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The main objective of this paper is to reduce the dimensionality of unknown coefficient vectors of finite-element (FE) solutions in two-grid (CN) FE (TGCNFE) format for the nonlinear unsaturated soil water flow problem by using a proper orthogonal decomposition (POD) and to design a new reduced-dimension iteration TGCNFE (RDITGCNFE). For this objective, a new time semi-discrete CN (TSDCN) scheme for the nonlinear unsaturated soil water flow problem is first designed and the existence, stability, and error estimates of TSDCN solutions are demonstrated. Subsequently, a new TGCNFE format for the nonlinear unsaturated soil water flow problem is designed and the existence, unconditional stability, and error estimates of TGCNFE solutions are demonstrated. Next, a new RDITGCNFE format with the same FE basis functions as the TGCNFE format is built by the POD method and the existence, unconditional stability, and error estimates of RDITGCNFE solutions are discussed. Ultimately, the rightness of theory results and the superiority of the RDITGCNFE format are verified by two sets of numerical tests. It is worth noting that the RDITGCNFE format differs completely from all previous reduced-dimension methods, including the authors' previous works. Therefore, the study of this paper can not only provide a new theoretical method for the dimensionality reduction of numerical models for nonlinear problems but also provide an algorithm implementation technology for the numerical simulation of practical engineering problems. [ABSTRACT FROM AUTHOR]

Adding different materials to soil can improve its engineering properties, but traditional materials such as cement, lime, fly ash, etc., have caused pollution to the environment. Recently, biopolymers have shown many advantages, such as economy and environmental protection, which make them applicable to geotechnical engineering. This study summarizes the effects of biopolymers on soil's engineering properties and the main directions of current research. Firstly, the advantages and disadvantages of a variety of widely used biopolymer materials and their effects on the specific engineering characteristics of soil (i.e., water retention characteristics, strength characteristics, permeability characteristics, microstructure) are introduced, as well as the source, viscosity, pH, and cost of these biopolymers. Then, based on the theory of unsaturated soil, the current research progress on the water retention characteristics of improved soil is summarized. The key factors affecting the strength of biopolymer-treated soil are introduced. Due to the actual environmental conditions, such as rainfall, the permeability and durability of biopolymer-treated soil are also worthy of attention. In summary, it is necessary to study the variation laws of the engineering properties of biopolymer-treated soil in the full suction range, and to predict such laws reasonably. The relevant results are conducive to the safer and more scientific application of biopolymers in geotechnical engineering practice.

An interlayer existed between the ballast layer and subgrade in the conventional railway substructure. Considering that the shear strength τ of the interlayer soil was influenced by the changes in the ballast grain content and water content, this aspect was explored in the present study. Monotonic triaxial tests were fulfilled, which considered five coarse grain contents f and three water contents of fine soil v and three water contents of fine soil w f of the soil mixture under both saturation and unsaturation. Conversely, in previous studies, the growth of f v contributed to an increment in τ of the soil mixture under both saturation and unsaturation. Conversely, in previous studies, the growth of f v induced a decline in the dry density of the fine soil fraction τ under saturation, but a decline in that under unsaturation. This was explained by the competing influences of f v and suction ψ : in previous studies, increasing f of the soil mixture decreased. Meanwhile, modelling of the v induced a decline in the dry density of the fine soil fraction ρ relationship in the soil mixture with varying d-f , which contributed to a decline in ψ . When the negative influence of declining ψ outweighed the positive influence of the incrementing f v , the τ of the soil mixture decreased. Meanwhile, modelling of the τ - ψ relationship in the soil mixture with varying f v was performed. This proposed model was examined using the test results from both the present and previous studies, which shows its reasonably good performance.

Unsaturated soil exhibits extremely complex engineering mechanical properties under the coupling effect of moistening and stress fields. Firstly, the effective stress principle and limit equilibrium conditions of unsaturated soil under the coupling effect of moistening and stress fields were discussed based on the basic principles of unsaturated soil. Secondly, a fractional-order model considering the viscoelasticity and strain hardening of unsaturated soil was established based on the fractional calculus theory. Then, based on the principle of damage mechanics, the damage variable evolution equation under the coupling effect of moistening and stress fields was established, and the fractional calculus-based statistical constitutive damage model of unsaturated soil under the coupling effect of moistening and stress fields was developed. In turn, parameters of the developed model were solved using a triaxial test of unsaturated loess, and the calculated data using the developed model were compared with the experimental data, which demonstrated that the developed model in this paper performed well in describing the whole strain hardening process of unsaturated soil under the coupling effect of moistening and stress fields. Finally, the sensitivity of the main parameters of the developed model was discussed under the coupling effect of moistening and stress fields, which showed that the proposed model performed well in reflecting the main mechanical properties of unsaturated loess. [ABSTRACT FROM AUTHOR]

Deep soil moisture data have wide applications in fields such as engineering construction and agricultural production. Therefore, achieving the real-time monitoring of deep soil moisture is of significant importance. Current soil monitoring methods face challenges in conducting the large-scale, real-time monitoring of deep soil moisture. This paper innovatively proposes a real-time prediction approach to deep soil moisture combining GNSS-R data and a water movement model in unsaturated soil. This approach, built upon surface soil moisture data retrieved from GNSS-R signal inversion, integrates soil–water characteristics and soil moisture values at a depth of 1 m. By employing a deep soil moisture content prediction model, it provides predictions of soil moisture at depths from 0 to 1 m, thus realizing the large-scale, real-time dynamic monitoring of deep soil moisture. The proposed approach was validated in a study area in Goodwell, Texas County, Oklahoma, USA. Predicted values of soil moisture at a randomly selected location in the study area at depths of 0.1 m, 0.2 m, 0.5 m, and 1 m were compared with ground truth values for the period from 25 October to 19 November 2023. The results indicated that the relative error (δ) was controlled within the range of ±14%. The mean square error (MSE) ranged from 2.90 × 10 − 5 to 1.88 × 10 − 4 , and the coefficient of determination ( R 2 ) ranged from 82.45% to 89.88%, indicating an overall high level of fitting between the predicted values and ground truth data. This validates the feasibility of the proposed approach, which has the potential to play a crucial role in agricultural production, geological disaster management, engineering construction, and heritage site preservation. [ABSTRACT FROM AUTHOR]

Due to cyclic wetting and drying, the hydro-mechanical behavior of unsaturated soil is impacted significantly. In order to assess the soil strength parameters, knowing the unsaturated behavior is important. Soil moisture content is an important parameter that can define the shear strength of the soil. Most of the highway slopes of Mississippi are built on highly expansive clay. During summer, the evaporation of moisture in the soil leads to shrinkage and the formation of desiccation cracks, while during rainfall, the soil swells due to the infiltration of water. In addition to this, the rainwater gets trapped in these cracks and creates perched conditions, leading to the increased moisture content and reduced shear strength of slope soil. The increased precipitation due to climate change is causing failure conditions on many highway slopes of Mississippi. Vetiver, a perennial grass, can be a transformative solution to reduce the highway slope failure challenges of highly plastic clay. The grass has deep and fibrous roots, which provide additional shear strength to the soil. The root can uptake a significant amount of water from the soil, keeping the moisture balance of the slope. The objective of the current study is to assess the changes in moisture contents of a highway slope in Mississippi after the Vetiver plantation. Monitoring equipment, such as rain gauges and moisture sensors, were installed to monitor the rainfall of the area and the moisture content of the soil. The data showed that the moisture content conditions were improved with the aging of the grass. The light detection and ranging (LiDAR) analysis was performed to validate the field data obtained from different sensors, and it was found that there was no significant slope movement after the Vetiver plantation. The study proves the performance of the Vetiver grass in improving the unsaturated soil behavior and stability of highway slopes built on highly expansive clay. [ABSTRACT FROM AUTHOR]

Describing the hydraulic conductivity of unsaturated soil is very important in predicting water transport. Most current models have complex forms and generally need to be calibrated by the measured unsaturated hydraulic conductivity curve. A simple model, by which it is possible to conveniently predict the unsaturated hydraulic conductivity, is proposed in this study. The soil–water characteristic curve and hydraulic conductivity curve are separated into three parts. The soil–water characteristic curve is represented by Fredlund and Xing's equation. A simple model composed of three lines is proposed for estimating the hydraulic conductivity of unsaturated soil. The model parameters can be conveniently calibrated from the measured soil–water characteristic curve and saturated hydraulic conductivity. Finally, the proposed model is validated by the experimental data from different soils. The proposed model provides a simple approach to estimating the hydraulic conductivity of unsaturated soil, which is more convenient for practical application. [ABSTRACT FROM AUTHOR]

Root reinforcement is an effective slope protection measure due to root water absorption and soil suction. However, the coupled effect of rainfall and root reinforcement remains unclear, resulting in a challenge to evaluate slope stability in complex environments. This paper regards the root–soil composite as a natural fiber composite and quantifies its reinforcement effect using direct shear tests. The unsaturated soil seepage–stress theory was employed to simulate the effect of rainfall on water migration and the stability of spoil, overburden, and vegetated slopes. Field measurements and pore water pressure tests verified the simulation results. Furthermore, the influences of the slope angle, rainfall parameters, and vegetation cover thickness on slope stability were analyzed. The results showed the following: (1) The root reinforcement enhanced the soil's ability to resist shear deformation, substantially improving soil shear strength. The cohesion of the root–soil composite (crs = 33.25 kPa) was 177% higher than that of the engineering spoil (ces = 12 kPa) and 32.21% higher than that of the overburden soil (cos = 25.15 kPa). (2) The overburden and vegetated slopes had lower permeability coefficients and a higher shear strength than the spoil slope, and the effect was more pronounced for the latter, resulting in lower landslide risks. The water migration trend of the vegetated slope was characterized by substantial runoff and a low sediment yield. The safety factors of the spoil slope, overburden slope, and vegetated slope were 1.741, 1.763, and 1.784 before rainfall and 1.687, 1.720, and 1.763 after rainfall, respectively, indicating a significantly higher safety factor of the vegetated slope after rainfall. (3) The slope angle significantly affected slope stability, with lower safety factors observed for higher rainfall intensities and durations. Under these conditions, the slope angle should be less than 30°, and the soil thickness should be 0.5 m for herbaceous vegetation and shrubs and 1.0 m for trees. [ABSTRACT FROM AUTHOR]

The design, energetic performance, and thermal impact of large-scale geothermal collector systems (LSCs) are dependent on the thermal conductivity of unsaturated soils (λ). The aim of this study was to investigate the benefits of two different λ measurement methods using single-needle sensor measuring devices on a laboratory scale. Since large-scale determinations are required in the context of LSCs, the potential for deriving λ from electrical resistivity tomography measurements (ERTs) was also examined. Using two approaches—the continuous evaporation method and the punctual method—thermal conductivities of soil samples from Bad Nauheim (Germany) were measured. The results were compared with averaged λ derived from three ERT sections. With the evaporation method, significant bulk density changes were observed during the experimental procedure, which were caused by the clay content and the use of repacked samples. The punctual method ensures a sufficiently constant bulk density during the measurements, but only provides a small number of measurement points. The thermal conductivities derived from ERTs show largely minor deviations from the laboratory measurements on average. If further research confirms the results of this study, ERTs could provide a non-invasive and unelaborate thermal exploration of the subsurface in the context of large-scale infrastructure projects such as LSCs. [ABSTRACT FROM AUTHOR]

Although many unsaturated soil experiments have successfully delivered positive outcomes, most studies just concisely illustrated sensor techniques, because their main objectives focused on bridging research gaps. Inexperienced research fellows might rarely follow up those techniques, so they could encounter very trivial and skill-demanding difficulties, undermining the quality of experimental outcomes. With a motivation to avoid those, this work introduces technical challenges in applying three sensor techniques: high precision tensiometer, spatial time-domain reflectometry (spatial TDR) and digital bench scales, which were utilized to measure three fundamental variables: soil suction, moisture content and accumulative outflow. The technical challenges are comprehensively elaborated from five aspects: the functional mechanism, assembling/manufacturing approaches, installation procedure, simultaneous data-logging configurations and post data/signal processing. The conclusions drawn in this work provide sufficient technical details of three sensors in terms of the aforementioned five aspects. This work aims to facilitate any new research fellows who carry out laboratory-scale soil column tests using the three sensors mentioned above. It is also expected that this work will salvage any experimenters having troubleshooting issues with those sensors and help researchers bypass those issues to focus more on their primary research interests.

Unsaturated soil mechanics, when applied to determine the soil shear strength, are crucial for accurately evaluating the safety of geotechnical structures affected by seasonal moisture variations. Over the past decades, multiple models have been formulated to predict the behavior of unsaturated soils in terms of water flow and shear strength individually. Building upon these foundational studies, this research introduces a model that couples an analytical solution for one-dimensional water infiltration with an unsaturated shear strength model. This model further incorporates the impact of void ratio fluctuations on soil properties and state variables related to shear strength. A parametric analysis is conducted to evaluate the effects of the initial void ratio on a representative soil profile during a water infiltration event. The model presented in this paper integrates various concepts from the field of unsaturated soil mechanics and is applicable to any homogeneous soil where expansion/collapse effects are negligible. It demonstrates how shear strength might be underestimated when using a saturated soil approach. Conversely, it may also lead to an overestimation of safety conditions if the soil approaches a saturated or dry state. The proposed model offers a more accurate prediction of unsaturated soil shear strength. It is useful for determining transient safety factors in geotechnical structures. Furthermore, when combined with field-installed instrument monitoring, this model contributes significantly to the functionality, safety, cost-effectiveness, and sustainability of geotechnical structures and projects. [ABSTRACT FROM AUTHOR]

Rammed earth has the potential to reduce the carbon footprint and limit the energy consumption in the building sector due to its sustainable characteristics. Still, its use is not generalized due to a lack of understanding of the material behavior, notably its sensitivity to water. The coupled hydro-mechanical behavior has been recently studied in the framework of unsaturated soil mechanics, using suction as the parameter to represent the hydric state. This dependency of the mechanical behavior on the hydric state leads to uncertainty of the drying period required to progress in the construction process. Notably, the drying period before building the next floor is unknown. To determine the drying period, thermo-hydro-mechanical coupled finite element method simulations were carried out on a single wall by using the unsaturated soil mechanics approach and safety criterion recommendations from the practical guide for rammed earth construction in France. It was determined that it takes significant time for the construction of additional floor both in 'summer-like' and 'winter-like' environmental conditions, whereas the walls were far away from the ultimate failure state. Thus the drying periods were overestimated. It was concluded that the safety criterion from the practical guide is very conservative and drying periods can be reduced without significantly compromising the safety factor.

The shear strength of unsaturated soils exhibits significant nonlinearity, while previous studies often simplified it with linear strength models. The objective of this paper is to investigate the distinctions in the stability of three-dimensional (3D) tunnel faces when using linear and nonlinear strength models. A new 3D rotational failure mechanism and an extended form of the Mohr–Coulomb (M-C) failure criterion were integrated into the kinematically limited analysis (KLA) framework to describe the failure characteristics of tunnel faces. Subsequently, the factor of safety (FS) of the 3D tunnel faces was calculated using the strength reduction method (SRM). In the discussion section, the impacts of nonlinear shear strength, matric suction in the unsaturated soils, and the 3D geometric parameters of the tunnel on the stability of the tunnel face were analyzed. The outcomes indicate that, in unsaturated soil conditions, diverse nonlinear strength calculation models and soil types exert disparate influences on the FS of 3D tunnel faces. The main novelty of this study lies in establishing an effective method for assessing the stability of tunnel faces in unsaturated soils. [ABSTRACT FROM AUTHOR]

Various electrochemical methods were used to understand the behavior of steel buried in unsaturated artificial soil in the presence of cathodic protection (CP) applied at polarization levels corresponding to correct CP or overprotection. Carbon steel coupons were buried for 90 days, and the steel/electrolyte interface was studied at various exposure times. The coupons remained at open circuit potential (OCP) for the first seven days before CP was applied at potentials of -1.0 and -1.2 V vs. Cu/CuSO 4 for the remaining 83 days. Voltammetry revealed that the corrosion rate decreased from ~330 µm yr -1 at OCP to ~7 µm yr -1 for an applied potential of -1.0 V vs. Cu/CuSO 4 . CP effectiveness increased with time due to the formation of a protective layer on the steel surface. Raman spectroscopy revealed that this layer mainly consisted of magnetite. EIS confirmed the progressive increase of the protective ability of the magnetite-rich layer. At -1.2 V vs. Cu/CuSO 4 , the residual corrosion rate of steel fluctuated between 8 and 15 µm yr -1 . EIS indicated that the protective ability of the magnetite-rich layer deteriorated after day 63. As water reduction proved significant at this potential, it is proposed that the released H 2 bubbles damage the protective layer.

Stratigraphic uncertainty is widely present in nature, but it has not been well considered in the stability analysis of unsaturated soil slopes in the past. In this study, the stability of the unsaturated soil slope is evaluated based on borehole data considering stratigraphic uncertainty. Firstly, an enhanced coupled Markov chain model is used to simulate stratigraphic uncertainty. Then, a finite element algorithm for automatically calculating the safety factor (FS) and the average groundwater table (AGT) of the unsaturated soil slope is developed. At last, a hypothetical slope located in the stratum from Perth, West Australia is analyzed using the proposed algorithm under different borehole schemes. The results show that with the increase in the borehole number, the statistics of FS and AGT will not monotonically increase or decrease. But the trend is that the mean values of FS and AGT gradually approach and eventually converge to the real values, and the standard deviations of FS and AGT decrease. There is a linear relationship between the standard deviation of FS (or AGT) and the average information entropy. The FS and AGT are negatively correlated considering stratigraphic uncertainty. [ABSTRACT FROM AUTHOR]

In the restoration of ancient water engineering, the loss of fine soil particles from the ancient riverbank can easily cause seepage instability problems such as piping during the flood transient process. This paper explores the influence mechanism of flood fluctuation on soil seepage stability based on indoor experiments, field monitoring and saturated-unsaturated soil seepage theory. The paper obtains the soil-water characteristic curve of unsaturated soil using laboratory tests, builds the transient seepage finite element model of porous media and modifies parameters monitoring data to verify the numerical analysis results. The results showed that the groundwater level, pore water pressure and seepage hydraulic gradient had changed. The maximum pore water pressure between the ancient riverbank and antiseepage structure increased by 13.4%, the maximum hydraulic gradient at the toe of the riverbank increased by 49.3% and the instability of seepage significantly increased. Through the modified lime grouting between the ancient riverbank and the antiseepage structure, the structure of the soil mass was changed, and the maximum hydraulic gradient was reduced by 55.6%, which restrains piping damage. This study can be used in the restoration of ancient riverbanks to solve piping problems. [ABSTRACT FROM AUTHOR]

Slope disasters often include soil erosion and shallow landslides. These types of slope failure can occur in unsaturated soil. In this study, artificial rainfall is applied to a compacted soil slope in a laboratory to investigate the effects of rainfall intensity, initial moisture content and relative compaction on infiltration and failure behavior of an unsaturated soil slope subjected to rainfall infiltration. Soil moisture probes installed in the soil slope were used to monitor soil moisture content during the experiment. Test results show that the soil saturation at the toe of the slope was observed to be higher than other areas of the slope following the onset of rainfall. Gradually, the saturation advanced towards the crest, resulting in the formation of a wetting band along the slope surface. With increasing rainfall duration, the wetting band progressed further downwards along the slope. The failure of the slope began at the crest and junction of the slope, and the primary cause was soil erosion resulting from high-intensity rainfall. The most significant variable affecting infiltration behavior is relative compaction, followed by the initial moisture content of the compacted slope. During the experiment, the low hydraulic conductivity of the saturated soil caused most of the rainfall on the slope to become surface runoff, which minimized the impact of rainfall intensity on infiltration behavior. Specifically, the hydraulic conductivity of soil compacted at optimal moisture content and 90 R.C. is only 7.041 × 10-5 cm/sec or 2.53 mm/h, which is much lower than rainfall intensities of 80 mm/h and 160 mm/h. In addition, soil saturation contours facilitated visualization and quantification of the infiltration behavior of slopes, enabling a more detailed analysis of experimental results. These results help understand the behavior of unsaturated soil slopes under artificial rainfall conditions and aid in designing effective slope stabilization measures to prevent slope failure and minimize the risk of landslides. [ABSTRACT FROM AUTHOR]

The uneven pore water distribution in unsaturated soil will cause water movement, change the hydraulic and mechanical characteristics of soil, and then cause soil damage. Therefore, it is important to study the hydraulic characteristics of unsaturated soil. In this paper, the law of conservation of mass and Darcy's law were used to analyze the unit soil after seepage to obtain a continuous equation. Combined with the soil-water characteristic curve (SWCC), the effect of matric suction and permeability coefficient of unsaturated soil on infiltration rate is substituted into the equation. Through the analysis of pore water stress of the unit soil, the function of the unsaturated permeability coefficient with the effective saturation degree is obtained, and the theoretical formula of the one-dimensional infiltration rate of unsaturated soil is derived. Compared with other models, this formula has fewer parameters and is easy to use. [ABSTRACT FROM AUTHOR]

Unsaturated soil is a three-phase medium with three interfaces, and the mathematical equations that represent its behavior must be developed in a fully coupled manner for accurately predicting its hydromechanical behavior. In this paper, a set of fully coupled governing equations was developed for the dynamics of unsaturated soil, considering the interaction among the bulk phases and interfaces. In addition to implementing the complete governing equations, a simplified formulation was developed for practical applications. The derivation of the finite element formulation considering all the terms in the partial differential equations resulted in a formulation called complete formulation and was solved for the first time in this paper. Another formulation called reduced formulation was derived by neglecting the relative accelerations and velocities of water and air in the governing equations. In addition, small and large deformation theories were developed and implemented for both formulations. To show the applicability of the proposed models, the dynamic behavior of an unsaturated soil embankment was simulated using both small and large deformation formulations by applying minor and severe earthquakes. The reduced formulation was found to be computationally efficient and numerically stable. The smaller displacements predicted by large deformation theories show that the results are consistent with the expected behavior. Large deformation theories are considered suitable when the geotechnical system undergoes large deformation and may lead to accurate prediction. [ABSTRACT FROM AUTHOR]

In geotechnical engineering, the consolidation of unsaturated soil is a common issue of great interest. Considering the multi-layered property and impeded drainage boundary condition of the soil stratum in real engineering, this study aimed to develop a general semi-analytical solution for assessing the one-dimensional (1D) consolidation behavior of multi-layered unsaturated soil that is subjected to a general impeded drainage boundary condition and a time-dependent loading. To achieve the final solution, the proposed consolidation system is firstly decoupled and solved in the Laplace domain. Then, the semi-analytical solutions for the excess pore-air pressure and excess pore-water pressures as well as the soil settlement are formulated. The Crump method is employed to provide their final results in the time domain. The correctness of the derived solutions was verified against the available analytical and numerical solutions, and excellent agreements were found for the two comparisons. Moreover, two studied examples are presented to illustrate the 1D consolidation behavior of multi-layered unsaturated soil and the influences stemming from the impeded drainage parameters are discussed. [ABSTRACT FROM AUTHOR]

An unsaturated soil constitutive model considering the influence of microscopic pore structure can more accurately describe the hydraulic–mechanical behavior of unsaturated soil, but its numerical implementation is more complicated. Based on the fully implicit Euler backward integration algorithm, the ABAQUS software is used to develop the established hydro-mechanical coupling constitutive model for unsaturated soil, considering the influence of micro-pore structure, and a new User-defined Material Mechanical Behavior (UMAT) subroutine is established to realize the numerical application of the proposed model. The developed numerical program is used to simulate the drying/wetting cycle process of the standard triaxial specimen. The simulation results are basically consistent with those calculated by the Fortran program, which verifies the rationality of the developed numerical program. [ABSTRACT FROM AUTHOR]

The variation of pore gas pressure caused by rainfall infiltration is an important factor that leads to slope failures. The purpose of this study is to propose a new slope stability analysis method that considers pore gas pressure and examines the effect of airflow on slope stability by using a numerical method. A water-air two-phase flow analysis was conducted to investigate the distribution of pore air pressure, pore water pressure, and water saturation triggered by rainfall infiltration. Then the variation of the load resulting from pore gas pressure was incorporated into the slope stability analysis method based on the unsaturated soil shear strength theory and the residual thrust method to simulate the influence of airflow on the Tanjiahe landslide in China. In order to study the infiltration behavior with respect to initial saturation, water and gas flow analyses were performed considering various initial states of saturation under similar settings. Results showed that the pore gas pressure between the slope surface and the slip band clearly varied and that it decreased during the process from the slide bed to the deep direction. Then, the pore water pressure formed in the saturated zone was transferred by the airflow to the slope toe. As a result, because the pore gas pressure gradient increased the thrust of the slide mass, the safety factor decreased over time. Moreover, in the first step, the magnitude of infiltration decreased with an increase in initial saturation, while when the magnitude dropped to the minimum value, it then went up with an increase in initial saturation. The maximum value was usually reached at a saturation degree of 0% or 100%. When evaluating slope stability, the safety factor obtained by the slope stability analysis method that considered the water-gas coupling effect was much lower than when it was not considered during the process of a similar seepage. The impact on the slope failure was significant and may provide a practical reference for hazard assessments to control rainfall-induced landslides. [ABSTRACT FROM AUTHOR]

Most soil mechanics theories are limited to strain hardening and shrinkage under high compressive stresses, and there are some shortcomings in the selection of suction or degree of saturation as the water content state varies in the constitutive models of unsaturated soil. Based on the triaxial shear tests of unsaturated compacted soil (a silt of high plasticity) with different water content and confining pressure (low-confining), a shear dilatancy model of unsaturated soil based on the mass water content is proposed in this paper. The influence of the water content on the shear deformation characteristics of the unsaturated soil is analysed. The stress–dilatancy relationship and the prediction equation of the minimum dilatancy rate of the unsaturated soil under different water content and different confining pressure are provided. Selecting the mass water content as the state variable, a constitutive model suitable for the dilatancy of unsaturated soil is established. The method of determining model parameters based on the mass water content is analysed. The applicability of the model is verified by comparisons between the predicted and experimental results. [ABSTRACT FROM AUTHOR]

The presence of unsaturated soil is critical in geotechnical engineering since the matric suction may aid in accommodating the pile shaft capacity. The design of piles can be optimized by incorporating unsaturated soil mechanics principles. Hence, the amount of waste materials can be reduced, the duration of pile installation can be expedited, and the amount of energy used for casting the pile can be optimized, resulting in more sustainable design and construction of piles. Conventional α, β, and λ methods and modified α, β, and λ methods are the common models that are used for calculating the shaft capacity by incorporating soil–water characteristic curves (SWCCs). However, in our opinion, we feel that the investigation of the influence of seepage infiltration due to rainfall on the shaft capacity of piles, calculated using both analytical means and numerical analysis, has been dealt with inadequately in past studies. The objective of this study is to investigate changes in the shaft pile capacity according to suction changes due to rainwater infiltration for the greater reliability of the pile design, using both analytical and numerical studies with the finite element method (FEM). Sand and kaolin, which are typical components of coarse-grained and fine-grained soil, are used in this study. The laboratory results were incorporated into PLAXIS 3D (Version 22), and a coupled analysis was carried out, utilizing the meteorological conditions in Astana. The results showed that the decreases in matric suction in sand and kaolin are similar after their subjection to rainfall, yet sand produces a higher shaft capacity compared to kaolin. The modified β method offers a higher shaft capacity compared to the other methods due to the effective stress factors being taken into account. The modified α and λ methods are recommended for short piles because they are more sustainable, whilst the modified β method is preferable for long piles. Overall, unsaturated soil conditions should be applied to optimize the foundation design since they generate a higher shaft capacity. [ABSTRACT FROM AUTHOR]

Genetic programming (GP) of four levels of complexity, including artificial neural networks of the hyper-tanh activation function (ANN-Hyper-Tanh), artificial neural networks of the sigmoid activation function (ANN-Sigmoid), evolutionary polynomial regression (optimized with genetic algorithm) (EPR), and intelligent techniques have been used to predict the erodibility of lateritic soil collected from an erosion site and treated with hybrid cement. Southeastern Nigeria and specifically Abia State is being destroyed by gully erosion, the solution of which demands continuous laboratory examinations to determine the parameters needed to design sustainable solutions. Furthermore, complicated equipment setups are required to achieve reliable results. To overcome constant laboratory works and equipment needs, intelligent prediction becomes necessary. This present research work adopted four different metaheuristic techniques to predict the erodibility of the soil; classified as A-7-6, weak, unsaturated, highly plastic, high swelling and high clay content treated with HC utilized in the proportions of 0.1–12% at the rate of 0.1%. The results of the geotechnics aspect of the work shows that the HC, which is a cementitious composite formulated from blending nanotextured quarry fines (NQF) and hydrated lime activated nanotextured rice husk ash (HANRHA), improves the erodibility of the treated soil substantially and consistently. The outcome of the prediction models shows that EPR with SSE of 1.6% and R2 of 0.996 outclassed the other techniques, though all four techniques showed their robustness and ability to predict the target (Er) with high performance accuracy. [ABSTRACT FROM AUTHOR]

Estimation of soil moisture distribution in desert regions is challenged by the deep unsaturated zone and the extreme natural environment. In this study, an entropy-based method, consisting of information entropy, principle of maximum entropy (PME), solutions to PME with constraints, and the determination of parameters, is used to estimate the soil moisture distribution in the 10 m deep vadose zone of a desert region. Firstly, the soil moisture distribution is described as a scaled probability density function (PDF), which is solved by PME with the constraints of normalization, known arithmetic mean and geometric mean, and the solution is the general form of gamma distribution. A constant arithmetic mean is determined by considering the stable average recharge rate at thousand year scale, and an approximate constant geometric mean is determined by the low flow rate (about 1 cm a year). Followed, the parameters of the scaled PDF of gamma distribution are determined by local environmental factors like terrain and vegetation: the multivariate linear equations are established to qualify the relationship between the parameters and the environmental factors on the basis of nineteen random soil moisture profiles about depth through the application of fuzzy mathematics. Finally, the accuracy is tested using correlation coefficient (CC) and relative error. This method performs with CC larger than 0.9 in more than a half profiles and most larger than 0.8, the relative errors are less than 30% in most of soil moisture profiles and can be as low as less than 15% when parameters fitted appropriately. Therefore, this study provides an alternative method to estimate soil moisture distribution in top 0-10 m layers of the Badain Jaran Desert based on local terrain and vegetation factors instead of drilling sand samples, this method would be useful in desert regions with extreme natural conditions since these environmental factors can be obtained by remote sensing data. Meanwhile, we should bear in mind that this method is challenged in humid regions since more intensive and frequent precipitation, and more vegetation cover make the system much more complex. [ABSTRACT FROM AUTHOR]

The purpose of this paper was to analyze the impact of extreme rainfall on the recurrence of slope instability using the Thulamela Municipality roads (R523) as a case study. To this end, the historical rainfall data of the area of study were analyzed between 1988 and 2018. The results show that a significant increase in rainfall is usually experienced in the summer months of December and January. Following this, the factor of safety (FoS) of slopes of silt clay, clay, and clay loam soils were estimated using the SLIDE simulator (Numerical software "Finite Element Method (FEM)") under sunny to rainy conditions of the area. A complementary model, FLACSlope (Numerical software "Finite Difference Method (FDM)"), was utilized to simulate FoS and pore water pressure in sunny and rainy conditions of the area. Simulation results show that extreme rainfall has the ability to reduce the shear strength and resistance of the soil slope material. This may explain the recurrent landslides noted in the area. Finally, the water pore pressure has been simulated to increase with the increased water table, which generally pushes the soil particles apart and reduces the stress between the particles resulting in soil slope failure. Extreme rainfall alters the phase of the material solid in a manner that may require further research for a better understanding. [ABSTRACT FROM AUTHOR]

Landslides or slope failure pose a significant risk to human lives and infrastructures. The stability of slopes is controlled by various hydrological processes such as rainfall infiltration, soil water dynamics, and unsaturated soil behavior. Accordingly, soil hydrological monitoring and tracking the displacement of slopes become crucial to mitigate such risks by issuing early warnings to the respective authorities. In this context, there have been advancements in monitoring critical soil hydrological parameters and slope movement to ensure potential causative slope failure hazards are identified and mitigated before they escalate into disasters. With the advent of the Internet of Things (IoT), artificial intelligence, and high-speed internet, the potential to use such technologies for remotely monitoring soil hydrological parameters and slope movement is becoming increasingly important. This paper provides an overview of existing hydrological monitoring systems using IoT and AI technologies, including soil sampling, deploying on-site sensors such as capacitance, thermal dissipation, Time-Domain Reflectometers (TDRs), geophysical applications, etc. In addition, we review and compare the traditional slope movement detection systems, including topographic surveys for sophisticated applications such as terrestrial laser scanners, extensometers, tensiometers, inclinometers, GPS, synthetic aperture radar (SAR), LiDAR, and Unmanned Aerial Vehicles (UAVs). Finally, this interdisciplinary research from both Geotechnical Engineering and Computer Science perspectives provides a comprehensive state-of-the-art review of the different methodologies and solutions for monitoring landslides and slope failures, along with key challenges and prospects for potential future study. [ABSTRACT FROM AUTHOR]

The soil–water characteristic curve (SWCC) is an important parameter of unsaturated soil, and almost all the engineering characteristics of unsaturated soil are more or less related to the SWCC. The SWCC contains important information for geotechnical engineering, water engineering, hydrogeology modelling and climate modelling. It is noted that the experimental measurement of SWCC is costly and time consuming, which limits the implementation of principles of unsaturated soil mechanics in practical engineering. The indirect method, which estimates the SWCC from the index properties of soil, can provide the SWCC with the errors which are within tolerance in practical engineering. In addition, the indirect method can determine SWCC very fast and almost with no cost. In this paper, the domestic sandy soils are selected and the index properties of those sands are used to correlate the SWCC fitting parameters. Consequently, mathematical equations are proposed to estimate SWCC from index properties of domestic sands. The proposed models are trained from 44 sets of experimental data and verified with another independent 8 sets of experimental data from published literature. It is observed that the results from the proposed model agree well with the experimental data from literature. [ABSTRACT FROM AUTHOR]

Relating soil moisture content to soil suction, the soil–water characteristic curve (SWCC) represents an essential feature in unsaturated soil mechanics that enables estimation of different unsaturated soil property functions and modeling of the macro-scale soil behavior. However, depending on the soil and processes under consideration, proper hydraulic characterization of a soil through direct laboratory measurements can be difficult, time-consuming, and involve many uncertainties. In the case of uniformly graded sands, there is a highly nonlinear and steep shape of the SWCC, with only a few kPa of soil suction separating saturated and residual soil moisture conditions, which makes measurements for determinations of SWCC especially challenging. This study encompasses an investigation of the sandy type of soil's behavior and presents some preliminary results and experiences on the determination of SWCC through the use of physical slope model tests. The 30 cm deep slope, inclined at 35 degrees and instrumented with soil moisture and pore water pressure sensors, was exposed to series of rainfall intensities, ranging from 37 up to 300 mm/h. The results indicated that the data on hydraulic response in monitored points are not only useful for the determination of SWCC, but that the approach is useful for investigation of hydraulic hysteresis phenomena, as well as its effects on soil moisture and pore water pressure conditions, which also affects the stability conditions of a slope. In particular, the best-fit parameters of the van Genuchten model suggested air entry values of 1.6 and 1.1 kPa for the drying and the wetting curves of the SWCC, respectively, with the two branches shifted by about 1 kPa of soil suction. [ABSTRACT FROM AUTHOR]

Unsaturated soil is a form of natural soil whose pores are filled by air and water. Different from saturated soil, the microstructure of unsaturated soil consists of three phases, namely, the solid phase (soil particle), vapor phase, and liquid phase. Due to the matric suction of soil pores, the hydraulic and mechanical behaviors of unsaturated soils present a significant dependence on the moisture condition, which usually results in a series of unpredictable risks, including foundation settlement, landslide, and dam collapse. Microbial-induced calcite precipitation (MICP) is a novel and environmentally friendly technology that can improve the water stability of unsaturated soft or expansive soils. This paper reviews the microscopic mechanisms of MICP and its effect on the mechanical properties of unsaturated soils. The MICP process is mainly affected by the concentration of calcium ions and urea, apart from the concentration of bacteria. The moisture-dependent properties were comparatively analyzed through mechanical models and influence factors on the experimental data among various unsaturated soils. It suggests that the variations in resilient modulus and permanent deformation are strongly related to the extent of MICP applied on unsaturated soils. Finally, the problems in the MICP application, environmental challenges, and further research directions are suggested. [ABSTRACT FROM AUTHOR]

The application of unsaturated soil mechanics in routine geotechnical engineering applications requires the determination of unsaturated soil properties. Unfortunately, the cost of direct measurement of unsaturated soil properties goes beyond the financial budget of most clients. A solution has arisen, however, that involves the measurement of two less costly soil properties functions that can be used in conjunction with a series of assumptions and estimation methodologies. The two laboratory tests involve measurement of the: (i) gravimetric water content versus soil suction, referred to as the soil-water characteristic curve (w-SWCC) and (ii) water content versus void ratio, referred to as the shrinkage curve (SC). These two unsaturated soil property relationships can be used along with saturated soil properties to extend unsaturated soil properties over the full range of soil suctions. "Estimation procedures" have been developed and verified for all physical properties of interest in unsaturated soil mechanics. The use of estimation procedures has meant that the geotechnical engineer must operate within a new paradigm. The new paradigm provides sufficient accuracy for most geotechnical engineering applications. The net result is an increased decision-making capability for geotechnical engineers. [ABSTRACT FROM AUTHOR]

In this paper, a state-dependent, bounding surface plasticity model that simulates the behavior of unsaturated granular soils is presented. An unsaturated, soil mechanics-compatible elastoplastic response is adopted in which no part of the response occurs in a purely elastic fashion. To create an appropriate hydro-mechanical coupling, a newer generation stress framework, consisting of the Bishop-type effective stress and a second stress variable, is used in conjunction with a soil-water characteristic curve function. Details regarding the model development, parameter estimation, and assessment of the model's predictive capabilities are outlined. With a single set of parameter values, the model realistically simulates the main features that characterize the shear and volumetric behavior of unsaturated granular soils over a wide range of matric suction, density, and net confining pressure. [ABSTRACT FROM AUTHOR]

Unsaturated soil shear strength is a significant topic in geotechnical engineering. The measurement of unsaturated soil properties such as shear strength and matric suction could be costly, hard, time-consuming, and often impractical to obtain. The purpose of this research is to propose a new shear strength model for unsaturated soil and to predict the shear strength by using the degree of saturation directly because the water saturation is easy to obtain. More specifically, this study focused on Bishop's shear strength theory and van Genuchten's soil–water characteristic curve to describe the effect of saturation on shear strength. The new shear strength model was expressed as a function of saturation degree, residual degree of saturation, effective shear strength indices, net normal stress, and five parameters. The performance of the presented model in this paper was verified by fitting it to data obtained by laboratory tests on silty sand including the triaxial shear test and soil–water characteristic curve test. From these two laboratory tests, we obtained the variation in the matric suction and shear strength with the degree of saturation under various dry densities. This proposed model was also validated against the shear strength results of the clayed-silty sand and Ankara clay recorded in the literature. Comparing the result of the shear strength under the variation of one parameter, this model was much more sensitive regarding parameter m, which was related to the material characteristic. The comparison between the predicted values and experimental points was particularly fine and showed the ability of this model to be applied to a wide range of soils. [ABSTRACT FROM AUTHOR]

This paper investigates rainfall-induced landslides on partially saturated soil slopes using the 2011 Umyeonsan landslides at the center of Seoul, Korea. An integrated analysis of rainfall-induced landslide was carried out through laboratory tests, field tests, and numerical analysis. The results of comprehensive investigations in the Umyeonsan region demonstrate that landslide activity is closely related primarily to rainfall and soil properties and slope geometry, and vegetation. Numerical analysis was also performed to confirm the effect of these factors on landslide occurrence. Special attention was given to rainfall penetration analysis to determine the wetting band depth for shallow and deep-seated slope failure in watershed-scale landslide. The simulated results agree closely with the investigation results, which indicates that the applied method is appropriate for use in the simulation of landslides in unsaturated soils. [ABSTRACT FROM AUTHOR]

Ignoring the role of soil suction and implementing unsaturated soil mechanics when assessing slope stability in Indonesia is a common practice. One of the main reasons is due to the precognition that incorporating soil suction means using higher shear strength, which leads to less conservative analysis, while it is difficult to practically obtain accurate input parameters for unsaturated soil analysis. However, a number of slope failures occur all over the world due to rainfall, which becomes evidence that ignoring the role of soil suction may not necessarily lead to a conservative result. On 9 January 2021, rainfall-induced slope failure occurred at Cimanggung after four hours of heavy rainfall and killed 32 people. Many of them were injured, and houses were destroyed. This event shows the significance of considering the interaction between infiltration and soil suction when conducting slope stability analysis. Difficulties in obtaining input parameters for unsaturated soil analysis experimentally hindered practitioners in applying unsaturated soil mechanics. While the parameters can be estimated, it is always of question whether the estimated parameters are sufficiently accurate for practical purposes. In this paper, conventional site investigations were carried out while unsaturated soil parameters were estimated to study the mechanism which triggers the landslide that occurred at Cimanggung. It will be shown that estimating unsaturated soil parameters can be practically accurate and manage to capture the failure mechanism such as critical rainfall duration and critical slip surface. [ABSTRACT FROM AUTHOR]

The hydrological and mechanical behavior of soil is determined by the moisture content, soil water (matric) potential, fines content, and plasticity. However, these parameters are often difficult or impractical to determine in the field. Remote characterization of soil parameters is a non-destructive data collection process well suited to large or otherwise inaccessible areas. A ground-based, field-deployable remote sensor, called the soil observation laser absorption spectrometer (SOLAS), was developed to collect measurements from the surface of bare soils and to assess the in-situ condition and essential parameters of the soil. The SOLAS instrument transmits coherent light at two wavelengths using two, continuous-wave, near-infrared diode lasers and the instrument receives backscattered light through a co-axial 203-mm diameter telescope aperture. The received light is split into a hyperspectral sensing channel and a laser absorption spectrometry (LAS) channel via a multi-channel optical receiver. The hyperspectral channel detects light in the visible to shortwave infrared wavelengths, while the LAS channel filters and directs near-infrared light into a pair of photodetectors. Atmospheric water vapor is inferred using the differential absorption of the on- and off-line laser wavelengths (823.20 nm and 847.00 nm, respectively). Range measurement is determined using a frequency-modulated, self-chirped, coherent, homodyne detection scheme. The development of the instrument (transmitter, receiver, data acquisition components) is described herein. The potential for rapid characterization of physical and hydro-mechanical soil properties, including volumetric water content, matric potential, fines content, and plasticity, using the SOLAS remote sensor is discussed. The envisioned applications for the instrument include assessing soils on unstable slopes, such as wildfire burn sites, or stacked mine tailings. Through the combination of spectroradiometry, differential absorption, and range altimetry methodologies, the SOLAS instrument is a novel approach to ground-based remote sensing of the natural environment. [ABSTRACT FROM AUTHOR]

Climate change is known to cause alterations in weather patterns and disturb the natural equilibrium. Changes in climatic conditions lead to increased environmental stress on embankments, which can result in slope failures. Due to wetting–drying cycles, expansive clayey soil often swells and shrinks, and matric suction is a major factor that controls the behavior. Increased temperature accelerates soil evaporation and drying, which can cause desiccation cracks, while precipitation can rapidly reduce soil shear strength. Desiccated slopes on embankments built with such soils can cause surficial slope failures after intense precipitation. This study used slope stability analysis to quantify how climate-change-induced extreme weather affects embankments. Historic extreme climatic events were used as a baseline to estimate future extremes. CMIP6 provided historical and future climatic data for the study area. An embankment was numerically modeled to evaluate the effect on slope stability due to the precipitation change induced by climate change. Coupled hydro-mechanical finite element analyses used a two-dimensional transient unsaturated seepage model and a limit equilibrium slope stability model. The study found that extreme climatic interactions like precipitation and temperature due to climate change may reduce embankment slope safety. The reduction in the stability of the embankment due to increased precipitation resulting from different greenhouse gas emission scenarios was investigated. The use of unsaturated soil strength and variation of permeability with suction, along with the phase transition of these earthen embankments from near-dry to near-saturated, shows how unsaturated soil mechanics and the hydro-mechanical model can identify climate change issues on critical geotechnical infrastructure. [ABSTRACT FROM AUTHOR]

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

Unsaturated soil covers a significant part of the world, and studying the behavior of deep foundations in this medium is an important step in increasing accuracy and economic efficiency in geotechnical studies. This paper presents an analytical solution to investigate the load-carrying characteristics of single piles embedded in unsaturated soils, accounting for the effect of groundwater level on the pile's response. For this purpose, relationships for shear modulus and Poisson's ratio for unsaturated soils were collected from the literature to consider their effects as key parameters on pile performance. A parametric study was conducted to evaluate the effect of soil moisture content on the behavior of the pile-soil system for different soil types, and the effect of pile slenderness on its load-settlement behavior was studied for varying soil moisture contents. The results indicate that the pile stiffness increases as the soil suction increases while below a critical slenderness value, hence increasing the pile load capacity. However, this improvement occurs within a limited range of soil suction that is narrower for coarse-grained soils. The pile settlement corresponding to soil failure was also evaluated by modifying the existing solutions for unsaturated soils. The developed solutions were verified against the predictions of published solutions as well as the results of finite element analysis and pile load tests. It was found that the system stiffness decreases by 50% when the water table rises from the pile toe level to the ground surface in the studied soil. [ABSTRACT FROM AUTHOR]

Tailing dams are commonly used to safely store tailings without damaging the environment. Sand tailings (also called Sediment tailings) usually have a high water content and hence undergo consolidation during their placement. As the sediment tailings are usually placed above the ground water level, the degree of saturation and permeability of the sediment tailing is associated with the unsaturated condition due to the presence of negative pore-water pressure or suction. Current practices normally focus on the analyses saturated conditions. However, this consolidation process requires the flow of water between saturated and unsaturated zones to be considered. The objective of this study is to investigate the stability and consolidation of sediment tailings for the construction of road pillars considering the water flow between saturated and unsaturated zones. The scope of this study includes the unsaturated laboratory testing of sediments and numerical analyses of the road pillar. The results show that the analyses based on saturated conditions overestimate the time required to achieve a 90% degree of consolidation. The incorporation of the unsaturated soil properties is able to optimize the design of slopes for road pillars into steeper slope angles. [ABSTRACT FROM AUTHOR]

In this paper, the discrete element method is used to simulate triaxial tests of unsaturated soil under a tension–shear state. A relationship between water content and uniaxial tensile strength with different void ratios is obtained, which is applied to uniaxial tensile discrete element simulations to establish a relationship between grain-scale parameters and water content from back analysis. A group of triaxial simulations for unsaturated soil under a tension–shear state is then conducted. The discrete element method is used to obtain the relationship between deviatoric stress and axial displacement with different water contents, and also to reveal the effects of water content on peak strength and dilatancy phenomena with different confining pressures. The displacement fields of numerical specimen are analyzed qualitatively, and the mechanism and process of failure are discussed from the prospective of energy and dissipation. [ABSTRACT FROM AUTHOR]

In order to reduce the construction cost of excessively wet silt roadbeds and improve the compaction quality, this paper studies the compaction characteristics and compaction technology of excessively wet silts through theoretical derivation, indoor experiments, numerical simulation, engineering verification, and other research methods. Based on the pendulum-type liquid bridge structure, this paper uses the Hertz–Mindlin theory, particle motion equation of state, and other theories to modify the force between particles of an unsaturated soil. Through the contact angle, which is a medium, the matrix suction is linked to the water content and is used to approximate the water content state of soil samples. Using PFC3D (particle flow code 3D) numerical simulation software, an unsaturated silt model was established based on Hill contact, and the model parameters were calibrated through basic geotechnical tests and triaxial compression tests. The correctness of the theory is verified by comparing the simulated triaxial compression test of wet silts with the indoor test. By simulating subgrade rolling, considering factors such as the distribution of contact force chain and the variation of compaction degree, the effects of rolling times, strong vibration, and weak vibration on the compaction effect were compared and analyzed. Finally, the optimal rolling and compaction process under the optimal water content is obtained through on-site engineering tests. The results show that: (1) The Hill contact model is reasonable for simulating the wet silt. (2) During the simulation of the roadbed compaction process using PFC3D software, it was found that the compaction degree change during the entire compaction process can be roughly divided into the initial compaction stage and the re-compaction stage. The reasonable number of compaction times was determined to be five through discrete element simulation. (3) It is found from the numerical simulation results that compared to static compaction, both strong and weak vibration compaction can effectively improve the compaction effect of subgrade compaction. The larger the vibration amplitude, the more obvious the improvement of the compaction effect. The compaction effect of strong vibration followed by weak vibration is stronger than that of weak vibration followed by strong vibration. (4) The optimal compaction process obtained under the optimal moisture content of 15% is static pressure once + strong vibration twice + weak vibration twice. [ABSTRACT FROM AUTHOR]

Cracks induced by evaporation or rainfall have a great influence on the stability of unsaturated soil slopes, which can lead to landslides during the rainfall process. In order to study the effect of crack characteristics on the evolution of stress and deformation of unsaturated soil slopes, a series of numerical analyses under different conditions were performed using a coupled elastoplastic finite element program that we developed for unsaturated soil. When carrying out the numerical analyses, the effective stress for unsaturated soil proposed by Bishop and an elastoplastic double-hardening constitutive model for the soil skeleton were employed. The varying parameters, including the crack location, the discharge speed, evaporation rate, infiltration rate, and tensile strength, were investigated to study the coupling process of pore water pressure and deformation in the process of evaporation and rainfall infiltration. The numerical results showed that the minimum pore water pressure of the soil slope at the end of evaporation/rainfall decreased gradually and the crack width increased gradually as the crack set closer to the slope; the larger the discharge speed of pore air, the greater the crack width. With the increase in the evaporation rate, the pore water pressure of the soil slope reduced and the crack initiated earlier and became wider. As the infiltration rate increased, the pore water pressure of the soil slope and the crack width increased, but the decreasing duration became shorter. The change of tensile strength had little effect on the pore water pressure, but the development of the crack width changed with evaporation and rainfall infiltration. [ABSTRACT FROM AUTHOR]

The soil moisture content at shearing and other factors, including dry density and grain size, influence its shear strength. This study investigated the effect of moisture content at compaction and grain size distribution on the unsaturated soil shear strength. Triaxial compression tests were performed in the laboratory using the modified triaxial apparatus on silica sands No. 3 and 6 without fines and with 20% fines to explore the unsaturated soil shear strength characteristics. Test samples were compacted and sheared at various combinations of the soil's optimum and residual moisture content. The analysis of the triaxial compression test results shows that moisture content at compaction and the grain size distribution influence the unsaturated soil shear strength. The test samples compacted at optimum moisture content showed higher peak shear strength when sheared at residual moisture content. Further, test results show that the test samples of soil without fines, when compacted at residual moisture content, show higher peak shear strength at optimum moisture content. The finding of this study endorses considering the moisture content at compaction for the geotechnical design of structures while predicting the soil shear strength. [ABSTRACT FROM AUTHOR]

The pseudo-static method was adopted to expand the application field of classical Rankine soil pressure theory to the circumstance of infinite inclined backfill acting on the retaining wall under earthquakes. The calculation formula of the active earth pressure for unsaturated soil was deduced by combining the shear strength model of unsaturated soil and the unified strength theory. The effects of matric suction, matric-suction-related friction angle and seismic coefficients on the intensity distribution of Rankine active earth pressure were evaluated, and the resultant action position was discussed. This research shows that rising seismic coefficients lead to a growth of the active earth pressure and an upward movement of the resultant action point; on the other hand, an increasing matric suction and its related friction angle causes a reduction of the active earth pressure and a downward movement of the resultant action. Examples indicate that the suggested formulas in this paper are in good agreement with those of the previous literature subjected to certain conditions. However, compared to the classic Rankine theory, the proposed method extends its application range by considering earthquake, unsaturated soil, inclined rough wall back, non-horizontal backfill surface, and the intermediate principal stress. [ABSTRACT FROM AUTHOR]

Natural and artificial slopes are frequently constituted, at least in part, by soils in unsaturated conditions. The authors discuss possible strategies to determine the hydro-mechanical parameters that can be used for a numerical analysis of such phenomena and the role of matric suction, which reflects soil infiltration, in the investigation of rainfall-induced landslides. Castelli et al. [[3]] report experimental data derived from laboratory and site tests, carried out with the aim of evaluating soil parameters for debris-flow run-out simulations. [Extracted from the article]

In order to conserve valuable soil and water resources and avoid problems related to frozen soil, it is important to study the migration of frozen soil water. A greater understanding of frozen soil–water migration can assist with sustainable development and utilization of soil and water resources in frozen areas. This study used an indoor soil column test device to conduct a one-way indoor freezing test of unsaturated soil and the response of soil sample water migration to different freezing temperatures, initial moisture contents, soil densities, freezing times, solute concentrations, and solute types. The experimental and analytical results showed that the temperature field of the soil sample could be divided into three stages: sharp cooling, slow cooling, and stability. After the soil sample had been frozen for 100 h, the temperature field stabilized. The freezing temperature, initial water content, soil density, and freezing time affected water migration in the soil sample. Lower freezing temperatures and greater initial water content resulted in higher levels of water migration. By contrast, greater soil density led to lower water migration levels. In addition, longer freezing times produced smoother soil–water migration curves. The solute concentration and solute type also affected water migration in frozen soil; the higher the solute concentration, the greater the water migration. Compared with CaCl2, NaCl had a stronger effect, causing more water migration and leading to a higher water content. The research findings will aid further studies on soil and water utilization, environmental maintenance, and restoration in areas with seasonally frozen soil, as well as promote the sustainable development of agriculture, water conservancy project development, and the social economy. [ABSTRACT FROM AUTHOR]