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.

Suction Stress Characteristic Curve for Unsaturated Soil

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

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)

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

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...

http://keshavarz.persiangig.com/document/Fredlund2006.pdf

Unsaturated Soil Mechanics in Engineering Practice

[1] We propose that the recently conceptualized suction stress characteristic curve represents the effective stress for the shear strength behavior of unsaturated soil. Mechanically, suction stress is the interparticle stress called tensile stress. The working hypothesis is that the change in the energy of soil water from its free water state is mostly consumed in suction stress. We demonstrate that the suction stress lies well within the framework of continuum mechanics where free energy is the basis for any thermodynamic formulation. Available experimental data on soil water characteristic curves and suction stress characteristic curves are used to test the hypothesis, thus validating a closed-form equation for effective stress in unsaturated soil. The proposed closed-form equation is intrinsically related to the soil water characteristic curve by two pore parameters: the air entry pressure and pore size spectrum number. Both semiquantitative and quantitative validations show that the proposed closed-form equation well represents effective stress for a variety of earth materials ranging from sands to clays. Of important practical implications are (1) the elimination of the need for any new shear strength criterion for unsaturated soil, (2) the elimination of the need for determining the Bishop's effective stress parameter χ because the new form of effective stress is solely a function of soil suction, and (3) the ready extension of all classical soil mechanics work on limit equilibrium analysis to unsaturated soil conditions.

/pdf/a-closed-form-equation-for-effective-stress-in-unsaturated-uw4ohcr7fm.pdf

A closed‐form equation for effective stress in unsaturated soil

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...

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

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...

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

Soils compacted at various “initial” water contents and to various densities should be considered as “different” soils from a soil mechanics behavioral standpoint even though their mineralogy, plasticity, and texture are the same. The engineering behavioral change from one specimen to another will vary due to differences in soil structure or aggregation. The shear strength of an unsaturated soil and the soil-water characteristic curve are dependent on soil structure or the aggregation, which in turn is dependent on the “initial” water content and the method of compaction. The laboratory preparation of specimens must, therefore, closely represent the physical conditions and the stress state conditions likely to occur in the field if a proper assessment of the shear strength parameters is to be achieved. This paper is primarily concerned with the study of the relationship between the shear strength of an unsaturated soil and its soil-water characteristic curve. Consolidated drained direct shear tests were conducted on statically compacted glacial till specimens, both under saturated and unsaturated conditions, representing three “initial” water contents and densities. The “initial” water contents and densities of the specimens were selected to represent the dry, optimum, and wet of optimum water content conditions with reference to the compaction curve. Multistage, unsaturated, direct shear tests were conducted under three different net normal stresses with varying matric suction values for each case. The soil-water characteristic curves were also developed on specimens with “initial” conditions similar to those used for the unsaturated shear strength tests. The shear strength variation with respect to matric suction was found to be nonlinear for all the tests. The rate of increase in the shear strength contribution due to matric suction, however, was found to be related to the rate of desaturation of the soil. The desaturation characteristics are a function of the “initial” water content of the compacted specimens. For any particular net normal stress and matric suction, specimens compacted wet of optimum water content offered more resistance to desaturation and exhibited a higher shear strength when compared to specimens compacted at dry of optimum or at optimum water content conditions. Under similar “initial” conditions, the soil-water characteristic curve bears a close relationship to the unsaturated shear strength behavior of the soil.

The relationship between the soil-water characteristic curve and the unsaturated shear strength of a compacted glacial till

Critical state models for unsaturated soils have been proposed in recent years; however, the proposed models have been based on limited experimental data. Compacted specimens have generally been us...

A study of critical state on an unsaturated silty soil

The shear strength of unsaturated soils can be computed using the soil-water characteristic curve and the saturated shear strength parameters of the soil. Closed-form solutions for the prediction of unsaturated shear strength are developed in this paper using simple soil-water characteristic curve equations available in the literature. These closed-form solutions are not suitable for all types of soils and large ranges of suction. A general form for the shear strength equation of unsaturated soils using a rigorous soil-water characteristic curve equation is also developed in this paper. The proposed models make use of the soil-water characteristic curve and the saturated shear strength parameters to predict the variation of shear strength with respect to suction. (A) For the covering abstract see IRRD 881647.

D E Pufahl

Papers

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

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

The relationship between the soil-water characteristic curve and the unsaturated shear strength of a compacted glacial till

A study of critical state on an unsaturated silty soil

Predicting the shear strength function for unsaturated soils using the soil-water characteristic curve