Mechanical Behaviour Of Unsaturated Soils Research Articles

Mechanical behavior of unsaturated soils from suction controlled ring shear tests

The shear strength behavior associated with a large shear deformation of both the fine- and coarse-grained unsaturated soils is important in interpreting and forecasting the initiation and movement of landslides. For this reason, a suction-controlled ring shear apparatus was designed by introducing modifications to the conventional Bromhead ring apparatus extending the axis translation technique. A series of suction-controlled ring shear tests were performed on fine- and coarse-grained unsaturated soil specimens subjecting to large shear deformation. The experimental results are presented and interpreted for highlighting: (i) the shear stress/void ratio-shear displacement relationships; (ii) the envelopes of the residual shear strength; (iii) the void ratio, water ratio and degree of saturation of the fine-grained soil specimens sheared to the residual state under different net normal stresses and matric suctions. These results provide valuable information toward understanding and interpreting the behaviors of landslides in unsaturated soils that experience the first failure and the reactivation along a pre-sheared slip surface.

Profiles of suction stress of layered soils under infiltration conditions and its applications

Suction stress is critical for understanding the hydrological and mechanical behavior of unsaturated soils in construction and engineering. Many soils encountered in these fields exhibit layering due to geological sedimentation or artificial reinforcement. This study establishes the vertical distributions of matric suction and degree of saturation for layered soils under one-dimensional (1D) steady-state seepage, using the Darcy law and constitutive relationships of unsaturated soils. A closed-form suction stress is derived for an arbitrary number of layers and soil types. Hypothetical layered soils with varying hydraulic properties at four infiltration rates are assessed. Results show that soil stratification affects the suction stress profiles, with the impact depending on infiltration rates, soil configuration, and layer thickness. Abrupt variations in suction stress occur at most layered interfaces due to differences in permeability between adjacent layers. The suction stress profiles can be used to evaluate the earth pressure and stability of layered slopes under seepage conditions. Several case studies of multi-layered slope failures in Southern Italy are presented to demonstrate the suitability of the proposed framework for evaluating the stability of layered slopes under rainfall conditions.

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.

Semi‐implicit material point method for simulating infiltration‐induced failure of unsaturated soil structures

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

Development of Wetting-Drying Curves from Elastic Wave Velocities Using a Novel Triaxial Test Apparatus

Abstract The relationship between the soil water characteristic curve (SWCC) and the mechanical behavior of unsaturated soil is imperative and has been well investigated. However, the correlation between elastic wave velocity along the wetting and drying paths of SWCC is largely unknown due to the nonavailability of a standard experimental setup for such a purpose. An ordinary triaxial apparatus has been modified for laboratory assessment of SWCCs under different Ko stresses, along with the measurement of shear and compression wave velocities in due course. The main aim of the study is to draw SWCC, wave velocity characteristic curve (WVCC), and a Poisson’s ratio characteristic curve (PRCC) and to establish the phenomenon that these curves possess hysteresis. The Poisson’s ratio was obtained indirectly by measuring Vp and Vs. Three soil samples with relative densities of 85%, 56%, and 39% were prepared and placed in a modified triaxial test apparatus under wetting and drying cycles. The test results showed that the newly developed apparatus is accurately capable of measuring SWCC. Owing to the similarity in the shape of wave velocity and Poisson’s ratio, response to SWCC, WVCC, and PRCC are drawn. The phenomenon of stress history and the effective stress of the soil affected the behavior during wetting and drying paths.

An effective stress-based approach to modeling the hydro-mechanical behavior of unsaturated soils

A constitutive model of unsaturated soils is developed by incorporating the concept of intergranular stress into the framework of the modified Cam-Clay model. Within this context, the degree of saturation is viewed as an internal state variable, so that the soil-water retention function appears naturally as an evolution equation for the volume fraction of water. The new model not only has a neat structure but also includes fewer material parameters compared to the other models. A new volumetric-hardening law is proposed by taking into account the effect of wetting-induced pore collapse. It is shown that the collapsing void ratio (the current void ratio minus the void ratio at full saturation under the same effective stress) is dominated by the degree of pore air saturation (one minus the degree of water saturation), and practically independent of intergranular pulling forces, highlighting the important effect of soil fabric on the wetting-induced pore collapse. The proposed model is applied to simulate different types of experiments on unsaturated soils subjected to various combined hydraulic and mechanical loadings, showing its capability and diversity in modeling the hydraulic and mechanical behavior of unsaturated soils.

Comparative study on suction obtained using the membrane filter method from triaxial apparatus and pressure plate apparatus

To investigate the stability of the earth embankments and slope failures during natural disasters, it is necessary to consider that soil is naturally unsaturated. Hence, it is essential to understand the variations of pore air pressure and pore water pressure, in terms of suction. The relationship between soil suction and soil moisture content is generally attained by soil water characteristic curve (SWCC), which is critically important in characterizing the mechanical behaviour of unsaturated soils. The SWCC is obtained from pressure plate apparatus by imposing the suction continuously by draining water from the saturated specimen and/or supplying water to unsaturated soil, which differs from the actual soil compaction process at the site. Therefore, the current study employs a methodology of suction measurement directly at a certain degree of compaction and degree of saturation using the membrane filter method in the triaxial apparatus, to meet the realistic conditions in the field. A comparative study has been done using SWCC with the imposed suction and the measured suction (Natural suction) at each compacted state. The results from both the methods exhibited an increase in the suction and air entry value with the increase in the degree of compaction, proving that an increasingly positive effect of suction contributed while increasing the density. Whereas the suction measured immediately after the compaction (Natural suction) in triaxial apparatus is more consistent with the suction imposed in the wetting process of SWCC. Further deep understanding of the microstructure behaviour of each compacted state and the SWCC is necessary.

Empirical Equations Expressing the Effects of Measured Suction on the Compaction Curve for Sandy Soils Varying Fines Content

To effectively apply various soil types for embankments, understanding their compaction characteristics is crucial. One crucial factor affecting compaction is suction, which plays a significant role as it is typically performed under unsaturated conditions. Suction varies with soil density, water content, and fines content. This study directly measures suction after soil compaction using the triaxial apparatus, unlike the Soil water characteristic curve (SWCC), assessing its impact on compaction characteristics. Immediate suction measurement after compaction provides apparent suction, resembling on-site conditions with open pore air pressure. Comparing SWCC with apparent suction at each compacted state reveals that suction and air entry value increase with initial density, positively impacting compaction. Notably, apparent suction aligns better with wetting process suction from the SWCC due to added water during specimen preparation. Empirical equations are derived to obtain suction contours across various density and saturation ranges, aiding in understanding suction variations on the compaction curve. Even slight variations in saturation causes noticeable changes in apparent suction during higher compaction efforts, affecting soil compaction characteristics. Therefore, the precise control of saturation control is needed to achieve desired properties of compacted soil, especially at higher compaction efforts and with various soil types. This understanding significantly impacts the mechanical behavior of unsaturated soils.

Numerical Modelling for effect of rainfall on unsaturated soil slopes

Expansive soils are predominant in many parts of the world. Expansive soil treated as problematic soils, as economic and design a safe slope on expansive soils is major challenge for civil engineers. Expansive soils were compacted under unsaturated condition, in various construction projects. Major failures reported in many countries around the world are due to swell shrinkage characteristics causing unstable slopes. Fills and compacted soils have an inherent suction when they are first compacted. The suction is the most important factor which affects the shear strength of unsaturated soil. variation of suction in unsaturated soil is an important factor considered for solving issues like slope stability and bearing capacity failure in unsaturated soil. Rainwater infiltration are major triggering factor for failureof expansive soil slopes. The strength of unsaturated soil changes with water evaporation and infiltration. Due to infiltration of rainwater into the soil a significant change in the pore water takes place. Hence, this fluctuation in pore water pressure responsible for the distinct mechanical behavior of unsaturated soil under different Geotechnical problems.A numerical modeling was done with Plaxis LE software, considering parameters of unsaturated soil. As the changes in pore-water pressure and moisture content are a function of the flow of moisture through the soil-atmosphere boundary.The effect of soil type, rainfall intensity, location of ground water table, permeability functionsareconsidered to reproduce the soil-atmosphere interaction. Obtain results illustrates the combine effect of suction and increase of saturation due to rainfall on slopes. It was observed that rainfall increase the induced matric suction and increase the positive pore water pressure finally results in slope failures.

A hydro-mechanical constitutive model for unsaturated soils over a wide saturation range

A hydro-mechanical constitutive model for unsaturated soils is proposed to describe the hydraulic and mechanical behaviour over a wide saturation range. Bishop’s effective stress and the effective degree of saturation are taken as the basic constitutive variables. A simple and novel loading-collapse yield curve is constructed to capture the variation of the yield stress with the hydraulic state over a wide saturation range. A non-orthogonal plastic flow rule is adopted to obtain the direction of the plastic strain increment. A simple water retention model is developed to describe the hydraulic behaviour of unsaturated soils. The proposed constitutive relation is presented through the calculation of the total strain increment and the calculation of the increment of the effective degree of saturation. All material parameters can be determined through conventional laboratory tests. The performance of the proposed model is illustrated through numerical examples of three classical stress paths, i.e., the isotropic compression path with constant suction, the wetting path with constant net stress, and the triaxial shearing path with constant suction and mean net stress. The capability of the proposed model to capture the hydraulic and mechanical behaviour of unsaturated soils is validated by simulating the test results from the literature.

Extending modified Cam-Clay to unsaturated soils by incorporating a scaled stress variable

This paper extends the modified Cam-Clay model to describe the behaviour of unsaturated soils by replacing the effective stress with the scaled stress. The scaled stress accounts for the mechanical effect of capillarity by factoring Bishop’s stress with a power function of degree of saturation. The extendedmodified Cam-Clay model postulates the existence of a yield curve in the mean scaled stress 𝑝̅– deviatoric scaled stress 𝑞̅ plane. Coherently with the modified Cam-Clay model, the stress-strain behaviour is predicted by means of an elastic law inside the yield curve. As the scaled stress state evolves onto the yield curve, the behaviour becomes elasto-plastic and this is modelled by a plastic flow rule combined with a volumetric hardening law. The extended modified Cam-Clay model is then validated against experimental data published in the literature. Results show that the proposed approach is capable of well predicting the mechanical behaviour of unsaturated soils during triaxial loading. Future work will be directed at further validating the present formulation along different stress paths as well as accounting for the combined effect of partial saturation and cementation within a critical state framework based on bounding surface plasticity.

Effects of microstructure on THM behaviour of geomaterials

In this paper, we discuss the thermo-hydro-mechanical behaviour of geomaterials at the light of their microstructure and its changes induced by multiphysics loading. After recalling the strong links between the microstructure and the water retention properties of unsaturated soils, the relation between themicrostructure and the physical properties ruling heat and mass transfers are discussed. The mechanical behaviour of unsaturated soils is then discussed focussing on the definition of an effective stress based on a microstructure description. The experimental determination of this microstructurally-based effective stressis presented, including recent advances to identify the stress coefficient from Mercury Intrusion Porosimetry data. Opening remarks towards macroscopic modelling of unsaturated geomaterials accounting for their microstructure and its changes is finally discussed.

Variation Mechanism and Prediction of Soil–Water Characteristic Curve Parameters of Low-Liquid-Limit Silty Clay under Freeze–Thaw Cycles

The soil–water characteristic curve (SWCC) is a key input for the numerical simulation of geotechnical engineering. It contains the basic information needed to describe the mechanical behavior of unsaturated soil. In order to study the variation of the SWCC characteristics and its mechanism after the freeze–thaw (F–T) process of low-liquid-limit silty clay in seasonally frozen regions, the SWCC of the soil samples subjected to zero, one, three, five, and seven F–T cycles at three dry densities were measured; then the microstructure was scanned by scanning electron microscopy (SEM) to analyze the relationship between the characteristic parameters of the SWCC and the microstructure after F–T cycles. Finally, according to the mathematical law of characteristic parameters, the prediction equation for the parameters of the SWCC considering the F–T cycles was established, which has a suitable fitting effect with the experimental data. The result shows that with the increase in F–T cycles, the air-entry value (AEV) and residual saturation (RS) decrease gradually, while the saturated water content and the moisture-losing rate of the transition section slightly increased. According to the microstructure analysis, it is due to the F–T process that the compacted soil sample has cracks, the entire plate structure is destroyed, and thus the arrangement between the soil particles becomes looser, and the porosity and average pore size increase. This study can provide data support and references for the design of low-liquid-limit silty clay foundation engineering in seasonally freezing regions.

Characterizing Suction Stress and Shear Strength for Unsaturated Geomaterials under Various Confining Pressure Conditions

This paper focuses on the theoretical representation of suction stress as an added confining pressure in evaluating the mechanical behavior of unsaturated soil under various confining pressure conditions. In general, the effect of the suction stress in interpreting the mechanical behavior of unsaturated soil under confining pressure up to 1 MPa is examined by using a triaxial compression test. The role of suction stress under low confining pressure conditions, however, has not yet been fully explained theoretically. The concept of the suction stress derived from the triaxial compression test results for unsaturated soil and examples of its application were described in this study. By adding suction stress to the confining pressure, the geometric relationship between the failure criteria of the unconfined compression test and existing theories on failure envelope for unsaturated soils was discussed. In addition, the unconfined compression test and constant water content compression test for silty soils were also carried out under different density conditions. By evaluating the results obtained in this study and the past results of silty soils, it was found that the interpretation of suction stress in terms of confining pressure is valid regardless of the density condition.

Simple Testing Method for Measuring the Triaxial Stress-Strain Relations of Unsaturated Soils at High Suctions

Abstract Unsaturated soils at high suctions are widespread in arid and semiarid areas. Test data on the mechanical behavior of unsaturated soils are scarce at high suctions. In this article, the effect of void ratio or density on the soil-water retention behavior was discussed on the basis of measured data of various compacted soils at high suctions. According to the test data, the soil-water retention behavior is independent of the dry density or void ratio over a high suction range, namely, if the water content of a soil specimen is kept constant, the suction will be constant and independent of the void ratio (deformation) at high suctions. Therefore, a simple testing method for measuring the stress-strain relations of unsaturated triaxial specimens at high suctions is proposed by combining the vapor equilibrium technique (VET) with saturated salt solutions and the triaxial shear tests at constant water contents. First, a known initial high suction is imposed on the specimen by using the VET with a saturated salt solution. Then, the specimen is triaxially sheared under a constant water content, thereby resulting in a constant high suction during triaxial shearing. The proposed testing method can substantially shorten the testing period for one triaxial test. Finally, the results of triaxial shear tests on silt, silty clay, and expansive soil at high suctions obtained by the proposed testing method are presented.

Effect of osmotic suction on the stability of microaggregations and aggregations in a lateritic soil

It has been established that the matric suction is the portion of the total suction that affects, more directly, the mechanical behaviour of unsaturated soils, being less considered the effects of the osmotic suction. Considering that in the deeply weathered soils will occur, depending on its chemical-mineralogical characteristics, more or less ionic exchange between the soil particles and the chemical products to which the soil has been exposed to, the study of the osmotic suction and its impact on the soil textural and structural stability becomes relevant. In natural terrain and slopes, the chemical products come mostly from agricultural inputs, while in road constructions they come from additives such as lime and cement that are incorporated to the soil with the objective of enhancing its mechanical behaviour. In the case of cast in situ concrete foundations, part of the concrete’s chemical compounds migrates to the surrounding soil. In all of the above mentioned cases, the matric suction acts on the chemical compounds’ migration process, affecting soil’s osmotic suction. Aiming at assessing the impacts of these chemical compounds on the soil properties and behaviour, soil-water characteristics curves of a lateritic soil mixed with a dolomitic limestone and a fertilizer were evaluated. The results show that, in the case of the fertilizer mixture, the variation of osmotic suction contributed to the disaggregation of the soil.

Determining soil water characteristic curve of lime treated loess using multiscale structure fractal characteristic

Soil–Water characteristic Curve (SWCC) is meant to describe the mechanical behavior of unsaturated soil. The present paper focuses on the internal multi-scale microstructure of Xining untreated loess and lime-treated loess with the use of scanning electron microscopy (SEM) and image processing technique. A new SWCC model was presented based on the fractal dimension of pore size distribution. The SWCC of untreated loess was calculated from fractal dimension and fitted well with curve tested from Fredlund SWCC device. The SWCC of lime-treated loess was then calculated. Two curves of Xining untreated loess and lime-treated loess have been compared and reasons for the difference have also been discussed. The results indicate that the content of large pores in lime-treated loess decreased and the content of micro-pore increased. The bracket pores were changed into cement pores. The pore fractal dimension D of Xining untreated loess is 1.39 and the pore fractal dimension D of Xining lime-treated loess is 1.53. Air-entry value of untreated loess is 12.16 kPa, while lime-treated Loess—35.15 kPa. In transition region, matric suction of lime-treated loess was in the range of 35.15 kPa ~ 4000 kPa, while matric suction of untreated loess—12.16 kPa ~ 2600 kPa. The range of the transition region in lime-treated loess is larger than that in the loess, while in the range of saturation region, the reverse applies. Under the condition of the same matrix suction, the saturation of lime-treated loess is greater than that of untreated loess. In the residual region, the difference of SWCC of soil samples is small.

Experimental study and constitutive modeling of volume change behavior in unsaturated soils

Unsaturated soils often show complex coupled hydro-mechanical behavior. We present a simple way to explore the 1D compression mechanical behavior of unsaturated soils under a constant degree of saturation by controlling the sample void ratio and suction. The experimental results presented here verify that the proposed method can control the degree of saturation during compression. The experimental data show that the hardening factor exhibits a linear relationship with the logarithmic degree of saturation. We apply a volume change equation using the hardening factor and effective stress as basic variables to simulate the hydro-mechanical behavior of unsaturated soils. We conducted numerical simulations to assess the model performance. Comparisons between the experimental data and simulations indicate that the model can well reproduce the mechanical and hydraulic response of unsaturated soils.

Modified State Surface Approach to Study Unsaturated Soil Hysteresis Behavior

Unsaturated soils are often used as a construction material in transportation infrastructures. In this situation, unsaturated soils are subjected to cyclic mechanical loading from traffic loads or wetting-drying cycles in seasonal climatic conditions. While mechanical hysteresis is a common feature of soils in general, hydraulic hysteresis is associated with unsaturated soils. Although several constitutive models for unsaturated soils have been proposed, the mechanical and hydraulic hysteresis behavior of unsaturated soils has been little studied. A modified state surface approach (MSSA) was first proposed for investigating the mechanical behavior of unsaturated soils. It was then extended to study the coupled hydro-mechanical behavior of unsaturated soils with a special focus on the consistency between different soil phases. However, hydraulic and mechanical hysteresis were neglected in MSSA formulations. In this paper, based on evidence from experimental results, the MSSA is extended further to study the coupled hydro-mechanical hysteresis behavior of unsaturated soils. The extended MSSA can reproduce several forms of mechanical and hydraulic behavior observed in experimental results that cannot be represented by existing constitutive models. To demonstrate the capabilities of the extended MSSA, typical behaviors are simulated and compared, qualitatively, with the characteristic trends of the behavior of unsaturated soils. Experimental results from the literature are then used to evaluate the model to predict, quantitatively, the observed behaviors. The agreement between measured and predicted results is considered satisfactory and confirms the possibility of the proposed approach to reproduce the hysteresis behavior of unsaturated soils.

Relationship between Water Vapor Sorption Kinetics and Clay Surface Properties

AbstractA complete understanding of soil–water interactions and the corresponding mechanical behavior of unsaturated soils requires differentiating adsorptive and capillary components of water rete...