Behaviour Of Unsaturated Soils Research Articles

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.

Consideration of microstructure in modelling the hydro-mechanical behaviour of unsaturated soils

The hydro-mechanical behaviour of unsaturated soils is often highly influenced by the microstructure; therefore, it can be beneficial to consider the effect of microstructure in a hydro-mechanical constitutive model. This paper considers the use of a microstructure-related model that adopts the effective degree of saturation as a microstructural index. The model can be used to reproduce the hydro-mechanical behaviour while the effect of the microstructure is considered. For comparison, a non-microstructuredependent model is also employed. The models are applied to simulate the behaviour of two different soilsand a comparison of the models’ performance in simulating triaxial test behaviour is made. Based on the comparison with experimental results and the non-microstructure-dependent model, it can be concluded thatthe adoption of the effective degree of saturation is beneficial to studying the hydro-mechanical behaviour of unsaturated soils affected by the microstructure.

Numerical implementation of BBM in FE package for solving unsaturated soil boundary value problems

Unsaturated soil mechanics is of keen relevance when dealing with soil above the water table orcompacted soils. Barcelona Basic Model (BBM) is one of the most widely used constitutive models for describing unsaturated soil behaviour and is available as a soil model in software like PLAXIS and CODE_BRIGHT (developed by UPC). The present study demonstrates a numerical framework to implement BBM within the ABAQUS package as a user-defined soil model using a user subroutine called UMAT (User Material). The backward Euler integration scheme coupled with Newton Raphson iterative algorithm is used to construct the model. A convergence test is conducted to check the accuracy and stability of the integration. UMAT's inability to manage pore pressure led to proxy diffusion to depict suction variation. The heat equation is used as a proxy for diffusion in three-dimensional space and time. Anoedometer test is simulated in which the soil is allowed to consolidate and then set to saturate at constant vertical stress, which leads to wetting collapse. The results demonstrate a reasonable behaviour of wettinginduced compression of soil subjected to loading with varying saturation level.

Constant void ratio vs. constant confining pressure tests on partially saturated silty soil

The presence of soil solids, water, and air complicates understanding unsaturated soil dynamics. A thorough understanding of unsaturated soil behavior is essential for the fruitful design of slopes, embankments, and retaining structures. A series of constant void ratio and constant confining pressure triaxial compression tests were carried out under constant water content conditions (pore air pressure drained and pore water pressure undrained) to study the shear strength and deformation characteristics of a partially saturated silty soil. Test specimens were prepared with a water content of 15%, 20% (Optimum water content), and 25%, with the corresponding degree of saturation of 37, 49, and 62.5%. The degree of compaction of all specimens was kept at 83% while the dry density was 1.29 g/cm3, and the void ratio was around 1.045. All the samples were isotropically consolidated under 500kPa by keeping deviatoric stress at 0kPa. The constant void ratio test results showed that the deviatoric stress reached a peak value followed by a sudden decrease within the axial strain of 0-1.5%. In contrast, the deviatoric stress increased continuously until reaching the critical state without depicting any peak for constant confining pressure tests. However, in both cases, the lower the water content at the time of specimen preparation, the higher the shear resistance.

Cyclic modeling of unsaturated sands using a pore‐scale hydromechanical approach

AbstractDetermining proper strain‐dependent shear modulus and damping ratio for soils is of utmost importance when investigating their response to cyclic loads. Experimental studies on the effect of the degree of saturation and suction on the shear modulus at large strain amplitudes are scarce due to the complexities involved in testing such soils. Furthermore, the previously presented micromechanical models for the cyclic behavior of unsaturated soils lack some of the required features of soil pore skeleton essential to model unsaturated soils’ hydromechanical behavior. This paper is, thus, aimed at addressing this shortcoming by incorporating a pore‐scale numerical method that couples the discrete element simulation of the solid phase and the fluid pore network to capture the cyclic behavior of unsaturated sands. To this end, the model is first calibrated to determine the micro‐scale parameters using experimental results from static drained triaxial compression tests on dry and saturated sand samples. Next, the coupled model is employed to simulate the observed behavior during cyclic triaxial tests on unsaturated sands. A linear dependence of the plastic rolling coefficient and the interparticle friction angle on the degree of saturation is proposed from simulations of suction‐controlled cyclic triaxial tests. Variation of shear modulus with shear strain amplitude and degree of saturation indicates a continuous increase in the shear modulus with the decrease in saturation. In contrast, the trend for the damping ratio is the opposite. Finally, closed‐form relationships are proposed based on the simulation results for the shear modulus and damping ratio of unsaturated soils.

X-ray-aided characterization of micro-hydro-mechanical behaviour of unsaturated sand using a suction-controlled mini-triaxial system

Despite the advancement of the knowledge of unsaturated soil behaviour developed over the past decades, the understandings remain largely at the macro or element scale. Existing testing systems allow simultaneous in situ loading and X-ray imaging to facilitate the studies at micro or pore scale, but the soils of concern are normally two-phase (i.e., dry or saturated). This paper develops a new suction-controlled miniature triaxial apparatus for studying the micro-hydro-mechanical behaviour of three-phase unsaturated soils via in situ quantification of the evolutions of soil microstructures, aided by micro-X-ray computer tomography imaging. The apparatus can independently control combinations of net mean stress and matric suction whilst permitting high-resolution full-field imaging of the entire samples through the image analysis via methods of pore network modelling and discrete digital image correction. Results reveal that at equilibrium, suction at the “transition zone” of the soil does not necessarily mean to have both the air and water phases continuous, as otherwise recognized at the element scale. This phenomenon leads to nonuniform distributions of local void ratio and degree of saturation. Post-peak strain localization and shear band development are explained by the movements of pore fluid and the associated influences on local soil void ratio change.

Cylindrical cavity expansion responses in anisotropic unsaturated soils under plane stress condition

In this paper, an anisotropic critical state model for saturated soils was extended to unsaturated conditions by introducing suction into its yield function. Combining this model with soil-water characteristic curves related to porosity ratio was employed to characterize the coupled hydromechanical behavior of unsaturated anisotropic soil. Based on the plane stress condition, the problem of the cylindrical cavity expansion in unsaturated anisotropic soils was transformed into first-order differential equations using the Lagrangian description. The equations were solved as an initial value problem using the Runge-Kutta algorithm, which can reflect the soil-water retention behavior during cavity expansion. Parametric analyses were conducted to investigate the influences of overconsolidation ratio (OCR), suction, and degree of saturation on the expansion responses of a cylindrical cavity in unsaturated anisotropic soil under plane stress condition. The results show that the above factors have obvious influences on the cavity responses, and the plane strain solution tends to overestimate expansion pressure and degree of saturation but underestimates suction around the cavity compared to the proposed plane stress solution. The theoretical model proposed in this paper provides a reasonable and effective method for simulating pile installation and soil pressure gauge tests near the ground surface of the unsaturated soils.

Microstructurally Related Model for Predicting Behavior of Unsaturated Soils with Double Porosity in Triaxial Space

Microstructurally Related Model for Predicting Behavior of Unsaturated Soils with Double Porosity in Triaxial Space

Contribution of capillary pressure to effective stress for unsaturated soils: Role of wet area fraction and water retention curve

Estimating the contribution of capillary pressure to the effective stress is critical for studying the mechanical and hydraulic behavior of unsaturated soils. Based on experimental evidence, we clarify that the degree of saturation (Sr) rather than matric suction (s) is more suitable as a variable for modeling Bishop's effective stress parameter (χ). The wet area fraction, defined as the ratio of the wet external area of soil particles to the total surface area, is then introduced as an alternative to the parameter χ. Because the contact area between soil particles and pore water is affected by both hydraulic (matric suction) and mechanical (volumetric deformation) states, the soil water retention curve (SWRC) is embedded in the proposed model to simulate the influences of pore size distribution and hydraulic hysteresis on shear strength. The proposed model requires only the material parameters of soils in the reference state and then can be used to predict the strength characteristics in other mechanical or hydraulic states. The shear strength data under different test conditions and the corresponding SWRC data are adopted to validate the new model. The results reveal that the wet area fraction could well predict the contribution of capillary pressure to the shear strength of unsaturated soils.

Numerical Implementation of the Barcelona Basic Model Based on Return-Mapping Integration

This paper implemented the Barcelona basic model (BBM) into the OpenGeoSys (OGS) platform for numerical modeling of the coupled hydro-mechanical (HM) behavior of unsaturated soil. Within the implicit integration approach in the OGS, the integration rule of the BBM was developed first and the integration form of the BBM under a return mapping algorithm was built. The closest point projection method was used for calculating the return mapping directions with the associative flow rule. The numerical simulation results show that the BBM is feasible in fitting the experimental results. The numerical integration algorithm can reflect the elastic–plastic mechanical behavior of materials, and improve the calculation accuracy. The material exhibits obvious elastic–plastic characteristics during numerical simulation and experiment, and the water absorption process can lessen the mixture’s compression stiffness while enhancing its recovery stiffness.

Experimental Study on the Effects of Matric Suction on Shear Properties of Polypropylene Fiber Reinforced Unsaturated Clay.

Matric suction has an important effect on the behavior of unsaturated soils, and polypropylene fibers are often used to improve soil. In order to probe into the mechanism of matric suction and fiber reinforcement, triaxial shear tests are carried out with changing matric suction and net confining pressure. Unsaturated clay in the Shaoxing section of East Zhejiang Grand Canal is selected with polypropylene fiber as a reinforcement material in the tests. The results show that the total cohesion intercept and effective internal friction angle of soil increase with the increase in matric suction, while the adsorption internal friction angle decreases gradually. Similarly, the contribution of matric suction to shear strength decreases. The total cohesion intercept is more sensitive to matric suction. As the length of fiber is 12 mm, the shear strength parameters of soil will be improved accordingly, which makes the fiber reinforcement achieve the best. The stress-strain relationship is approximately hyperbolic and strain hardening. The characteristics of strain hardening are more obvious with the increase in matric suction, and the soil specimens present plastic failure. The volumetric strain of specimen is more sensitive to the changing net confining pressure. It increases with the increase in net confining pressure, and increases linearly as the matric suction is zero. The failure modes of triaxial tests are divided into tensile failure and friction failure.

The Influence of Particle Separation Distance on the Behavior of Unsaturated Soils

The Influence of Particle Separation Distance on the Behavior of Unsaturated Soils

Effect of Saturation Degree on Mechanical Behaviors of Shallow Unsaturated Expansive Soils

In the southwest of China, there are widely distributed expansive soils. However, to save costs and manage the speed of construction, these shallow expansive soils are often filled with subgrade materials. Therefore, it is necessary to clearly understand the mechanical behaviors of unmodified shallow expansive soils. Current research on the mechanical behaviors of shallow expansive soils is mainly focused on shear and compressive strengths but rarely on the tensile strength since general tests are costly, time consuming, and difficult to conduct. Therefore, uniaxial tensile, unconfined compression and direct shear tests were carried out to study the mechanical behavior of shallow unsaturated expansive soils under different saturation degrees, and the tests analyzed the change mechanism of its mechanical behavior. The following were found: (1) with an increase in saturation degree, the uniaxial tensile strength, unconfined compressive strength, shear strength, cohesive force, and internal friction angle first increased and then decreased; (2) when the saturation degree increased from 18.7% to the saturation degree corresponding to the peak, the uniaxial tensile strength, unconfined compressive strength, cohesive force, and internal friction angle increased by about 11 times, 3.24 times, 2.34 times, and 0.52 times, respectively; (3) when the saturation degree increased from the saturation degree corresponding to the peak to 80.3%, they decreases by about 42%, 51.4%, 36%, and 50%, respectively; (4) with the increase in dry density, the saturation degree corresponding to the peak of uniaxial tensile strength gradually increased, while the saturation degree corresponding to the peak of unconfined compressive and shear strength did not significantly change.

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.

Coupled hydro-mechanical modelling of unsaturated soils; numerical implementation and application to large deformation problems

This paper presents coupled hydro-mechanical modelling of unsaturated soil problems by incorporating some advanced numerical and constitutive models in a general-purpose commercial software package, Abaqus. Two different strategies for the interpretation of unsaturated soil behaviour with respect to the soil volume change are considered and the relevant constitutive models are implemented through user-defined subroutines. The first approach that is commonly used in geomechanics considers suction as an additional variable and uses a constitutive model in the space of effective stress–suction assuming that soil compressibility is a function of suction. The second approach treats the constitutive model in the space of effective stress and degree of saturation assuming that the soil compressibility is a function of the degree of saturation. The hydro-mechanical behaviour regarding the change in the degree of saturation caused by both suction and net stress changes is also considered, together with the effect of hydraulic hysteresis. Validation of the implemented algorithms is presented through several benchmark problems. Finally, the application and utility of the implemented procedures are illustrated by simulations of two challenging problems of unsaturated geomechanics, including a slope failure due to seepage and rainfall infiltration as well as cone penetration tests in unsaturated soil. A suitable mesh optimisation scheme is also incorporated to handle finite deformations.

Closed-form modeling of matric suction in unsaturated soils under undrained heating

Matric suction is an important parameter controlling the hydro-mechanical behavior of unsaturated soils. Several emerging geotechnical and geoenvironmental applications involve unsaturated soils subject to undrained heating. This study presents a closed-form model to determine the matric suction in unsaturated soils under undrained heating. The final expression is derived based upon the Young–Laplace capillary principle and only needs the enthalpy of immersion at the reference temperature and as primary input parameters. Under undrained (constant water) conditions, the closed-form model incorporates the impact of temperature on matric suction by considering the temperature dependency of the liquid–gas interfacial tension, the contact angle of the solid–liquid–gas interface, and the enthalpy of immersion. The model is validated against six sets of experimental data for clay, silt, and sand, reported in the literature. The results show a very good agreement between measured and predicted values for all soil types over a wide range of temperatures. The matric suction monotonically decreases with temperature under undrained heating. The proposed closed-form model can readily be employed in numerical and analytical analyses to simulate the undrained behavior of unsaturated soils under elevated temperatures.

Modelling the thermal–hydro-mechanical behaviour of unsaturated soils with a high degree of saturation using an extended precise integration method

Unsaturated soils with a high degree of saturation (HDS) are commonly encountered in marine and lacustrine sediments. In these soils, the gas phase generally exists in the state of discrete bubbles, which is sensitive to stress and temperature changes and can dramatically change the soil's engineering properties. This paper explores the thermal-induced behaviour of HDS soils using an efficient extended precise integration method (XPIM). Biot poroelasticity theory, extended to include thermal effects and compressibility of gas–water mixture, is employed to analyze the soil behaviour under non-isothermal conditions. Based on the Laplace–Fourier transform and Taylor series expansion, such problems can be solved by XPIM. The robustness of XPIM was confirmed by comparing the present results with analytical solutions and test data. Extensive parametric studies are undertaken to examine both the effects of soil grain thermal expansion and anisotropic permeability on soil behaviour and temperature effects on the degree of saturation ( Sr). The thermal-induced variations in Sr are more pronounced with lower initial values (e.g., 90% compared to 99%). These quantitative results demonstrate the benefits of the proposed method, which proves to be extremely efficient and several orders more precise than conventional numerical approaches, with its precision limited only by the computational effort used.

Characterizing and Modeling the Coupled Hydro-Mechanical Cyclic Behavior of Unsaturated Soils Using Constant Water Content Oedometer and Direct Shear Tests

Most transportation infrastructures are constructed on compacted soils that are typically unsaturated above the groundwater table. The soils are subjected to cyclic traffic loadings and seasonal wetting–drying cycles. Although problems associated with unsaturated soils are ubiquitous in the US, coupled hydro-mechanical analysis is rarely included in the design/analysis of transportation geosystems. This can be attributed to two main reasons: (a) there are no simple devices/methods which can be used to rapidly characterize stress–strain behavior for unsaturated soils, and (b) there is a lack of a constitutive model to study coupled hydro-mechanical cyclic behavior for unsaturated soils in a consistent way. This paper takes advantage of the recent advancements in suction measurement and constitutive modeling to overcome these limitations. In this paper, conventional oedometer and direct shear tests for saturated soils are modified to characterize stress–strain behavior of unsaturated soils under cyclic undrained loading conditions. Then, series of constant water content cyclic oedometer and direct shear tests are performed to characterize the hydro-mechanical behavior of a silty soil. Moreover, methods are developed to use results from both tests to calibrate parameters for a recently proposed hydro-mechanical constitutive model. The paper also compares the model predictions with the measured results. Results indicate that the model is able to satisfactorily predict the hydro-mechanical hysteresis behavior of the soil. With developments in equipment, constitutive models, and analysis, it is now possible to fully characterize the hydro-mechanical behavior of unsaturated soils in a more efficient manner.

Wetting-Induced Collapse Behavior of Unsaturated Soils in Disturbed State Concept Framework

Wetting-Induced Collapse Behavior of Unsaturated Soils in Disturbed State Concept Framework

Study on the frost heave behavior of the freezing unsaturated silty clay

The frost heave behavior of unsaturated soils is related to the water-vapor redistribution, whereas the water-vapor migration processes cannot be directly observed due to the limitations of measurement techniques. To reveal the frost heave behavior of the unsaturated silty clay, five specimens for unsaturated silty clay were prepared. One-side freezing tests for the five specimens were carried out under uniform conditions. The test durations are 12 h, 24 h, 48 h, 96 h, and 192 h, respectively, and the water content profiles in the freezing process were studied by testing final water content profile of each specimen as the profile at that time. The heat-water-vapor migration and frost heave behavior of the freezing unsaturated silty clay were interpreted. The results showed that the temperature variations can be divided into three stages (a sharply cooling stage, a slowly cooling stage, and a steady stage) according to the change rate of temperature, and the duration times of the first and the second stage are short. A mix migration of liquid water and water vapor in unsaturated soils can be experimentally determined during the freezing, and a thin ice forms at the top of the specimens due to the vapor migration and condensation. Meanwhile, the liquid water in unfrozen zone mainly migrates to frozen fringe, and segregated ice forms in the freezing unsaturated soil once enough water accumulates at the frozen fringe. In addition, the frost heave of the specimens is discrete under the uniform test conditions. We selected the relative error and absolute error of the frost heave as evaluation indexes, and further evaluated the repeatability of the test results. Evaluation results showed that the one-side freezing test results are repeatable, and it is an useful testing method to study the heat-water-vapor migration through one-side freezing tests with different test durations under uniform testing conditions. Moreover, the confusion that the frost heave mainly occurred in the top layer of high-speed railway subgrade was explicit solved by the behavior revealed in this paper.