Behaviour Of Unsaturated Soils Research Articles

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.

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.

Unified elastoplastic framework for the volumetric behavior of unsaturated soils during drained, undrained, and static compaction tests

When unsaturated soils are subjected to drained or undrained compression tests, they approach the saturated compression line with different slopes. During compression, four main phenomena occur in these materials: first, with the reduction in volume, the degree of saturation increases; second, with the reduction in the size of pores, the soil-water retention curve shifts on the axis of suction; third, these two phenomena produce an increase in the suction stress; and finally, this increase in suction stress produces a certain amount of collapse on the sample. In this paper, a unified elastoplastic framework is developed and coupled to a hydraulic model to simulate the volumetric behavior of unsaturated soils under different stress paths. This approach is based on a single equation with singled values for the compression and recompression indexes, resulting in a unified compression theory for soils. The comparison between experimental and numerical results shows the pertinence of the model.

Effect of wetting stress paths on mechanical behavior and instability of unsaturated soil in stress state space

In current study, a test program is designed to observe the influence of wetting stress paths on behavior and instability of unsaturated soil in stress state space. The mechanical behavior was studied by performing three test series in constant matric suction plane. In first series, water infiltration was performed together with shearing starting from zero (0 kPa) deviatoric stress. In second series, first water was infiltrated at zero (0 kPa) deviatoric stress then samples were sheared in drained pore water pressure conditions. In third series, water was first infiltrated at zero deviatoric stress then specimens were sheared in undrained pore water pressure conditions. Instability was studied in constant shear stress plane by first shearing soil up to predefined level of deviatoric stress then infiltrating water keeping deviatoric stress constant on the specimen. This research also describes how unsaturated soil behavior and instability is varied with variation in net confining stresses when suction is decreased from initial measured value to start water infiltration with different wetting stress paths and their effect on shear state and failure surface in stress state space. The test results revealed that the movement of stress paths was independent of net confining stress, matric suction and shearing conditions employed in the study.

Experimental investigation on strength behavior and microstructure of unsaturated net-like red soil

Experimental investigation on strength behavior and microstructure of unsaturated net-like red soil

Estimating soil water suction from texture, bulk density and electrical resistivity

Information on soil water suction (h) is essential to study water flow and solute transport in soils, and to understand engineering behaviors of unsaturated soils. Due to limited availability of field methods that can accurately measure h, numerous studies have been performed to estimate h from more readily available soil properties. In this study, a new relationship between h and soil electrical resistivity (ρ), developed from the Gardner water retention model and Archie’s second law, was used to estimate h values lower than the air entry value and a formation factor (expressed on log-scale) less than 1. The two model parameters (A and B), which are functions of soil texture and bulk density, were obtained by fitting the model to ρ and h data measured on soil columns of eight textures and various bulk densities. Laboratory and field evaluations with independent h and ρ data showed that the model estimated h values agreed well with the measured values, with root mean square errors less than 0.85 kPa. The model provides a new opportunity to evaluate in situ h dynamics and study coupled transport of water and solutes in the field.

Soil Water Characteristic Curves of Compacted Marine Clay

The soil water characteristics play an important role in predicting the engineering behaviour of unsaturated soils. An experimental programme was performed using pressure plate apparatus to investigate the effect of moulding water content, compactive effort and cycles of wetting and drying on Soil Water Characteristic Curves (SWCC) of compacted marine clay. The specimens were prepared with moulding water contents (optimum, dry of optimum and wet of optimum) for both Standard Proctor and Modified Proctor compactive efforts. The results obtained were fitted to Brooks and Corey (BC) and Van Genuchten (VG) models. The study indicated that SWCC for specimens with higher compactive efforts plotted above those with lower compactive efforts. It was observed that the variation in moulding water content had no significant effect in the resulting SWCC. With the increase in the number of cycles of wetting and drying, a decrease in the water retention capacity was observed.

Modelling the behaviour of unsaturated non-active clays in saline environment

The chemical composition of pore fluid and matric suction rule the mechanical behaviour of soils. In case of clays, their fabric changes in line with those variables. Since both the increase in matric suction and salinity cause a transition from an open to a close microstructure of low and medium activity clayey materials, a unique framework could tackle problems where salinity and saturation changes are expected.This paper presents a simple elasto-plastic model capable of reproducing the behaviour of unsaturated clayey soils in saline environments. Changes in the pore fluid composition are addressed through the use of osmotic suction as a variable. The proposed model extends the Barcelona Basic Model for partially saturated soils to consider the effect of osmotic suction.The model, implemented in the Thebes code, is calibrated for Boom Clay. The reproduced tests include mechanical loading at different matric and osmotic suctions in oedometric conditions, as well as more complex chemo-mechanical stress paths. Despite the simplicity of the formulation, the agreement between the experimental results and the simulations is encouraging. It seems that the modelling approach addresses the most important features of partially saturated soils with saline pore fluid which are slightly or moderately expansive.

One-dimensional consolidation of unsaturated soils considering self-weight: Semi-analytical solutions

For the newly deposited soil, the consolidation will occur under the self-weight and external load jointly if the soil is treated by the stack pre-pressure method. The soil stress caused by self-weight has a significant influence on its consolidation process no matter the soil is either saturated or unsaturated. However, the soil’s self-weight is ignored in the one-dimensional (1D) consolidation theory for unsaturated soils (Fredlund and Hasan, 1979). In this paper, we derived the semi-analytical solutions for excess pressures and normalized settlement in 1D dimensional consolidation of unsaturated soils considering self-weight. The analytical solutions were obtained by Crump’s method (Crump, 1976), through a computing program coupling the calculation of initial excess pore pressures with the inverse Laplace transform of the obtained solutions. Then, the proposed solutions are verified through a degradation case. Several 2D charts are presented to visualize the solutions, and various examples are employed to discuss the key factors that affect one-dimensional consolidation behavior of unsaturated soils when self-weight is considered. It can be found that the relative excess pore pressure will be restrained for the one-dimensional consolidation when considering self-weight.

Unsaturated soils in the context of tropical soils

The practice of geotechnical engineering in tropical climate regions must consider the use of unsaturated soil concepts. However, these concepts must also take into account the specific behavior traits of tropical soils, particularly those related to soil aggregation, pore structure, and mineralogy. This paper will initially present considerations on the typical properties of unsaturated tropical soils as well as fundamental concepts. Throughout the article, several engineering problems will be presented alongside reflections on the complex interaction between the numerous variables involved in the modeling and engineering practice of tropical unsaturated soil behavior. The paper addresses issues related to soil formation, chemical and mineral composition, physical properties, tropical soil classification, and structural characteristics of soils. Issues related to compaction and the influence of weathering, geomorphology and bioengineering are also addressed.

A general semi-analytical solution for predicting axisymmetric consolidation behavior of multi-layered unsaturated soil

A general semi-analytical solution for predicting axisymmetric consolidation behavior of multi-layered unsaturated soil

Numerical and experimental modeling of geomechanical behavior of partially saturated soils

The Barcelona Basic Model (BBM) has been implemented in a finite difference-based computer program to simulate the behavior of unsaturated soils subjected to wetting. The BBM implementation was verified using analytical solutions, and the proposed model has been used to simulate the response of a compacted embankment under complete saturation and suction induced conditions. Numerical analyses indicate that considerable amount of total and differential settlements could develop at the top surface of the embankment. BBM is executed into FLAC2D extending a defined module for modified Cam Clay (MCC) and has been set up an analytical solution for suction-dependent stress and strain. Evaluating the effect of anisotropy and nonlinear apparent tensile strength in unsaturated soils, a modification to BBM formulation has been proposed and optimized by developing numerical analyses to reduce the size of elastic region of loading collapse (LC) curve. Then, an experimental study in the literature is investigated by utilizing comparative curves from BBM and modified BBM indicating well agreement with natural circumstances. As a result of the work presented in this research, finite difference codes with BBM and modified BBM has the capability of simulating the real behavior and is operational being applied to problems associated with earthen structures in unsaturated or partially saturated of expansive soils as a three-phases medium.

Hydromechanical behaviour of two unsaturated silts: laboratory data and model predictions

This paper presents the results from a campaign of unsaturated and saturated isotropic tests performed on two compacted silts of different coarseness, namely a clayey silt and a sandy silt, inside triaxial cells. Some tests involved an increase or a decrease of mean net stress at constant suction or an increase or a decrease of suction at constant mean net stress. Other tests involved an increase of mean net stress at constant water content with measurement of suction. During all tests, the void ratio and degree of saturation were measured to investigate the mechanical and retention behaviour of the soil. The experimental results were then simulated by the bounding surface hydromechanical model of A.W. Bruno and D. Gallipoli (2019. Computers and Geotechnics, 110: 287–295. doi:10.1016/j.compgeo.2019.02.025), which was originally formulated to describe the behaviour of clays and clayey silts. Model parameters were calibrated against unsaturated tests including isotropic loading stages at constant water content with measurement of varying suction. Loading at constant water content is relatively fast and allows the simultaneous exploration of large ranges of mean net stress and suction, thus reducing the need of multiple experiments at distinct suction levels. Predicted data match well the observed behaviour of both soils, including the occurrence of progressive yielding and hysteresis, which extends the validation of this hydromechanical model to coarser soils. Specific features of the unsaturated soil behaviour, such as wetting-induced collapse, are also well reproduced.

Investigating the salinity effect on water retention property and microstructure changes along water retention curves for lime-treated soil

Lime treatment is a widely applied technique in improving the workability and geotechnical properties of soil. The water retention property and microstructure are highly related to the hydro-mechanical behaviour of unsaturated lime-treated soil. In this study, the water retention property and the evolution of pore size distribution (PSD) along the water retention curves (during drying) were studied for a lime-treated soil, with emphasis put on the curing time and salinity effects. Two soil powders with different soil salinities were prepared and stabilised by 2% lime. The chilled-mirror dew-point hygrometer and the contact filter paper method were used to measure the total and matric suctions, respectively. The PSD of lime-treated soil at various water contents was obtained using mercury intrusion porosimetry (MIP). Results showed that the matric suction increased significantly, while the total suction varied slightly during curing. At a given curing time, the specimens with higher salinity exhibited higher matric and total suctions. The difference between the soil water retention curves (SWRCs) determined by the filter paper method and from the PSD became more significant at longer curing time, as the production of cementitious compounds did not contribute to the SWRC from PSD, but contributed to the increase of matric suction measured by the filter paper method. The PSD of lime-treated soil changed from bi-modal characteristics (w > 14 %) to tri-modal pattern (w ≈ 8 %), and finally recovered to bi-modal characteristics (w ≈ 3 %), due to the shrinkage-related cracking of the clay fraction. The lime treatment inhibited the clay shrinkage, whereas the curing time and salinity effects on the drying-induced microstructure were insignificant.

An elastoplastic solution for cylindrical cavity expansion under constant water content conditions in anisotropic unsaturated soils

In this paper, an elastoplastic solution is developed for cylindrical cavity expansion in anisotropic unsaturated soils under constant water content conditions. The most prominent features of the solution can be attributed to 1) the stress–strain behaviour of unsaturated soil is subtly coupled with the saturation-suction behaviour; 2) both the mechanical and hydraulic behaviours are modelled as elastoplastic processes; and 3) an anisotropic critical state soil model for saturated soil is extended to an unsaturated situation to consider the effects of the initial stress anisotropy and stress-induced anisotropy of the natural unsaturated soils. The problem is formulated as a system of first-order differential equations and solved as an initial value problem. A numerical code that can judge the hydraulic yield condition of soil particles during the cavity expansion process is developed based on the Runge-Kutta algorithm to solve the governing equations. Parametric analyses, which cover a wide range of suctions, overconsolidation ratios, degrees of initial stress anisotropy, are conducted to investigate the effects of these important factors on the expansion responses in unsaturated soils. The proposed solution could potentially be applied to interpret pressuremeter tests and model pile installation effects in anisotropic unsaturated soils.

Behavior of unsaturated cohesive-frictional soils over a whole range of suction/thermo-controlled stress paths and modes of deformation

The present work documents some of the most recent experimental evidence of the thermohydro-mechanical behavior of compacted soils over a whole range of suction- and/or thermo-controlled stress paths and modes of deformation, including data from a series of triaxial, true triaxial, plane strain, ring shear, and resonant column tests conducted on different types of cohesive-frictional soils in the low-to-medium matric suction range under either room temperature or thermally controlled conditions. The work has been accomplished at the Advanced Geomechanics Laboratory of the University of Texas at Arlington, focusing primarily on the following essential features of unsaturated soil behavior: (1) Loading-collapse and apparent tensile strength loci assessed from suction-controlled triaxial and true triaxial testing on clayey sand, (2) Critical state lines from suction-controlled plane strain testing on silty soil, (3) Peak and residual failure envelopes from suction-controlled ring shear testing on clayey soil, (4) Frequency response curves and cyclic stress-strain hysteretic loops from thermo-controlled, constant-water content resonant column testing on clayey soil, and (5) Residual failure envelopes from suction/thermo-controlled ring shear testing on clayey soil. The work is intended to serve as a succinct yet reasonably thorough state-of-the-art paper contribution to PanAm-UNSAT 2021: Third Pan-American Conference on Unsaturated Soils, Rio de Janeiro, Brazil, July 21-25, 2021.

A bounding surface plasticity model for unsaturated structured soils

A bounding surface plasticity model based on the effective stress concept is presented to describe the behaviour of unsaturated structured soils subjected to hydro-mechanical loadings. The structural degradation effects on the compressive and tensile strength are considered through controlling the size of the bounding surface, allowing for a smooth transition of the response from structured to unstructured states. The structural degradation is modelled using a work hardening approach, considering both the effects of stress magnitude and accumulated plastic strain on the degradation process. A void ratio-dependent water retention model is adopted, taking the effect of hydraulic hysteresis into account. Attention is also given to the stiffening effect of a decrease in the degree of saturation on the mechanical response of unsaturated structured soils and the wetting-induced collapse. A radial mapping rule with a mobile centre of homology is adopted to capture the response of the soil under unloading–reloading conditions. The predictive capability of the model is demonstrated through the comparison of the model simulations with experimental data for different conventional laboratory tests including constant-suction oedometer and triaxial shearing and wetting tests.

A General Analytical Solution for Axisymmetric Consolidation of Unsaturated Soil with Impeded Drainage Boundaries

The consolidation of soil is one of the most common phenomena in geotechnical engineering. Previous studies for the axisymmetric consolidation of unsaturated soil have usually idealized the boundary conditions as fully drained and absolutely undrained, but the boundaries of unsaturated soil are actually impeded drainage in most practical situations. In this study, we present a general analytical solution for predicting the axisymmetric consolidation behavior of unsaturated soil that incorporates impeded drainage boundary conditions in both the radial and vertical directions simultaneously. The impeded drainage boundary is modeled using the third kind boundary, and it can also realize fully drained and absolutely undrained ones by changing the drainage parameter. A general analytical solution is developed to predict the excess pore-air and pore-water pressures as well as the average degree of consolidation in an unsaturated soil stratum using the common methods of eigenfunction expansion and Laplace transform. The newly developed solution is expressed in the product of the terms of time, depth, and radius, which are derived using Laplace transform, usual Fourier, and Fourier-Bessel series, respectively. The eigenfunctions and eigenvalues are evaluated from the impeded drainage boundaries in both radial and depth dimensions. Then, the correctness of the proposed analytical solution is verified against the existing analytical solution for the case of traditional boundaries and against the finite difference solution for the case of general impeded drainage boundaries, and excellent agreements are obtained. Finally, the axisymmetric consolidation behavior of unsaturated soil involving impeded drainage boundaries is demonstrated and analyzed, and the effects of the drainage parameters are discussed. The results indicate that the larger drainage parameter generally expedites the dissipations of the excess pore pressures and further promotes the soil settling process. As the drainage parameter increases, its influence gradually diminishes and even can be neglected when it is larger than 100. The general analytical solution and findings of this study can help for better understanding the axisymmetric consolidation behavior of the unsaturated soil stratum in the ground improvement project with vertical drains as well as the gas-oil gravity drainage mechanism in the naturally fractured reservoirs.