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A method for an a priori estimation of the frost heave ratio for clayey soils, considering the degree of their water saturation upon a closed system of freezing, is proposed. The proposed approach involves the introduction of a new parameter in the mechanics of frozen soils, namely, the frost heave index of clayey soils, which characterizes the degree of frost heaving of soil relative to the maximum height of the frost heave of soil that is fully saturated with water. An introduced parameter is used in the final formula to estimate the frost heave ratio of clayey soils. Laboratory experiments were performed to determine the height of frost heaving of clayey soils with a variation in their moisture and porosity to verify the proposed approach. A comparison of the experimental and calculated results showed a suitable convergence sufficient for the practical use of the formula to determine the frost heave ratio in a closed system of freezing. An example of a calculation to justify antiheaving measures using the proposed estimation of the frost heave ratio is given. [ABSTRACT FROM AUTHOR]

A soil water unit weight of 9.8 kN/m3 has been universally considered to quantify soil volumetric phase properties such as void ratio and degree of saturation, but has been found to greatly vary depending on soil type and the volume scale with which it is defined. Recent experimental and theoretical evidence has indicated that the unit weight of soil water can be significantly greater than 9.8 kN/m3 for clayey soils when gravimetric water content is less than 30%. A soil water unit weight as high as 18.8 kN/m3 is evident for some expansive soils at low water content. The significance of abnormally high water unit weight in quantifying soil phase volumes, saturation, and void ratio was experimentally assessed for various clayey soils and theoretically interpreted as a function of water content and soil type. For clayey soils with low liquid limit, average soil water unit weight can be as high as 12.5 kN/m3 for gravimetric water content less than 10%. This leads to an overestimation of liquid-phase saturation and void ratio by as much as 8% if a soil water unit weight of 9.8 kN/m3 is used. For clayey soil with a high liquid limit, the average soil water unit weight can be as high as 18.8 kN/m3 for water content less than 18%, leading to overestimation of liquid-phase saturation by as much as 36% and void ratio by as much as 20% if 9.8 kN/m3 is used. Charts were developed to estimate average soil water unit weight as a function of soil specific surface area and water content, and as a function of liquid limit and water content. The commonly used value of 9.8 kN/m3 for water unit weight can lead to significant errors in estimating phase volumes, void ratio, and saturation for clayey soils. [ABSTRACT FROM AUTHOR]

Pore-water pressure in soil is caused by three physically distinguishable sources: ambient (environmental) pressure, surface tension–induced capillary pressure, and the soil's electromagnetic potential–induced adsorptive pressure. The former two form the conventional concept of pore-water pressure, which is considered a constant within a soil-water-air representative elementary volume and can be directly measured by piezometer (under saturated and compressive states) or tensiometer (under unsaturated and tensile states). The third one can be called adsorption-induced pore-water pressure and is localized within a certain distance to the particle surface of soil or intercrystalline surface of swelling clay. The adsorption-induced pore-water pressure is always compressive and dictates the water phase transition in soil by altering water's freezing point, density, and viscosity, among other physical properties. A framework of quantifying the adsorption-induced pore-water distribution via the measured soil water isotherm is presented for any soil type under any given water content. It is demonstrated that the adsorption-induced pore-water pressure can be up to 1.6 GPa in the first few layers of hydration, but will diminish to zero at a distance equivalent to the gravimetric water content >1% for sandy soil and greater than a few percent for silty soil. In clayey soil, the adsorption-induced pore-water pressure can sustain tens of megapascals even at much farther distance, equivalent to ∼30% water content. In expansive clay, the adsorption-induced pore-water pressure inside the crystalline lamellae can exceed 800 MPa. The soil water density functions of a silty soil and a bentonite clay predicted by the proposed framework matched well with that measured independently from the conventional consolidation testing, validating the framework to determine the spatial distribution of the adsorption-induced pore-water pressure in soil. [ABSTRACT FROM AUTHOR]

Smart irrigation is one application of digital agriculture that can be used to achieve improved crop yields while saving water and energy. Many variations of smart irrigation can be applied, but there is no consensus among the scientific community as to which method is the best practice. The objective of this research was to evaluate two smart irrigation methods: a soil moisture–based method using sensors and an evapotranspiration- (ET-) based technique. The selected crop was sweet corn. Growth variables (biomass, yield, harvest index, and water productivity) were compared for each of the two methods. Three irrigation regimes were applied for each method: 60%, 90%, and 120% of crop evapotranspiration (ETc) for the ET-based method, and 25%, 30%, and 35% of the total soil moisture for the soil moisture–based method. Corn sap flow was measured in response to the three treatments in the soil moisture–based experiment to measure transpiration. The results showed that the ET-based method is easier to implement with less infrastructure, and it can result in similar yields compared to the soil moisture–based method. Although the fresh yield was 16% higher using the soil moisture–based method, grain yield can be sustained with the ET-based method using 8% less water. Selection of an appropriate irrigation scheduling method should be based on marketable yield: keeping the soil water content near field capacity will result in a higher fresh yield but will not translate into more dry matter. If grain yield is the target, the ET-based method is less expensive and more farmer friendly, but care should be taken to properly estimate irrigation losses to avoid underirrigation. An evaporative stress index (determined using sap flow measurements) of 70% will cause a 22% reduction in fresh sweet corn yield but only a 7% reduction in grain yield. Transpiration peaked when the soil moisture was above 85% of the available water (i.e., at 38% in the calcareous clay soil of the experiment). [ABSTRACT FROM AUTHOR]

The curve number (CN) method is the most widely used method to estimate the depth of runoff resulting from a given depth of rainfall. However, there are still doubts about the effect of rainfall characteristics and soil moisture conditions on the method's parameters (CN, and the standard initial abstraction ratio, λ). In this study, design rainfall distributions were used along with Hortonian infiltration characteristics to calculate CN and λ values for a highly clayey soil (>80%). Prerainfall infiltration conditions were characterized by the measured initial soil moisture content, and Hortonian infiltration model was obtained from experimental infiltration test measurements. The main results show that there is no influence of initial infiltration rates on CN values, and rainfall duration generates substantial influence on CN. The CN is inversely related to rainfall depth, and λ is dependent on the initial soil moisture. Further, neither tabulated CN nor standard λ corresponds to the simulated values obtained in this case. Therefore, these findings suggest that the method's parameters should be specifically adapted to local hydrological conditions. [ABSTRACT FROM AUTHOR]

Frost heave of soil is a highly nonuniform problem, in which the local deformation is much more severe and meaningful compared with the overall deformation. When soil is upon freezing, a water–ice phase change occurs and induces the migration of pore water. Subsequently, the distribution of temperature, water content, ice content, and fabric is highly nonuniform in frozen soil. The local frost deformation of soil mainly depends on the local temperature, soil water content, and ice content. In this study, the particle image velocimetry (PIV) method is introduced into the soil column test to monitor the local frost deformation. Several tracer particles are tested to overcome the difficulty that the deformation field is not clear in clay soil. The results demonstrate that the mixed sand is a suitable choice of tracer particles and can lead to clear contours of vertical displacement and strain. Such local deformation information is valuable and can be synthetically analyzed with the local temperature, soil water content, and ice content. The PIV technique is considered to be a good choice for monitoring the local frost deformation field in frozen soil. [ABSTRACT FROM AUTHOR]

Desiccation of soil is the gradual process of moisture loss from a fully or partially saturated condition to a dry state. If the soil is constrained against shrinkage during desiccation, it will begin to crack when tensile stresses in the soil exceed the tensile strength. Fundamental to understanding desiccation crack development in clayey soils is the accurate determination of the tensile strength during desiccation. Although numerous published studies have focused on soil desiccation and soil tensile strength, very little has been focused on experimental determination of tensile strength during desiccation. This paper presents the first generation of a novel apparatus designed to determine tensile strength during desiccation. During drying the tensile forces in a thin compacted soil layer are measured while simultaneously monitoring moisture content changes and crack formation via time-lapse photography. Results are presented from desiccation tests performed on two clayey compacted soils of medium to high plasticity. Soil water characteristic curves (SWCCs) were determined to relate the tensile strength to suction. The results demonstrate that the tensile strength of compacted soil is influenced by initial water content, suction, dry unit weight, soil pore structure, and soil type. Measured tensile strengths compare favorably with results of a theoretical model that considers microstructural and macrostructural aspects of unsaturated compacted clayey soil. [ABSTRACT FROM AUTHOR]

he mechanical behavior of a sandy clay soil was investigated by a series of constant water content triaxial tests on unsaturated samples with suction measurements. The tests were carried out in double-cell triaxial cells on compacted samples and also on samples wetted and dried from the as-compacted conditions. A series of tests on saturated samples was also performed to provide a reference state for the unsaturated tests. Because the specimens were at high degrees of saturation (generally >80%), calculations based on effective stress showed a reasonable interpretation of the data for the critical state parameters M and λ. However, the intercept of the critical state line in the ν-axis (Γ) differed and increased with an increase in water content (at compaction). The data were also analyzed using the Bishop stress (p*) approach, and a better fitting was achieved in the stress plane, where it was possible to define a unique critical state line, where M was 0.91. However, in the ν-p* plane, different critical state lines were still obtained for different compaction water contents. The mechanical behavior of the sandy clay soil was found to be governed by the initial conditions, the as-compacted conditions. Although the critical state stress ratio, M, itself was not affected by the initial water content, the differences in volumetric behavior led to differences for samples subjected to wetting and drying after compaction. Samples that were dried back to a particular water content had a higher strength than samples compacted at that water content; in contrast, a sample wetted to achieve that water content had a lower strength. [ABSTRACT FROM AUTHOR]

The relationship between soil suction and degree of water saturation (also known as the soil-water characteristic curve or SWCC) for a given soil depends on a number of variables such as the soil type, mineral type, fluid type, pore size distribution, and anisotropy. In addition, the SWCC is found to exhibit a form of hysteresis, in that it is different in wetting and drying. Design of structures founded on unsaturated soils requires an in-depth microscopic understanding of the variables controlling SWCC and the underlying hysteresis. Most of the past basic studies have focused on granular materials, with solid particles represented by spheres. The present study is focused on clayey soils involving platelike solids. The analysis method of molecular dynamics (MD) is used to understand the nature of clay-water capillary meniscus. On the basis of the observations from the MD analysis and the Young-Laplace equation, a model is proposed for predicting SWCC and the associated hysteresis. The microscopic parameters needed for the model are obtained by analyzing the fabric of numerical specimens prepared by the discrete-element method (DEM) of analysis of an assembly of clay particles. The results are shown to be consistent with experimental observations. [ABSTRACT FROM AUTHOR]

This study evaluated the constituent make up of simulated rainwater runoff from Class C fly ash–stabilized and nonstabilized clay soil using laboratory test pads to assess the potential for impacts to surface water from the use of uncovered fly ash–stabilized soils as potential roadbed material. Recirculated runoff from test pads was sampled and tested during three simulated rainfall events over an 84-day trial period. All samples were analyzed for trace metals. Analytical results from the simulated runoff were screened to identify five indicator parameters in the runoff that were used as the basis for assessing potential environmental effects to surface waters. Runoff water results from fly ash–stabilized test pads for these indicator parameters were compared to water quality benchmarks. Based on the low concentrations measured in runoff relative to applicable criteria, and on the conservative nature of the experimental methods relative to typical field conditions, we concluded that surface runoff from fly ash–stabilized soil would not present significant adverse effects to surface water if used uncovered on low traffic exposed surfaces. [ABSTRACT FROM AUTHOR]

Volume changes due to wetting may occur in naturally deposited soils as well as earthen construction (e.g., compacted fills or embankments). Depending on the stress level, some soils exhibit increase in volume upon wetting (swell) while others may exhibit decrease in volume upon wetting (collapse). The work described in this paper focused on wetting-induced volume changes in compacted soils. Motivation for this work stemmed from observations of earthen structures that exhibit problematic behavior under wetting conditions, even though soils were compacted to engineering specifications (i.e., at or above minimum density and within moisture content ranges). Not only is this problematic behavior a concern but also the laboratory tests used to predict settlement of constructed facilities may not properly model the actual behavior of soil compacted under field conditions. For example, settlements experienced by compacted fills may be different from settlement predictions based on one-dimensional oedometer tests. These differences are partly related to the variations in the soil structure in tested specimens that arise because soil clods compacted in the laboratory are smaller than soil clods compacted in the field. The term “soil structure” includes the combined effects of soil fabric and interparticle forces. “Fabric” generally refers to the geometric arrangement of particles, whereas interparticle forces include physical and physicochemical interactions between particles. The soil structure in this case is associated with specimen preparation methods and is influenced by several factors including soil composition (including pore water chemistry), compaction method, clod sizes, initial moisture condition of clods, dry density or void ratio, and compaction moisture content. A laboratory research study was conducted to investigate the influence of variations in clod-size and structure on one-dimensional volume change, with emphasis on wetting-induced volume change, for nine different fine-grained soils. The results of the study suggest that the influence of structure in one-dimensional oedometer tests depends on soil type and nature of the clods in the compacted soil. Clayey soils appear to be influenced more by differences in structure, whereas silts or clayey sands of low plasticity (PI<10) do not appear to suffer as much from structure effects in one-dimensional oedometer tests. This is attributed to more extensive clod development in clayey soils. Furthermore, the moisture condition of clods appears to have an important influence on volume change behavior. [ABSTRACT FROM AUTHOR]

A theoretical expression for clay soil-water characteristic curves (SWCC) is developed for cases in which the osmotic potential is small relative to the matrix potential, for the range of water potential lower than the cavitation value, based on the following foundations. Soil structure consists of saturated soil pods containing parallel soil particles, and angular interpod macropores possibly containing both air and water. The pod water is adsorbed to the particle surfaces, and subject to adsorption while the macropore water is subject to capillary potential (commonly referred to as suction). It is suggested that the capillary potential cannot be lower than the cavitation potential of the capillary water; at lower potentials, the macropore water drains into the pods and the matrix potential is represented solely by the adsorption potential. The paper examines the hypothesis that the adsorption potential is dominated by van der Waal forces, and this forms the basis for the theoretical SWCC. While the theoretical curve reasonably fits experimental data from several soils, kaolin soils depart from it, and pure montmorillonites fit only over a range of low potential, indicating that van der Waal adsorption forces may control adsorption potential for some, but not for all soils. [ABSTRACT FROM AUTHOR]

This note describes a triaxial apparatus that applies liquid infiltration under automatically controlled boundary conditions while using the new Xeritron sensor to measure suction. Three different infiltration boundary conditions of interest including constant mean stress (CMS), constant volume (CV), and constant stiffness (CS) are applied to examine the influence of boundary conditions on the mechanical and hydraulic behavior of unsaturated clay materials. CMS and CV tests provide limits of infiltration boundary conditions while CS tests represent a flexible spring-type boundary condition. Spatial distribution of gravimetric water content and bulk density are measured to calculate dry density and degree of saturation following testing to determine internal changes to the specimens. The results from initial testing show that the apparatus is providing experimental evidence to evaluate unsaturated flow models and elastic-plastic constitutive models to examine the behavior of swelling clay soils under varying confinement conditions. [ABSTRACT FROM AUTHOR]

The present contribution reports on a mixed numerical experimental study of the time dependent processes in saturated soils with reference to isotropic consolidation. Owing to its relative simplicity, isotropic consolidation is often viewed as a basic tool for inferring the material parameters of various constitutive models. Within this modeling framework, the paper advocates the use of critical state models for an accurate description of the time dependent deformation of clayey soils. Material models, in which the critical state model applies only to the subcritical region, have been also addressed. Examples include models where the supercritical region is cut off, e.g., by a Mohr–Coulomb surface. Starting with the modified Cam clay model, as a classical representative, the paper revisits a simple isotropic consolidation test. It is suggested that such a conventional laboratory experiment, when combined with a suitable optimization procedure, is sufficient in deriving the desired material parameters, including the initial void ratio, deformation dependent variation of the coefficient of permeability and the preconsolidation pressure representing here an important measure of the structural strength of soils. Several numerical and experimental results are presented to support the theoretical grounds. [ABSTRACT FROM AUTHOR]

The soil–water characteristic curve (SWCC) of fine-grained soils is usually determined experimentally. In the design of mine waste covers and landfill liners, the unsaturated hydraulic conductivity function, k(h), is often derived theoretically from the measured SWCC. Implicit in these derivations is the transformation of the SWCC to a pore-size distribution (PSD), typically assumed to be constant and monomodal. However, PSD measurements of a clayey till compacted at various water contents after compaction, after flexible-wall permeability testing and before and after SWCC tests show that the PSD of the same material varies significantly under the stated physical conditions. Predictions of the SWCCs using PSDs measured both before and after the SWCC tests significantly underpredicted the values measured. By applying a simple transformation to the PSD to account for the scaling effect from the porosimetry samples (approximately 1 g dry weight) to the SWCC test samples (approximately 200 g dry weight), the predicted SWCCs were found to envelop the measured values. A simple model that simulates the change in PSD during the SWCC test predicted water contents close (1% root mean square error) to the measured SWCCs. [ABSTRACT FROM AUTHOR]

Tests were conducted to determine the variation of water content and pore water suction for compacted clayey soils. The soils had varying amounts of clay fraction with plasticities ranging from low to high plasticity. The unsaturated soil behavior was investigated for six conditions, covering a range of compactive efforts and water contents. The experimental data were fit to four commonly used models for the water content-pore water suction relationship. Each model provided a satisfactory fit to the experimental data. However, the individual parameters obtained from the curve fits varied significantly between models. The soil water characteristic curves (SWCCs) were more sensitive to changes in compaction effort than changes in compaction water content. At similar water contents, the pore water suction increased with increasing compaction effort for each compaction condition and soil type. For all compaction conditions, the lowest plasticity soils retained the smallest water content and the highest plasticity soils retained the highest water content at a specified suction. In addition, SWCCs for soils compacted in the laboratory and in the field were similar. [ABSTRACT FROM AUTHOR]

Although widely used for field measurements of soil moisture, problems continue with the use of the neutron probe in clay soils comprised of over 40% clay fractions, and in the determination of the intensity of field sampling necessary to characterize a site. We examine issues associated with field practice and calibration, data analysis and estimation of variance, and error in soil moisture measurements for silty clay soils in the Imperial Valley, California, using large calibration data sets from several different layouts of neutron access tubes. Our analysis indicates that for field calibrations: (1) bulk density corrections of the count ratio data do not necessarily improve the standard errors of the water content measurements; (2) soil sampling for calibration within the access tube hole at the time of tube installation results in smaller measurement errors; (3) calibrations improve as the average soil moisture of the profile at the time of the original sampling decreases; and (4) field-wide calibrations improve, and the sampling intensity required to characterize soil moisture content decreases with an increasing soil depth. We also address aspects of the neutron probe use associated with soil salinity and polyvinyl chloride (PVC) access tubing (chloride interference), inclusion of shallow sampling data in calibrations, and instrument, calibration, and spatially related standard variances affecting field measurements. [ABSTRACT FROM AUTHOR]

The article presents the authors' comments about the article "Parameters Affecting Soil-Water Characteristic Curves of Fine-Grained Soils," by Vikas K.S. Thakur, S. Sreedeep and Devendra N. Singh, published in the April 2005 issue of the journal. According to the authors, the findings presented in the article show that soil moisture suction in fine-grained soils such as clay soils is primarily governed by the physicochemical interactions between water and clay.

Presents information on a study which described soil-water characteristic curves for compacted clay barrier soils. Analysis of soil specimens; Equations to estimate parameters of soil-water characteristic curves. INSET: Closure, by James M. Tinjum et al.