Experimental and Applied Modeling of Unsaturated Soils

Soil-water retention properties and compressibility are closely related to the stability of geotechnical engineering structures made of compacted soils. In the present study, the water retention curve of a compacted soil from Northern France was characterized using either suction control method or suction measurement. The results obtained showed a good correspondence between the two methods. Then, these two experimental methods were used in oedometer compression test to highlight the factors influencing the compression properties, especially in terms of changes in compression index as a function of initial suction and initial structure (i.e. density). In the case of low density (1.1 Mg/m³), the results evidenced different zones for the variations of compression index, separated by a stress threshold. Compression index did not change significantly when suction values were relatively high and the applied vertical stresses were lower than the identified threshold stress.

A new prediction model for unsaturated soil shear strength parameters with respect to degree of saturation is presented in this paper. Details of the model formulation and the determination of model parameters are described and reported. Introducing shear strength parameters of saturated soil, the model can be used in practice engineering involving both unsaturated soils and saturated soils. Moreover, more soil engineering characteristics can be captured in the new model compared with previous analogous models. The validity of this model is verified through a number of shear strength data available in the published literature. To verify the model against Yunnan red clay from China, a series of constant-water content triaxial tests have been carried out on both saturated and unsaturated specimens. Details of the experimental program and results are presented in this paper.

Though stress-strain characteristics of unsaturated soils are complicated and rather different from those of saturated soils, it should be described properly by a constitutive model for soils because soil usually stays under an unsaturated condition in actual field. In the current study, a simple elastoplastic model for saturated soil is extended to one applicable to unsaturated soils. The proposed model is formulated using the Bishop's effective stress and the residual strength is, therefore, assumed to be constant. In the proposed model, the decrease (or increase) in the degree of saturation is linked with upward (or downward) movement of normally consolidated line in the compression plane of mean effective stress and void ratio, by which the typical volumetric and distortional behaviors of unsaturated soils are properly described. In addition, a simple method to extend classical water retention curves such as van Genuchten's equation to be able to incorporate the influences of suction histories and density is proposed and applied to the proposed model. In the present paper, the outline of the proposed model is explained and applicability of the model is discussed through typical results of simulations.

To study the effects of stress paths on shear strength characteristics of unsaturated sand, a series of conventional triaxial compression (CTC), reduced triaxial compression (RTC), and reduced triaxial extension (RTE) tests were conducted on shallow gassy sand from the Hangzhou Metro project, China. Tests were carried out by using a stress path unsaturated triaxial test system, capable of reproducing stress paths that are normally induced on soils in subway construction projects. Results show that the stress-strain relationship of unsaturated sand under various stress paths has different characteristics and the strength parameters are closely related to the particular stress path followed. Effective cohesion and apparent cohesion are affected significantly by the stress path, but effects on the internal friction angle are not important. Relationships between apparent cohesion and matric suction from various stress paths induced on unsaturated sand can be described via power functions. A unified shear strength model was then devised for gassy sand along different triaxial stress paths.

Today, it is well known that most geotechnical infrastructure resting on or made of unsaturated soils experience a wide range of deformations. Calculation of foundation settlement, for instance, requires a good estimation of soil stiffness at relatively small strains. Analyses of slopes, embankments, and soil bearing capacity, on the other hand, require adequate estimation of shear strength, from peak to residual. To date, there is limited experimental evidence of unsaturated soil response at mid-to-large strain levels corresponding to residual shear strength, and virtually no research work is available that spans the whole range of deformations in unsaturated soils using ring shear testing technique under controlled-suction states. This paper introduces a novel ring shear apparatus suitable for testing unsaturated soil samples over a whole range of deformations via axis-translation technique. The lower annular platen features a full set of 5-bar ceramics for control of pore-water pressure during constant-suction RS testing. The paper outlines the full development of the apparatus, including details of its main components and the test procedures. Results from a preliminary series of suction-controlled ring shear tests on compacted silty sand are presented.

This paper introduces a servo-controlled true triaxial (cubical) device that has been developed to test 3-in (7.5-cm) side, cubical specimens of unsaturated soil under controlled-suction states for a wide range of stress paths not achievable in a conventional cylindrical apparatus. The cell is a mixed-boundary type of device, with the specimen seated on top of a high-air-entry disk and between five flexible (latex) membranes on the remaining sides of the cube. The new cell is an upgraded, more elaborate version of the one previously reported by Hoyos et al. (2005), featuring two independent pore-air pressure and pore-water pressure control systems via a PVC-100-UNSAT pressure panel, as well as a fully computer-driven stress application/control system via a PCP-5000-UNSAT pressure panel. Suction states, s = (u_a – u_w), are induced in the specimens via axis-translation technique. The technique is implemented by using the s = u_a testing concept (i.e., u_w = 0). Results from a preliminary series of hydrostatic compression and triaxial compression tests are presented.

Seismic compression is the accumulation of contractive volumetric strains in unsaturated soils during earthquake loading. Cyclic simple shear tests were conducted on several unsaturated soil specimens to evaluate the role of low-plasticity fines content and the degree of saturation on vertical strains from seismic compression. Sands with low plasticity fines generally show larger vertical strains than clean sands and decreased vertical strains at intermediate saturations (S ~ 30%) relative to dry (S = 0%) and higher saturations (S ~ 60%). At saturation levels where vertical strains are minimized, the specimens have relatively high matric suction head, which maximizes the effective confining stress in the soil sample. Accordingly, it is postulated that saturation levels that give rise to high matric suction increase the moduli of the specimen, which in turn reduces volume change susceptibility.

Unsaturated soil moisture diffusivity parameter is important to variety of engineering applications, such as design and analysis of pavements, highway embankments and slopes, and shallow foundations. The unsaturated diffusivity coefficient controls transient moisture flow conditions within a soil in response to suctions or fluxes imposed at the boundaries of the soil mass. This research paper presents laboratory test results for wetting and drying unsaturated diffusion coefficients, and investigates the hysteresis effect between the wetting and drying processes on the measured parameters. Undisturbed soil specimens were obtained from a compacted highway embankment site in Oklahoma. Soil specimens were exposed to drying and wetting cycles, and the resulting wetting and drying coefficients were evaluated. The wetting diffusion coefficients were generally higher than the drying diffusion coefficients by factors between one and two.

Collapse of loess is the combined effect of a number of factors acting together. The water content is the most important of these factors. Based on a series of consolidation tests on Yuncheng loess, the influence of initial water content on the collapsibility of loess was investigated. As the initial water content increases, the compressibility of the loess increases, while the collapsibility coefficient, the peak collapsibility coefficient and the peak collapse pressure all decrease. Overall, the collapsibility of the loess decreases. Collapse will likely not occur if the initial water content yields a collapsibility coefficient of less than 0.015. In addition, scanning electron microscopy analysis indicated that the void area ratio of the loess specimens decreases as the initial water content increases, while the density of the loess increases and the collapsibility of the loess decreases.

Collapsible soils are unsaturated soils which present a potential for large deformation and a complete change to the whole particle structure after wetting, with or without loading. These soils are characterized with loose structures composed of silt to fine-sand-size particles. Collapsible soils are deposited in arid and semi-arid regions. Due to the expansion of human activities, these regions are occupied aggressively leading to the use of large quantities of water, which creates favorable conditions for soil collapses. Theses soil failures lead to severe damage and large distress to man-made structures. The objective of this experimental study is to illustrate that the mechanical resistance of collapsible soil can be improved. This study demonstrates that it is possible to minimize the collapse potential C_p to an acceptable level after chemical treatment with salts (ammonium sulfates (NH₄)₂SO₄ and potassium chlorides KC1) at different concentrations (i.e., 0.5, 1.0, 1.5 and 2.0 mole/liter) and under different compaction energies. The method used in this study is based on oedometric tests with variable normal stresses.

Lateritic soil treated with up to 12% bagasse ash by dry weight of soil was prepared at –2, 0 and +2% of the optimum moisture content (OMC) and compacted with standard Proctor energy, for use as a suitable material in waste containment application. The treated soil was assessed to determine its soil water characteristic curve (SWCC) parameters that were used to predict the measured laboratory SWCC with Brooks - Corey (BC) and Van Genuchten (VG) models. VG n fitting parameter increased to a peak at OMC and thereafter decreased with higher molding water content, while α fitting parameter generally increased with higher molding water content. BC pore size distribution index (λ) increased with higher molding water content, while no clear trend was observed for air entry suction (Ψ_a) with higher molding water content. VG n increased, while α decreased with higher bagasse ash content. Higher BC Ψ_a and λ were obtained with higher bagasse ash content. BC model tends to over predict the volumetric water content (θ), particularly at low matric suction values, while VG model under predicts θ. Generally, the measured and predicted SWCC did not show great variation from each other but are fairly close.

Soil water characteristic curve (SWCC), which defines a soil's ability to store and release water as it is subjected to various soil suctions, is an important tool to estimate and interpret unsaturated sol property functions. The purpose of this study was to estimate SWCC data for lateritic soil - bentonite mixtures proposed as a barrier material in waste containment systems. Also, the effect of various compaction states, namely dry, optimum and wet moisture conditions, using standard Proctor energy on SWCCs of the mixtures was examined. The SWCCs were measured in the laboratory by sequential desorption of saturated compacted lateritic soil — bentonite mixtures using pressure plate extractors. The experimental SWCC data were fit to the Brooks — Corey and Van Genuchten parametric equations. Variations in SWCCs due to higher bentonite contents reflected in the fitting parameters of the two equations. For example, higher air entry pressures (Ψ) were obtained for mixtures containing higher bentonite content when compacted wet of optimum water content.

Alonso et al. (1990) proposed the first elasto-plastic model for unsaturated soils, which was subsequently called the Barcelona Basic Model (BBM). The BBM successfully explained many features of unsaturated soils and received extensive acceptance. At present, the BBM remains one of the fundamental elasto-plastic models for unsaturated soils. The original BBM was proposed in terms of an incremental formulation. Though the incremental formulation is useful for programming in the constitutive modeling, it is complicated for understanding soil behavior. This paper derives an analytical formulation of the BBM. Its significances in the constitutive modeling of unsaturated soils are also discussed.

An unsaturated soil is not a special type of soil, rather a state of the soil. All soils can be unsaturated with water. There are many concepts specifically related to constitutive models for unsaturated soils, e.g., suction, net stress, apparent consolidation, loading-collapse yield surface, suction-increase yield surface, etc. These concepts are not always well understood and also seem to make unsaturated soils very special. This paper gives a critical review of these concepts, in an attempt to clarify some common confusion in constitutive modelling of unsaturated soils.

The Barcelona Expansive Model (BExM) for unsaturated expansive soils is complicated and the micro parameters and the coupling function from micro-structural strain to macro-structural strain are difficult to be determined. This paper presents an elastoplastic constitutive model from the macroscopic observation for predicting the hydraulic and mechanical behaviour of unsaturated expansive soils. The model predictions are performed on the results of triaxial tests on compacted unsaturated expansive clay available in the literature. The comparisons between measured and predicted results indicate that the model offers great potential for quantitatively predicting the hydraulic and mechanical behaviour of unsaturated expansive soils.

This paper presents a field experimental study on the influence of slope terrain on the distribution of matric suction of the soil in unsaturated slopes subjected to rainfall. Instrumentation and testing programs were carried out in a slope with different terrains, i.e. planar slopes, gullied slopes, and ridge-like slopes. The matric suctions of the soil at a depth greater than 1.2m were measured using jet-filled tensiometers, and those less than 0.9m were measured using the filter paper method. The matric suctions of the soil along the depth in the slope were measured prior to and after the rainfall event, and data in two major rainfall events (accumulated precipitations with small and large amount) were presented. Major findings concluded from this research are: (1) the matric suctions of the soil at shallow depths in gullied slopes in dry season are significantly less than those in planar slopes and ridge-like slopes, and those in ridge-like slopes are slightly greater than those in planar slopes; (2) the amount of drop in the matric suctions of the soil in gullied slopes is considerably small compared with that in planar and ridge-like slopes; (3) after the rainfall event, the matric suctions of the soil at the shallow depth in the ridge-like slope are considerably greater than those in the planar and gullied slopes no matter what the amount of the accumulated precipitation is; (4) the influence of the rainfall with a low accumulated precipitation on the behavior of the matric suction of the soil in different slope terrains is similar to that with a high accumulated precipitation.

Water infiltration is an important actor causing a bank slope failure, and the development of unsaturated soils mechanics has provided a new theoretical basis and corresponding analysis method for studying the effects on slope stability of reservoir water infiltration. The three-dimensional terrain and structure of slope are also the essential influencing factors in its stability analysis. In the paper, the pore water pressure fields are computed by simulation of unsaturated seepage to ascertain the stress and strain distributions of three-dimensional slope based on the constitutive model of unsaturated soils, then the contribution of matric suction to shear strength is considered and three-dimensional stability analysis methods of unsaturated soils slope are performed to evaluate the safety factor (Fs). Taking Xietan landslide in Three Gorges reservoir area as the example, the result shows that its deformation and stability are seriously affected by reservoir water level change, and variation rules of slope stability are consistent with the dispatch of reservoir water.

The effect of negative pore-water pressure (suction) is often ignored in slope stability analyses. Based on the theory of infiltration and seepage through an unsaturated soil system, transient numerical analyses for an engineering case of slope instability, caused by the loss of soil suction under the condition of continuous rainfall, are conducted. The results of the analysis show that the unsaturated soil suction has an important effect on the stability of the unsaturated slope. The suction profile depends on the magnitude of the flux, frequency, duration of rainfall and evapotranspiration. In most cases, the simultaneous effect of several factors must be considered in assessing the magnitude of soil suction. The paper highlights the importance of monitoring soil suction in engineering practice for slope stability purposes.

Collapse of volcanic slopes induced by rainfall has been frequently induced in Hokkaido, Japan, especially the middle southern area. It has been known that volcanic slopes in Hokkaido have a strong potential to cause such failure. The objective of this study is to reveal failure mechanism of volcanic slopes caused by both rainfall and freeze-thaw actions. A series of model tests was performed on volcanic slopes having several moisture contents. In particular, the effect of freeze-thaw actions on mechanical behavior at failure was investigated detailedly.

Failures of soil slopes are often triggered by rainfall, and weak layers tend to accelerate the failure process significantly. Centrifuge model tests were conducted to investigate the behavior of cohesive soil slopes with weak layer under rainfall conditions. Deformation process and change in soil suction were observed and recorded during the tests. Comparison between soil suction and strain showed that the rainfall infiltration, the decrease of suction and the increase of strain correspond in time. The analysis of displacement showed that dislocation zones appeared in the slopes during the rain precipitation process. The rainfall induced a discontinuous dislocation zone in the slope with a weak layer, and a continuous one in the homogeneous slope.

The low-volume road pavement systems on high plasticity index (high PI) clay subgrades are perceived to fail prematurely by some highway agencies. The most common distress, at least in Texas, is longitudinal shrinkage cracking. Several models have been developed to estimate the shrinkage characteristics of various types of clays from their index properties. The process of estimating the shrinkage strain, modulus and tensile strength from index properties of clays is presented in this paper. The estimation of these parameters from their index properties can be reasonably carried out.

It is well known that vegetation influences slope stability in following ways: (1) By mechanical reinforcement from the plant roots; (2) by increasing in soil suction or reduction of pore water pressure; (3) by surcharge from the weight of trees; and (4) by"wind throwing." The main beneficial effect includes root reinforcement. Roots increase the shear strength of soil primarily by transferring shear stresses that develop in the soil matrix into tensile resistance in the roots via interface friction along the imbedded roots.

Longitudinal cracks often develop along the slope of embankments on shrink-swell soils due to continuous saturation. The presence of montmorillonite clay mineral in the soil sample considered for the study, exhibited binary phenomenon. Charge distribution in the montmorillonite clay mineral indicates the presence of both expansion and expansion preventing bonding forces in the same mineral structure. The study presents the use of Cohesive Non-Swelling Layer technology to reduce the formation of cracks. Structures constructed with this technology have shown superior performance for more than thirty seven years.

Predicted values for free-field heave and foundation movement are basic parameters for the design of foundation systems for buildings sited on expansive soil. Current design procedures generally incorporate only the maximum predicted heave that will occur at a site. This can introduce an impractical design situation when the depth of potential heave and the associated heave is large. Under some site conditions the design life of the structure may be less than the time required for the entire amount of predicted heave to occur. In such cases, it is desired to consider the rate of heave and the amount of heave to be expected at the end of the design life. This paper presents a methodology for extending observed heave data accumulated over a period of time in order to predict the progression of heave with time. The methodology incorporates curve fitting of survey data to a hyperbolic function. The method of analysis is validated and demonstrated using actual data accumulated over a period of several years on a large building that was constructed with a drilled pier foundation. The results show that in that case future movement of the foundation could be accurately predicted.

This paper evaluates the effect of precipitation events, irrigation practice, and deep wetting from underground sources on migration of water in soils and bedrock. The water migration analyses were conducted using the VADOSE/W software for the TRACON building at Denver International Airport, Denver, Colorado, USA. Measurements of water content and density from surface nuclear gauge tubes installed around the building were used to calibrate and validate input parameters and boundary conditions specified in the VADOSE/W models. Long-term water migration within the soils and bedrock was evaluated for the cases with various water sources. It was concluded that the presence of deep water bearing stratum and irrigation on the lawn can be significant in the migration of the wetting front within the soils and bedrock.

In coastal processes, the strong water movements due to short periodic waves (such as sea swell) can induce irregular water level fluctuations in the swash zone and within the sandy beach. In this paper, the measured water level fluctuations in a wave canal with a sloping sand beach were analyzed by using seven capacitive sensors. Fourier spectrum and multi-resolution wavelet analyses were used to study these water level fluctuations. Several dominant periods for the entry water level were identified with Fourier analysis. The corresponding amplitudes and phases were obtained by wavelet analysis. A simplified (bimodal) Fourier approximation was then determined for the irregular water level fluctuations, with a good agreement with the original signals, which, therefore, could be used as the entry condition for numerical simulations.

Surface ground-penetrating radar (GPR) was assessed as a non-destructive method to measure the temporal and spatial variability of unsaturated seepage flow under uniform wetting conditions on a homogeneous sandy soil. GPR profile survey with 400MHz antenna was conducted. GPR reflected wave from advancing wetting front during uniform field infiltration experiments was used to map the wetting front depth variability with time. The dielectric constant of soil measured by vertical soil moisture probe below the ground surface was used as standard measures of soil moisture to compare with the GPR estimated unsaturated seepage flow behavior. The utility of our proposed GPR profile survey was demonstrated by using numerical seepage flow FEM analysis of field infiltration experiments. There was good agreement between computed and measured wetted zone movements in two-dimensional unsaturated seepage flow. Traceability of the wetting front depth variability with time was found by GPR profile survey.

The unified solution for shear strength for unsaturated soil in terms of two independent state stress variables is presented based on both Unified Strength Theory (UST) and Fredlund's shear strength theory for unsaturated soil, which can be especially versatile in reflecting the effect of intermediate principal stress. Considering different distributions of matric suction, the unified solution of active earth pressure for unsaturated soil is deduced, which could avoid some shortages of Rankine's active earth pressure. The influence of the intermediate principal stress and matric suction on the active earth pressure is also investigated. It is shown that the active earth pressure decreases significantly with an increase in the unified strength theory parameter or matric suction.