Unsaturated Fine-grained Soils Research Articles

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.

Experimental and modeling study of shear strength of unsaturated fine-grained soils with bi-modal water-retention characteristics

The mechanical behavior and strength characteristics of unsaturated fine-grained soils with dual-porosity are of crucial importance in geotechnical designs. Nanyang fine-grained soils have been selected as typical dual-porosity structure soils to perform experimental tests under a wide range of suction and different initial densities while studying its stress-strain-strength properties constitute the main scope of this study. Axial translation and vapor equilibrium techniques are jointly employed to apply a wide suction range. Our data suggest that soil behavior transits from strain-hardening with shear-induced contraction to strain-softening with shear-induced dilation as suction and density increase. By exploiting a bi-modal soil–water retention curve (SWRC) explicitly separating capillarity and adsorption mechanisms, the shear strength is allowed to be analyzed in the capillary suction stress-shear stress space. The strength envelop exhibits bi-linear characteristics. Building upon these findings, we propose a bi-linear shear strength criterion specifically for dual-porosity fine-grained soils. We utilize the obtained test data to evaluate existing strength criteria based on effective stress and dual stress variables that consider the bi-modal SWRC characteristics. The comparison indicates that the proposed bi-linear shear strength criterion can more reasonably represent the variation of shear strength under a wide range of suction for unsaturated dual-porosity fine-grained soils.

Enhancement of a hydro-mechanical hypoplastic model for unsaturated fine-grained soils accounting for small strain stiffness

Due to global climate change, larger extreme seasonal and daily moisture variations have been occurring more frequently in the last decades. This phenomenon can affect geotechnical structures by inducing cyclic coupled hydro-mechanical loads. However, reproducing this behaviour from a numerical point of view requires robust constitutive models that can predict the coupling between the hydraulic and mechanical behaviour of fine-grained soils, combined with predictions of history-dependent stiffness evolution at small strains. For this reason, in the present work the hypoplastic model for unsaturated finegrained soils was further modified to better predict the water retention behaviour of unsaturated soils incorporating a smoothed hysteretic Water Retention Curve (WRC). In addition, the constitutive model wascalibrated using experimental data available on the literature of a completely decomposed tuff (CDT) from Hong Kong. At the end, the capabilities of the extended model to predict cyclic behaviour of unsaturated soils were evaluated using cyclic constant water triaxial tests at different suctions The results indicate that the extended model is able to describe with more accuracy the cyclic hydro-mechanical behaviour of the decomposed tuff if additional suction-dependency of one of its small-strain parameters is considered. Without this, the model can be calibrated to data at a given suction but its cyclic predictions for different suctions are not reasonably accurate.

Effect of Water Content on Apparent Cohesion of Soils from Landslide Sites

There are many empirical equations published for unsaturated fine-grained soils. However, there is only one empirical equation established for silty sand using the shear-box test and filter-paper-based suction test, but with the suction range of 0 to 200 kPa. It is reported that there is a significant discrepancy between the predicted values and test results of apparent cohesion within the range of 0 to 100 kPa for unsaturated coarse-grained soils. The purpose of this research is to study the effect of water content on apparent cohesion and predict apparent cohesion for coarse-grained soils within the range of 0 to 100 kPa using shear-box test and suction test results without much inconsistency. In this research, soil samples from the rainfall-induced landslide sites were obtained; laboratory tests such as soil-classification tests, shear-box tests and consolidated undrained triaxial tests were carried out. Test results were analyzed, and the findings are presented. When the water content is increased from 0% to 30%, there is a reduction of 89% in apparent cohesion on average. A newly developed prediction model for apparent cohesion based on the low range of matric suction from 0 to 100 kPa for unsaturated coarse-grained soils is introduced in this paper and compared with published models.

A Temperature-Dependent Model for Ultimate Bearing Capacity of Energy Piles in Unsaturated Fine-Grained Soils

AbstractThis study presents an analytical framework to estimate the change in ultimate bearing capacity of energy piles in unsaturated fine-grained soils under drained mechanical loading conditions...

A unified small-strain shear stiffness model for saturated and unsaturated soils

This paper presents a model for calculating the small-strain shear stiffness of saturated and unsaturated fine-grained soils as the product of a dimensionless stiffness index and four individual functions of the mean average skeleton stress, the over-consolidation ratio, the reference saturated void ratio and the degree of saturation. The main element of novelty of the model resides in the introduction of a reference saturated state, which results in a dependency of the small-strain stiffness on the degree of saturation. The reference saturated state is calculated according to a recently published constitutive law that relates the quotient between the unsaturated void ratio and the reference saturated void ratio to the degree of saturation. The model requires only two extra parameters for unsaturated states, i.e. one parameter for the volumetric behaviour and one for the stiffness behaviour. These two parameters may also be correlated to the saturated parameters, which simplifies the calibration of the model. The proposed framework is validated against laboratory experiments on three different materials resulting in generally accurate predictions compared to other published models. The good predictive capabilities and the simplicity of the formulation justify the implementation of the model into numerical codes for the analysis of geotechnical problems.

Plane strain shear strength of unsaturated fiber-reinforced fine-grained soils

Discrete randomly distributed fibers are commonly used to improve the engineering characteristics of the soil and thus soil properties such as shear strength, compressibility, density, and hydraulic conductivity. Most studies have so far focused on describing the behavior of soils containing randomly distributed fibers under dried or saturated conditions. However, the water table may seasonally fluctuate, thus generating unsaturated soil conditions. Therefore, a better understanding of the hydro-mechanical properties of unsaturated improved soils is of high necessity. In this research, the shear strength parameters of fine-grained soils were evaluated using the biaxial device available at Ruhr Universität Bochum. The applied device was modified to test unsaturated fine-grained soils with various degrees of saturation using axis translation and vapor equilibrium techniques. The experiments were conducted on fine soils containing 0, 0.5, and 1% fiber contents under a wide range of matric suctions. The ductile behavior was more noticeable in samples with lower suctions and higher straw contents. Furthermore, the shear strength of both unreinforced and reinforced fine-grained soils considerably increased by an increase in the suction. Finally, shear band inclination increased by the suction while decreasing by straw content.

Application of Mathematical Function to Estimate the Compaction Characteristics of Unsaturated Soils

The study aims to propose a mathematical approach to determine the optimum moisture content (<img src=image/14820636_01.gif>) and the corresponding maximum dry unit weight (<img src=image/14820636_02.gif>) of unsaturated fine-grained clay soils with accuracy. Laboratory tests such as grain size distribution, Atterberg limits, specific gravity, Proctor compaction test, and soil suction measurement are conducted to assess soil properties. The WOP and <img src=image/14820636_02.gif> are determined using the mathematical approach based on differential function (∂) and the graphical method. The differences in optimum moisture content values between the mathematical approach and the graphical method (<img src=image/14820636_03.gif>) values are 0.43 %, 0.36 %, 0.42 %, 0.24 %, respectively for soils PES, BFS, WIS, BES, and induced differences in total suction of 179.17 kPa, 144.00 kPa, 175.00 kPa, 96.00 kPa, respectively for soils PES, BFS, WIS, BES. Moreover, the differences in matric suction are 148.27 kPa, 116.13 kPa, 144.83 kPa, 80.00 kPa, respectively for soils PES, BFS, WIS, BES. <img src=image/14820636_02.gif> and <img src=image/14820636_03.gif> values are smaller than 0.5 % and marginal in the context of saturated soil mechanics. However, the total suction and matric suction values induced by <img src=image/14820636_02.gif> values are significant for unsaturated soils. An accurate estimation of <img src=image/14820636_03.gif> and <img src=image/14820636_01.gif> can be performed on unsaturated compacted soils using the mathematical approach.

A novel procedure to determine shear dynamic modulus and damping ratio for partial saturated compacted fine-grained soils

This paper presents a new procedure for the characterisation of the dynamic mechanical properties of unsaturated fine-grained soils (i.e., dynamic shear modulus and phase angle). The procedure uses a rheometer, which is a conventional device for the characterisation of viscous and viscoelastic materials but with limited used in the area of geotechnical engineering. Tests were carried out on cylindrical samples following a dynamic loading scheme under strain-controlled conditions. This research is divided into two phases. The first phase aims at validating the proposed experimental methodology. Thus, one sample of kaolin was tested using the proposed methodology, and the results were compared against those obtained from the conventional Resonant Column test. During the second phase, samples of kaolin were compacted at the optimum Proctor Standard; conditioned at different degrees of saturation, and tested using the proposed procedure to show its applicability on partially saturated soils. The dynamic shear modulus and phase angle of these samples were measured using a strain level ranging from 6×10-6 to 1×10-3 with different suction conditions and without confinement pressure. The results suggest that the newly developed procedure could be more versatile and accurate than available standardized tests.

The effect of microbial calcite precipitation on the retention properties of unsaturated fine-grained soils: discussion of the governing factors

In recent years, biogeotechnology has been introduced as a novel and environmentally friendly technique for soil improvement. The need to address global warming and the adverse environmental effects of the chemical additives have led to the emergence and development of the techniques which use calcite producing microorganisms in order to improve soil mechanical properties. While the effects of microbial induced calcite precipitation (MICP) on the hydraulics and mechanics of saturated coarse-grained soils have been well examined and studied, there is not yet much information on the effects these microorganisms would have on the unsaturated soil mechanical behaviour. The first step, in this regard, is to understand the effect of the processes involved in the MICP on the soil retention properties. Soil water suction is a key factor controlling soil hydraulic and mechanical behaviour. In this study, the influence of MICP on the soil total suction in an unsaturated fine-grained soil sample has been explored using filter paper experiment. The results of this study revealed that by increasing the amount of bacterial solution, the soil saturation-total suction curves are significantly affected. The soil water retention changes are attributed to the change in double layer thickness as well as the precipitation of calcite crystals.

Numerical modelling of the response of an unsaturated silty soil under wetting and gravitational loading processes

This paper presents the results of a numerical study aiming at simulating the response of an unsaturated fine-grained soil under wetting and gravitational loading processes. This study is based on the results of some centrifuge tests carried out to assess the influence of partial saturation on the laterally loaded pile response. The hydro-mechanical behaviour of the silty soil is described using a constitutive model adapted to unsaturated conditions. The model predictions are compared with the measurements provided by LVDTs and laser transducers in the first phases of the experimental study. Besides validating the model, the numerical study aimed at investigating the influence of the after-compaction conditions on both the displacement field and the evolution of the more significant state variables during imbibition and gravitational loading processes. Finally, an additional analysis is conducted to determine the effects of the pile installation on the soil response.

Volume change behaviour of swelling and non-swelling clays upon inundation with water and a low dielectric constant fluid

Studies of the volume change behaviour of saturated and unsaturated fine-grained soils upon exposure to various types of fluid are of significant interest while dealing with contaminated soils. In this study, the one-dimensional volume change behaviour of two clays (Spergau kaolin and Calcigel bentonite) with dominant minerals as kaolinite and montmorillonite was studied by inundating several initially unsaturated and saturated clay specimens with water and a non-polar organic liquid (heptane). For both cases, applied vertical pressures of 25 and 100 kPa were considered under oedometric conditions. The magnitude of compression and swelling deformations of the clays were found to be strongly dependent upon the mineralogy of clays, dielectric properties of the pore-fluid, initial compaction conditions, and applied stress during the wetting process. The test results indicated distinct interactions between unsaturated clays and water. Specimens of Spergau kaolin exhibited compression with water, whereas specimens of Calcigel bentonite exhibited swelling. Compression and swelling deformations of the clays with heptane as the inundating liquid remained within about ±1.0% for both clays indicating a very minor interaction between unsaturated clays and molecules of this non-polar organic liquid. The test results emphasized the significance of attractive and repulsive forces and their impact on the volume change behaviour of clays of different mineralogy.

On modelling the compressibility of unsaturated fine-grained soils

An empirical expression linking the slope λ of the normal compression line (NCL) for unsaturated soils to the effective degree of saturation Se is studied in this technical note. The parameters in this empirical expression are intrinsic soil parameter α, the slope of the normal compression line (NCL) at full saturation, λ100%, and the slope of the normal compression line at the driest state achievable by a soil, λd (a state at which only adsorbed water remains in the pores). The determination of λ at any other level of saturation usually requires a considerable amount of test data. This technical note explores the relationship between regression parameter α and the particle size, quantified by the D10 of the soil, based on test data for five different soils taken from the literature. A regression equation is developed that relates α to D10. Based on the proposed regression equation, the slope of the normal compression line for unsaturated fine-grained soils at any degree of saturation can be estimated when the test data are limited.

Effect of specimen preparation techniques on dynamic properties of unsaturated fine-grained soil at high suctions

The seismic response of soil depends on proper evaluation and use of soil dynamic properties, including shear modulus and damping ratio at various strain levels. Despite extensive studies on the shear modulus and damping ratio of saturated soils, research on the dynamic properties of unsaturated fine-grained soils — especially at high suction — is limited. This study aims to investigate the dynamic properties of loess at a variety of initial states resulting from different specimen preparation techniques (reconstituted, recompacted, and intact) and their evolutions due to suction-induced desiccation. Results of resonant column tests show that at initial states, the specimen preparation technique has a negligible effect on the normalized modulus (G/Gmax) and damping degradation pattern, while the influence becomes significant at a high suction (40 MPa). This is attributed to the microstructural evolution of specimens with different initial states that were subjected to suction-induced desiccation. More specifically, the elastic shear strain threshold decreases (reduction of elastic range) while shear modulus increases as suction reaches 40 MPa. Furthermore, the rate of increase in the damping ratio as well as degradation of the shear modulus for specimens at high suction is faster than their initial states. Based on the scanning electron microscopy observations, these findings may be attributed to the aggregation of larger silt–clay assemblies induced by suction increase. Consequently, soil with larger aggregates behaves more like granular sandy soil than saturated silt or clay.

Approach to Rapidly Determining the Water Retention Curves for Fine-Grained Soils in Capillary Regime Based on the NMR Technique

AbstractA new procedure is developed for rapidly determining the water retention curves of unsaturated fine-grained soils based on the nuclear magnetic resonance (NMR) technique. From a well-define...

New Classification and Hydraulic Property Testing for Wet Unsaturated Fine-Grained Soils

New Classification and Hydraulic Property Testing for Wet Unsaturated Fine-Grained Soils

Influence of Poisson’s Ratio on the Stress vs. Settlement Behavior of Shallow Foundations in Unsaturated Fine-Grained Soils

Poisson’s ratio is typically assumed to be constant for both saturated and unsaturated soils. However, the back-calculated Poisson’s ratio using the relationship between elastic and shear modulus (i.e. the equation for homogeneous, isotropic and linear elastic continuum) published in the literature showed that the Poisson’s ratio is not constant but decreases with the degree of saturation (or increasing suction). In the present study, more focused investigations are undertaken to study the influence of Poisson’s ratio on the stress vs. settlement (SVS) behaviour of shallow foundations in an unsaturated fine-grained (UFG) soil. The FEA are carried out using the software, SIGMA/W (GeoStudio 2007) to better understand the SVS behavior taking account of the influence of Poisson’s ratio for different matric suction values and compared with the model footing tests conducted in an UFG soil. Several suggestions are made with respect to the influence of Poisson’s ratio on the SVS behavior of shallow foundations in the UFG soils based on the results of this study.

Effects of loading frequency on cyclic parameters of unsaturated Zenoz kaolin

According to the considerable effects of the loading frequency on the cyclic behaviour of the soils and lack of the sufficient and precise data in this field for the unsaturated soils, more attention needs to be paid to the experimental investigations in various loading paths and frequencies. Meanwhile the unsaturated cyclic tests require advanced equipment and need more precision and time. In this paper, the results of two groups of tests which were performed on an unsaturated fine grained soil in the same initial condition are presented and compared during specific stress paths (pre-cycles of loading and during cycles). The tests are performed using the same suction-controlled cyclic triaxial apparatus in the same stress path before loading cycles, but two conditions of the cyclic loading frequencies (slow cyclic tests with the constant suction condition during the cycles and rapid cyclic tests with the unsteady suction condition during the cyclic path). The results show that increase in the loading frequency leads to the considerable increase in the shear modulus of the samples in the same strain levels.

Effects of mean net stress and cyclic deviatoric stress on the cyclic behavior of normally consolidated unsaturated kaolin

Experimental study of the cyclic behavior of unsaturated materials is more complex than that of the saturated materials due to the required equipment, experience and time. Furthering investigations in the field of unsaturated materials is necessary to better understand its complexity and sensitivity of unsaturated cyclic parameters to different determinants such as suction path, stress path, loading speed, deviatoric stress amplitude, physical specifications, and etc. To this end, the main focus of this study has been to analyze the effects of factors such as mean net stress and deviatoric stress levels in fast cyclic loading on the cyclic behavior of a normally consolidated unsaturated fine-grained trade soil, namely the Zenoz kaolin. Various unsaturated tests were performed in three mean net stress levels and three amplitudes of cyclic deviatoric stress levels. Results showed that increase of suction in the same strain level leads to increase in stiffness in normally consolidated samples (i.e. increase in elastic modulus and shear modulus and decrease in damping ratio). Also, in the same suction value and strain level, increase of the mean net stress during the isotropic consolidation causes to the denser normally consolidated samples and results to increase of elastic modulus and shear modulus, and decrease of damping ratio.

Incorporating the Effect of Moisture Variation on Resilient Modulus for Unsaturated Fine-Grained Subgrade Soils

Moisture content and stress state, which includes the effects of matric suction for unsaturated soil, exert a significant effect on the resilient modulus (MR) values of fine-grained subgrade soils. The moisture content and, consequently, matric suction vary periodically in subgrades in response to seasonal variation. The relationship between MR and matric suction is investigated through repeated load triaxial testing on fine-grained soils to obtain MR values and evaluation of the soil–water characteristic curves (SWCCs). The SWCCs, which represent the relationship between suction and degree of saturation, were evaluated with a combination of axis-translation and chilled-mirror hygrometer techniques to allow SWCC representation across the entire range of saturation. The plasticity index (PI) had a significant impact on the matric suction–water content relationship such that the SWCC shifted to the left as the soil PI decreased. The test results indicated a significant relationship between MR and matric suction. The results also were used to analyze the relationship between MR and moisture, which showed that the degree of saturation was superior to gravimetric water content in its ability to capture the effect of moisture variation on MR values. A modified constitutive MR-matric suction model was proposed to capture the effect of moisture variation on MR while accounting for the stress state of unsaturated soils. The proposed model performed better than existing models in reducing the number of regression constants and providing a better fit to the measured MR data.