Sai K. Vanapalli

Bio: Sai K. Vanapalli is an academic researcher from University of Ottawa. The author has contributed to research in topics: Expansive clay & Soil water. The author has an hindex of 34, co-authored 162 publications receiving 4557 citations. Previous affiliations of Sai K. Vanapalli include University of Texas at Arlington & University of Saskatchewan.

Model for the prediction of shear strength with respect to soil suction

Abstract: Experimental studies on unsaturated soils are generally costly, time-consuming, and difficult to conduct. Shear strength data from the research literature suggests that there is a nonlinear increas...

The influence of soil structure and stress history on the soil-water characteristics of a compacted till

Abstract: The soil–water characteristic defines the relationship between the soil suction and gravimetric water content, w, or the volumetric water content, θ, or the degree of saturation, S. Theoretical and...

Review of collapse triggering mechanism of collapsible soils due to wetting

Abstract: Loess soil deposits are widely distributed in arid and semi-arid regions and constitute about 10% of land area of the world. These soils typically have a loose honeycomb-type meta-stable structure that is susceptible to a large reduction in total volume or collapse upon wetting. Collapse characteristics contribute to various problems to infrastructures that are constructed on loess soils. For this reason, collapse triggering mechanism for loess soils has been of significant interest for researchers and practitioners all over the world. This paper aims at providing a state-of-the-art review on collapse mechanism with special reference to loess soil deposits. The collapse mechanism studies are summarized under three different categories, i.e. traditional approaches, microstructure approach, and soil mechanics-based approaches. The traditional and microstructure approaches for interpreting the collapse behavior are comprehensively summarized and critically reviewed based on the experimental results from the literature. The soil mechanics-based approaches proposed based on the experimental results of both compacted soils and natural loess soils are reviewed highlighting their strengths and limitations for estimating the collapse behavior. Simpler soil mechanics-based approaches with less parameters or parameters that are easy-to-determine from conventional tests are suggested for future research to better understand the collapse behavior of natural loess soils. Such studies would be more valuable for use in conventional geotechnical engineering practice applications.

Comparison of Different Procedures to Predict Unsaturated Soil Shear Strength

Abstract: Several procedures have been proposed in the recent years to predict the shear strength of an unsaturated soil. The soil-water characteristic curve has been used as a tool either directly or indirectly in the prediction of the shear strength along with the saturated shear strength parameters in these procedures. This paper provides comparisons between the measured and predicted values of unsaturated shear strength using these procedures for three soils both for limited and large suction ranges. The three soils used in the study for comparisons have different gradation properties, percentages of clay and plasticity index, Ip values. The advantages and limitations associated with predicting the shear strength of unsaturated soils using the procedures is discussed in the paper. INTRODUCTION A theoretical framework for unsaturated soil mechanics that parallels saturated soil mechanics is available in terms of stress state variables, namely; net normal stress, (σn ua), and matric suction, (ua uw) where σn is the normal stress, ua is the pore-air pressure an uw is the pore-water pressure. (Fredlund and Rahardjo, 1993). The framework is based on experimental studies that are costly and time consuming. Several advancements have been made in the prediction of the engineering behavior of unsaturated soils in recent years. The soil-water characteristic curve has been found to be a useful tool in the estimation of engineering properties for unsaturated soils. Examples are the coefficient of permeability and the shear strength functions. Shear strength forms an important engineering property in the design of numerous geotechnical and geo-environmental structures such as earth dams, retaining walls, pavements, liners, covers, etc. Several procedures have been proposed in the literature during the past five years to predict the shear strength of an unsaturated soil. __________________________________________________________________________ Department of Civil Engineering, University of Saskatchewan, SK, Canada, S7N 5A9 These procedures use the soil-water characteristic curve as a tool either directly or indirectly along with the saturated shear strength parameters, c’ and φ’, to predict the shear strength function for an unsaturated soil (Vanapalli et al. 1996, Fredlund et al. 1996, Oberg and Sallfors 1997, Khallili and Khabbaz 1998 and Bao et al. 1998). The philosophy used in each of the prediction procedures proposed by these investigators is different. Comparisons have been provided between predicted and measured values of shear strength for a limited suction range for various soils (i.e., between 0 to 500 kPa). Escario and Juca (1989) measured the soil-water characteristic curves and the shear strength of three soils prior to the time when any proposals had been made for the shear strength functions. The three soils have different gradation properties, percentages of clay and plasticity indices, Ip. These results are used in this paper to provide comparisons between the predicted and measured shear strength values both for a limited suction range and a large suction range. The study presented in the paper highlights the advantages and limitations associated with the various procedures for predicting the shear strength of unsaturated soils. The simple procedures discussed in this paper are of value in bringing the shear strength theories for unsaturated soils into engineering practice. EQUATIONS FOR INTERPRETING THE SHEAR STRENGTH OF UNSATURATED SOILS Bishop (1959) proposed shear strength equation for unsaturated soils by extending Terzaghi’s principle of effective stress for saturated soils. Bishop’s original equation can be arranged as shown below. ( ) ( ) ( )( ) [ ] ' tan ' tan ' φ χ φ σ τ w a a n u u u c − + − + = [1] where: τ = shear strength of unsaturated soil, c’ = effective cohesion, φ’ = angle of frictional resistance, (σn ua ) = net normal stress, (ua uw ) = matric suction, and χ = a parameter dependent on the degree of saturation The value of χ was assumed to vary from 1 to 0, which represents the variation from a fully saturated condition to a total dry condition. Several investigators found limitations with respect to the quantification of the parameter χ both theoretically and experimentally. Fredlund et al. (1978) have proposed a relationship to explain the shear strength of unsaturated soils in terms two independent stress state variables as shown below: ( ) ( ) b w a a n u u u c φ φ σ τ tan ' tan ' − + − + = [2] The shear strength contribution due to matric suction, φ , was initially assumed to be linear based on the analysis of limited results published in the literature. Later experimental studies performed over a large range of suction values have shown that the variation of shear strength with respect to soil suction is nonlinear (Gan et al. 1988 and Escario and Juca 1989). Equation [1] can be applied for both the linear and non-linear variation of shear strength with respect to suction. Figure 1. Typical soil-water characteristic curve showing zones of desaturation. The Relationship between the Soil-Water Characteristic Curve and the Shear Strength of Unsaturated soils The soil-water characteristic curve defines the relationship between the soil suction and either the degree of saturation, S, or gravimetric water content, w, or the volumetric water content, θ (Figure 1). The soil-water characteristic curve provides a conceptual and interpretative tool by which the behavior of unsaturated soils can be understood. As the soil moves from a saturated state to drier conditions, the distribution of the soil, water, and air phases change as the stress state changes. A typical soil-water characteristic curve with various zones of desaturation are shown in Figure 1. The wetted area of contact between the soil particles decreases with an increase in the soil suction. There is a relationship between the rate at which shear strength changes in unsaturated conditions to the wetted area of water contact between the soil particles or aggregates. In other words, a relations hip exists between the soil-water characteristic curve and the shear strength of unsaturated soils. Different Procedures for Predicting the Shear Strength of an Unsaturated Soil Vanapalli et al. (1996) and Fredlund et al. (1996) have proposed a more general, nonlinear function for predicting the shear strength of an unsaturated soil using the entire soilwater characteristic curve (i.e., 0 to 1,000,000 kPa) and the saturated shear strength parameters as shown below: ( ) [ ] ( ) ( )( ) { } [ ] ' tan ' tan ' φ φ σ τ κ Θ − + − + = w a a n u u u c [3] where: κ = fitting parameter used for obtaining a bestfit between the measured and predicted values, and Θ = normalized water content, θw/θs. The shear strength contribution due to suction constitutes the second part of [Eq. 3], which is: ( ) ( )( ) { } [ ] ' tan φ τ κ Θ − = w a us u u [4] Equation [3] can also be written in terms of degree of saturation, S, or gravimetric water content, w, to predict the shear strength yielding similar results. The entire soil-water characteristic curve data (i.e., 0 to 1,000,000 kPa) is required along with the saturated shear strength parameters in the use of Equation [3]. A bestfit soilwater characteristic curve can be obtained in terms of a, n, and m parameters using the equation proposed by Fredlund and Xing (1994) which is shown below:

Volume Change Behaviors of Expansive Soils Stabilized with Recycled Ashes and Fibers

Abstract: In this paper, class F fly ash, bottom ash, polypropylene fibers, and nylon fibers were evaluated as potential stabilizers in enhancing volume change properties of sulfate rich expansive soils. As a part of the research evaluation, a comprehensive laboratory experimental program was designed and conducted on two different subgrade soils from two locations in Texas. Four dosage levels of each stabilizer, two compaction moisture content levels, and 14 days curing period were investigated. Volume change behavioral tests including volumetric free swell, volumetric shrinkage strain, and vertical swell pressure tests were conducted on both isolated stabilizer treated and combined ash-fiber stabilized soils. Ash stabilizers showed improvements in reducing swelling, shrinkage, and plasticity characteristics by 20–80% whereas fibers treatments resulted in varied improvements. In combined treatments, class F fly ash mixed with nylon fibers was the most effective treatment on both Dallas and Arlington soils, where the soil property enhancements were considered average-to-moderate. Possible mechanisms that resulted in the soil property improvements are discussed along with the recommended stabilizers and their dosages for expansive soil treatments.

Suction Stress Characteristic Curve for Unsaturated Soil

Abstract: The concept of the suction stress characteristic curve (SSCC) for unsaturated soil is presented. Particle-scale equilibrium analyses are employed to distinguish three types of interparticle forces: (1) active forces transmitted through the soil grains; (2) active forces at or near interparticle contacts; and (3) passive, or counterbalancing, forces at or near interparticle contacts. It is proposed that the second type of force, which includes physicochemical forces, cementation forces, surface tension forces, and the force arising from negative pore-water pressure, may be conceptually combined into a macroscopic stress called suction stress. Suction stress characteristically depends on degree of saturation, water content, or matric suction through the SSCC, thus paralleling well-established concepts of the soil–water characteristic curve and hydraulic conductivity function for unsaturated soils. The existence and behavior of the SSCC are experimentally validated by considering unsaturated shear strength data for a variety of soil types in the literature. Its characteristic nature and a methodology for its determination are demonstrated. The experimental evidence shows that both Mohr–Coulomb failure and critical state failure can be well represented by the SSCC concept. The SSCC provides a potentially simple and practical way to describe the state of stress in unsaturated soil.

A unique relationship for χ for the determination of the shear strength of unsaturated soils

Abstract: In this note, a unique relationship between the effective stress parameter chi and the ratio of suction over the air entry value (suction ratio) is proposed, based on the shear strength data reported in the literature. Comparisons are made between the measured and the calculated shear strengths for two laboratory prepared unsaturated soils. Extremely good agreement has been obtained between the measured and the calculated values using the proposed relationship. (A)

Unsaturated Soil Mechanics in Engineering Practice

Abstract: Unsaturated soil mechanics has rapidly become a part of geotechnical engineering practice as a result of solutions that have emerged to a number of key problems (or challenges). The solutions have emerged from numerous research studies focusing on issues that have a hindrance to the usage of unsaturated soil mechanics. The primary challenges to the implementation of unsaturated soil mechanics can be stated as follows: (1) The need to understand the fundamental, theoretical behavior of an unsaturated soil; (2) the formulation of suitable constitutive equations and the testing for uniqueness of proposed constitutive relationships; (3) the ability to formulate and solve one or more nonlinear partial differential equations using numerical methods; (4) the determination of indirect techniques for the estimation of unsaturated soil property functions, and (5) in situ and laboratory devices for the measurement of a wide range of soil suctions. This paper explains the nature of each of the previous challenges and...