

Rehabilitation following open-cast mining aims to build a long-term functional and sustainable soil cover for a stable landscape development. The objective of this study was to assess changes in soil recovery of rehabilitation performed at different times (1980, 1998, 2009, 2016, 2017) measured as soil hydraulic and mechanical properties (shear stress) at an open-cast Yallourn mine site in south-east Australia (Victoria) in 2021. Soil hydraulic properties (SHP) were determined using the extended evaporation method and the water retention and hydraulic conductivity curves were fitted using the van Genuchten-Mualem model. The vane shear tests were performed at two depths (0-8 and 10-18 cm) combined with soil water content measurements. The results of the SHP showed a shift in the soil water retention curve when comparing 1980 and 2017 sites. While the saturated water content i.e., total porosity was the same, the saturated hydraulic conductivity (Ks) decreased from 36.7 to 1.02 cm day-1, respectively. This was mostly connected to the textural pore size distribution, as large differences in clay and sand content among the sites were observed. The vane shear test showed also large differences with rehabilitated sites indicating a larger variation compared to the reference site (exception 2016 site) and having generally higher shear resistances. The observed small-scale heterogeneity of the rehabilitated soils is most likely explained by soil heterogeneity and disturbance due to excavation activities and rehabilitation as well as availability of uniform soil material. Inevitable heterogeneity of the soil hydraulic and mechanical properties should be taken into consideration during the design and construction of various landforms as well as when implementing soil monitoring schemes.

A three-scale constitutive model for unsaturated granular materials based on thermodynamic theory is presented. The three-scale yield locus, derived from the explicit yield criterion for solid matrix, is developed from a series of discrete interparticle contact planes. The three-scale yield locus is sensitive to porosity changes; therefore, it is reinterpreted as a corresponding constitutive model without phenomenological parameters. Furthermore, a water retention curve is proposed based on special pore morphology and experimental observations. The features of the partially saturated granular materials are well captured by the model. Under wetting and isotropic compression, volumetric compaction occurs, and the degree of saturation increases. Moreover, the higher the matric suction, the greater the strength, and the smaller the volumetric compaction. Compared with the phenomenological Barcelona basic model, the proposed three-scale constitutive model has fewer parameters; virtually all parameters have clear physical meanings. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V.

Hydromechanical behaviour of unsaturated expansive soils is complex, and current constitutive models failed to accurately reproduce it. Different from conventional modelling, this study proposes a novel physics-informed neural networks (PINN)-based model utilising long short-term memory as the baseline algorithm and incorporating a physical constraint (water retention) to modify the loss function. Firstly, a series of laboratory tests on Zaoyang expansive clay, including wetting and drying cycle tests and triaxial tests, was compiled into a dataset and subsequently fed into the PINN-based model. Subsequently, a specific equation representing the soil water retention curve (SWRC) for expansive clay was derived by accounting for the influence of the void ratio, which was integrated into the PINN-based model as a physical law. The ultimate predictions from the PINN-based model are compared with experimental data of unsaturated expansive clay with excellent agreement. This study demonstrates the capability of the proposed PINN in modelling the hydromechanical response of unsaturated soils and provides an innovative approach to establish constitutive models in the unsaturated soil mechanics field.