Unsaturated Soil Research Articles

Evaluation of the Impact of Climate Variability on the Soil-Water Characteristics Curve

The hydro-mechanical behavior of unsaturated soil, particularly expansive soil, is influenced significantly by cyclic wetting and drying. Understanding the soil parameters is crucial when evaluating the performance of infrastructures constructed on expansive clay. As a result of extreme rainfall events, highway slopes containing highly expansive Yazoo clay in Mississippi, U.S., become vulnerable to volume change. The phenomenon creates perched water zones within the slopes and poses a risk of slope failure. The soil-water characteristic curve (SWCC) defines the relationship between water content and soil suction, which can be obtained from different laboratory procedures. However, conventional laboratory methods have some limitations. To address this, various analytical and predictive models have been developed, but they can only offer estimates based on soil characteristics and lack seasonal variations occurring in field conditions. Studying seasonal SWCC through field measurements can help understand soil responses to changing moisture conditions. The current study utilized field data from six highway slopes in Mississippi and classified the data into different seasons: spring, summer, and fall. After obtaining van Genuchten parameters from the fitted curve for each season, the finite element method was applied to evaluate the parameters for accurate numerical analysis of infrastructures containing expansive clay. The study observed the variations in flow parameters with seasonal change that cannot be achieved when data from only one season is considered. The findings underscore the importance of field instrumentation data for developing SWCC and the significance of seasonal flow parameters in infrastructure design.

A framework for estimating the matric suction in unsaturated soils using multiple artificial intelligence techniques

AbstractImplementation of the state‐of‐the‐art understanding of the mechanics of unsaturated soils into geotechnical engineering practice is partly limited due to the lack of quick, reliable, and economical techniques for matric suction measurement. Matric suction is one of the key stress state variables that significantly influences the hydro‐mechanical behavior of unsaturated soils. In this paper, to address this objective, two artificial intelligence (AI) models were developed for estimating matric suction in unsaturated soils based on the particle swarm optimization support vector regression (PSO‐SVR) and multivariate adaptive regression spline (MARS) algorithms. The results suggest that both these models can reasonably estimate matric suction. Compared to the MARS model, the PSO‐SVR model can achieve higher accuracy. Nonetheless, the MARS model facilitates the sensitivity analysis and the selection of essential inputs. A novel integrated framework is proposed and validated, leveraging the strengths, and alleviating the limitations of the PSO‐SVR and MARS algorithms for reliable and rapid estimation of matric suction in the range of 0–1500 kPa for low plastic soils (0 < Ip ≤ 7). Six inputs are required to use this model successfully; some can be measured using conventional laboratory tests, and others can be calculated from mass‐volume relationships.

Behavior of model pile in unsaturated soil subjected to cyclic loading

Behavior of model pile in unsaturated soil subjected to cyclic loading

Mechanism of rainfall‐induced toppling in loess collapses

AbstractTo study the formation mechanism of rainfall‐induced soil erosion, collapse and landslides in the Loess Plateau based on the unsaturated soil seepage theory and the effective stress principle, the failure process of collapse with cracks under heavy and long‐term rainfall was investigated using the finite element method. The results showed that under the action of heavy rain, surface run‐off quickly gathered in cracks to form a dominant flow, resulting in a rapid increase in the water level and hydrostatic pressure in the cracks. The strength of the soil at the tip of the crack decreased rapidly under the softening effect of the water, and tensile failure occurred. The crack accelerated downward to the foot of the slope, causing the steep slope to collapse. Long‐term rainfall acts directly on the soil near the crack, resulting in faster saturation than the soil inside the slope body. When the rainfall time exceeded a certain value, the soil near the crack reached a saturated state, and the soil strength was minimized. Simultaneously, the seepage force generated when rainwater inside the collapsed body seeps from its outer edge increases the overturning moment of the collapsed body. Under the continuous action of rainfall, the cracks gradually extended to the foot of the slope, causing the collapsed body to fail. These research results provide a theoretical basis for the prevention and control of land degradation and collapse disasters on the Loess Plateau.

Microscale Mechanical Behaviour Evaluation of Unsaturated Granular Soils via 4D in situ Triaxial Test on μ-CT

Owing to the complex structure and stress history of unsaturated soils, the micro-mechanism of effective stress theory in unsaturated soil mechanics remains unelucidated. In this study, we developed a miniature triaxial test apparatus for a micro-focus computed tomography (μ-CT) scanning system for the microscopic study of the mechanical behaviour of unsaturated soil. By comparing the results from the developed mini-triaxial test device system with those from a conventional triaxial device, its experimental accuracy was verified. Micron-level high-resolution CT images of sheared specimens at different strain stages were obtained using the device for in situ suction-control triaxial compression 4D (3-Dimensional+Dynamic) tests with a μ-CT scanning system. The multiscale evolution behaviours of the physical parameters (porosity and degree of saturation) of the specimens corresponded to the stress–strain relationship revealed by the in situ test. Triaxial shearing drains out the pore water, and the decomposition of the largest water clusters causes the quantity of small-volume liquid clusters to increase, shifting the distribution of the interfacial areas. The degree of saturation scale indicates the presence of liquid bridges within the shear band that provide interparticle cementation effect and thus enhance the global strength of unsaturated granular soil specimens.

Micromechanics of multiphase geomaterials based on micro-CT image analysis: unsaturated soil, vegetated soil, and bio-cementation

Under the influence of climate change, extreme weather events such as rainstorms may induce fluctuations in soil water content above the groundwater level, thereby causing geological disasters (e.g., landslides). To mitigate these hazards, green and low-carbon engineering measures such as vegetation reinforcement and bio-cementation are proposed. This study investigates the micromechanical properties of multiphase geomaterials—unsaturated soil, vegetation-reinforced sand, and bio-cemented sand—utilizing high-resolution computed tomography (CT) scanning technology and associated image analysis techniques. The findings are presented as follows: (1) a 4D examination of the macroscopic and microscopic behaviors of unsaturated granular materials under triaxial loading conditions was conducted using self-designed in situ CT scanning equipment. (2) Triaxial loading alters the distribution of phase interfacial surfaces in unsaturated soil, affecting its overall strength. This loading also increases the anisotropy of the solid skeleton and suction stress. (3) The global saturation of the root–soil complex diminishes with root growth, with pore distribution significantly influencing local saturation. (4) In the unsaturated MICP process, low-saturation conditions are preferable for effective cementation, although the saturation levels should be restricted to 20%. After calcification, the particle contact coordination number increases, maintaining the isotropy in the contact between the soil particles.

Bearing characteristics of a model piled raft foundation in unsaturated soil subjected to variations in the groundwater level

The bearing characteristics of a piled raft foundation (PRF) in saturated or dry soil ground have been investigated in previous studies. However, soils near the ground surface are generally in an unsaturated state. Matric suction occurs in unsaturated soils and influences the mechanical properties of soil. For more reasonable designs of PRFs, it is undoubtedly necessary to study the bearing characteristics of PRFs in unsaturated soil. In this study, model tests were conducted to investigate the bearing characteristics of a PRF in unsaturated soil subjected to variations in the groundwater level. The experimental results showed that, when the groundwater level was increased and the suction decreased, the load carried by the raft increased under dead load, leading to a significant settlement of the model PRF.

Numerical modeling bearing capacity in wetting unsaturated expansive soils

To address the load-bearing deterioration of expansive soils uplift pile foundation after moisture absorption, a coupled seepage-soil deformation model for unsaturated expansive soils is implemented into ABAQUS. This modeling work consists primarily of two aspects: (1) The total strain of expansive soils has been decoupled into two parts, i.e., one caused by external loads and calculated by using Mohr-Coulomb plasticity model, and the other induced by wetting swelling and modeled by defining the moisture-swelling relation curve of expansive soils; (2) User-defined field subroutine (USDFLD) is developed by defining suction-dependent soil’s parameters (elastic modulus, cohesion and internal friction angle), and employed to simulate softening behaviors of expansive soils after moisture absorption. he above model is subsequently used to perform 3D finite element modeling and hydraulic-stress coupling analysis on the bearing capacity of pile in expansive soils before and after rainfall. Eventually, the proposed numerical method is validated by simulating a group of full-scale tests.

Constitutive and experimental study of unsaturated fine-grained soils over a wide suction range

The instabilities exhibited by unsaturated fine-grained soil, stemming from coupled hydro-mechanical phenomena, represent crucial engineering characteristics essential for geotechnical designs. This study introduces a constitutive model for unsaturated fine-grained soils, incorporating the framework of sub-loading surface plasticity. The model integrates Bishop’s effective stress and capillary degree of saturation as basic variables, while also addressing the distinct influences of soil volume change on capillarity and adsorption mechanisms. Validation of the proposed model is conducted using a diverse set of experimental data derived from suction-controlled triaxial compression tests conducted under low and high suction conditions. The results demonstrate that the new constitutive model can achieve a reasonable agreement with the observed data.

Numerical study on the rupture behavior of funicular liquid bridge between three spheres

Owing to the increase in saturation, unsaturated soil slopes are prone to experiencing landslides and mudflows during the rainy seasons. As liquid volume rises, the funicular regime between multiple particles gradually replaces the pendular regime within unsaturated granular soil. This study establishes a numerical model of a funicular liquid bridge between three spherical particles using the Surface Evolver software. The equilibrium state of the funicular bridge was examined using energy-minimization methods. The effects of liquid volume, contact angle, upper particle-pair gap, and separation distance on the capillary force and rupture distance were investigated. Finally, the relationship between capillary force and rupture distance is further elucidated based on meniscus profiles.

Hydro-mechanical model for small-strain stiffness of unsaturated soil

This study aims to investigate a novel method for predicting the small-strain stiffness of unsaturated soil. A hydro-mechanical model is proposed to estimate the small-strain stiffness of unsaturated soil with various matric suction and stress states. A series of saturated and unsaturated bender element tests are conducted, and the test results indicate that the small-strain stiffnesses of saturated and unsaturated soil have similar changing trends with the variation of the stress state. The small-strain stiffness of unsaturated soil is calculated by the proposed model based on the test results of saturated soil. The model has two parameters that have a physical meaning and can be measured by tests. Thus, combined with the mature prediction model and measurement method of saturated soil and the proposed hydro-mechanical model, the complex unsaturated bender element tests are unnecessary. The proposed model is anticipated to be of practical value for solving engineering problems during the determination of small-strain parameters in the design of pits and foundations.

Impact of Vetiver Plantation on Unsaturated Soil Behavior and Stability of Highway Slope

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.

Thermomechanical Behavior of Energy Piles with Different Roughness Values in Unsaturated Soil

Thermomechanical Behavior of Energy Piles with Different Roughness Values in Unsaturated Soil

Slope stability analysis based on rate-dependent soil-water characteristic curve

Rainfall-induced landslides are a common geological hazard. Analyzing this problem necessitates the use of the unsaturated soil theory, with the soil-water characteristic curve (SWCC) being a crucial component. Most existing unsaturated soil seepage models are established based on the static soil-water characteristic curve model. However, the dynamic capillary pressure has been observed in experiments conducted in previous studies. In this study, experiments were performed to explore the dynamic impact of the SWCC. The influence of different infiltration rates was studied, and a rate-dependent SWCC model under dynamic conditions was established. The model was validated through experiments. The behavior of an unsaturated soil slope under different rainfall intensities was analyzed based on the equivalent model. The equation for the slope safety factor was then derived using the Bishop method. The slope safety factors based on both the rate-independent and rate-dependent SWCC models were compared. The results indicated that the safety factor continuously decreased with increasing rainfall intensity. The dynamic effect reduced the safety factor, making the slope more susceptible to instability.

Temperature-Dependent SWCC Model for Unsaturated Soil

Temperature-Dependent SWCC Model for Unsaturated Soil

Effect of volumetric deformation on saturation of unsaturated soils under constant suction

The soil–water retention curve (SWRC) represents a fundamental relationship in the study of unsaturated soils, where volumetric deformation plays a crucial role in shaping the water retention characteristics by impacting the pore size distribution. This study conducted a thermodynamic analysis to derive a relationship between the effective stress parameter and the water retention properties of unsaturated soils. Consequently, a streamlined formula was introduced to forecast the impact of volumetric deformation on saturation at constant suction. This formula, incorporating a singular parameter from the effective stress parameters equations, can be seamlessly integrated with any prevailing SWRC model to characterize the influence of volume deformation on the SWRC. The effectiveness of the proposed model was corroborated through seven sets of experimental data spanning various soils, affirming its capability to precisely describe the water retention behavior of deformable soils under deformation.

Simple method to determine coefficient of permeability of unsaturated soil

This study introduces a novel transient method for determining the unsaturated coefficient of soil permeability, requiring the measurement of volumetric water content and corresponding pore-water pressures at three distinct points. This distinctive advantage of the method lies in eliminating the need to measure or control the flux at any point within the soil profile. The proposed method was validated through numerical simulations that performed one-dimensional seepage analysis, followed by a comparison of the calculated unsaturated coefficients of permeability with the input data. The strong agreement between the compared results validates the feasibility of the proposed method for determining the unsaturated coefficient of soil permeability. Moreover, a spatial interval of less than 5 cm is recommended to ensure accurate results for the unsaturated coefficient of permeability when employing this method.

A New Multiphase Apparatus for Testing Soil Permeated with Three Immiscible Fluids

Abstract Oil-contaminated unsaturated soil is a complex multiphase material. In this study, a new triaxial apparatus was developed to independently control/measure its air, oil, and water pressures. Therefore, the suctions associated with air–water, air–oil, and oil–water interfaces can be determined. The apparatus utilizes the hanging column and axis-translation techniques to control suctions in 0∼5 kPa and 5∼600 kPa. It is suitable for investigating water/oil retention, isotropic compression, and triaxial shear behavior of oil-contaminated soil. Using this new apparatus, five tests were conducted to analyze the saturation–pressure (S–P) relationship of sand and sandy silt at various oil and water contents and to evaluate the validity of the axis-translation technique for soil permeated with three nonimmersible fluids. Results showed that the artificial increase of air pressure in the axis-translation technique does not obviously alter the air–water and air–oil suctions at constant oil and water contents. A popular suction–saturation model, which was proposed based on test results of clean sands, showed a good prediction of the air–oil suction. However, the oil–water suction results of sandy silt do not align with the model prediction, suggesting that the model may need to be revised for soils with some silt and clay particles.

Experimental study on compression behaviours of compacted sandy loam using a Modified State Surface Approach

It is essential to study the deformation of unsaturated soil to predict the settlement of constructed embankments and foundations under precipitation and external loadings. This study conducted suction-controlled and stress-controlled oedometer tests on compacted sandy loam. The void ratio and degree of saturation were measured to study the deformation behaviours of unsaturated soil. The acquired data were analyzed using a modified state surface approach (MSSA). It was found that for the samples with an identical initial state, a unique state surface can be employed to describe the change of void ratio in an e-σ v’-s space, which consists of an elastic session and a plastic collapsible session. After yielding, the LC yield curve expands along the plastic collapsible surface with increasing loading or decreased suction, accompanied by irrecoverable plastic deformation. Along the hydraulic wetting paths (suction decreasing), the soil exhibits expansive deformation in the elastic surface but collapses into plastic collapsible after yielding. Differing from deformation behaviours, this study also demonstrated that the degree of saturation (S r) cannot be plotted in a unique surface in S r-σ v’-s space regarding hydraulic wetting and mechanical compression. This can be interpreted by different mechanisms of increasing S r by adsorbing water and mechanical compression.

Multi-phase modelling of loose, unsaturated soil’s continuous motion using a three-phase Eulerian approach

This paper develops a numerical model for loose, unsaturated soil to represent its mixing and motion using Computational Fluid Dynamics. It was found that existing models are not suitable for loose, unsaturated soil. The Herschel–Bulkley model is more appropriate for sludge and mud-like materials, and the Kinetic Theory of Granular Flow is inapt because it lacks yield stress. To mitigate these shortcomings, a novel three-phase Eulerian model is proposed, which combines these two models with an air phase. Each phase includes a physical characteristic of loose, unsaturated soil: the Herschel–Bulkley phase includes yield stress, a solid phase includes granular behaviour, and the air fraction adds porosity. To identify the model’s parameters, density is measured, and a slump test is performed. The radial run-out of the slump test is used to determine the required yield stress in the model. Test simulations showed clear improvements over the existing models by combining yield stress and granular, lumpy behaviour successfully. Application of the new model to a 3D screw conveyor simulation showcases its ability to simulate continuous soil motion phenomena, such as avalanching caused by a recirculatory flow, soil spilling over the flight and soil flowing back to the previous pitch.