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- Unsaturated soil, Unsaturated Soil (Engineering), Testing, Unsaturated Soils

The paper presents an elasto-plastic model for unsaturated soils that takes explicitly into account the mechanisms with which suction affects mechanical behaviour as well as their dependence on degree of saturation. The proposed model is formulated in terms of two constitutive variables directly related to these suction mechanisms: the average skeleton stress, which includes the average fluid pressure acting on the soil pores, and an additional scalar constitutive variable, 'csi', related to the magnitude of the bonding effect exerted by meniscus water at the inter-particle contacts. The formulation of the model in terms of variables closely related to specific behaviour mechanisms leads to a remarkable unification of experimental results of tests carried out with different suctions. The analysis of experimental isotropic compression data strongly suggests that the quotient between the void ratio, e, of an unsaturated soil and the void ratio es, corresponding to the saturated state at the same average soil skeleton stress, is a unique function of the bonding effect due to water menisci at the inter-particle contacts. The same result is obtained when examining critical states at different suctions. Based on these observations, an elastoplastic constitutive model is developed using a single yield surface the size of which is controlled by volumetric hardening. In spite of this simplicity, it is shown that the model reproduces correctly many important features of unsaturated soil behaviour. It is especially remarkable that, although only one yield surface is used in the formulation of the model, the irreversible behaviour in wetting–drying cycles is well captured. Because of the behaviour normalisation achieved by the model, the resulting constitutive law is economical in terms of the number of tests required for parameter determination.

- by Radhey Sharma and +2
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- Numerical Modeling, Unsaturated soil, Geotechnical Engineering, Genetic Algorithms
- by Lupii Hakim
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- Unsaturated Soils, Rainfall, Soil Suction, Failures

Triaxial tests have been widely used to evaluate the soil behavior. In the past few decades, several methods have been developed to measure the volume changes of unsaturated soil specimens during triaxial tests. Literature review indicates that until now it remains a major challenge for researchers to measure the volume changes of unsaturated soil specimens during triaxial testing.
This paper presents a noncontact method to measure the total and local volume changes of unsaturated soil specimens using a conventional triaxial test apparatus for saturated soils. The method is simple and cost-effective, requiring only a commercially available digital camera to take images of an unsaturated soil specimen during triaxial testing from which accurate 3D model of the soil specimen is reconstructed. In this proposed method, the photogrammetric technique is utilized to determine the orientations of the camera where the images are taken and the shape and location of the acrylic cell, multiple optical ray tracings are employed to correct the refraction at the air-acrylic cell and acrylic cell-water interfaces, and a least–square optimization technique is applied to estimate the coordinates of any point on the specimen surface. The paper first discusses the theory of proposed method. An image analysis on a caliper was then used to evaluate the accuracy of photogrammetric analysis in the air. A series of isotropic compression tests on a stainless steel cylinder were used to demonstrate the procedure and evaluate the accuracy of the proposed method, while triaxial shearing tests on a saturated sand specimen were used to exam the capacity of the proposed method for measuring the total and localized volume changes during triaxial testing. Based upon the analysis of the test results, some conclusions are drawn.

- by Fernando A M Marinho
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- Unsaturated Soils

Mounding often occurs beneath engineering structures designed to infiltrate reuse water. AQTESOLV software and a spreadsheet solution for Hantush, together with soil moisture water balance (SWAGMAN farm model), were used for quantitatively predicting the height and extent of groundwater mounding underground to assess the groundwater-flow simulations of infiltration from a hypothetical irrigation site. Horizontal and vertical permeability, aquifer thickness, specific yield, and basin geometry are among the aquifer and recharge properties inputs. For 2.2 ha sites, the maximum heights of the simulated groundwater mound ranges up to 0.29 m. The maximum areal extent of groundwater mounding measured from the edge of the infiltration basins of 0.24 m ranges from 0 to 75 m. Additionally, the simulated height and extent of the groundwater mounding associated with a hypothetical irrigation infiltration basin for 2.2 ha development may be applicable to sites of different sizes, using the recharge rate estimated from the SWAGMAN farm model. For example, for a 2.2 ha site with a 0.0002 m/day recharge rate, the irrigation infiltration basin design capacity (and associated groundwater mound) would be the same as for a 1.1 ha site with a 0.0004 m/day recharge rate if the physical characteristics of the aquifer are unchanged. The study claimed that the present modelling approach overcomes the complications of solving the Hantush equation for transient flow. The approach utilised in this study can be applied for other purposes such as measuring the feasibility of infiltrating water, attenuation zone, risk mitigation essential for decision-makers and planning regulators in terms of environmental effects and water use efficiency.

High-suction tensiometers are able to measure suctions up to 2 MPa. Direct calibration at such high suctions requires the imposition of negative water pressures, which are difficult to achieve using facilities commonly available in soil mechanics laboratories. For this reason, tensiometers are usually calibrated in the positive pressure range, and such calibration is subsequently extrapolated to negative pressures. This paper examines different experimental techniques to assess the accuracy of such extrapolation. Any error in the calibration process would be directly reflected in the measured values of suction, and might be particularly significant (in relative terms) for the measurement of low suctions. In addition, the results of this study show that calibration in the positive range is affected both by the physical configuration of the tensiometer during calibration and by aspects of its design. The paper concludes that linear extrapolation of the calibration from the positive to the negative range is sufficiently accurate provided that calibration is done under conditions that closely match the conditions in which the tensiometer will be used. Owing to structural differences between tensiometers, and also to suction induced ‘calibration hysteresis’, at least one check on the accuracy of the extrapolated calibration equation over a range of negative pressure should be performed, even if at low values of suction.

- by Domenico Gallipoli and +1
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- Numerical Modeling, Unsaturated soil, Geotechnical Engineering, Genetic Algorithms
- by Jie Li
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- Unsaturated Soils, Expansive soilss

The paper presents a bounding surface model that describes the gradual yielding of unsaturated soils subjected to isotropic loads. The model originates from consideration of the capillary bonding between soil grains, which leads to the definition of a “unified normal compression line” that is valid in both saturated and unsaturated conditions. This line has the same slope and intercept of the saturated normal compression line but is formulated in terms of a “scaled stress” variable, which takes into account the mechanical effect of capillarity by factoring the average skeleton stress (also known as Bishop’s stress) with a power function of degree of saturation. The normal compression behaviour of unsaturated soils is therefore described by only one additional parameter, which is the exponent of the degree of saturation in the scaled stress expression. For over-consolidated soils, the occurrence of gradual yielding is introduced by assuming that, as the soil state moves towards the unified normal compression line, the slope of the loading curve tends towards the slope of the unified normal compression line according to an expression requiring only one extra parameter. Interestingly, this expression can be integrated in a closed form to provide a general equation for all loading paths in saturated and unsaturated conditions. Different loading curves are simply distinguished by the different values of the integration constants. Unloading paths are also simulated in a similar way. The proposed model requires a total of five parameters, which include the three standard parameters for saturated soils (i.e. the slope and intercept of the saturated normal compression line and the slope of the swelling line) plus one parameter to describe unsaturated behaviour and one parameter to describe the gradual yielding of over-consolidated soils subjected to loading.

- by Domenico Gallipoli
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- Civil Engineering, Soil Science, Unsaturated soil, Geotechnical Engineering

The paper presents a soil water retention model that takes into account the effects of void ratio and hydraulic hysteresis on the variation of degree of saturation. Based on a modified form of van Genuchten equation, the model defines two bounding surfaces, i.e. a main drying surface and a main wetting surface, which delimit the region of admissible soil states in the space of degree of saturation, suction and void ratio. Suction and void ratio are then combined into a single auxiliary variable, named scaled suction, and the main surfaces are recast as main curves in the plane of degree of saturation and scaled suction. The effects of both suction and void ratio on the drying/wetting behaviour of the soil are simply incorporated by relating degree of saturation to scaled suction. The soil is dried when the scaled suction is increased and is wetted when the scaled suction is decreased. The model assumes that, inside the region of admissible soil states, the derivative of degree of saturation with respect to the scaled suction depends on the distance of the soil state from the main curves. This assumption ensures a smooth transition of the drying and wetting paths towards their respective main curves. Interestingly, the derivative of degree of saturation with respect to scaled suction can be integrated in a closed form and all wetting and drying paths can therefore be described by two explicit equations (one for drying paths and one for wetting paths), where different wetting or drying paths are characterized by different values of the integration constant. The integration of the model in a closed form facilitates its implementation into numerical codes. The model requires seven parameters, whose values can be obtained from a single drying-wetting test. Predictions are validated against two different data sets published in the literature, which shows the capability of the model to capture the behaviour observed during laboratory tests on fine grained soils.

- by Domenico Gallipoli
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- Agricultural Engineering, Civil Engineering, Hydrogeology, Soil Science

Compacted clay fills are generally placed at the optimum value of water content and, immediately after placement, they are unsaturated. Wetting might subsequently occur due, for example, to rainfall infiltration, which can cause volumetric deformation of the fill (either swell or collapse) with associated loss of shear strength and structural integrity. If swelling takes place under partially restrained deformation, due for example to the presence of a buried rigid structure or a retaining wall, additional stresses will develop in the soil and these can be detrimental to the stability walling elements and other building assets. Factors such as dry density, overburden pressure, compaction water content and type of clay are known to influence the development of stresses. This article investigates these factors by means of an advanced stress path testing program performed on four different clays with different mineralogy, index properties and geological histories.
Specimens of kaolin clay, London clay, Belfast clay and Ampthill clay were prepared at different initial states and subjected to “controlled” wetting whereby the suction was reduced gradually to zero under laterally restrained conditions (i.e. Ko conditions). The results have shown that the magnitude of the increase in horizontal stresses (and therefore the increase of Ko) is influenced by the overburden pressure, compaction water content, dry density at the time of compaction and mineralogy.

- by Vincent Martin and +3
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- Unsaturated Soils, Geotechnical

The net stress plus suction and the average skeleton stress plus modified suction are two alternative sets of energetically consistent stress variables for modelling the hydro-mechanical behaviour of unsaturated soils. When used in conjunction with their work-conjugate strains, both sets of stress variables correctly calculate the first-order term of the hydro-mechanical work input into a soil element subjected to infinitesimal changes of deformation and water content. They therefore also correctly calculate the increment of internal energy along a given stress-strain path, that is the integral of the first-order term of the infinitesimal work input. This paper shows, however, that the above two sets of stress variables lead to different values of the second-order term of the hydro-mechanical work input. They are therefore no longer equivalent with respect to other aspects of material behaviour governed by the second-order work such as the flow rule of elasto-plastic models. The flow rule assumes the normality between plastic strains and equipotential surfaces defined in the conjugate stress-strain space. This normality is however lost when an elasto-plastic model originally formulated in terms of net stress plus suction is recast in terms of average skeleton stress plus modified suction (or vice versa) by means of standard mapping relationships between stress variables. To restore normality in both stress spaces, it is necessary to impose specific forms of elastic and plastic behaviour.

- by Domenico Gallipoli and +1
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- Civil Engineering, Soil Science, Materials Science, Thermodynamics

Seven teams took part in a benchmarking exercise on selection of parameter values for the Barcelona BasicModel (BBM) from experimental data on an unsaturated soil. All teams were provided with experimental results from tests performed on a compacted soil in order to determine values for the ten BBM soil constants and an initial value for the hardening parameter. The coordinating team then performed simulations (at stress point level) with the 7 different sets of parameter values, in order to explore the implications of the differences in parameter values and hence to investigate the robustness of existing BBM parameter value selection procedures. The major challenge was found to be selection of values for the constants λ(0), r, β, N(0) and pc and an initial value for the hardening parameter p0(0), with the various teams proposing significantly different values for some of these key parameters. A key lesson emerging from the exercise is the importance of choosing a method for selecting values for the parameters β and pc which places the main emphasis on attempting to optimise the match to the experimental spacing of normal compression lines at different values of suction.

- by Domenico Gallipoli
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- Civil Engineering, Soil Science, Unsaturated soil, Geotechnical Engineering

Temperature Effects on Suction Characteristic Curve of Bentonite‐Sand Mixtures. [ASCE Conference Proceedings 189, 108 (2006)]. YF Arifin, SS Agus, T. Schanz. Abstract. This study focused on the effects of temperature on the ...

- by Yulian Firmana Arifin
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- Chemistry, Unsaturated Soils
- by Domenico Gallipoli
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- Unsaturated Soils
- by Vincenzo De Gennaro
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- Unsaturated Soils
- by Naser Al-shayea
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- Unsaturated Soils
- by Juan Jorge Muñoz and +2
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- Unsaturated Soils
- by Antonio Gens
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- Environmental Science, Unsaturated Soils

The occurrence of unsaturated soils in the field has been well documented worldwide by measurements of the degree of saturation on samples taken during geotechnical investigations. On the other hand, the suction of unsaturated soils in the field, especially as part of long-term measurements, is documented very rarely and references on the subject are very few and for very few places around the world. The scarceness of this kind of measurements denies researchers perception of the anticipated suction and its possible loss or retention as a result of climatic conditions, especially in countries with warm temperate climate. Suction measurements from temporary and permanent stations in Greece and Cyprus are presented in the paper. From these-admittedly few-measurements until today, the large magnitude of suction that may occur both during summer and winter is presented, along with the possibility of long periods of these suctions being maintained. Another consequence is the range of suction values that sensors should be able to measure in regions of similar climatic conditions in order to cover the range of expected values as obtained from the measurements available so far.

- by Paul Michaels
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- Damping, Comparative studies, Unsaturated Soils, Wave propagation
- by Jorge Zornberg
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- Unsaturated Soils
- by Michael Bardanis and +1
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- Unsaturated Soils

The filter paper method is probably the simplest of the methods available for estimating a wide range of soil suction. The method is highly dependent on the suction calibration curve employed (i.e., the water retention or characteristic curve of the filter paper). Several calibration functions for ashless filter paper have been published in the literature, and significant discrepancy exists among them. Most of the functions are called “bilinear” (i.e., two different equations). The possibility of using a smooth function representing the best-fit curve for fictional points obtained from seven published calibration curves is evaluated. The suggested function incorporates a power law and a logarithmic dependence of water content on suction as observed in most published soil water retention functions. The variability of the suggested function compared to the seven published calibration curves for Whatman 42 filter paper for different levels of suction is quantified

- by Adelmo Bertolde and +1
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- Unsaturated Soils

In high suction tensiometers, cavitation occurs when the tension in the water inside the device exceeds a certain threshold. The threshold corresponds to the formation of air bubbles either by air entry into or formation within the porous stone or reservoir. The standard procedure to inhibit air bubble formation in
the device is to flood under vacuum and apply water at high pressure to force any remaining air bubbles to dissolve. A series of factors are, however, likely to influence the cavitation behaviour of high suction tensiometers. This paper demonstrates that: (1) there are no precursory phenomena leading to cavitation, (2) higher pressures are not essential for saturation, (3) cooler temperatures and tensiometer usage decrease the pore water pressure at cavitation, and (4) high suction tensiometers record increasing pressures when plunged dry in free water. The findings provide useful guidance to researchers using these devices for suction measurement.

- by Domenico Gallipoli
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- Civil Engineering, Unsaturated soil, Geotechnical Engineering, Soil Mechanics

The main objective of the current research is to analyze through the use of elemental simulations, unsaturated tests executed on soils with metastable characteristics (Brasilia porous clay), which due to high porosity and kind of cementation, presents a structure temporally unstable when undergoing an increasing of wetting and/or variation of the stress state, resulting in the most of cases in a significant change of volume denominated collapse. The tests were done in the Geotechnical Lab from the University of Brasilia and Geological Institute from Charles University in Prague, considering characterization tests for the identification of the basic properties, tests for determination of the characteristic curve, consolidation, direct shear and triaxial, in both conditions saturated and unsaturated, as well as the porosimetry test, for the study of the mechanical behavior. The tests were reproduced using numerical modeling to elemental level, involving a hypoplastic model for the unsaturated soils behavior.

- by René Salgueiro
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- Materials Science, Unsaturated Soils
- by Buddhima Indraratna
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- Prediction, Behaviour, Unsaturated Soils, Vegetation
- by H. Ghasemzadeh
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- Geomechanics, Unsaturated Soils
- by Jubert Pineda
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- Unsaturated Soils

This article presents an overview of hydrological studies undertaken and published by a Senior Scientist working at National Institute of Hydrology (A Government of India Society under Ministry of Jal Shakti), Roorkee-247667 (Uttarakhand), India. It covers a wide variety of research outcomes related to groundwater assessment; seawater intrusion in coastal aquifers; numerical modelling of unsaturated flow, groundwater flow and contaminant transport; management of aquifer recharge; and impact of climate change on groundwater etc.

- by C. P. Kumar
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- Hydrogeology, Climate Change, Hydrology, Water resources

Understanding the settlement behavior of different types of soil is important for almost every geotechnical structure. Many efforts have been done in the field of geotechnical engineering to evaluate and theorize the consolidation behavior of the soil in both saturated and dry conditions. However, similar efforts in the case of unsaturated soils are fairly limited. This research details the developed theories for the long-term deformation of unsaturated soils with a particular emphasis on the importance of flow law and elasticity at different phases of the soil. The state of the art in the field of Multiphysics consolidation is summarized in this paper. A series of the widely used mechanical concepts for unsaturated consolidation disintegrated in this study and elaborated in detail. This discretization of the current theories allowed further investigation of the pros and cons of each theory. A critical review of the literature assessed the applicability of the unsaturated consolidation theories in solving real-world problems.

- by David Toll
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- Unsaturated Soils

The paper gives a brief description of double wall cell triaxial equipment for testing soil samples under unsaturated conditions and presents results of some of the calibrations necessary to achieve accurate measurements of inner cell water volume, pore water volume, pressure, load and displacement. The calibration of the measurement devices showed linear relationships between the raw readings and applied values with the regression constants differ from 1 and 0. The result of apparent volume change calibrations showed significant correlation between cell water volume fluctuation and temperature fluctuation inside testing room. A correction to the measured volume was therefore required to reduce the effects of long-term temperature fluctuation on the measured volume. Investigation of the accuracy of volume change measurement with the double wall cell showed excellent matching between the inner cell volume change and pore water volume change under saturated conditions, indicating the high reliability of the double wall system for apparent volume change measurements.