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AbstractRainfall infiltration is a primary cause of slope failure. Analyzing the entire process of a landslide triggered by rainfall is crucial for the design of infrastructure. However, the complexity associated with simulating large deformations has often hindered previous studies from modeling the entire landslide process subsequent to rainfall infiltration. This study aims to provide a method to simulate the entire process of landslides considering rainfall and more insight into the mechanism of landslides. The discrete element method (DEM) is first applied to model the entire process of shallow landslides triggered by rainfall. The numerical results indicate that the intensity and duration of rainfall infiltration have significant impacts on the slope stability. The particle displacement field, resulting from landslides due to rainfall, corresponds with three distinct stages within the landslide process. The increased rainfall intensity causes the area of the particles within failure mass grows rapidly during the initial stage of the landslide. The closer to the slope surface the faster the particle velocity. The simulations demonstrate that the slope surface transitions from a linear to a concave downward geometry during the landslides. Larger particles display greater displacement and velocity compared to smaller particles. [ABSTRACT FROM AUTHOR]

ABSTRACT Slopes in nature usually present layered characteristics, and its stability is susceptible to rainfall events. Considering that current analytical solutions are only suited to simulate the rainfall infiltration of double‐layered infinite unsaturated slopes, an analytical procedure is hence proposed in this study to tackle the consideration of multiple layers. The variable separation method and transfer matrix method are combined to derive the analytical solution of pore water pressure (PWP) for simulating rainfall infiltration in layered infinite unsaturated slopes. After having validated the proposed model and analytical solutions by comparing with existing literature and numerical simulation, the closed‐form solution of PWP is incorporated into the limit equilibrium for assessing slope stability. A three‐layer slope is selected as an example for further discussion. PWP distribution and factor of safety are calculated, considering the effects of saturated hydraulic conductivity and thickness of the upper layer, intensity of antecedent and subsequent rainfall, and varied soil unit weight along the depth. The slope stability subjected to rainfall effects is consistent with the variation of PWP. The proposed analytical solutions provide a simple and practical avenue for computing PWP distribution and evaluating the stability of multi‐layered slopes under rainfall conditions, which can also serve as a benchmark for numerical solutions. [ABSTRACT FROM AUTHOR]

Periodic wave barriers have been widely used to manipulate elastic waves propagating in saturated and single-phase soil due to their attenuation zone properties. However, it is difficult to promote application of periodic barriers in unsaturated soils due to their complex constitutive relationship. In this study, manipulation of surface waves by periodic in-filled trench barriers in unsaturated soil has been studied based on the periodic theory. The dispersion relations of a periodic structure for surface waves in unsaturated soil are determined. The attenuation mechanism of evanescent surface waves is revealed. Next, the effects of several key parameters of unsaturated soil on the attenuation zones of the periodic in-filled trench barriers are comprehensively discussed. It is found that in a particular range for material parameter, the surface waves are attenuated over the entire frequency range due to the viscosity of fluid. Finally, a periodic in-filled trench barrier is designed according to a field test of ground vibration induced by a train, and its performances in mitigating surface waves propagating in unsaturated and saturated soils are conducted and compared by conducting analysis in time domain. This investigation provides a new insight for manipulating surface waves by periodic barriers. This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 1)'. [ABSTRACT FROM AUTHOR]

The issue of rainfall-induced slope failure has attracted more attention from geotechnical engineers as a consequence of global warming. Current cumulative waste disposal has generated scientific interest in the utilization of waste materials in geotechnical design for climate change adaptation measures. Taking into consideration the effect of slope height and angle, steel slag—a waste product derived from the production of steel—was investigated as a slope cover against rainfall. To assess the stability of the slope and the infiltration of water into the soil, numerical analyses were conducted using both SEEP/W and SLOPE/W software in conjunction with rainfall conditions. Based on the findings, it can be concluded that increasing the slope's elevation and inclination will have an adverse effect on its safety factor. Steel slag can nevertheless be utilized for minimizing rainwater infiltration into the slope, as indicated by the pore-water pressure variations and graphs of the safety factor versus time. For a 20-m slope height, steel slag slopes have demonstrated a lower factor of safety difference in comparison to the initial slope without remediation. Regardless of slope angle and slope height, the safety factor reduces marginally during rainfall. [ABSTRACT FROM AUTHOR]

Accurate knowledge of hydraulic properties of unsaturated soil is vital to modelling water and solute transport process and addressing soil salinisation problems. Due to the defect that the van Genuchten (VG) model cannot represent soil water characteristics (SWC) during the drying stage, we have introduced a modified model, which combines capillary flow with film flow models to describe the unsaturated hydraulic conductivity. The capillary SWC is obtained by assuming the specific water capacity function with lognormal distribution. The film SWC is modified using the logarithmic nonlinear expression by introducing a modification coefficient into the Campbell equation, and the film relative hydraulic conductivity (RHC) is obtained based on the logarithmic nonlinear relationship. The calculated modification coefficient values range from 0.99 to 2.49. Finally, the proposed model's rationality and applicability are validated through root mean square error (RMSE) analysis with measured values and comparison with the VG model. While the calculation RMSEs of SWC in the two models are basically consistent, the RMSEs of RHC in the proposed model are around 1–10 times smaller than those of the VG model, indicating its superiority in calculating the hydraulic conductivities over the entire matric head. Highlights: A modified model describing the variably saturated hydraulic conductivity is presented combining the capillary with film flow model. A logarithmic nonlinear expression with a modification coefficient is introduced in the film soil water characteristic model. The fitting parameters for relative hydraulic conductivity are determined and root mean square errors are chosen for evaluating the modified model. [ABSTRACT FROM AUTHOR]

Suffusion refers to the loss of small particles in a soil matrix without volume changes, caused by hydrodynamic forces and alternation of pore fluid chemistry. This study investigated the ionic concentration (IC)-induced suffusion of sand–clay mixtures under saturated and unsaturated conditions through laboratory soil-column experiments. The impacts of clay mineralogy and sand-grain size on suffusion were investigated based on the breakthrough curves (BTCs) obtained from the soil-column experiments. In addition, the degree of saturation during the experiments was measured to assess the impact of the saturation state on suffusion in the sand–clay mixtures. The observed BTCs in this study demonstrated that suffusion was more significant under saturated conditions than unsaturated conditions for the sand–kaolinite and sand–illite mixtures. In contrast, the sand–bentonite mixtures exhibited higher susceptibility to suffusion under unsaturated conditions. Moreover, for the sand–kaolinite mixtures, a higher susceptibility to suffusion was observed with smaller sand-grain sizes under saturated conditions, whereas the opposite trend was observed under unsaturated conditions. This study provides insights into the suffusion behavior of sand–clay mixtures, considering influential factors such as clay type, sand-grain size, and IC gradient under both saturated and unsaturated conditions, which highlight the importance of considering the saturation state for a comprehensive understanding of suffusion in the sand–clay mixtures. [ABSTRACT FROM AUTHOR]

The soil is usually unsaturated in practical engineering, and the slope failure surface generally presents three-dimensional (3D) characteristics. Therefore, the traditional plane problem calculation method does not accurately describe the slope's stability and safety assessment results strengthened through frame beam anchor plates (FBAPs). This work introduces a novel approach based on the limit analysis (LA) technique for assessing the 3D stability of unsaturated soil slopes supported by FBAPs to address this issue. The slope safety factor (Fs) expression was derived based on the energy balance principle. The soil slope stability the FBAP reinforces was quantitatively expressed as a function of slope parameters. Nonlinear distribution features of soil matrix suction and effective unit weight, as well as the internal energy dissipation rate created by the anchor plate, must be taken into account when calculating the internal energy dissipation and external work rate. The impact of the 3D effect, seismic parameters, unsaturated soil characteristics, and supporting structure on slope stability was discussed. Some design suggestions for slope reinforcement are put forward. According to the findings, the suction effect's beneficial effects on stability should be considered. Slopes with a width-to-height ratio (B/H) of less than 10 should consider the 3D effect's beneficial effects. The seismic action significantly negatively impacts slope stability, while using FBAP reinforcement structures can dramatically improve slopes' static and seismic stability. [ABSTRACT FROM AUTHOR]

The soil‒water characteristic curve (SWCC) defines the relationship between the amount of water in soil and soil suction. The SWCC is commonly used to estimate the hydraulic conductivity function (HCF) and the shear strength function (SSF). Therefore, an accurate determination of the SWCC is crucial for implementing the principles of unsaturated soil mechanics. The SWCC is commonly determined from a limited number of experimental data because SWCC measurements are time-consuming and costly. As a result, the minimum number of required soil specimens is crucial for a SWCC test when considering the accuracy of the determined SWCC and the experimental expenses. In this study, both engineered from sand and kaolin mixtures and residual soils from Bukit Timah Formation in Singapore are selected to prepare soil specimens for SWCC measurements. The SWCCs obtained from the specimens with engineered soil are consistent, while those from specimens with residual soil are slightly scattered. This indicates that one specimen is sufficient to determine the SWCC for engineered soil samples, while a minimum of two specimens should be prepared for the determination of SWCC for residual soil samples from Bukit Timah Formation. [ABSTRACT FROM AUTHOR]

When remediating contamination in the vadose zone, which is a potential source of continuous groundwater and surface water pollution, technical challenges are often encountered. In this study, the feasibility of delivering sodium persulfate (SPS) oxidant to unsaturated soils was explored by integrating porous pipes, commonly used in agriculture, with in-situ chemical oxidation (ISCO). An evaluation of the operating characteristics of the pipe determined a recommended influent rate of < 0.1 L min−1 and an initial influent water pressure < 20 kPa to prevent percolation pore enlargement. Two-dimensional sandbox experiments revealed that pulse infiltration at an influent rate of 0.05 L min−1 was effective in creating matrix flow and lateral movement, minimizing preferential flow induced by continuous infiltration. Application of SPS (10% by wt.) under these conditions resulted in approximately 90% wetting of the sandbox area SPS flow at the experimental time that infiltrated from the pipe. After a 14-d post infiltration resting state, residual SPS concentrations ranged from 6,000 -12,000 mg/kg in soil, with soil pH levels of 3–4, ORP around 600 mV, and 40–60% decreases in soil organic carbon. This developed method helps retain the oxidant and promote chemical reactions, making ISCO a viable option for remediating unsaturated soil contamination. [ABSTRACT FROM AUTHOR]

Weathered rock (especially granite) slopes are prone to failure under the action of rainfall, making it necessary to study the response of weathered rock slope to rainfall infiltration for landslide prevention. In this study, a series of model tests of weathered rock slope under different conditions were conducted. The matric suction, volumetric water content, earth pressure and deformation of slope were monitored in real time during rainfall. The response of the slope to rainfall infiltration, failure process and failure mode of slope under different conditions were analyzed, and the early warning criterion for the failure of weathered rock slope caused by rainfall was studied. The results show that the slope deformation evolution process under rainfall condition was closely related to the dissipation of matric suction. When the distribution of the matrix suction (or water content) of slope met the condition that the resistance to sliding of the slip-mass was overcome, the displacement increased sharply and landslide occurred. Three factors including rainfall process, lithologic condition and excavation condition significantly affect the response of weathered rock slope to rainfall. It can be found from the test results under different conditions that compared with intermittent rainfall condition, the rainfall intensity and infiltration depth were smaller when the slope entering accelerated deformation stage under the condition of incremental rainfall. The accumulated rainfall when weathered clastic landslide occurring was greater than that of weathered granite, which results in greater disaster risk. The excavation angle and moisture distribution of a slope were the main factors affecting the stability of a slope. In addition, the evolution processes and critical displacement velocities of slopes were studied by combining the deformation curves and matrix suction curves, which can be used as reference for early warning of rainfall-induced weathered rock landslide. [ABSTRACT FROM AUTHOR]

Unsaturated soils with bimodal pore structures, such as many residual and colluvial soils, are widespread in nature. Bimodal pore structures can noticeably influence their soil–water retention behaviour. Estimating the bimodal soil–water retention curve (SWRC) accurately has become critical in both theoretical research and engineering practice. In this paper, a novel equation to simulate the bimodal SWRC of unsaturated soils is proposed. The proposed bimodal SWRC equation not only provides higher simulation accuracy for bimodal SWRCs compared to other equations, but it also shows good agreement with experimental data from different types of soils. Moreover, the relationships between bimodal SWRC variables (i.e., air entry value, inflection point, and slopes at transition zone) and the best fitting parameters are presented. Finally, a practical approach is developed to estimate the bimodal relative permeability of unsaturated soils based on the proposed bimodal SWRC equation. The results are convincingly demonstrated that there is good agreement between the predicted and measured relative permeability coefficients of unsaturated soils in the literature. [ABSTRACT FROM AUTHOR]

This study investigates experimentally the water retention behavior of granular soils from saturation to oven-dry state. The soil–water retention curve (SWRC) tests were conducted on a well-graded sand with clay using tensiometer and chilled-mirror hygrometer techniques. The soil samples were statically compacted at various water contents to different initial densities. The results showed that individual linear segments in the log–log graph could characterize the desorption process of capillary and adsorption water. A novel water retention model in a simple mathematical form was developed by conceptualizing the total water content as the sum of the suction-dependent capillary and adsorption components. The model parameters possess an unambiguous physical meaning. They can be easily calibrated based on the graphical properties of the test data using simple linear regression, which is a significant advantage over conventional SWRC models. The model was validated using the water retention data of the tested soil in this study and the available data of granular soils in the literature. The reproduced curves agree well with the experimental results. This study also analyzed the influence of compaction water content, initial density, and clay content on the capillary and adsorption components of the water retention curve. Additionally, the proposed framework provides a quick approximation method for the adsorption capacity, which plays an essential role in assessing the effective stress and the simulation of liquid film flow in unsaturated granular soils. [ABSTRACT FROM AUTHOR]

Torrential rainfall is one of the significant factors contributing to shallow slope failures. This study discusses the effect of the different filter gabion sizes against slope instability due to torrential rainfall numerically. The experimental investigation of effects of different filter gabion sizes was extensively investigated. The investigations were carried out in a two-dimensional domain using the saturated–unsaturated consolidation analysis method with an elastoplastic model for unsaturated soils considering two suction effects. Four different sizes of filter gabion were used in this investigation. The results of numerical simulations of slope model tests showed that, as rainwater accumulated at the slope toe, the pore water pressures (PWPs) near the slope toe increased to be positive or larger than the surface soil’s air entry PWP. The phreatic surface that appeared on the slope surface as the positive PWPs increased with the elapsed time. In the simulations, large displacement, considered surface failure, started once the phreatic surface reached the slope surface. The simulated phreatic surface appearance time on the slope surface extended as the filter gabion size became larger. Nevertheless, the results showed that the applied analysis method is powerful and reliable for simulating slope failure behaviours and their proper countermeasures under torrential rainfall. [ABSTRACT FROM AUTHOR]

Pile response in unsaturated soils is an area of interest due to its prevalence in real-world scenarios. Significant differences exist in the lateral behavior of single piles in partially saturated soils due to increased soil stiffness. In this study, a single pile was monotonically loaded laterally to investigate the performance of piles in soils with variable water table levels. Pile head lateral response showed that a mix of unsaturated-saturated soil led to an increase in lateral loads on the pile at a similar displacement compared with the fully saturated or dry soil conditions. The lateral response of the pile was obtained based on the curvature data from the strain gauges. A derivative method was used to generate the shear force and lateral resistance along the pile length based on finite differences at the seven strain gauge locations, thereby eliminating errors associated with curve fitting. Various lateral response plots, such as the normalized p-y curves, M-y curves, and maximum bending moment response plots, were used to assess the response to lateral load. Further, the effect of two different loading rates were assessed on the fully saturated and unsaturated-saturated soil, and the range of loading rates evaluated had minimal influence on the pile lateral response. Results indicated that considering the influence of the unsaturated soil could increase lateral resistance under similar lateral displacements. However, there is a risk of inducing large pile bending moments, potentially leading to pile flexural failure if the pile plastic moment is reached. [ABSTRACT FROM AUTHOR]

This study presents a semi‐implicit MPM to adequately characterize the mechanical behavior of unsaturated soil based on Biot's mixture theory. To represent the dependency of the degree of saturation on the suction, we employ the VG model along with a soil‐water characteristic curve, which determines a functional form of permeability called the Mualem model. Hencky's hyperelastic model and the Drucker‐Prager model assuming nonassociativity are adopted for elastic and plastic deformations, respectively. The novelty of this study is the incorporation of the fractional‐step method into the MPM framework so that the pore water pressure is obtained by implicitly solving the pressure Poisson's equation, which reduces numerical instability and improves computational efficiency. Also, because the drag force between solid and liquid phases is evaluated using the intermediate velocity of pore water relative to the intermediate velocity of solid skeleton, the time increment can be chosen without considering the magnitude of water permeability. In addition, to suppress "odd‐even" oscillation, we employ a sub‐grid method in which two grids with different spatial resolutions are used for the velocities and pore water pressure. Furthermore, considering the advantages and disadvantages of two different interpolation schemes for pore water pressure, we suggest switching the schemes depending on the model conditions. Several numerical examples are presented to demonstrate the performance of the proposed method. Specifically, unidirectional consolidation and leak flow analyses are performed for verification purposes, followed by validation analysis of a model experiment of infiltration‐induced landslides. [ABSTRACT FROM AUTHOR]

This study proposes a closed‐form solution for axisymmetric electro‐osmotic consolidation of unsaturated soil under semi‐permeable boundary conditions. The governing equations are formulated to allow for vertical and radial flows of liquid and air phases. The techniques of eigenfunction expansion and Laplace transformation are employed to develop the exact solution for excess pore‐air (EPAP) and pore‐water pressures (EPWP). The proposed solution is first validated by comparing it to an existing solution, followed by verification through finite element simulations. Both methods of validation confirm the accuracy of the analytical solution. Then, based on the obtained solution, the effects of vertical flow, semi‐permeable boundary conditions, electrical voltage, electro‐osmosis conductivity and spacing ratio re/rw on the consolidation profile have been further investigated. Parametric studies show that the EPWP at the steady state depends on the electro‐osmosis conductivity and applied electricity gradient. In addition, the dissipation rates of EPWP and EPAP in the axisymmetric electro‐osmotic consolidation would be underestimated if the vertical flows are neglected. The semi‐permeable boundary conditions have great influences on the dissipation rate and the steady‐state solution. The proposed solution could serve as a theoretical basis for axisymmetric electro‐osmotic consolidation of unsaturated soil. [ABSTRACT FROM AUTHOR]

The physical meaning of the neutral stress and effective stress for soils is still unclear, which leads to the neutral stress for unsaturated soils being totally ignored and increasingly complicated expressions for effective stress. To solve these problems, we establish an extended three-phase physical model for unsaturated soils, including capillary water, pore air, and generalized soil skeleton (GSS) and then derive a generalized Terzaghi's effective stress equation, stating that the effective stress is equal to total stress minus neutral stress. In our equation, the neutral stress consists of three components, i.e. the homogenized pore fluid pressure, the stress acting on the cross sections of soil particles, and the stress acting on the contacts between particles. The mechanical effects of the neutral stress can be appreciable, especially in unsaturated clays at a relatively low saturation. The effective stress is found to be the stress that is transmitted through GSS. The local stress, induced by physicochemical forces, cementation, and surface tension, is not transmitted through GSS, and thus, it should not be added into effective stress equation. The mechanical effects of the effective stress are controlled by the mechanical parameters of GSS expressed as a function of suction besides the effective stress itself. Integrating the mechanical effects of neutral stress and effective stress, a general framework is proposed for describing the mechanical behaviour of unsaturated soils. Under the framework, the proposed shear strength equation is in very good agreement with the experiment data reported in the literature. [ABSTRACT FROM AUTHOR]

Unsaturated soil desiccation cracking is a coupled hydro-mechanical problem. It seriously weakens the hydraulic and mechanical characteristics of soil, causing various natural disasters potentially. For the unsaturated soil, the mechanism of moisture diffusion and deformation is more complicated compared with saturated soil, attracting wide attention. Thus, a coupled hydro-mechanical bond-based peridynamic (BB PD) model was proposed to explore the moisture diffusion and crack propagation in unsaturated soil. Specifically, the moisture diffusion equation for unsaturated soil was recast with the peridynamic differential operator, and an improved micro-modulus was adopted to revise the bond force density in the BB PD. In addition, a hybrid algorithm combining the explicit difference scheme for solving the diffusion equation and the implicit scheme for solving the motion equation was adopted, to avoid the incongruity of time steps for two types equations under the same explicit scheme. The validity of the proposed model and algorithm was verified by the examples on the desiccation of an unsaturated soil block and the desiccation cracking of a 3D unsaturated soil plate. The results show the potential of peridynamics in capturing desiccation cracks of unsaturated soil. [ABSTRACT FROM AUTHOR]

To address the engineering problem of wall collapse and destabilization during the construction of the slurry trench wall, the stability of the slurry trench wall under the condition of three-dimensional (3D) unsaturated stable seepage is studied using the upper limit analysis method and in combination with the three-dimensional rotating failure mechanism. The expression of the factor of safety (FS) for the unsaturated soil slurry trench was deduced by the gravity increase method. Additionally, a genetic algorithm was used to search for the minimum FS and the critical slip surface, and the obtained results were compared and validated with existing literature results. The effects of relevant parameters on the stability of the slurry trench under the condition of unsaturated seepage were analyzed in detail. The results of this paper indicated that ignoring the 3D effect of the slurry trench will underestimate the stability of the slurry trench. The presence of suction in unsaturated soil is beneficial to the stability of the slurry trench, and the impact of suction is closely associated with seepage conditions and the characteristics of the soil. The increase in surcharge load can lead to a decrease in the FS, and its effect on slurry trench stability increases with the enhancement of the three-dimensional effect. The impact of different seepage conditions on the sliding surface of the channel wall of clay strata is more significant, while the effect on the trench wall of sand strata is negligible. [ABSTRACT FROM AUTHOR]

Rainfall, evaporation, tides and waves make the soil at coastlines show dynamic unsaturated characteristics. In order to cope with landslides, collapses, and coastline receding, microbially induced calcium carbonate precipitation (MICP) was recently implemented to reinforce the soil in slopes and coastal zones using a spraying method. However, most of the researches on MICP focus on soil with a static saturation degree, and there are few numerical researches about the MICP reactions under dynamic saturation degree. Therefore, a coupled numerical model capable of describing the evolutions of bacteria, substance, calcium carbonate (CaCO3), porosity, and permeability in unsaturated soil over time reasonably was developed based on the convective–diffusion–reaction theory and the Richards' equation. The effect of MICP reactions on the parameters α, n, θs, θr in van Genuchten-Mualem (VG) model was considered. A large cylinder test (30 × 75 cm) about reinforcing sand based on the spraying method was carried out to validate the model. The CaCO3 distribution of the MICP infiltration and mineralization was fully reproduced and the concept of critical depth (Zcr) was proposed. The initial porosity, initial bacterial concentration, and spraying rate were the main parameters affecting the CaCO3 distribution and critical depth Zcr. When the bacterial concentration increased, the maximum CaCO3 content, critical depth Zcr, and normalized permeability coefficient (k/k0) indicated exponential increase, exponential increase, and exponential decrease. The saturated permeability coefficient of the sand was determined by the maximum calcium carbonate. The normalized permeability coefficient (k/k0) and critical depth Zcr showed an exponential decrease and a power function increase with the increase of the maximum CaCO3 content. [ABSTRACT FROM AUTHOR]

Realistic estimates of evaporation from unsaturated soils, which are important for many geotechnical and geoenvironmental applications, need to be considered in the context of soil settlement, and require thermohydromechanical (THM) analysis. Evaporation also depends on environmental parameters, including air temperature, air relative humidity, net radiation, and wind speed. Therefore, a consideration of atmospheric coupling in predicting evaporation is also necessary. In this study, the two-dimensional model EVAP1––which numerically estimates evaporation from unsaturated soil using THM and employing a soil–atmosphere model––was used to conduct a parametric study in order to investigate the effects of variation in environmental parameters and to study the effects of climate change on potential and actual evaporation and soil settlement. We found that the evaporation rate increased nonlinearly with increases in air temperature, net radiation, and wind speed, but decreases with an increase in relative humidity. However, the effect of a change in wind speed was less than the effect of a change in the three other environmental parameters. In addition, the change in evaporation rate differed in different regions with different air temperatures. For example, the temperature change had more of an effect on the evaporation rate at higher temperatures. In addition, neglecting soil settlement led to an overestimation of evaporation, albeit the amount of evaporation was almost the same whether soil settlement was considered or not, both at the beginning of the evaporation process, when potential evaporation was dominant, and at the end, when the water content in both cases had decreased almost to the residual water content, and the evaporation was minimized. The difference in the amount of evaporation was greater in the middle of the evaporation process, when actual evaporation dominated. [ABSTRACT FROM AUTHOR]

• Develop fiber-reinforced steel slag-bentonite mixture to construct hydraulic barrier. • This new material reduces desiccation cracks and ensures a low water permeability. • Fiber content and length affect hydro-mechanical properties and crack behavior. • Optimal use 0.3 % 9 mm-fiber to reinforce steel slag (90 %) mixed with bentonite (10 %) Desiccation-induced cracks in a compacted clay liner significantly deteriorate the hydraulic barrier performance of landfill covers. The present study explores the effects of polypropylene (PP) fiber reinforcement on the hydrological response and crack resistance of compacted steel slag (SS; 90 wt%) – bentonite (10 wt%) mixtures under drying and wetting cycles. Comprehensive tests were conducted to explore the impact of different fiber lengths (6–12 mm) and contents (0–0.4 % wt.%), including hydraulic conductivity tests for measuring the saturated hydraulic conductivity (k s), unconfined-penetration tests for measuring the tensile strength, small-sized plate tests for quantifying crack development, and large-sized bucket tests for studying the hydrological response and crack characteristics. Higher fiber contents and longer fiber lengths increased the k s -value of the specimens. For a 0.3 % fiber content, the tensile strength peaked for the 9-mm fiber. Consistently, the specimen reinforced with the 9-mm fibers exhibited significantly fewer cracks than those reinforced with the 6-mm and 12-mm fibers. It was because the 6-mm fibers had a shorter anchorage length, while the 12-mm fibers tended to agglomerate. The large-sized bucket tests showed that fiber reinforcement limited crack development significantly under wetting and drying cycles, reducing the rainfall infiltration by 40 % and enhancing the soil water retention capacity. Finally, a 0.3 wt% of 9-mm PP was recommended to reinforce the compacted SS-bentonite mixtures. [ABSTRACT FROM AUTHOR]

The effective interaction mechanisms in the pullout resistance of reinforcements include skin friction mobilized at the soil-solid surface, soil-soil shear resistance, and compressive resistance created against transverse elements. The third component is obtained from passive lateral pressure (LPM) or bearing capacity (BCM) methods. An analytical solution is proposed to determine the pullout capacity of geocell, geogrid, and strengthened geogrids embedded in ordinary and unsaturated soils. For unsaturated soils, the effective stress approach was employed. The solution-predicted results were compared with those obtained from large-scale pullout tests reported in the literature. Results indicated that considering LPM for 2D and 3D reinforcements better agrees with experimental results. The mobilized frictional rib-soil interfaces and the soil-soil shear resistance components generally contribute more to the pullout capacity of the geocell and geogrid, respectively. For the extensibility represented by m p i and flexibility of geocell denoted by α p i , the values of m p i = 1, 0.7, and 0.3 for the first, second, and third row of geocell, α p i = 0.4 for the first row of geocell and 0.25 for the second and subsequent rows are suggested to be considered. Parametric studies showed that the optimum transverse rib spacing is over 50 times the equivalent rib thickness (B eq). • To develop a solution based on the lateral earth pressure and bearing capacity methods to determine the pullout resistance of three-dimensional (3D) reinforcement such as geocell and two-dimensional (2D) reinforcement like geogrid and strengthened geogrids. • Validating the proposed equation for reinforcement pullout resistance in unsaturated soil, based on effective stress. • Investigating the present solution outcomes with 162 pullout tests performed on 28 different types of reinforcement, extracted from 10 case studies. • Proposing a method to reduce the passive resistance of geocell transverse ribs, related to the effects of extensibility and flexibility. • Presenting an evaluation method considering the geocell geometry and soil matric suction on the pullout capacity of geocells and the effects of the transverse rib spacing and width on the pullout capacity of geogrids. [ABSTRACT FROM AUTHOR]

The limit support pressure significantly influences the stability of the shield tunnel face and deformation control during tunneling. Current research predominantly focuses on soils under dried or saturated conditions. However, unsaturated soils, which are more commonly encountered in engineering practice and widely distributed, have not been as extensively studied. Based on limit equilibrium theory, a modified analytical wedge-prism model is proposed in this paper to predict the limit support pressure acting on the shallow shield tunnel face in unsaturated sandy soil where the soil arching effect is considered. Taking advantage of finite-element software ABAQUS (version 2021), a series of numerical trapdoor tests are conducted first to study the failure behavior of unsaturated sandy soil. Then, by combining the rotation trajectory of major principal stress with the shear strength theory of unsaturated soil, the loosening earth pressure solution above the shallow trapdoor can be derived analytically. The limit support pressure can be finally obtained through a force equilibrium analysis of the wedge body. The accuracy of the proposed model is validated by comparing the calculated results with current model tests and numerical simulations. Parameter analysis results reveal that both the loosening earth pressure and limit support pressure initially decrease and then increase with the rising degree of soil saturation. A critical saturation point is identified where the loosening earth pressure and the limit support pressure reach their minimum values, attributed to the full development of the soil arching effect. [ABSTRACT FROM AUTHOR]

Thermal conductivity is a key parameter governing heat transfer in rocks and soils with applications to geothermal systems and groundwater studies. Its accurate measurement is crucial to understand energy exchange in the Earth's subsurface. This study explores the application of the percolation-based effective-medium approximation (P-EMA) model to a broad range of soil types using a database including 158 soil samples. The P-EMA model for soil thermal conductivity, introduced by Ghanbarian and Daigle, is validated through robust optimization of its parameters and by comparing with the laboratory measurements where we find an excellent match between the theory and the experiments. A regression-based model is developed to estimate the P-EMA model parameters directly from other soil properties, such as sand, clay, bulk density, and thermal conductivities at completely dry and full saturation. The proposed regression-based relationships are evaluated using unseen data from two databases: one from Kansas containing 19 soil samples and another from Canada containing 40 soil samples. These regression-based relationships offer an approximation for the P-EMA model parameters, providing a practical approach to estimate the thermal conductivity of soils. Furthermore, a curve-clustering approach is proposed to classify soil thermal conductivity curves based on their similarities, providing insights into the heterogeneity of samples. We find seven clusters for each of which the average P-EMA model parameters are reported. The classification and regression models generally extend the seamless applicability of the P-EMA model. [ABSTRACT FROM AUTHOR]

Suction stress is one of the most significant factors affecting the serviceability and stability of soil structures. This study presents a framework for analyzing the stability of geosynthetic reinforced soil (GRS) walls and slopes under unsaturated conditions. An analytical formulation of suction stress-based effective stress was implemented into a limit equilibrium solution, namely, the top-down procedure. The developed framework enables the prediction of the tensile load distribution and connection load between the reinforcement and wall face considering the pullout resistance of reinforcements in GRS walls backfilled with granular and marginal soils under unsaturated conditions. The applicability of the framework was demonstrated by providing an illustrative example followed by three series of parametric studies to understand the effects of pore size distribution, air entry pressure, and infiltration rate on the performance of an unsaturated GRS wall. The results quantify the impact of suction, showing that as it increases, the maximum tensile loads and connection loads decrease while pullout resistance increases. Mostly affected by the suction effect are upper reinforcement layers, where combined effects of reduced tensile load and increased pullout resistance decrease connection load and reinforcement length requirements. The current study strongly discourages counting on the contribution of suction for the design of new GRS walls. The suction value cannot be accurately and reliably determined for the entire lifespan of the GRS wall, and it may decrease or diminish in an uncontrolled and random manner under infiltration. However, by quantifying the effect of suction, the proposed framework in this study provides a valuable tool for analysis purposes, enabling a rigorous interpretation of field-measured reinforcement loads during wall service as well as evaluation of the forensics of failed GRS walls. [ABSTRACT FROM AUTHOR]

AbstractExcess pore water pressure (EPWP) induced by shield tunneling has a significant influence on the stability of the tunnel face, post-construction settlement, and the mechanical behavior of the tunnel lining. However, the three-dimensional and unsaturated property of the soil field is seldom considered in current research. Considering the non-uniform radial convergence model, a modified three-dimensional displacement solution induced by shield tunneling was established first. Then based on unsaturated EPWP elastic theory, a reliable and efficient method is developed to expedite the evaluation of EPWP distribution in three-dimensional saturated and unsaturated clay soil. The validity of the method is confirmed through comparison with field test and numerical outcomes. The analysis examples demonstrate that negative and positive EPWP are generated above the tunnel crown and beneath the tunnel invert, respectively. In the vertical direction, the negative EPWP exhibits a decreasing trend ahead of the heading face and an increasing trend behind it. Along the longitudinal direction, the influence zone of EPWP extends to 1D ahead of and 6D behind the heading face. With the decrease of soil saturation, the EPWP values tend to diminish. The maximum EPWP values observed in saturated conditions can be 16.13 times higher than those under unsaturated conditions. [ABSTRACT FROM AUTHOR]

Taking Bingham slurry as the research object, an infiltration diffusion model of Bingham slurry in unsaturated soil is established based on the seepage theory of unsaturated porous media, which takes into account the displacement effect and the viscosity time-varying characteristics of slurry. Combined with the seepage motion equation, relevant boundary conditions and interface conditions, the Bingham slurry diffusion equation considers grouting parameters and mechanical parameters is obtained. The research results are degraded to the saturated state and compared with the existing theoretical results. Numerical examples are used to study the influence of the above parameters on grouting pressure and slurry diffusion radius, then the diffusion mechanism of Bingham slurry is verified and analysed by model test. The above analysis and test results confirm the effectiveness and applicability of the research results, which can provide reliable theoretical support and technical reference for the design and construction of grouting engineering in unsaturated soil areas. [ABSTRACT FROM AUTHOR]

Limited experimental and theoretical research has been conducted on the shear strength of unsaturated soils over a wide suction range. This study aimed to investigate the shear strength characteristics of a weakly expansive soil over a wide suction range (0.05–367 MPa) through a series of triaxial tests. The results of the experiments demonstrate that, under the same net confining stress, the shear strength increases with the suction over the entire suction range. Furthermore, an increase in suction leads to a change in the shape of the stress–strain curve, from strain-hardening to strain-softening. The effect of net confining stress on the stress–strain curve and volumetric strain-axial strain curve varies with suction. An additional analysis indicates that the cohesions and angles of internal friction acquired through net stress differ from those obtained through effective stress. The angles of internal friction at different suctions remain the same on the shear strength-net normal stress (τf−(σ−ua)) plane, while the cohesions increase exponentially with increasing suction. Based on these test results, a novel equation for the shear strength of unsaturated soil over a wide suction range is proposed by incorporating suction-dependent cohesion. The results of the study show excellent agreement between the predicted and measured shear strength results over a wide suction range, and its superior accuracy is demonstrated through comparisons with several existing models in the literature. [ABSTRACT FROM AUTHOR]

Energy piles are often partially or fully embedded in unsaturated soils, particularly in arid and semiarid regions; however, suction effects on their thermomechanical behavior have not been fully understood. In this study, a series of physical modeling tests were carried out to investigate the behavior of energy piles in unsaturated soils. The soil used for testing is completely decomposed granite, which is a type of clayey sand sampled from Hong Kong. Five constant-temperature pile load tests and three constant-load heating and cooling tests were conducted under different conditions of pile surface roughness and initial soil suction. It is observed that the ultimate capacity of the energy pile (Qult) increases as the initial suction increases from 0 to 90 kPa, the pile roughness increases from 0.05 to 1, and the temperature increases from 10°C to 21°C. Moreover, the effects of roughness are more significant in unsaturated conditions than in saturated states. A suction increment can increase both shaft and toe resistance, while temperature and roughness mainly affect the shaft resistance. During cyclic heating and cooling, suction and roughness increment reduces the irreversible pile head settlement due to the increment of shaft resistance. Furthermore, the irreversible pile head settlement is greatly affected by vertical load. When the working load is relatively small (smaller than 0.5Qult in this study), the settlement increases with the number of thermal cycles but at a decreasing rate for both saturated and unsaturated conditions, and finally reaches a stable state. When the working load is relatively high (0.7Qult in this study), the response of the pile does not reach a stable state in saturated conditions even after five thermal cycles. These observations should be treated with caution because they may be specific and only applicable to the case of no overburden pressure. Exercising caution is crucial when interpreting these observations, particularly at a quantitative level, because they may be limited to the test soil and the low stress level utilized in the physical model tests. [ABSTRACT FROM AUTHOR]

The combination of permeable and impermeable columns is commonly employed to enhance the stability of natural ground and expedite the consolidation process while effectively reducing foundation settlement. This study presents a comprehensive investigation of the consolidation characteristics of unsaturated composite foundations (USCF) using various methods. Firstly, a mathematical model is developed under the assumption of equal strain, considering depth‐dependent initial stress. To obtain more generalized analytical solutions for excess pore pressures and foundation settlement, separation of variables, Laplace transformation, and inversion Laplace transformation are employed. Secondly, the accuracy of the proposed analytical solutions is validated by comparing them with two specific cases reported in the existing literature, achieved by reducing the compression modulus of columns. Lastly, a series of parameters are investigated to examine the consolidation characteristics of USCF considering depth‐dependent initial stress. The results demonstrate that the final settlement of the foundation will be overestimated while the variation of vertical stress along the depth is neglected. Furthermore, adjusting the values of area replacement ratios proves to be more effective and straightforward than modifying the compression modulus of columns in the reinforcement treatment of USCF. [ABSTRACT FROM AUTHOR]

Precipitation is one of the most important factors inducing shallow slope failures, and the shallow slope covering bedrocks is prone to instability after heavy rainfall. In one-dimensional (1D) seepage–deformation coupling issues, permeability coefficient and moisture vary with matric suction in unsaturated soil. Combining mass conservation, Darcy's law, and elastic theory, an analytical solution for coupled seepage–deformation in unsaturated soil slopes during rainfall infiltration is derived using the Fourier integral transformation method. The analytical solution can be applied to a 1D seepage problem in a soil slope with flux at the top and impervious bedrock in the base under heavy precipitation, and is conducive to study infiltration into the slope under rainfall conditions. To validate the accuracy of the proposed analytical solution in this study, it is compared with monitored pore-water pressure data from the Gufenping Landslide in the red-bed region located in Nanjiang, Sichuan, China. The compared result shows a good consistency between the analytical solution and the measured results, with a minor relative error. Investigation of the parameters demonstrates that the water-level rise is closely related to the coupling, which is influenced by precipitation duration, precipitation intensity, soil properties, and slope angle. The bottom boundary of the slope is considered to be impermeable in this study, which leads to rainfall accumulation at the base over time, and the coupled effect becomes more pronounced at the bottom boundary. [ABSTRACT FROM AUTHOR]

Understanding the moisture migration of unsaturated soil in response to temperature changes is essential in soil science and geosystems applications. The coupling of thermohydro (TH) processes is a result of changes in fluid density and viscosity; however, gravity can disrupt the moisture migration of unsaturated sand initiated by a thermal gradient when the sand is considerably porous. Moreover, the moisture redistribution caused by the gravity effect is influenced by the imposed boundary temperature and initial moisture content of the unsaturated sand. Therefore, this paper presents heating tests performed on sand specimens of different gradations and subjected to various thermal gradients in a modified and well-controlled soil testing cell to evaluate the coupling behavior of TH processes in unsaturated sand. Two sands were tested in the soil testing cell, which was equipped with thermocouples (TCs) to measure the temperature and time-domain reflectometry (TDR) sensors to measure the apparent dielectric constant (Ka); three temperature boundary conditions were imposed on the soil testing cell to create two thermal gradients. This study shows that gravity, thermal boundary conditions, initial moisture content, and matric suction influence the migration of moisture: the silty sand, as compared with pure sand, more properly displayed the phenomenon of coupled TH processes in unsaturated soil when tested at a temperature gradient of 200°C/m and initial moisture content of 6%. The study provides an advanced understanding of the paired migration of heat and moisture in porous media under various conditions, including thermal gradient, initial moisture content, and gradation. When unsaturated soils are under a temperature gradient, the pore water tends to move from the heat source to a cooler end in search of moisture equilibrium. Temperature-driven moisture flow is commonly investigated in soil science, agriculture, and geosystems. For example, the soil surrounding a geosystem (electrical cables and ground heat exchangers) experiences heating and cooling cycles when heat energy is exchanged from buildings to the soil via the geosystem. This heat exchange can effectively cool the buildings in the summer and heat the buildings in the winter. To maintain effective heat transfer in the soil, it is necessary to investigate the temperature-driven moisture flow, as it has been established that redistributed moisture also significantly impacts heat transport. At the same time, the moisture flow is also influenced by gravity, particularly for coarse sands. Moisture flow in coarse sands is more prone to the effect of gravity than that in fine sands. This paper evaluates how the physical and hydraulic properties of the soil, such as gradation and suction, can affect the coupled thermal and hydraulic processes. The gravitational effect can dominate the moisture redistribution depending on the direction of the temperature gradient. [ABSTRACT FROM AUTHOR]

The rock walls of Valles Marineris (VM) valleys on Mars reveal significant gravitational failures, resulting in a sequence of massive landslides spanning several hundred cubic kilometers in volume. For further Mars exploration missions, it is critical to understand which characteristics impact the stability of these rock walls. In this work, ArcGIS is used to identify 30 steep slopes. Utilizing the finite element method, we calculate the proposed seven possibly landslide-prone slopes based on geometry in VM. Using Strength Reduction Method (SRM) in Midas GTS NX, the impacts of variations in cohesion, internal friction angle, unit weight, and elastic modulus of soil and rock on the slope safety factor against landslides are evaluated. The Strength Reduction Method (SRM) is a widely used approach in geotechnical engineering to assess slope stability. It involves systematically reducing the strength parameters of the soil and rock materials within the slope until failure occurs. By iteratively reducing the strength parameters, the SRM calculates the factor of safety against landslides. Internal friction angle is the most critical factor in determining the stability of a slope under low gravity circumstances since it has the widest range of possible alterations. Furthermore, the material's cohesion and unit weight significantly impact the safety factor, although elastic modulus barely affects slope stability. A modulus of elasticity of more than 35 GPa will not enhance the factor of safety. There is no significant difference in soil suction between Earth and Martian gravities near the surface water table. However, as the groundwater depth increases, soil suction under Martian gravity becomes notably lower than that on Earth. Additionally, consistent with prior investigations, the Vadose zone on Mars is positioned at higher elevations relative to Earth, indicating the presence of a higher capillary fringe. Furthermore, the factor of safety for slope stability consistently outperforms Earth for equivalent slope configurations under unsaturated conditions, with approximately 2.5 times higher factor of safety for higher suctions and approximately 1.5 times higher factor of safety for lower suctions compared to Earth. [ABSTRACT FROM AUTHOR]

This paper aims at proposing a calculation model to deal with the face stability of shield tunnels in unsaturated soil. Based on the existing results of centrifugal test and numerical simulation, a new failure mechanism is proposed, which combines the logarithmic spiral with upper loosening arch mechanisms. The failure mechanism considers the influence of the soil arching effect and the logarithmic spiral height coefficient. Formulas for calculating the loosening earth pressure of unsaturated soil are deduced. Subsequently, this model of loosening earth pressure is introduced into an analytical model to compute the limit support pressure on the tunnel face. Compared with other height coefficients and the degree of soil arching effect, when the height coefficient is 0.95 and the degree of soil arching effect is 10%–30%, the limit support pressure proposed in this paper is in good agreement with the centrifugal test results. Finally, comparisons are performed with the proposed model and classical models as well as typical centrifuge model tests. The comparative analysis shows that the proposed theoretical model is in good agreement with the numerical simulation. The outline of the failure zone by the theoretical model in this paper is in good agreement with the centrifuge results. [ABSTRACT FROM AUTHOR]

Permeability tests on unsaturated frozen sand and gravel In artificial ground freezing applications, frozen saturated soil is assumed to be practically impervious. However, in the case of partial initial saturation, this is no longer the case. As part of a pilot study, permeability tests were carried out in the laboratory on cohesionless, loose, partially saturated, frozen soil samples. Mineral oil was used as the flow medium. Since no standardized apparatus exists, suitable experimental techniques and evaluation procedures had to be developed. The permeability of unfrozen, fully saturated soil was also determined for reference. Due to the loose packing and the low degree of saturation, the partially saturated soils exhibited a significant permeability, which is in the same order as the value of the corresponding water‐saturated, unfrozen soils. [ABSTRACT FROM AUTHOR]

It is important to understand the shear behavior of oil-contaminated soils in many geotechnical problems, such as the analysis of pipelines and storage tanks affected by oil leakage. In this study, the shear behavior of a silt permeated with silicone oil/water (denoted by OS and WS, respectively) was investigated through direct shear tests. The residual shear strength was measured at various degrees of liquid saturation (65–100%) and net normal stresses (50 to 400 kPa). The oil and water retention curves were measured and used to explain the shear behavior. The results show that the residual friction angle of OS is 30% larger than that of WS. This is likely because OS and WS show aggregated and matrix structures, respectively, as evidenced by the Scanning Electron Microscope (SEM) test. The former structure includes more particle contacts than the latter one. These structural differences can be attributed to varying fluid properties, such as wettability, viscosity, and dielectric constant. When the volumetric degree of water saturation decreases, the total cohesion of WS increases substantially because of the strengthening effects of air-water interfaces on the soil skeleton. In contrast, the degree of oil saturation has minuscule effects on these variables, mainly because OS has a low oil retention ability and the air-oil suction remains very low (i.e., less than 2 kPa) during the desaturation process. [ABSTRACT FROM AUTHOR]

Borehole thermal energy storage (BTES) is an innovative renewable energy technology for building heating and cooling. The lack of studies about BTES in unsaturated soils acts as a barrier to further implementation. In this study, the research obstacles, progress, hotspots, and differences between countries of BTES systems in unsaturated soils were described in detail based on a literature review, bibliometric analysis, technology roadmap, and data visualizations. The results show that the process of coupled heat and moisture transfer has been a research hotspot in recent years, and the thermo‐hydraulic properties increase the complexity of analysis. There are few studies on experimental data and engineering practices of BTES systems in unsaturated soils. China is leading in publications number, but European and North American countries perform better in international cooperation and policy incentives. Technology development will be directly driven by policy momentum, followed by the gradual establishment of a complete research, design, and evaluation system, which helps accelerate the replacement of traditional fossil energy with renewable energy. In an attempt to improve society's awareness and move forward with scientific foresight of BTES systems in unsaturated soils, this review has revealed the evolutionary process, identified its current status, and depicted the future development from political, economic, social, and other dimensions. [ABSTRACT FROM AUTHOR]

Field compaction control is arguably the most common yet critical quality control procedure in geotechnical engineering. Since the early 1930s, the systematic process for performing quality control of compacted soils has often been implemented by measuring the in-place dry unit weight (or density) and as-compacted soil moisture content after placement in a fill. However, the current practice overlooks several facts resulting from comparing soil prepared and compacted in the laboratory to soils placed and compacted in the field. These issues include comparing the compaction energy in the lab versus what is applied in the field, and the behavior of saturated soils in the laboratory to the performance of unsaturated soils in the field. To address some of these gaps, this study presents a new saturation-based framework for compaction quality control. The aim of this new framework is to reduce the uncertainties and assumptions of the compaction control process and provide practicing engineers with further insight into the key engineering attributes of compacted soils. The proposed saturation-based approach compares a degree of saturation difference with a normalized dry unit weight ratio, making saturation upon compaction the controlling diagnostic variable and the focus of the monitoring effort. In essence, the optimal compaction conditions will be referenced to a characteristic saturation state near 80%. Compared with the conventional quality control system for field compaction, the saturation-based approach is developed with the same field and reference data collected for most earth fill projects. The results of this approach enhance the engineering judgment required to match the laboratory reference values to the field conditions. For the purposes of illustration, the proposed saturation-based framework is applied to compaction control data of a large earth dam and compared against the conventional method side by side. [ABSTRACT FROM AUTHOR]

We develop an analytical solution to the problem of one‐dimensional consolidation of unsaturated soil subjected to cyclic loads with arbitrary waveforms. The solution predicts the excess pore water and pore air pressures and the accompanying vertical compression in a poroelastic, unsaturated soil material. Cyclic loading occurs in a variety of engineering applications and often generates higher excess pore fluid pressures and larger vertical compression than does a time‐invariant load. In the present study, the loading function is allowed to take on any arbitrary waveform represented by a Fourier trigonometric series. Analytical solution to the boundary‐value problem in one dimension is given in closed form describing the frequency‐independent and frequency‐dependent components of the poroelastic response. We verify the analytical solution through representative examples involving cyclic loads with square and triangular patterns. Apart from the shape of the forcing function, we also investigate the effects of initial water saturation, soil texture, and excitation frequency on the system response. [ABSTRACT FROM AUTHOR]

A viscoplastic constitutive model is developed to describe the viscoplastic behavior of unsaturated soils. The proposed model accounts for the stain‐rate and suction effects on yield by adopting an unsaturated isotach concept. The nonstationary flow surface theory (NSFS) is applied for modeling the viscoplastic behavior, with the yield surface which can evolve with the viscoplastic strain, viscoplastic strain rate and suction. Meanwhile, the progressively hardening concept is adopted for reproducing the viscoplastic behavior of soil at an over‐consolidated state. For the validation, a series of loading conditions are considered based on the data from the literature. Results show that the proposed model is able to reproduce the main viscoplastic behaviors of unsaturated highly compacted Gaomiaozi (GMZ) bentonite, Glenroy silt and Qianjiangping landslide (QL) soil, including CRS compression tests, rate‐dependent triaxial shear tests, and triaxial creep tests. [ABSTRACT FROM AUTHOR]

The stratigraphic uncertainty and rotated anisotropy of soil properties exist widely in nature. Recent studies have shown that the slope stability was significantly influenced by these two uncertainties. However, there is no proper method for simulating these two uncertainties at the same time, and the influence of the two uncertainties has not been considered in previous unsaturated soil slope stability analysis. This paper aims to propose a coupled method for characterizing the stratigraphic uncertainty and rotated anisotropy of soil properties, and investigate the unsaturated soil slope stability considering the two uncertainties. Through a slope case, the proposed method for characterizing the two uncertainties is examined. The effect of rotational angle on the slope stability and groundwater table is studied. In addition, four different uncertainty considerations are chosen to compare their influence on the slope stability and groundwater table. The results show that the proposed method can well characterize the two uncertainties at the same time. The rotational anisotropy of soil properties has a substantial impact on the slope stability and groundwater table. The rotational angles corresponding to the maximum and minimum reliability index of slope depend on the uncertainty considerations in the slope stability analysis. The slope reliability index only considering stratigraphic uncertainty is the highest, and the slope reliability index considering stratigraphic uncertainty and rotated anisotropy of soil properties is the lowest. [ABSTRACT FROM AUTHOR]

Landslides due to catastrophic weather events, especially heavy rainfall, have risen significantly over the last several decades, causing significant damage and affecting the health and livelihoods of millions of people. Using tree roots to bio-engineer shallow slopes has been proven to be a cost-effective, sustainable measure and thus has gained increasing popularity. As slope failure often occurs under heavy precipitation, it is important to understand the mechanical interactions in the soil matrix surrounding a root to better estimate the reinforcement capacity of a root system, especially as the soil undergoes wetting from drier conditions. However, very few studies of root reinforcements have considered the effects of degree of saturation on behaviour. In this study, steel wires are used as a root analogue to explore the impact of root geometry, soil dilation and soil saturation on the pull-out behaviour of a root and three commonly used unsaturated soil strength models have been used to interpret the pull-out results. It was found that roots with larger diameter did not contribute to additional resistance. Also, a linear relationship between degree of saturation and pull-out strength was identified over a large range of suctions and one of the unsaturated soil strength models seemed to provide a more reasonable interpretation. The results will help future bioengineering slope design by improving the understanding of soil-root interface behaviour, including the effect of root diameter in slippage failure and greater emphasis on the importance of taking degree of saturation into account in unsaturated soil strength models. [ABSTRACT FROM AUTHOR]

Purple soil is widely distributed in southwestern China and is one of the important natural resources. Because of the loose parent rock and low degree of differentiation, purple soil is characterized by a loose structure that is easily eroded and high soil fertility. Soil erosion is very serious in the purple soil region, which is one of the main sources of sediment in the upper reaches of the Yangtze River, and has a certain impact on the agricultural production and engineering development in the purple soil region. After erosion, purple soil shows the characteristics of reduced dry density of soil and serious loss of fine particles, which affects its water-holding characteristics. In order to explore the changes of soil water-holding properties of purple soil after rainfall erosion, this study was carried out to analyze the effect of missing grain groups on the water-holding properties of sandy clayey purple soil under rainfall conditions by selecting three test methods, namely, axial translation technique, filter paper method, and saturated salt saturated saline solution vapor equilibrium technique for the three grain groups which are the most susceptible to changes in the rainfall conditions, and carrying out the Soil-Water Characteristic Curve (SWCC) test with the absence of grain groups within the full suction range to analyze the effect of the missing grain groups on the water-holding properties of sandy clayey purple soil. The results show that: 1) the missing grain group reduces the water-holding characteristics of sandy clayey purple soil, which is most obvious in the boundary effect zone and transition zone, and has less effect on the residual zone. The mass of the missing grain group is inversely proportional to the air intake value and the residual water content. 2) Using the grain size parameters (Constrained diameter、Average diameter、Median diameter、Effective diameter) to characterize the soil grading condition, the relationship between the soil grading curves and the characteristic values of SWCC was established. It was found that except for effective diameter, the other parameters were linearly inversely related to the air intake value and residual water content; 3) The data obtained from different methods were fitted using the Fredlund-Xing model, and it was found that the results of the data obtained from a single test method fitted better, but the SWCC significantly deviated from the test data points in the boundary effect zone and the transition zone. Root mean square error (RMSE)、 percent bias (PB) and mean absolute percent relative error (MAPRE) were introduced as evaluation indexes, and it was found that the suction range was the main factor affecting the fitting effect of SWCC, and within a certain suction range, the larger the range of data points involved, the better the model fitting effect was, and after a certain suction value was reached, the suction range had very little effect on the model fitting effect. It is proposed that to obtain the accurate SWCC of purple soil quickly, only the maximum suction value needs to be determined, and the number of test points with high suction is reduced, which can effectively shorten the SWCC test period. The results lay a foundation for the subsequent study of the role of soil erosion and water-holding characteristics of purple soil, and also provide reliable ideas for the prevention of soil erosion and the application of soil-water characteristic curves in the Three Gorges Reservoir area. [ABSTRACT FROM AUTHOR]

This paper presents a coupled hydro-mechanical peridynamic (PD) model to simulate unsaturated seepage and crack propagation due to moisture change in unsaturated expansive soils. The PD form of the nonlinear moisture diffusion equation is derived by using the PD differential operator (PDDO). The bond-based PD equilibrium equation includes the effect of elastic strain due to moisture change. These coupled field equations are solved by employing explicit schemes. Their accuracy is established by simulating the transient flow in the unsaturated soil pillar, steady flow in the unsaturated soil dam, and crack propagation in the cylinder soil specimens. The PD fluid pressure predictions are in good agreement with the analytical and numerical results. The crack propagation process and crack patterns are similar to the experimental observation. The results show that coupled hydro-mechanical PD model can capture realistic hydraulic and mechanical behaviors of unsaturated expansive soils. [ABSTRACT FROM AUTHOR]

The soil–water characteristic curve (SWCC) plays an important role in solving the stability and deformation problems of unsaturated soils. In many practical situations, soils are usually experienced by both deformations and thermal conditions. In this interest, the paper proposes a simple and effective model to predict the combined effect of initial density and temperature on the SWCC and to be able to quantify the changes in thermal-hydro-mechanical behavior of unsaturated soils. In the first step, an initial density-dependent SWCC model is presented using the translation principle between particle-size distribution curve and soil–water characteristic curve. In the second part, a non-isothermal model is proposed to predict the effect of temperature on the SWCC. The key to the non-isothermal model is considering five different temperature-dependent functions, which are surface tension, contact angle, particle-size expansion, void ratio, and water density. On the basis of 22 data sets of thermal volume change, this study also developed further a theoretical correlation between void ratio and temperature that is directly related to soil plasticity. It was observed that the value of the thermal void ratio increases as soil plasticity increases, and there is a nonlinear relationship between the plasticity index and the void ratio. Because of this, soils with high plasticity are more susceptible to volume changes caused by temperature fluctuations than soils with low plasticity. A coupled mechanical–thermal model is then produced which is capable to predict separately or simultaneously the effect of temperature and initial density on SWCC. The proposed model is validated against several test data sets available in the literature. The results show that the proposed model has a good performance in predicting the variation in SWCC with arbitrary temperature and initial density. [ABSTRACT FROM AUTHOR]

It is well-recognised that stress can greatly affect the pore characteristics of unsaturated soil, such as pore size distribution (PSD) and pore shape (PS). So far, the effects of PSD and PS on the drying and wetting water retention curve at different stresses have not been well understood. To fill the knowledge gap, eight water retention tests were carried out to evaluate the effects of PSD and PS on hysteretic water retention behaviour at different stresses. Soil specimens purposely prepared at two compaction water contents and two initial void ratios were subjected to drying and wetting under two different net stresses (0 and 50 kPa). Moreover, mercury intrusion porosimetry (MIP) and micro-X-ray computed tomography (µ-XCT) were used to quantify the microstructure of specimens. The results showed an important role of pore structure in the hysteretic SDSWRCs. Compared to the specimens prepared on the dry side, specimens compacted on the wet side of the optimum are highly aggregated, possessing a lower water retention ability but a higher degree of hysteresis. The higher hysteresis observed in the highly aggregated specimens can be explained by the irregular-shaped pore characteristics observed in µ-XCT tests. [ABSTRACT FROM AUTHOR]

The analytical representation of kinematic interaction between pipe pile and unsaturated soil subjected to vertical SH-waves is revisited using three-dimension continuum modelling. The fractional-order standard line solid (FSLS) model is employed in the governing equation of unsaturated soil to refine the characterization of the flow-independent viscosity of the soil skeleton. The Timoshenko beam theory is utilized to describe the horizontal dynamic behavior of pipe pile. A closed series form solution of horizontal kinematic response for a pipe pile embedded in unsaturated soils under the action of vertical SH-waves is deduced theoretically with boundary conditions of pile-soil system. The solutions of the proposed model are compared with those captured from existing solutions. Finally, the effects of physical parameters in pile-soil system on the horizontal kinematic response of pipe pile under SH-waves are evaluated with analytical examples and parametric study. The results indicate that FSLS model parameters have significant impact on the horizontal kinematic response of the pipe pile, changes in the soil saturation have a relatively slight effect on the horizontal kinematic response of the pipe pile, and the length and radii of pipe pile should be moderate rather than too large or too small to make the pipe pile play a better seismic performance. [ABSTRACT FROM AUTHOR]

A fully-automated unsaturated triaxial device with continuous measurement of all variables is developed to investigate the hydromechanical behavior of unsaturated soils under different stress/strain paths, especially under anisotropic stress states. An experimental program including nine wetting paths was carried out to study the effect of initial shear stress on the hydromechanical behavior of unsaturated specimens of Gorgan loess, corroborating the functionality of the developed device. The results vividly show the important role of initial shear stress on the water absorption and deformation behavior of the reconstituted loessial soil under different mean net stresses. The application of initial shear stress on the tested collapsible soil specimens resulted in a higher collapse potential and larger volume decrease (i.e., up to a maximum increase of 4.6% in the volumetric strain) when the specimens were subjected to wetting, compared to the specimens wetted under an isotropic stress state with the same initial mean net stresses. Furthermore, in the case of the specimens wetted under the highest level of initial shear stress, at suctions lower than 15 kPa corresponding to the axial strains of about 30%, the critical state condition was achieved. Visual inspections of the failed specimens showed a bulging-type failure. Based on the effective stress theory, the analyzed results illustrated that for the same levels of initial mean effective stress, the initial shear stress caused a slight increase in suction stress values, except for the tests with the lowest level of initial mean effective stress in which this effect was negligible. [ABSTRACT FROM AUTHOR]

Aims: This study experimentally investigated the short-term (about 6 months) effects of roots of Cynodon dactylon on the gas permeability and gas diffusion coefficient of soils with different degrees of compaction. Methods: Compacted soil planted with Cynodon dactylon were left outdoors for about 6 months for plant growth. The measurements of rooted and bare soils included gas permeability, gas diffusion coefficient, root characteristics and soil microstructure. The relative effects of different root characteristic parameters on gas permeability and gas diffusion coefficient were compared through grey relational analysis. Results: The root volume ratio had a greater effect on gas permeability and gas diffusion coefficient, compared with root area index, root area ratio, root length density, and root biomass ratio. When the degree of compaction ≥ 85% (porosity ≤ 0.41, bulk density ≥ 1.56 g cm−3), the macro-pores at the root–soil interface increased gas permeability and gas diffusion coefficient, while negligible effects of roots on gas movement existed under degree of compaction of 80%. The increase in gas permeability by roots was more significant than that in gas diffusion coefficient. However, roots' increase of gas movement generally decreased at higher root volume ratio due to roots-occupied soil pores. Finally, gas permeability and gas diffusion coefficient of rooted soil were well predicted by newly-developed empirical models considering the effect of root volume ratio. Conclusions: Macro-pores at the root–soil interface tended to increase gas permeability and gas diffusion coefficient of soil with a degree of compaction ≥ 85%, while it is the opposite for root volume ratio. [ABSTRACT FROM AUTHOR]