Unsaturated Soils

The collapsible soils are localized in arid and semi-arid regions. Such soils are in nature unsaturated soils, which are characterized by a metastable structure and undergo an abrupt collapse when they are inundated with or without loading. These changes provoked a dangerous problem in the constructions. Several soil collapse classifications based on parameters such as water content, dry density, relative density, Atterberg limits and clay fraction have been proposed in the literature for the prediction of the collapse potential. Based on the use of the Proctor tests, the experimental study presented, about reconstituted soils in laboratory, allow the factors which influence collapse to be predicted confirmed with oedometer tests. The results obtained clearly show the effect of water content and the energy of compaction on the rate and magnitude of collapse. However, a new experimental approach to the prediction of collapsible soils is determined.

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.

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

The experimental work presented in the paper explores the soil-water characteristic curve and the evolution of tensile and unconfined compression strength of drying slurried soils. The axis translation technique and the saturated salt solutions method were employed for matric and total suction control respectively. Tensile strength was determined using the splitting tensile (brazilian) test.

RESUMEN La presencia de plaguicidas en aguas subterráneas y acuíferos representa una situación frecuente, especialmente en aquellas regiones que tienen un elevado consumo de plaguicidas (Pasquarell y Boyer, 1996). La hipótesis aceptada de que la fracción no saturada del suelo constituye una defensa efectiva contra la penetración de plaguicidas es por lo tanto cuestionable (Giuliano, 1995). Los modelos numéricos son herramientas que mejoran el conocimiento del transporte de pesticidas como atrazina y Acetoclor. Para determinar el nivel de contaminación del suelo y del agua debido al uso de productos agroquímicos, es necesario caracterizar tanto el movimiento del agua y el transporte de solutos en la zona vadosa, e identificar los parámetros característicos, así como ajustar el transporte de productos químicos a través de la zona no saturada. Hay una amplia variedad de instrumentos que pueden medir la humedad del suelo como las sondas de neutrones, tensiómetros, sensores de capacitancia, TDR, y otros. Se presentan los diferentes tipos de equipos y valores. Estos valores se compararon para definir los rangos de fiabilidad para las técnicas más simples y se utilizaron en los estudios de infiltración. Finalmente los resultados de los modelos matemáticos se compararon con los valores medidos en el campo. ABSTRACT The presence of pesticides in groundwater and aquifers is a common situation, especially in those regions which have a high consumption of pesticides (Pasquarell and Boyer, 1996). The accepted hypothesis that the unsaturated soil fraction is an effective defense against the penetration of pesticides is therefore questionable (Giuliano, 1995) by. Numerical models are essential tools to improve knowledge of the unsaturated in relation to transport of pesticides such as atrazine and Acetochlor area. This requires obtaining soil moisture data field that can be purchased with capacitance sensors (requiring prior calibration), which it will be used to validate the numerical model parameters. To determine the level of contamination of soil and water due to the use of agrochemicals, it is necessary to characterize both the movement of water and solute transport in the vadose zone, and identify the characteristic parameters and adjust the transportation of chemicals through the unsaturated zone. There is a wide variety of instruments that can measure soil moisture neutron probes including, tensiometers, capacitance sensors, TDR, and others. different types of devices and valuesare presented. These values were compared to define ranges for reliability and simpler techniques used in the studies infiltration. At the end the results of the mathematical models were compared with the measured field values. PALABRAS CLAVES: medición de humedad en suelo, estudios de campo, sensores y sonda de neutrones

Soil water retention curves were determined using four different techniques for suction measurement. Transistor psychrometer and non‐contact filter paper techniques were used for total suction determinations; pressure plates and in‐contact filter paper techniques were used for matric suction measurements. The soil water retention curves (WRC) were determined for bentonite, speswhite kaolin and a mixture of kaolin and sand (70 percent: 30 percent by mass). Specimens were prepared at their liquid limits and the WRC were ...

The authors have presented a new suction control system for unsaturated soils with independent drying and wetting systems and direct measurement of suction. This new technique has several advantages: (1) as stated by the authors "the principal advantage of these systems is that high suction can be controlled without raising the ambient air pressures as is required with the axis-translation technique"; (2) it measures suction directly by means of a high suction tensiometer able to reach 1500 kPa while, for example, techniques based on the relative humidity equilibrium of the system calculate suction indirectly via a thermodynamic relation with the water vapour pressure (e.g. ; (3) the suction range (up to 1500kPa depending on the tensiometer) provided by this new technique is of most

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.

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.

Tailing deposits are singular geotechnical structures, which are intended to last indefinitely in time. One of the most important challenges in relation to such waste deposits is to predict their behaviour in the long term, e.g. several centuries. In this context, it seems useful to observe the behaviour of old abandoned tailings deposits. Since tailings are very fine grained materials, the unsaturated phenomena play a significant role in the hydraulic and mechanical behaviour of the deposit. In this paper, some examples of old abandoned tailing deposits are examined, located in low precipitation regions. The in situ measured water content distributions show that, the degree of saturation remains relatively high, even though the low rain amounts and high evaporation rates. It is concluded that the evolution of water content should be taken into account in the deposit design process. management of the decant pond. Moreover, the severity of the consequences of failure is also related to tailings water content. Catastrophic mud avalanches developed only in high water content tailings or when special circumstances determined that large amounts water entered the deposit.

To better understand the behaviour of unsaturated clays for retaining wall design in Adelaide, South Australia, a trial soldier pile retaining wall supporting an 8 m deep excavation was constructed for a proposed 3 km long underpass. The full scale trial retaining wall was constructed prior to the start of summer to allow the soil to shrink over an extended dry period, typical of an Adelaide summer. At the end of summer, the soil was wetted up via a controlled inundation program that replicated a pipe burst or leaking water main, critical considerations for retaining wall design. As part of an extensive testing and monitoring program, in situ moisture probes were used to quantify the change in soil moisture content before, during and after inundation. This paper presents a practical means to estimate soil suction based on the calibration of moisture probe data and the use of a site specific soil-water characteristic curve. Limitations of using moisture probes to infer suction are discussed, with suggestions provided for the future use of similar moisture probes in very stiff to hard clay soils as encountered on this site.

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.

The stability of civil engineering constructions is a statically indeterminate problem when they were built on collapsible soils. These soils present significant changes due to sudden decrease in volume after wetting with or without loading. This paper is aimed at studying the treatment by cement of a collapsible soil, which was reconstituted in laboratory. Oedometer tests were carried out by using the double consolidation method. The main aim of this experimental study is to investigate the influence of this kind of treatment and the initial conditions on the collapse of the soils. The results obtained show that it is possible to reduce the soil collapse potential to an acceptable level by adding low cement contents to a test specimen at various initial water contents and under the effect of several compacting energies. These experimental results contribute in solving the problems of soil collapse by treating to enhance its resistance.