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CHAPTER 5

Soil Fabric and ItsMeasurement

5.1 INTRODUCTION

Although soils are composed of discrete soil particlesand particle groups, a soil mass is almost alwaystreated as a continuum for engineering analysis anddesign. Nonetheless, the specific values of propertiessuch as strength, permeability, and compressibility de-pend on the size and shape of the particles, their ar-rangements, and the forces between them. Thus, tounderstand a property requires knowledge of these fac-tors. Furthermore, new theories of particulate mechan-ics and computational methods based on these theoriesare now becoming available. With these theories andmethods it may ultimately be possible to predict themechanical behavior of soil masses in terms of thecharacteristics of the particles themselves, although at-taining this goal appears somewhat far off.

Particle arrangements in soils remained largely un-known until suitable optical, X-ray diffraction, andelectron microscope techniques made direct observa-tions possible starting in the mid-1950s. Interest thencentered mainly on clay particle arrangements andtheir relationships to mechanical properties. In the late1960s, knowledge expanded rapidly, sparked by im-proved techniques of sample preparation and the de-velopment of the scanning electron microscope. In theearly 1970s attention was directed also at particle ar-rangements in cohesionless soils. From this work camea realization that characterization of the properties ofsands and gravels cannot be done in terms of densityor relative density alone, as had previously beenthought. Particle arrangements and stress history mustbe considered in these materials as well.

In the 1970s and 1980s, micromechanics theorieswere developed that aimed to relate microstructure tomacroscopic behavior. Various homogenization tech-

niques that incorporate small-scale features such asinhomogeneity and microfractures into continuummodels became available (Mura, 1987; Nemat-Nasserand Hori, 1999). Increased computational speeds al-lowed simulation of an assembly of individual soil par-ticles by modeling particle contact behavior, and thisled to the development of numerical methods such asthe discrete/distinct element method and contact dy-namics (Cundall and Strack, 1979; Moreau, 1994;Cundall, 2001). In the early developments, simulationswere limited to an assembly of two-dimensional cir-cular disks. However, it is now possible to performsimulations with various three-dimensional particleshapes, complex contact models, and pore fluid inter-actions. These ‘‘digital’’-type studies offer possibilitiesfor systematic investigation of soil fabric effects onmechanical properties in comparison to ‘‘laboratory’’-type studies, which contain inherent errors associatedwith measuring soil fabrics of different specimens.Furthermore, mechanical responses under the stresspaths that are difficult to apply in the laboratory canbe investigated using distinct element methods.

Other innovations in the past two decades have ledto improved material measurement techniques andtheir interpretation using computers. These include theenvironmental scanning electron microscopy (ESEM),nanoindentation and probing, complex digital imageanalysis, magnetic resonance imaging (MRI), X-Raytomography, and laser-aided tomography. Some ofthem have been used to characterize the microscopicproperties of soils (Oda and Iwashita, 1999).

The more established methods for studying and,where possible, quantifying the arrangements of par-ticles, particle groups, and voids in different soils aredescribed and illustrated in this chapter. Some ele-

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Copyright © 2005 John Wiley & Sons Retrieved from: www.knovel.com

110 5 SOIL FABRIC AND ITS MEASUREMENT

Figure 5.1 Modes of particle associations in clay suspen-sions and terminology. (a) Dispersed and deflocculated, (b)aggregated but deflocculated (face-to-face association, orparallel or oriented aggregation), (c) edge-to-face flocculatedbut dispersed, (d ) edge-to-edge flocculated but dispersed, (e)edge-to-face flocculated and aggregated, (ƒ ) edge-to-edgeflocculated and aggregated, and (g) edge-to-face and edge-to-edge flocculated and aggregated. From An Introduction toClay Colloid Chemistry, by H. van Olphen, 2nd ed., Copy-right � 1977 by John Wiley & Sons. Reprinted with per-mission from John Wiley & Sons.

ments and applications of the newer methods are in-troduced in later chapters.

5.2 DEFINITIONS OF FABRICS AND FABRICELEMENTS

The term fabric refers to the arrangement of particles,particle groups, and pore spaces in a soil. The termstructure is sometimes used interchangeably with fab-ric. It is preferable, however, to use structure to referto the combined effects of fabric, composition, and in-terparticle forces. Methods for determination of soilfabric are described and examples of different fabrictypes are given in the following sections. The impor-tance of soil fabric as a factor determining soil prop-erties and behavior is discussed and illustrated inChapter 8. In practice, special problems, unusual soils,and the need to ensure that measured properties prop-erly reflect the in situ conditions may require appli-cation of these testing and interpretation methods.

It is necessary to consider the size, the form, andthe function of different fabric units and to keep inmind the scale at which the fabric is of interest. Forexample, a carefully compacted clay liner for an im-poundment may have uniformly and closely packedparticle groups within it, thus giving a material withvery low hydraulic conductivity. If, however, the linerbecomes broken into sections measuring a meter or soin each direction as a result of shrinkage cracking, thenleakage through it will be dominated totally by flowthrough the cracks, and the small-scale fabric is un-important. Similarly, the strength of intact, homoge-neous soft clay will be influenced greatly by theparticle arrangements on a microscale, whereas that ofstiff fissured clay will be controlled by the propertiesalong the fissures.

Particle Associations in Clay Suspensions

Many soil deposits are formed by deposition fromflowing or still water. Accordingly, knowledge of par-ticle associations in suspensions is a good startingpoint for understanding how soil fabrics are formedand changed throughout the history of a soil. Cleansands and gravels are usually comprised of single grainarrangements, and these are discussed in Section 5.3.Particle associations in clay suspensions may be morecomplex. They can be described as follows and as il-lustrated in Fig. 5.1 (van Olphen, 1977):

1. Dispersed No face-to-face association of clayparticles

2. Aggregated Face-to-face (FF) association ofseveral clay particles

3. Flocculated Edge-to-edge (EE) or edge-to-face(EF) association of aggregates

4. Deflocculated No association between aggre-gates

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DEFINITIONS OF FABRICS AND FABRIC ELEMENTS 111

Figure 5.2 Schematic representation of elementary particlearrangements (Collins and McGown, 1974). (a) Individualclay platelet interaction, (b) individual silt or sand particleinteraction, (c) clay platelet group interaction, (d ) clothed siltor sand particle interaction, and (e) partly discernible particleinteraction.

Thicker and larger particles result from FF associa-tion. The EF and EE associations can produce card-house structures that are quite voluminous untilcompressed.

The terms flocculated and aggregated are often usedsynonymously in a generic sense to refer to multipar-ticle assemblages, and the terms deflocculated and dis-persed are used synonymously in a generic sense torefer to single particles or particle groups acting in-dependently.

Particle Associations in Soils

Particle associations in sediments, residual soils, andcompacted clays assume a variety of forms; however,most of them are related to combinations of the con-figurations shown in Fig. 5.1 and reflect the differencein water content between a suspension and a densersoil mass. Fine-grained soils are almost always com-posed of multiparticle aggregates. Overall, three maingroupings of fabric elements may be identified (Collinsand McGown, 1974):

1. Elementary Particle Arrangements Singleforms of particle interaction at the level of indi-vidual clay, silt, or sand particles

2. Particle Assemblages Units of particle organi-zation having definable physical boundaries anda specific mechanical function, and which consistof one or more forms of the elementary particlearrangements

3. Pore Spaces Fluid and/or gas filled voidswithin the soil fabric

Schematic illustrations of each of the fabric featuresin these three classes are shown in Figs. 5.2 through5.4. Electron photomicrographs illustrating some of thefeatures are shown in Fig. 5.5. Figure 5.6 shows theoverall fabric of undisturbed Tucson silty clay, a fresh-water alluvial deposit. The features shown in the fig-ures are sufficient to describe most fabrics, although anumber of additional terms have also been used to de-scribe the same or similar features.

Cardhouse is an edge-to-face arrangement formingan open fabric similar to the edge-to-face flocculatedbut dispersed arrangement of Fig. 5.1c (Goldschmidt,1926). A domain (Aylmore and Quirk, 1960, 1962) orpacket or book (Sloane and Kell, 1966) is an aggregateof parallel clay plates. An array of such fabrics istermed a turbostratic fabric and is similar to the inter-weaving bunches of Fig. 5.3h. An edge-to-face asso-ciation of packets or books is termed a bookhouse andis similar to the arrangement of Fig. 5.1e. A cluster isa grouping of particles or aggregates into larger fabric

units (Olsen, 1962; Yong and Sheeran, 1973). In a fab-ric composed of groupings of clusters, it is useful torefer to intracluster and intercluster pore space and tocluster and total void ratios. The term ped (Brewer,1964) has a similar meaning to cluster.

Fabric Scale

The fabric of a soil may be viewed relative to threelevels of scale. From smallest to largest they are:

1. Microfabric The microfabric consists of theregular aggregations of particles and the verysmall pores between them. Typical fabric unitsare up to a few tens of micrometers across.

2. Minifabric The minifabric contains the aggre-gations of the microfabric and the interassem-

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Figure 5.3 Schematic representations of particle assemblages (Collins and McGown, 1974).(a) connectors, (b) connectors, (c) connectors, (d ) irregular aggregations by connector as-semblages, (e) irregular aggregations in a honeycomb, (ƒ ) regular aggregation interactingwith particle matrix, (g) interweaving bunches of clay, (h) interweaving bunches of clay withsilt inclusions, (i) clay particle matrix, and ( j ) granular particle matrix.

blage pores between them. Minifabric units maybe a few hundred micrometers in size.

3. Macrofabric The macrofabric may containcracks, fissures, root holes, laminations, and thelike that correspond to the transassemblage poresshown in Fig. 5.6.

Soil mechanical and flow properties depend on de-tails of these three levels of fabric to varying degrees.For example, the hydraulic conductivity of a fine-grained soil is almost totally dominated by the macro-

and minifabrics. Time-dependent deformations such ascreep and secondary compression are controlled moststrongly by the mini- and microfabric.

5.3 SINGLE-GRAIN FABRICS

Sand and gravel particles are sufficiently large andbulky that they ordinarily behave as independent units.Attempts to describe the stress–deformation behaviorof granular soils using particulate mechanics theories

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SINGLE-GRAIN FABRICS 113

Figure 5.4 Schematic representation of pore space types (Collins and McGown, 1974).

[e.g., Newland and Allely (1957), Rowe (1962, 1973),Horne (1965), Matsuoka (1974), Murayama (1983),Nemat-Nasser and Mehrabadi (1984), and Wan andGuo (2001)] have met with some success. The devel-opment of discrete element methods for numericalmodeling of granular soils has greatly extended thepotential for these methods as discussed in Section 5.1.These theories are based on elastic distortion of par-ticles and the sliding and rolling of particles, usuallyassumed of spherical or disk shape. In real granularsoils, the irregular particle shapes and distribution ofsizes mean that packing is usually far from regular.Nonetheless, the theories and computations can pro-vide valuable insights into behavior, and knowledge ofthe characteristics of ideal systems can be useful forinterpreting data on real soils (see Chapter 11).

Direct Observation of Cohesionless Soil Fabric

The study of the fabric of a cohesionless soil is usuallydone by optical means. The particles are large enoughto be easily seen in the petrographic microscope. Thinsections can be made after impregnation of a sampleby a suitable resin or plastic. Water-soluble materialsare available for use in initially saturated sands. Afterthe resin or plastic has hardened, thin sections can beprepared.

In some cases, sand samples can be dried prior toimpregnation since sand fabrics are not generally af-fected by capillary stresses. A procedure for doing thisto enable study of the fabrics produced in MontereyNo. 0 sand by different methods of compaction isgiven by Mitchell et al. (1976).

Packing of Equal-Sized Spheres

Regular packing of spheres of the same size providesinsight into the maximum and minimum possible den-sities, porosities, and void ratios that are possible insingle-grain fabrics. Five different possible packing ar-rangements are shown in Fig. 5.7, and properties of thearrangements shown are listed in Table 5.1. The rangeof possible porosities is from 25.95 to 47.64 percent,and the corresponding range of void ratios is from 0.35to 0.91.

Random packings of equal size spheres can be con-sidered to be composed of clusters of simple packings,each present in an appropriate proportion to give theobserved porosity. The relationship between coordi-nation number N and porosity n in such systems is

N � 26.486 � 10.726/n (5.1)

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Figure 5.5 Scanning electron photomicrograph features of undisturbed soil fabrics (Collinsand McGown, 1974). (a) Partly discernible particle systems in Lydda silty clay, Israel (fresh-water alluvial deposit); (b) grain–grain contacts in Ford silty loess, England (aeolian deposit);(c) connector assemblages in Breidmerkur silty till, Iceland (glacial ablation deposit); (d )particle matrix assemblage in Immingham silty clay, England (estuarine deposit); (e) regularaggregation assemblage in Holon silty clay, Israel (consisting of elementary particle arrange-ments interacting with each other and silt) (freshwater alluvial deposit); (ƒ ) interweavingbunch assemblage in Hurlford organic silty clay, Scotland (freshwater lacustrine deposit);and (g) irregular aggregation assemblage in Sundland silty clay, Norway (marine deposit).

Glass balls allowed to fall freely form an anisotropicassembly, with the balls tending to arrange themselvesin chains (Kallstenius and Bergau, 1961). The numberof balls per unit area in contact with a vertical planecan be different from the number in contact with ahorizontal plane. The same behavior is observed forsand pluviated through air and water.

Spontaneous segregation and stratification has beenobserved when granular mixtures of particles of twodifferent predominant sizes are dumped into a pile(Makse et al., 1997; Fineberg, 1997). When a mixtureof sizes is poured into a pile, the larger particles tend

to accumulate near the base. Makse and co-workers’(1997) experiments produced the interesting additionalresult that if the large grains in a binary mixture havea greater angle of repose than the small grains, thenthe mixture stratifies into alternating layers of smalland large grains. If the small grains have a larger angleof repose than the large grains, then segregation with-out stratification results. This type of behavior is rel-evant to such geoengineering problems as the stabilityof dumped mine waste piles, geological formationssusceptible to static liquefaction, and the processingand transport of granular materials.

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Figure 5.5 (Continued )

Particle Packings in Granular Soils

Particle sizes in soil vary, and as a result, smaller par-ticles can occupy pore spaces between larger particles.This results in a tendency toward higher densities andlower void ratios than for uniform spheres. On theother hand, irregular particle shapes produce a ten-dency toward lower densities and higher porosities andvoid ratios. The net result is that the range of porositiesand void ratios in real soils with single-grain fabricsmay not be much different from that for uniform

spheres shown by the values in Table 5.1, that is, po-rosity in the range of 26 to 48 percent and void ratioin the range of 0.35 to 0.91. This is illustrated by thedata in Table 5.2. The lower values of porosity anddensity and higher unit weight for silty sand and gravelcan be attributed to silt filling the large voids betweenthe gravel particles.

Many studies have shown that a given cohesionlesssoil can have different fabrics at the same void ratioor relative density. Characterization of this fabric can

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Figure 5.6 Overall microfabric in Tucson silty clay, United States (freshwater alluvial de-posit) (Collins and McGown, 1974).

be done in terms of grain shape factors, grain orien-tations, and interparticle contact orientations (Lafeber,1966; Oda, 1972a; Mahmood and Mitchell, 1974;Mitchell et al., 1976). More recently, application ofimage analysis techniques (Section 5.8) has led to bet-ter understanding and quantification of fabric features.

The orientation of grains in a sand deposit can bedescribed in terms of the inclination of the particleaxes to a set of reference axes. For example, the ori-entation of the particle shown in Fig. 5.8 can be de-scribed by the angles � and �. In most studies,however, a thin section is studied to give the orienta-tions of apparent long axes. The long axes of particlesare referred to a single horizontal reference axis by anangle �.1 The spatial orientation of the thin section it-

1 This method underestimates the value of L /W for elongate particleshaving their long axis out of the plane of the thin section.

self with respect to the sample and to the field depositis also an essential part of the fabric description.

Orientations of long axes for a large number ofgrains can be expressed by a histogram or rose dia-gram. A frequency histogram for a sand having a meanaxial ratio equal to 1.65 and placed by tapping the sideof a vertical, cylindrical mold is shown in Fig. 5.9. Theorientation of each grain was assigned to one of the15� intervals between 0� and 180�. The V-section refersto a thin section from a vertical plane (oriented parallelto the cylinder axis). The H-section refers to orienta-tions in the horizontal plane.

Orientations of long axes in the vertical plane fortwo samples of well-graded crushed basalt [mean(length)/(width) � 1.64] are shown by the rose dia-grams in Figs. 5.10 and 5.11. In this study, the orien-tations of at least 400 grains were measured for eachsample, and the orientation of each was assigned to

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Figure 5.7 Ideal packings of uniform spheres: (a) simple cubic, (b) cubic tetrahedral, (c)tetragonal sphenoidal, (d ) pyramidal, and (e) tetrahedral.

Table 5.1 Properties of Ideal Packings of Uniformly Sized Spheres

Type of PackingCoordination

NumberLayer Spacing(R � radius) Volume of Unit

Porosity(%) Void Ratio

Simple cubic 6 2R 8R3 47.64 0.91Cubical–tetrahedral 8 2R R343 39.54 0.65Tetragonal–sphenoidal 10 R3 6R3 30.19 0.43Pyramidal 12 R2 4 R32 25.95 0.35Tetrahedral 12 R22/3 R342 25.95 0.35

one of the eighteen 10� intervals between 10� and 180�.A completely random distribution would yield thedashed circles shown in the figures. There is a strongpreferred orientation in the horizontal direction in thesample prepared by pouring (Fig. 5.10). Dynamic com-

paction, however, resulted in a more nearly randomfabric (Fig. 5.11).

Interparticle contact orientations and their distribu-tion influence deformation and strength properties andanisotropy. These orientations can be described in

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Table 5.2 Maximum and Minimum Void Ratios, Porosities, and Unit Weights for Several Granular Soils

Void Ratio

emax emin

Porosity (%)

nmax nmin

Dry Unit Weight(kN m�3)

d min d max

Uniform spheres 0.91 0.35 47.6 26 — —Standard Ottawa sand 0.80 0.50 44 33 14.5 17.3Clean uniform sand 1.0 0.40 50 29 13.0 18.5Uniform inorganic silt 1.1 0.40 52 29 12.6 18.5Silty sand 0.90 0.30 47 23 13.7 20.0Fine to coarse sand 0.95 0.20 49 17 13.4 21.7Micaceous sand 1.2 0.40 55 29 11.9 18.9Silty sand and gravel 0.85 0.14 46 12 14.0 22.9

Modified from Lambe and Whitman (1969).

Figure 5.8 Three-dimensional orientation of a sand particle.

Figure 5.9 Frequency histograms of long particle axis orientations in two planes for auniform fine sand. Reprinted from Oda (1972a), with permission of The Japanese Societyof SMFE.

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CONTACT FORCE CHARACTERIZATION USING PHOTOELASTICITY 119

Figure 5.10 Particle orientation diagram for crushed basalt.Vertical section through a sample prepared by pouring. Den-sity is 1600 kg/m3 and the relative density is 62 percent.

Figure 5.11 Particle orientation diagram for crushed basalt.Vertical section through a sample prepared by dynamic com-paction. Density is 1840 kg/m3 and the relative density is 90percent.

terms of a perpendicular Ni to the tangent plane at thepoint of contact. As most fabric characterization stud-ies are done in a two-dimensional plane, and actualparticle contact points rarely occur in the analyzedplane, measurement of contact normals can be proneto detection errors.

The orientation of Ni is defined by angles �� and ��as shown in Fig. 5.12. A procedure for determinationof the angular distributions of normals E(��, ��) isgiven by Oda (1972a). For a fabric with axial sym-metry around the vertical axis, the function E(��, ��)is independent of �, so the distribution of E(��) as afunction of �� can be used to characterize the distri-bution of interparticle contact normals. Contact normaldistributions for four sands deposited in water andcompacted by tapping on the sides of their containersare shown in Fig. 5.13. The horizontal dashed linesrepresent the distributions for an isotropic fabric. Ineach case there is a greater proportion of contact planenormals in the near vertical direction; that is, there isa preferred orientation of contact planes near the hor-izontal.

Various methods to quantify long axis and contactdistributions are available (Oda, 1972a; Fisher et al.,1987; Shih et al., 1998). The measured statistical dis-tributions can be converted to a tensor that has thesame dimensionality as stresses and strains (Satake,1978; Kanatani, 1984; Oda et al., 1985; Kuo et al.,1998). One notable measure is the fabric tensor (Odaet al., 1982b) that characterizes the contact normal di-rections. This tensor and its evolution with plasticstrains are used in development of micromechanicstheories as well as continuum-based constitutive mod-els (e.g., Tobita, 1989; Muhunthan et al., 1996; Yimsiriand Soga, 2000; Wan and Guo, 2001; Li and Dafalias,2002).

The mean value of the particle coordination numberand its standard deviation are additional important fab-ric features in granular soils. The coordination numberis the number of adjacent particles in contact with anygiven particle, and it is dependent on particle size,shape, size distribution, and void ratio. Relationshipsbetween the different orientation and packing param-eters and mechanical properties of cohesionless soilsare given in Chapter 8.

5.4 CONTACT FORCE CHARACTERIZATIONUSING PHOTOELASTICITY

Photoelasticity is a phenomenon in which light goingthrough a photoelastic material (such as glass, rubber,and polymer) is polarized by the internal stresses ofthe material. The basic concept is that the speed oflight depends on the direction of the plane of oscilla-tion due to stress-induced optical anisotropy of the ma-terial. The planes of the limiting velocities coincidewith the direction of the principal stresses. Utilizingthis technique, the analysis of a photoelastically sen-sitive particle assembly under different boundary load-ing conditions gives information about the internalforce structure through particle contacts. Averaging thecontact forces over a number of particles in a regionof interest gives the average effective stress. The down-side of this technique is that actual soil particles cannotbe used. However, the force information obtained froma transparent particulate assembly is useful for under-standing how actual soil particle systems are likely tobehave.

Light propagates in a vacuum or in air at a speed Cof 3 � 108 m/s. In other transparent materials, thespeed V is lower and the ratio C /V is called the re-fractive index. In photoelastic materials, the change inrefractive index in the i direction (ni) is proportional tothe change in normal stress �i in the same direction; ni � Kso �i, where Kso is the stress-optical material

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Figure 5.12 Characterization of interparticle contact orientation.

Figure 5.13 Probability density functions of E(�) for (a)crushed chert, (b) Toyoura sand, (c) Soma sand, and (d ) To-chigi sand. The crushed chert and Toyoura sand are mainlyrodlike or flat particles. Tochigi sand has spherical particles.Soma sand is intermediate in particle shape (from Oda,1978). Reprinted by permission.

constant. Hence, the velocity becomes direction depen-dent when the material is stressed in an anisotropicmanner.

Using a polarizer, the incoming light is polarizedalong a well-defined plane. If another polarizer isplaced along the polarized light, complete extinctionof the light can be achieved by making the filteringdirection perpendicular to that of the first polarizer.When the polarized light goes through a stressed trans-parent material, two polarized lights are generated inthe direction of principal strains (also the principalstress directions in an elastic material). The velocity ofeach component is inversely proportional to the differ-ent refractive indices of its particular plane, and therewill be a relative retardation �:

� � (n � n )l � K (� � � )l (5.2)max min so max min

where l is the material thickness, nmax and nmin are therefractive indices of the two polarized lights, and �max

and �min are the maximum and minimum principlestress, respectively.

A polarizing analyzer can be placed along the po-larized lights and it will transmit only one componentof each of these waves. The polarized waves will in-terfere, and the light intensity of the polarized lightcoming out of the analyzer will be a function of � andthe angle between the analyzer and direction of prin-

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MULTIGRAIN FABRICS 121

-1 -0.5 0 0.5 10

0.5

1

1.5

2

(a) (b)

Figure 5.14 Photoelastic image of a circular disk squeezed between two contacts: (a) the-oretically expected image and (b) actual image (from Howell et al., 1999).

cipal strains. The light intensity becomes zero whenthe angle becomes zero and hence the principal strainsdirections can be determined. Optical filters known asquarter-wave plates can be added in the path of lightpropagation to produce circularly polarized light. Bydoing so, the image observed is not influenced by thedirection of principal strains, but the intensity I viewedby a circular polariscope depends on � by the follow-ing equation:

2I � I sin (�� /�) (5.3)0

where I0 is a constant and � is the wavelength of thelight. The light intensity becomes zero when � � N�(N � fringe order � 1, 2, ...), and hence the magnitudeof principal stress difference at a given point can beevaluated from Eq. (5.3). Photoelastic images of a cir-cular disk squeezed between two contacts are shownin Fig. 5.14 (Howell et al., 1999).

The forces applied to particles are not equal. Instead,the spatial distribution of forces varies significantly dueto random positions of the particles. Figure 5.15 showsimages in an assemblage of pentagonal-shaped disksunder (a) geostatic stresses by gravity and (b) bothgravity loading and point loading at the center of themodel (Geng et al., 2001). A chainlike force distribu-tion, indicated by large light intensity paths, existseven under geostatic stress conditions. Strong forcechains can develop in an assembly of pentagonal-shaped polymer particles as shearing progresses (Geng

et al., 2003). A complicated network of force chainsdevelops in the direction of the maximum principalstress.

Microscopic investigations of the development ofcontact force distribution under different loading con-ditions provide physical insights to understand defor-mation behavior of granular materials. Further detailsare given in Chapter 11.

Photoelasticity investigations can also be performedusing three-dimensional particle assemblages. Al-though the actual material may be transparent, the par-ticles become opaque due to refraction and reflectionof light at the particle surfaces, which are often opti-cally damaged. This adds difficulty in examining thecontact force distributions. However, if the pores arefilled with a fluid that has the same refractive index asthe photoelastic material, the assembly becomes moretransparent. Figure 5.16 shows the force distribution incrushed glass particles when a cone penetrometer ispushed into the material (Allersma, 1999). Again, de-velopment of a strong force network is evident.

5.5 MULTIGRAIN FABRICS

In Section 5.2, it was emphasized that single-grain fab-rics are rare in soils containing clay-size particles. Thisis often true also for silts (particle sizes in the rangeof 2 to 74 �m). For example, experiments have shownthat silt-size quartz particles sedimented in water can

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Figure 5.15 Photoelastic images of pentagonal shape diskassembly under (a) geostatic stresses by gravity and (b) bothgravity loading and point loading at the center of the model(from Geng et al., 2001).

Figure 5.16 Cone penetration test in photoelastic particles(from Allersma, 1999).

Figure 5.17 Schematic diagram of a honeycomb fabric insilt.have a void ratio as large as 2.2. Quartz particles in

this size range may be somewhat platy and can accountfor a part of this high void ratio as compared to anupper limit of about 1.0 for single-grain assemblagesof bulky particles. However, silt-size particles formmultigrain arrangements during slow sedimentation,because they are sufficiently small that their arrange-ments can be influenced by surface force interactions.An open honeycomb type of arrangement, as shownschematically in Fig. 5.17, is thought to exist in somesilts (Terzaghi, 1925a). Loose fabrics such as this aremetastable and subject to sudden collapse or liquefac-tion under the action of rapidly applied stresses.

Multigrain fabrics of clays and clay–nonclay mix-tures form because clay particle surface forces are sig-nificant relative to clay particle weight; clays can ad-

sorb on nonclay particle surfaces, and clay surfaces areoften chemically reactive. In addition, clay particlegroups in many soils may be remnants of a preexistingrock from which the soil was derived.

5.6 VOIDS AND THEIR DISTRIBUTION

Different types of pores are illustrated in Figs. 5.4 and5.6. The pore sizes and their distribution complementthe particle and particle group sizes and their distri-bution. Emphasis is usually placed on the solid phaserather than the liquid and gas phases when describing

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SAMPLE ACQUISITION AND PREPARATION FOR FABRIC ANALYSIS 123

properties and behavior. However, the pores and voidsdetermine the fluid and gas conductivity propertiesthat, in turn, control such important processes as therate of fluid and chemical transport, generation of ex-cess pore pressures during deformation, consolidationrate, the ease and rate of drainage, capillary pressuredevelopment, and the potential for liquefaction underdynamic loading. Methods for determining and char-acterizing pore sizes and their distribution are de-scribed in Section 5.9.

5.7 SAMPLE ACQUISITION ANDPREPARATION FOR FABRIC ANALYSIS

Obtaining representative soil samples with minimaldisturbance is essential if reliable measurements of en-gineering properties are to be made. The same consid-erations apply in the selection and preparation ofsamples for the study of fabric. Accordingly, the sam-pling and preparation phases of fabric study are criti-cal, and special methods are many times needed.Proven methods for reliable determination of fabriccan also be used for evaluation of the effects of dif-ferent sampling procedures used in engineering prac-tice, although there does not appear to be much recordof this having been done.

Both direct and indirect methods are used to studythe fabric and fabric features of soils, as listed in Table5.3. An illustrative schematic diagram prepared byR. N. Yong that summarizes methods for analysis ofsoil composition and fabric using various parts of theelectromagnetic spectrum is shown in Fig. 5.18. In in-terpreting the results from any of these methods, judg-ment is required to be sure that the conclusions pertainto properties and behavior of interest. For example,discontinuities, fractures, and anisotropy on a macro-scale can override the influences of microfabric details.

Of the methods listed in Table 5.3, optical and elec-tron microscopy, X-ray diffraction, and pore size dis-tribution offer the advantage of providing direct(usually) unambiguous information about specific fab-ric features, provided the samples are representativeand the sample preparation procedures have not de-stroyed the original fabric. On the other hand, thesetechniques are limited to small samples, and they aredestructive of the samples studied. The other tech-niques are nondestructive, at least in principle, and canbe used for the study of soil fabric in situ and for thestudy of changes in fabric that accompany compres-sion, shear, and fluid flow. However, with most of thesemethods interpretation is seldom straightforward or un-ambiguous. The use of several methods of fabric anal-

ysis may be appropriate in some cases in order toobtain information of more than one type or level ofdetail.

Sample Preparation for Fabric Analysis

Acoustical, dielectric, thermal, and magnetic measure-ments can be made directly on samples in their undis-turbed, wet state. Optical and electron microscopy,X-ray diffraction, and porosimetry require that the porefluid be removed, replaced, or frozen. To do this with-out disturbance of the original fabric is difficult, andin most cases there is no way to determine how muchdisturbance there may have been.

Pore Fluid Removal Air drying without significantdisruption of the natural fabric may be possible forsoils that do not undergo much shrinkage. For softsamples at high water content, oven drying may causeless fabric change than air drying, evidently becausethe longer time required for air drying allows forgreater particle rearrangement (Tovey and Wong,1973). On the other hand, the stresses induced duringoven drying may result in some particle breakage.

Water removal by drying at the critical point has alsobeen used. If the temperature and pressure of the sam-ple are raised above the critical values, which for waterare 374�C and 22.5 MPa, respectively, the liquid andvapor phases are indistinguishable. The pore water canthen be distilled off without the presence of air–waterinterfaces that can lead to shrinkage. The high tem-perature and pressure may change the clay particles,however. To avoid this, replacement by carbon dioxidehas been used. The critical temperature and pressureof carbon dioxide are 31.1�C and 7.19 MPa, respec-tively. The procedure requires prior impregnation ofthe sample with acetone, which may cause swelling inpartly saturated and expansive soils (Tovey and Wong,1973). Both critical point and freeze-drying cause lesssample disturbance and shrinkage than do air or ovendrying, but they are more difficult and time consuming.

Freeze-drying can be used for removal of water.Sublimation of the ice in a soil that has been rapidlyfrozen avoids the problem of air–water interfaces andshrinkage that accompany water removal by drying.Sample size must be small, usually thinner than about3 mm, if disruption due to nonuniform freezing is tobe avoided. Quick freezing is best done in a liquid thathas been cooled to its melting point in liquid nitrogen,such as isopentane at �160�C or Freon 22 at �145�C.This avoids gaseous bubbling caused by direct immer-sion in liquid nitrogen at �196�C (Delage et al., 1982).The freezing temperature should be less than �130�Cto avoid formation of crystalline ice. Sublimation of

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124

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125

Polarized LightMicrograph

Replica TransmissionElectron Micrographor Diffraction Pattern

Scanning ElectronMicrograph

Figure 5.18 Methods for examining mineralogy, fabric, and structure of soils using parts ofthe electromagnetic spectrum (prepared by R. N. Yong, McGill University Soil MechanicsLaboratory).

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Table 5.3 Techniques for Study of Soil Fabric

Method BasisScale of Observations and Features

Discernable

Optical microscope(polarizing)

Direct observation of fracturesurfaces of thin sections

Individual particles of silt size andlarger, clay particle groups,preferred orientation of clay,homogeneity on a millimeterscale or larger, large pores, shearzones

Useful upper limit of magnificationabout 300�

Electron microscope Direct observation of particlesor fracture surfaces throughsoil sample (SEM)observation of surfacereplicas (TEM)

Resolution to about 100 A; largedepth of field with SEM; directobservation of particles; particlegroups and pore space; details ofmicrofabric; environmental SEMcan be used to observespecimens containing waterand gas

X-ray diffraction Groups of parallel clay platesproduce stronger diffractionthan randomly orientedplates

Orientation in zones several squaremillimeters in area and severalmicrometers thick; best in singlemineral clays

Pore size distribution (1) Forced intrusion of anonwetting fluid (usuallymercury)

(1) Pores in range from �0.01 to�10 �m

(2) Capillary condensation (2) 0.1 �m maximumWave propagation Particle arrangement, density,

and stress influences wavevelocity

Anisotropy; measures fabricaveraged over a volume equal tosample size

Dielectricdispersion andelectricalconductivity

Variation of dielectricconstant and conductivitywith frequency

Assessment of anisotropy,flocculation and deflocculation,and properties; measures fabricaveraged over a volume equal tosample size

Thermal conductivity Particle orientations anddensity influence thermalconductivity


Magneticsusceptibility

Variation in magneticsusceptibility with change ofsample orientation relativeto magnetic field


Mechanical Propertiesstrength moduluspermeabilitycompressibilityshrinkage and swell

Properties reflect influences offabric; see Chapter 11

Fabric averaged over a volumeequal to sample size; anisotropy;macrofabric features in somecases

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METHODS FOR FABRIC STUDY 127

the water is then done at temperatures between �50and �100�C rather than at the initial freezing temper-ature to increase the rate of water vapor removal. Attemperatures less than �100�C the vapor pressure ofthe ice, about 10�5 torr, may be less than the capabilityof the vacuum system.

The freezing process may produce fabric changes invery high water content systems such as a 10 percentby weight slurry of bentonite in water (Kumai, 1979).However, with more typical saturated clays at consis-tencies likely to be encountered in geotechnical inves-tigations, the effects of freeze-drying on the fabric aresmall. Additional considerations in sample preparationby freeze-drying are given by Tovey and Wong (1973)and Gillott (1976).

Pore Fluid Replacement If thin sections are re-quired, as for optical microscopy or when dryingshrinkage must be minimized, but the presence of amaterial in pore spaces is not objectionable, replace-ment of the pore water may be necessary. Various res-ins and plastics have been used for this purpose.High-molecular-weight ethylene glycol such as Car-bowax 6000 is miscible with water in all proportionsand has been used for many studies. Carbowax 6000melts at 55�C but is solid at lower temperatures.

Impregnated samples are prepared by immersing anundisturbed cube sample, 10 to 20 mm on a side, inmelted Carbowax at 60 to 65�C. The top surface of thespecimen should be left exposed to vapor for the firstday of immersion to allow escape of trapped gases andprevent specimen rupture. The wax should be changedafter 2 or 3 days to ensure water-free wax in the samplepores. Replacement of all water by the Carbowax isusually complete in a few days. After removal fromthe liquid wax and cooling, the sample is ready forsectioning.

Thin sections are prepared by grinding using emerycloth or abrasive powders and standard thin-sectiontechniques. However, heat, water, or other water-soluble liquids cannot be used at any stage of thegrinding or section mounting process. Measurementsby X-ray diffraction have shown that Carbowax re-placement of water has essentially no effect on the fab-ric of wet kaolinite (Martin, 1966).

Gelatins or water-soluble resins may be used in lieuof Carbowax, or the sample may be impregnated withmethanol or acetone before replacement with resins orplastics. Further details on resin impregnation aregiven by Smart and Tovey (1982) and Jang et al.(1999).

Preparation of Surfaces for Study

Surfaces chosen for study should reflect the originalfabric of the soil and not the preparation method.

Grinding or cutting air-dried and Carbowax-treatedsamples may result in substantial particle rearrange-ment at the surface, thus making them of little valuefor study by the electron microscope. To overcome thisproblem, successive peels from the surface of a driedspecimen using adhesive tape can be used to exposethe original fabric. Alternatively, the surface may becoated with a resin solution that partly penetrates thesample. After hardening, the resin is peeled away re-vealing an undisturbed fabric. A comparison of sur-faces before and after this procedure is shown in Fig.5.19.

The disturbed zone at the surface of Carbowax-treated samples extends to a maximum depth of about1 �m in kaolinite (Barden and Sides, 1971). As thinsections used for polarizing microscope study are ofthe order of 30 �m thick, this disturbed zone is of littleconsequence. It is also insignificant for X-ray diffrac-tion studies.

Fracture surfaces in dried specimens are sometimestaken as representative of the undisturbed fabric. Ad-ditional preparation, such as gentle blowing of the sur-face or peeling is needed following fracture because(1) there may be loose particles on the surface, and (2)a fracture surface may be more representative of aplane of weakness than of the material as a whole. Analternative approach to avoid these problems is to frac-ture a frozen wet specimen as described by Delage etal. (1982).

The method of sample preparation should be se-lected after consideration of scale of fabric features ofinterest, method of observation to be used, and the soiltype and state as regards water content, strength, dis-turbance, and so forth. With these factors in mind, theprobable effects of the preparation methods on the fab-ric can be assessed.

5.8 METHODS FOR FABRIC STUDY

Once suitable samples and surfaces have been pre-pared, direct study of different fabric features is pos-sible using one or more of several methods, asindicated in Fig. 5.18. Details of these methods arediscussed in this section as well as the advantages andlimitations of each.

Polarizing Microscope

Individual particles of silt and sand can be seen usingpetrographic and binocular microscopes, and the sizes,orientations, and distributions of particles and porespaces can be described systematically. Thin sectionsor polished surfaces can be used for two-dimensional

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Figure 5.19 Effect of surface preparation on fabric seen by the scanning electron microscope(a) before peeling and (b) after one peeling, �5000 (from Tovey and Wong, 1973).

Figure 5.20 Pore pattern of a section from a stony tableland soil from Woomera, Australia.Pores in white, clay matrix in gray, and silt sand grains in black (from Lafeber, 1965).Reprinted with permission of AJSR.

analyses. Three-dimensional analyses require a seriesof parallel cross sections.

Many petrographic techniques and special treat-ments are available to aid in identification of featuresof interest (e.g., Stoopes, 2003). Rose diagrams can beused to represent two-dimensional planar patterns.Three-dimensional patterns can be represented usingstereo net projections. As an illustration of two-dimensional representation, Fig. 5.20 shows the porepattern in a section of a stony desert tableland soil fromnear Woomera, Australia, which suggests some degree

of preferred orientation. Rose diagrams are shown inFig. 5.21 of both pore orientation (white figure) andsilt and sand grain orientation (black figure). Consid-erable preferred orientation of both pores and particlesis evident.

It is not usually possible to see individual clay par-ticles with the polarizing microscope because of limi-tations in resolving power and depth of field. Practicalresolution is to a few micrometers using magnificationsup to about 300 times. If, however, clay plates arealigned parallel to each other in a group, then they

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L1

L1

RR

s1

s1

s2

s2

Figure 5.21 Distribution of elongated pores (white figure) and of elongated skeleton grains(black figure) in different directions for the pattern in Fig. 5.20. The broken circle representsan even distribution of lengths over all directions. s1 and s2 are the major maxima of theelongated pores, L1 is the major maximum of elongated grains, and R is the reference di-rection (from Lafeber, 1965). Reprinted with permission of AJSR.

Figure 5.22 Thin section of varved clay under polarizedlight (courtesy of Division of Building Research, NationalResearch Council, Canada).

Table 5.4 Orientation Scale for Clay AggregatesViewed in Plane Polarized Light

Birefringence Ratio Particle Parallelism

1.0 Random1.0–0.9 Slight0.9–0.5 Medium0.5–0.1 Strong

0 Perfect

From Morgenstern and Tchalenko (1967c).

behave optically as one large particle with definite op-tical properties.

The optical axes and the crystallographic axes of theclay minerals are almost coincident. For plate-shapedparticles, the refractive indices in the a and b directionsare approximately equal, but different from that in thec-axis direction. The difference in refractive indicesalong different optical axes of a crystal determines theoptical property termed the birefringence.

If a group of parallel particles is viewed in planepolarized light looking down the c axis, a uniform fieldis seen as the group is rotated around the c axis. If thesame particle group is viewed with the c axis normalto the direction of the light, no light is transmittedwhen the basal planes are parallel to the direction ofpolarization, and a maximum is transmitted when theyare at 45� to it. Thus there are four positions of ex-tinction and illumination when the sample is viewedusing light passed through crossed nicols and the mi-croscope stage is rotated through 360�. For rod-shapedparticles in parallel orientation, a uniform field is ob-served looking down the long axis, whereas illumina-tion and extinction are seen when looking normal tothis axis. Use of a tint plate in the microscope is oftenhelpful because the resulting retardation of light wavesresults in distinct different colors for extinction andillumination.

If particle orientation is less than perfect or if the c-axis direction of a group of parallel plates is other thannormal to the direction of light, then the minimum in-tensity is finite and the maximum intensity is less thanfor perfect orientation. The ratio of minimum intensityImin to maximum intensity Imax is called the birefrin-gence ratio �.

Photometric measurements of the birefringence ratiocan be used to quantify clay particle orientation (Wu,1960; Morgenstern and Tchalenko, 1967a). Althoughthere may be difficulties in photometric methods whendealing with other than monomineral materials with

singular orientations of particles (Lafeber, 1968), thesemiquantitative scale proposed by Morgenstern andTchalenko (1967c) given in Table 5.4 is useful.

A vertical section taken through varved clay isshown in Fig. 5.22. The upper half shows the winter-deposited clay varve and the lower half the summer-deposited silt varve. Strong preferred orientation of the

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Figure 5.23b Details of Fiddler’s Ferry shear zone (Mor-genstern and Tchalenko, 1967c). F is a fragment of ambientmaterial; the hatched areas indicate the shear matrix wherethe birefringence ratio � � 0.45; and the direction of hatch-ing is the average particle orientation over the stippled areaswhere � � 1.00.

Figure 5.23a Photograph of Fiddler’s Ferry shear zone(from Morgenstern and Tchalenko, 1967c).

clay is evident by comparison of illumination on theleft and extinction on the right. Were the clay platesoriented randomly throughout, the thin section wouldhave had the same appearance at both orientations. Theupper portion of the silt varve is also seen to containsome zones of well-oriented clay. A series of planarpores is also visible. These pores probably were de-veloped during impregnation of the sample or prepa-ration of the thin section.

Optical microscope study of fabric provides a viewof some features that are too small to be seen by eye,too large to be appreciated using an electron micro-scope, but important to understanding soil behavior.Some of these features include distributions of silt andsand grains, silt and sand particle coatings, homoge-neity of fabric and texture, discontinuities of varioustypes, and shear planes (e.g., Mitchell, 1956; Morgen-stern and Tchalenko, 1967b, 1967c; McKyes andYong, 1971; Oda and Kazama, 1998). A thin sectionfrom a shear zone through a soft silty clay at the siteof a foundation failure under an embankment at Fid-dler’s Ferry on the floodplain of the Mersey River,England, is shown in Fig. 5.23a. Details of the shearzone deduced from the photomicrograph are shown inFig. 5.23b.

Electron Microscope

The electron microscope can reveal clay particles andtheir arrangements directly. The practical limit of res-

olution of the transmission electron microscope (TEM)is less than 10 A, and atomic planes can be seen. Thepractical limit of the scanning electron microscope(SEM) is about 100 A; however, lesser magnificationis sufficient to resolve details of clay particles andother very small soil constituents. The major advan-tages of the SEM relative to the TEM are the muchgreater depth of field, the wide, continuous range ofpossible magnifications (about 20� to 20,000�), andthe ability to study surfaces directly. Either surface rep-licas or ultra-thin sections are needed for the TEM. Themain advantage of the TEM relative to the SEM is itshigher limit of resolution. Historical developmentsalong with its application to clay minerals and aggre-gates examination are given by McHardy and Birnie(1987) for SEM and Nadeau and Tait (1987) for TEM.

Both types of electron microscopy require an evac-uated sample chamber (1 � 10�5 torr), so wet soilscannot be studied directly unless they are housed in aspecial chamber. Cold stages are available, so frozenmaterials may be studied. It is usually necessary tocoat SEM sample surfaces with a conducting film toprevent surface charging and loss of resolution. Gold

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Figure 5.24 Microfabrics of artificial clay sediments. Scalebar � 2 �m for all micrographs: (a) kaolinite in distilledwater, (b) kaolinite in 0.5 N NaCl, (c) illite in distilled water,(d ) illite in 0.5 M NaCl, (e) montmorillonite in distilled wa-ter, and (ƒ) montmorillonite in 0.5 NaCl (from Osipov andSokolov, 1978).

Figure 5.25 Honeycomb microfabrics: (a) recent lacustrinesilt from Lake Vozhe and (b) recent marine silt from theBlack Sea (from Sergeyev et al., 1980). Reprinted with per-mission from Blackwell Scientific Publications, Ltd.

placed in a very thin layer (20 to 30 nm) in a vacuumevaporator is often used.

The main difficulty in the electron microscope studyof fabric is the preparation of sample surfaces, surfacereplicas, or ultra-thin sections that retain the undis-turbed fabric of the original soil. In general, the higherthe water content and void ratio of the original sample,the greater the likelihood of disturbance. Soils contain-ing expansive clay minerals may undergo changes inmicrofabric as a result of removal of interlayer water,or there may be shrinkage. The dry–fracture–peeltechnique and the freeze–fracture technique appear thebest of the available methods for obtaining represen-tative sample surfaces.

That careful techniques are successful in preservingdelicate fabrics is evidenced by Fig. 5.24, which showsthe microstructures of six artificial clay sediments (Os-ipov and Sokolov, 1978). These samples were obtainedby gradual sedimentation of clay particles �1 �m in

size from a 1 percent suspension followed by freeze-drying. When sedimented in distilled water, the sedi-ment porosities were kaolinite 96 percent, illite 90percent, and montmorillonite 83 percent. When sedi-mented in electrolyte solution, the porosities were 97,98 and 99 percent, respectively. The photomicrographsreflect the very high porosities of all samples and thatthe flocculating effect of the salt solution had a signif-icant effect on the initial microfabric.

Undisturbed silt microfabrics are shown in Fig. 5.25.These silty clay microfabrics are formed under condi-tions of uninterrupted sediment accumulation and havequite high porosities (60 to 90 percent). Sediments ofthis type are very compressible and weak.

Progressive collapse of microfabric of a sensitiveChamplain clay with increasing vertical loading isshown in Fig. 5.26 (Delage and Lefebvre, 1984). Thepreconsolidation pressure of the clay was 54 kPa. TheSEM photos were taken along the vertical plane andthe distribution of macropores at each loading stagewas derived from the photos as shown in the figure.Aggregate structure is apparent at the intact stage be-low the preconsolidation pressure. At a loading of 124

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(a) (b) (c) (d)

10 μm intact 10 μm 124 kPa 10 μm 421kPa 10 μm 1452 kPa

Pores Solid particlesVoids due topulling out ofparticles

Figure 5.26 SEM photographs of a sensitive Champlain clay under consolidation at (a)intact state, (b) 124 kPa, (c) 421 kPa, and (d ) 1452 kPa. The preconsolidation pressure ofthe clay is 54 kPa (from Delage and Lefebvre, 1984).

kPa, the collapse of macropores in the horizontal di-rection is observed. Aggregates are also aligning in thehorizontal direction. As the loading increases (421 and1452 kPa), aggregates become less apparent by thecomplete collapse of macropores and the particles arealigning in the horizontal direction. Although the fieldof view at high magnification is limited, mosaics ofphotomicrographs may be prepared to show larger fab-ric features. Such a composite is shown in Fig. 5.6.

Accessories are available for the SEM to enable de-termination of the elemental composition of specificmaterials under observation (McHardy and Birnie,1987; Bain et al., 1994). Further details on the tech-niques of electron microscopy used to examine thestructures of soils can be found in Smart and Tovey(1981, 1982).

Environmental SEM

Conventional SEM samples have to be dry, vacuumcompatible, and electrically conductive. To examineliquid and hydrated samples, the pressure has to be atleast 612 Pa, the minimum vapor pressure required tomaintain liquid water at 0�C. An environmental scan-ning electron microscope (ESEM) allows wet, natural,and nonconductive samples to be examined by havingthe specimen chamber at higher pressure separatedfrom the high-vacuum electron optical regions in

which the SEM electromagnetic lens must exist. Thispressure differentiation is achieved by a special devicecalled a pressure-limiting aperture. Examination ofsamples can be done under a range of gaseous envi-ronments (H2O, CO2, N2, etc.), relative humidities (0to 100 percent), pressures (up to 6.7 kPa), and tem-peratures (�180 to 1500�C). ESEM images are takenusing an electrical current detector that collects andprocesses signals generated by ionized gas molecules(usually water vapour) in the specimen chamber. Sec-ondary electrons emitted by the sample collide withgas molecules, which then cause ionization of the gas,creating positive ions and additional secondary elec-trons. The cascading amplification of the signal fromthe original secondary electrons enables the secondaryelectron detector to create an image. The positive ionsare attracted to the negatively charged sample surfaceand suppress the charging artefacts. This charge sup-pression allows imaging of nonconductive samples.

A significant feature of ESEM is its ability to ob-serve liquids inside the samples. The rate of sublima-tion and condensation of water can be controlled bymanipulating the pressure and temperature. Figure 5.27is an ESEM image of a sample containing illite clays(left side) and quartz grains (right side). Water dropletswere placed on the sample by condensation of distilledwater present as a gaseous phase in the testing cham-

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Figure 5.27 ESEM image of illite clay (left side) and quartzgrains (right side). Water droplets placed on the samplesshow that the quartz surface is hydrophilic and the illite sur-face is hydrophobic (from Buckman et al., 2000).

Figure 5.28 ESEM images showing swelling process of bentonite clay in a sand–bentonitemixture (from Komine and Ogata, 2004).

ber. The photo shows the wettability of fluids on soilminerals. Spherical water droplets are observed on theclay surface, indicating that this illite is hydrophobic.Quartz sand, on the other hand, is hydrophilic as lowdomed droplets of water are formed on the surface.

As pressure and temperature can be varied in thespecimen chamber, the ESEM allows studies of dy-namic changes in samples such as wetting, drying, ab-sorption, melting, corrosion, and crystallization. Figure5.28 shows ESEM images of the swelling of bentonitein a sand–bentonite mixture (Komine and Ogata,2004). Initially, the bentonite particles are attached tothe sand grains and macropores can be observed. As

water is added to the specimen, the bentonite swells tocompletely fill the macropores.

Image Analysis

Image analyzers can be used with both optical andelectron microscopes for quantification of fabric fea-tures. Digital imaging cameras can resolve reflected ortransmitted light from the sample into pixels. Theamount of light per pixel is then converted into ananalog signal. After the entire image is acquired, theanalog signal for each pixel is converted to digital formfor analysis, manipulation, and storage. Image analysisoffers greatly increased potential for quantitative de-scription of different fabric elements. Details of themethod are beyond the scope of this book. Examplesof image analysis of soil specimens are given by Frostand Wright (1993), Tovey and Hounslow (1995), andFrost and McNeil (1998).

X-ray Diffraction

As discussed in Section 3.22, crystallographic planesin minerals refract X-rays at an intensity that dependson (1) the amount of mineral in the volume of soilirradiated and (2) the proportion of the mineral grainsthat are properly oriented. For clay minerals, parallelorientation of plates enhances the basal reflections butdecreases the intensity of reflection from lattice planesoriented in other directions. The intensity of (001) re-flections provides a measure of clay particle orienta-tion.

The relative heights of basal peaks for different sam-ples of the same material give a measure of particle

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orientation differences. A fabric Index (FI) based onareas of diffraction peaks is defined as (Gillott, 1970):

FI � V / (P � V) (5.4)

where V is the area of the basal peak in a section cutperpendicular to the orientation plane, and P is the areaof the same peak from a section cut parallel to theplane of parallel orientation of particles. The value ofFI ranges from zero for perfect preferred orientation to0.5 for perfectly random orientation. A similar proce-dure that retains the concept of peak areas, but doesnot require their exact measurement, is given by Yo-shinaka and Kazama (1973).

The peak ratio (PR), defined as the ratio of the (002)reflection to that of the (020) reflection, can also beused as a measure of orientation. The PR has the ad-vantages of being independent of the particle concen-tration within the total soil and of minimizing theeffects of mechanical and instrumentation variables(Martin, 1966). The PR of kaolinite with completelyrandom particle orientations is about 2.0. For maxi-mum parallel orientation the PR is about 200. The rea-sons for choosing the (002) and (020) reflections arethat (1) they are strong and (2) the corresponding 2�values are not too far apart, thus ensuring that aboutthe same sample volume will be irradiated for deter-mination of both peaks.

X-ray diffraction methods had the advantage ofquantification of data in a way that was not possiblewith optical and electron microscope methods. How-ever, the development of image analysis techniques foruse with the latter has largely overcome this problem.X-ray methods have some disadvantages, including (1)difficult interpretation in multimineral soils, (2) thedata are weighted in favor of the fabric nearest thesample surface, and (3) the soil volume irradiated willusually include both microfabrics and minifabrics, andthe results will average rather than distinguish them.

Thus, X-ray diffraction is best suited for fabric anal-ysis of single mineral clays in which particle orienta-tions over regions the size of the X-ray beam (a fewmillimeters) are of interest or in conjunction with othermethods that can provide detail on the character of themicrofabric.

Transmission X-Ray and Computed TomographyScan

By detecting differences in electron density in mate-rials, transmission X-ray is a useful and nondestructivemethod for the study of soil stratigraphy, homogeneity,and macrofabric. X-radiographs of samples while stillin sample tubes provide information about the above

features as well as on texture and disturbance (Kenneyand Chan, 1972). A number of laboratories routinelyX-ray sample tubes prior to selection of samples forremoval and testing for determination of deformationand strength properties. The procedure is simple, rapid,and inexpensive (apart from the initial cost of theequipment).

X-radiography is also useful for the study of defor-mation patterns in soils. Lead shot is placed in regularpatterns in samples or in blocks of soil used for modeltests. The positions of the shot are determined at var-ious stages throughout a test by comparison of succes-sive radiographs. The results can be used to locateshear zones and compute strains and their variationthroughout the material.

X-ray computed tomography (CT) allows construc-tion of a three-dimensional density profile insidea material by assembling X-ray radiographic two-dimensional images taken at different angles. The res-olution of a CT scanner is determined by thedimensions of a source and a detector as well as theirpositions in relation to the test specimen. The tech-nique has been used to examine the locations of shearzones within a specimen as local dilation inside theshear band gives low electron density (Desrues et al.,1996; Otani et al., 2000; Alshibi et al., 2003; Otaniand Obara, 2004). Figure 5.29 shows the locations ofshear zones in cylindrical sand specimens that weresheared to different axial strains in triaxial compres-sion. The specimens showed strain-softening behaviorand exhibited uniform bulging with no apparent singleor multiple shear bands. The CT images were taken atstrains greater than the peak axial strain of approxi-mately 2 percent. No apparent shear zones are ob-served at an axial strain of 4.6 percent, indicating thatthe strain softening was due to dilation throughout thespecimens. As the axial strain increased, however,shear zones with large local void ratio appeared insidethe specimens. The following two shear zone structuresare apparent (Desrues et al., 1996; Alshibi et al., 2003):

1. Cone-Shaped Shear Zone The images of thehorizontal plane show black circles appearing atthe center, and they become smaller in diameterfrom the boundary toward the middle height ofthe specimen (Fig. 5.29a). This suggests a cone-shaped shear zone from the midheight to theboundary. The tip of the cone is at the midheightand the symmetry exists at the central axis of thespecimen.

2. Conjugate-Inclined Shear Zones The horizon-tally sliced images show radially oriented linesgenerating outward from the circle (Fig. 5.29a).These are the inclined lines in the vertically

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PORE SIZE DISTRIBUTION ANALYSIS 135

Figure 5.29 CT scans of a dense sand specimen under triaxial compression: (a) Horizontalslice at the midheight, (b) vertical slice, and (c) 3D image (from Alshibi et al., 2003).

sliced images (Fig. 5.29b). Close examination ofthese lines reveal that there are several pairs ofconjugate shear bands at two different inclinedangles as shown in Fig. 5.29c.

Further details of shear bands are given in Chapter11. Other noninvasive techniques reported to observeparticle packing arrangements include nuclear mag-netic resonance imaging (Ehrichs et al., 1995; Ng andWang, 2001) and laser-aided tomography (Matsushimaet al., 2002).

5.9 PORE SIZE DISTRIBUTION ANALYSIS

The shape and distribution of voids are one of the threemost important measures of fabric, along with contactdistributions and particle orientations. Pore informationcan be obtained by volumetric pore size distribution

determinations and from image analysis of thin sec-tions and SEM pictures.

Volumetric Pore Size Distribution Determinations

Volumetric pore size distributions can be determinedusing forced intrusion of a nonwetting fluid, a capillarycondensation method based on interpretation of ad-sorption and desorption isotherms, and by removal ofwater by suction or air pressure.

The maximum pore size that can be measured usingthe capillary condensation method is about 0.1 �m.With the possible exception of intraaggregate poresmost soil pores are larger, so this method is of limitedusefulness. The mercury intrusion method, however, isuseful for measurement of pore sizes from about 0.01�m to several tens of micrometers. The basis of themethod is that a nonwetting fluid (fluid-to-solid contactangle �90�) will not enter pores without application of

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Figure 5.30 Pore size distributions in crushed basalt as affected by compaction method.

pressure. For pores of cylindrical shape, the capillarypressure equation applies, and

4� cos �d � � (5.5)

p

where d is the diameter of pore intruded, � is the sur-face tension of the intruding fluid, � is the contact an-gle, and p is the applied pressure.

The volume of mercury intruded into an evacuateddry sample that is about 1 g in weight is measuredusing successively higher pressures. The total volumeof mercury intruded at any pressure gives the total vol-ume of pores with an equivalent diameter larger thanthat corresponding to that pressure. The surface tensionof mercury is 4.84 � 10�4 N/mm at 25�C. The contactangle � is about 140�; measurements by Diamond(1970) gave 139� for montmorillonite and 147� forother clay mineral types.

Limitations of the mercury intrusion method are:

1. Pores must be dry initially. Freeze-dried samplesare often used to minimize the effect of volumechange upon drying.

2. Isolated pores are not measured.3. Pores accessible only through smaller pores will

not be measured until the smaller pores are pen-etrated.

4. The apparatus may not have the capacity to pen-etrate the smallest pores in a sample.

In spite of these limitations, pore size distributionsdetermined by the mercury intrusion method can pro-vide useful information about factors influencing fabricand fabric–property interrelationships. An example isshown in Fig. 5.30. The data are in the form of cu-mulative volumes of pore space intruded for a pore ofthe indicated size and larger. It may be seen that thepores cover a range of sizes and that changes in densityand sample preparation method result in changes inpore size distributions.

Pore size distributions may be estimated for sands,which are too coarse for mercury intrusion, by deter-mination of the pore water volume that is drained ei-ther by application of suction to the sample or byapplication of air pressure to the pore water. Equation(5.5) applies. The surface tension of water, 7.5 � 10�5

N/mm at ordinary temperature, and a contact angle �of 0� should be used.

Image Analysis

The spatial distribution of local voids inside a soilspecimen can be obtained by analyzing the images ob-tained from thin sections. Generally, two image anal-ysis methods are available: (1) method of polygons and(2) mean free path. In the first method the centroids of

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INDIRECT METHODS FOR FABRIC CHARACTERIZATION 137

Figure 5.31 Image analysis methods to determine void fabric: (a) polygon method (afterBhatia and Soliman, 1990) and (b) mean free path method (Kuo et al., 1998).

particles are located and linked to produce polygons,representing individual void elements as shown in Fig.5.31a. Using this method, Bhatia and Soliman (1990)found that looser specimens of sand exhibited a greatervariability in local void ratio than denser specimens.Frost and Jang (2000) used this method to quantify thevariation of local void distribution produced by differ-ent preparation methods. Moist tamped specimens hada higher standard deviation of local void ratio for thesame mean void ratio than air-pluviated specimens.

The mean free path method measures the mean freepath between particles by use of a scanning line thatpasses through both particles and voids as shown inFig. 5.31b. The spacing and orientation of the line arevaried, and a representative void is then produced bysumming over the void lines found on a number ofscanned lines in each direction (Kuo et al., 1998). Us-ing this method, Masad and Muhunthan (2000) foundthat larger local voids exist in the horizontal directionthan the vertical for a pluviated specimen.

5.10 INDIRECT METHODS FOR FABRICCHARACTERIZATION

All physical properties of a soil depend in part on thefabric; therefore, the measurement of a property pro-vides indirect measure of the fabric. Some of the mea-surements that are particularly useful are listed in Table5.3 and are discussed briefly in this section.

Elastic Wave Propagation

The propagation velocities of compression and shearwaves through a soil depend on the density, confining

stress, and fabric of the soil. According to elastic the-ory, which is applicable to soils for the small defor-mations associated with wave propagation, the shearwave (S-wave) velocity Vs and the compression wave(P-wave) velocity Vp are related to the shear modulusG and the constrained modulus M by

V � G /� (5.6)s

and

V � M /� (5.7)p

where � is the mass density.The constrained modulus M is related to the more

familiar Young’s modulus according to

1 � �M � E (5.8)

(1 � �)(1 � 2�)

in which � is Poisson’s ratio. Young’s modulus andthe shear modulus are related to each other by

E � 2(1 � �)G (5.9)

The moduli depend on the applied effective stresses,stress history, void ratio, and plasticity index. For co-hesionless soils the modulus varies approximately asthe square root of the effective confining pressure. Forcohesive soils the modulus varies as the effective con-fining pressure to a power between 0.5 and 1.0. Thesmall strain shear modulus of soil depends on contactstiffness and fabric state. Therefore, the change inshear wave velocity with confining pressure provides

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0 0.2 0.4 0.6 0.8 1

B-value

Vs = 212 (ms)

vb

= 0.25v

b= 0.5

vb

= 0.35v

b= 0.4

Vw = 1492 (m/s)

Dr = 30%σo' = 98 kPa

Vp

Vs

Toyoura sandAir pluviation

Vp2 = Vs

2[ 43– –––––––––]+

2(1 + vb)

3(1 – 2νb) (1 – B)

Vp

& V

s(m

/s)

2000

1500

1000

500

0

Figure 5.32 Variation in P- and S-wave velocities with Bvalue in loose Toyoura sand under an isotropic compressionstress of 98 kPa (after Tsukamoto et al., 2002).

insight on the pressure dependency of contact stiffness.Equations (5.6) and (5.7) assume isotropic elasticity. Ifthe material is viscoelastic, the wave velocities becomefrequency dependent. Solutions for various viscoelasticmodels are given by Santamarina et al. (2001).

If two samples of the same soil have the same massdensity and are under the same effective confiningpressure but have different fabrics, they will have dif-ferent modulus values. This difference will be reflectedby differences in shear and compression wave veloci-ties. These velocities can be measured, and this pro-vides a means for assessing fabric. The shear wavevelocity is the more useful of the two because shearwaves are only transmitted through the solid grainstructure of the soil mass, that is shear waves cannotbe transmitted through water. Anisotropic soil structureand stress states can be detected on the basis of dif-ferent shear wave velocities in different directions. Fur-ther details of the relationships between small strainmoduli and compositional and environmental factorsare given in Chapter 11.

If the material is dry, the bulk modulus of the skel-eton can be derived using both shear wave and com-pression wave velocity measurements. If the materialincludes water, the P-wave velocity depends on theelastic properties of soil solids and water, saturation,and porosity. For fully saturated conditions, solutionsare available for two-phase media (Biot, 1956a, 1956b;Stoll, 1989; Mavko et al., 1998; Santamarina et al.,2001). The solutions show that there are two P-wavesand one S-wave. The fast P-wave and S-wave are thestandard waves and the velocities have weak depend-ency on frequency. The slow P-wave (or Biot wave),which is associated with the diffusional process of wa-ter flow in deforming porous media, especially at lowfrequency, and is very difficult to detect (Plona, 1980;Nakagawa et al., 1997). Hence, the fast P-wave and S-wave are commonly used to characterize the soil.

In fully saturated condition, the fast P-wave propa-gates with a velocity that is 10 to 15 percent fasterthan the velocity through water. This is because thestiffness of the soil skeleton contributes to increasingP-wave velocity. In very loose saturated soil, the P-wave velocity is essentially controlled by the bulkmodulus of water and has a value of about 1500 m/s.When air is introduced, P-wave velocity decreases.Even with a small amount of air, the reduction is dra-matic due to a large decrease in bulk modulus of thefluid–air mixture. The effect of B-value (or water sat-uration ratio Sw) on P- and S-wave velocities of Toy-oura sand specimen (Dr � 30 percent) is shown in Fig.5.32 (Tsukamoto et al., 2002). The fast P-wave veloc-ity at B � 0.95 (Sw � 100 percent) is 1700 m/s,

whereas that at B � 0.05 (Sw � 90 percent) is only500 m/s. The S-wave velocity, on the other hand, isindependent of the water saturation. Kokusho (2000)derives the following relationship that relates the fastP-wave velocity to B value:

4 2(1 � � )bV � V � (5.10)p s 3 3(1 � � )(1 � B)b

where �b is Poisson’s ratio of soil skeleton. Equation(5.10) is plotted in Fig. 5.32 for different �b values.There is a dramatic decrease in P-wave velocity witheven a very small decrease in B value from fully sat-urated conditions.

Dielectric Dispersion and Electrical Conductivity

The flow of electricity through a soil is a composite of(1) flow through the soil particles alone, which issmall, because the solid phase is a poor conductor, (2)flow through the pore fluid alone, and (3) flow throughboth solid and pore fluid. The total electrical flow alsodepends on the porosity, tortuosity of flow paths, andconditions at the interfaces between the solid and liq-uid phases. These factors are, in turn, dependent on theparticle arrangements and the density. Thus, a simple

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INDIRECT METHODS FOR FABRIC CHARACTERIZATION 139

Figure 5.33 Dielectric and conductivity dispersion charac-teristics of saturated illite (Grundite) (from Arulanandan etal., 1973).

measurement of electrical conductivity would seem arapid and reliable means for evaluation of soil fabric.

However, electrical measurements in soils are com-plicated by the fact that if direct current is used, thenthere will be electrokinetic coupling phenomena, suchas electroosmosis, and electrochemical effects that cancause irreversible changes in the system, as discussedin Chapter 9. On the other hand, if alternating current(AC) is used, then the measured responses depend onfrequency. Thus the application of electrical methodsand interpretation of the data require careful consid-eration of how the measurement method may influencewhat is being measured. At the same time, however,measurement of the frequency dependence of electricalproperties can be useful for evaluation of fabric and asan index for engineering properties.

The capacitance C and the resistance R can be mea-sured relatively easily. If electrical flow is in one di-mension only, then the electrical conductivity � isgiven by

� � L / (RA) (5.11)

where L is the sample length and A is the cross-sectional area.

The capacitance can be converted to the relative di-electric constant D (see Chapter 6) using

D � CL / (A� ) (5.12)0

where �0 is the permittivity of vacuum (8.8542 � 10�12

C2 J�1 m�1).In fine-grained materials such as clays, the applica-

tion of an AC field causes the electrical charges thatare concentrated adjacent to particle surfaces to moveback and forth with amplitude dependent on such fac-tors as type of charge, association of charge with sur-faces, particle arrangement, and strength and frequencyof the field. These oscillating charges contribute to apolarization current that can be measured. The numberof charges per unit volume times the average displace-ment is the polarizability. The magnitude of the po-larizability is determined by the composition andstructure of the material and is reflected by the dielec-tric constant.

Phenomena contributing to polarization include di-pole rotation, accumulation of charges at interfaces be-tween particles and their suspending medium, ionatmosphere distortion, coupling of flows, and distortionof a molecular system. The extent to which polariza-tion can develop depends on ease of charge movementand time available for displacement. With increase infrequency the dielectric constant may decrease and the

conductivity may increase. These changes are termedanomalous dispersion. Several regions of anomalousdispersion may develop over the frequency range fromzero to microwave (�1011 Hz). Different polarizationmechanisms cease to be effective above different fre-quency values, thus accounting for the successiveregions of anomalous dispersion. Electrolyte solutionsalone do not exhibit dispersion effects at frequenciesless than 108 Hz, but clays do in the radio frequencyrange. For example, the conductivity and dielectric dis-persion behavior of saturated illite are shown in Fig.5.33.

The electrical response characteristics in the low-frequency range depend on particle size and size dis-tribution, water content, direction of current flowrelative to the direction of preferred particle orienta-tion, type and concentration of electrolyte in the porewater, particle surface characteristics, and sample dis-turbance. Relationships between dielectric propertiesand compositional and state parameters such as poros-ity, particle shape, fabric anisotropy, and specific sur-face area are given by Arulanandan (1991). The theoryis based on Maxwell’s (1881) relationship between po-rosity and the dielectric properties of a mixture of so-lution and spherical particles, and its extension toellipsoidal particles that are all oriented in one direc-tion by Fricke (1953). Extensive discussion of electro-magnetic properties of soils is given in Santamarina etal. (2001).

The formation factor appears in the relationshipsused to describe soil properties and state in terms ofelectrical properties. The formation factor is the ratioof the electrical conductivity of the pore water to theelectrical conductivity of the wet soil. It is a nondi-mensional parameter that depends on particle shape,

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long axis orientation, porosity, and degree of satura-tion. If a soil has an anisotropic fabric, then the for-mation factor is different in different directions.

Thermal Conductivity

Heat transfer through soils is through soil grains, wa-ter, and pore air. As the thermal conductivity of soilminerals is about 2.9 W/(m � �C), and the values forwater and air are 0.6 and 0.026 W/(m � �C), respec-tively, heat transfer is mainly through the soil particles.Accordingly, the lower the void ratio, the greater thenumber and area of interparticle contacts and thehigher the degree of saturation, the higher is the ther-mal conductivity. The thermal conductivity of a typicalsoil is likely to be in the range of 0.5 to 3.0 W/(m ��C). This property is considered in more detail in Sec-tion 9.6.

Thermal conductivity can be determined using a rel-atively simple transient heat flow method in which aline heat source, called a thermal needle, is insertedinto the soil. The needle contains both a heating wireand a temperature sensor. When heat is introduced intothe needle at a constant rate, the temperatures T2 andT1 at times t2 and t1 are related to the thermal conduc-tivity k according to

4 ln(t ) � ln(t )2 1k � � (5.13)Q T � T2 1

where Q is the heat input between t1 and t2. Thismethod and factors influencing the results are de-scribed by Mitchell and Kao (1978).

Differences in thermal conductivity in different di-rections provide a measure of soil anisotropy. Forexample, the ratios of thermal conductivity in thehorizontal direction kh to that in the vertical directionkv for three clays with preferred particle orientations inthe horizontal direction were in the range of 1.05 to1.70, depending on the clay type, consolidation pres-sure, and sample disturbance (Penner, 1963b). For theprobe in the vertical position in a cross anisotropicfabric, the value of k determined from Eq. (5.13) is kh.For the probe in the horizontal direction, a value of ki

is measured that is related to kv and kh according to(Carlslaw and Jaeger, 1957)

2kik � (5.14)v kh

Thermal probe measurements can also be used todetect differences in density at different locations inthe same material (Bellotti et al., 1991) and for eval-uation of changes in density, water content, and struc-

ture caused by mechanically and environmentallyinduced changes in state of the soil.

Mechanical Properties

The mechanical properties of soil, including stress–deformation behavior, strength, compressibility, andpermeability, depend on fabric in ways that are rea-sonably well understood, as considered in Chapter 8.Therefore, information about fabric can be deducedfrom measurements of these properties and known in-terrelationships between properties and fabric.

5.11 CONCLUDING COMMENT

Fabric analyses are useful in research to show howmechanical properties are dependent on particle asso-ciations and arrangements. Fabric information can beused to deduce details of the depositional and postdep-ositional history of a deposit. The effects of differentsampling methods can be assessed through the studyof fabric changes. Insights can be gained into the me-chanics of strength mobilization, the nature of peakand residual strengths, and the stress–strain behaviorof soils from fabric studies.

The indirect methods for fabric study are often use-ful for determination of properties, homogeneity, andanisotropy in situ. They may be of value also for as-sessing whether reconstituted samples used for labo-ratory testing correctly duplicate the field conditions.The particulate nature of soil and the many possibleassociations of discrete particles and particle groupsmean that a soil of given composition can have manydifferent fabrics and exist over a very wide range ofstates, each having its own unique set of geotechnicalproperties.

QUESTIONS AND PROBLEMS

1. Two samples of the same remolded clay have beenconsolidated from the liquid limit to the same watercontent. One was consolidated under an isotropicset of stresses and the other under anisotropicstresses. What differences in fabric would you an-ticipate? Why?

2. Two slurries of the same clay, one with flocculatedclay particles and the other with deflocculated par-ticles, have been consolidated under an effectivestress of 100 kPa. Which will have the higher (a)void ratio, (b) sensitivity, (c) strength? Explain youranswer.

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QUESTIONS AND PROBLEMS 141

Exhibit 5.1 Soil fabrics.

3. A series of shrinkage tests was done on a fine-grained soil mass, and it was found that the shrink-age was a maximum in the Z direction and was aminimum in all directions lying in a plane perpen-dicular to the Z direction.a. Was the soil mass likely to have been isotropi-

cally consolidated or anisotropically consoli-dated?

b. If anisotropically consolidated, what was the ma-jor principal stress direction?

c. Would you expect the soil to be isotropic withrespect to hydraulic conductivity? Why? If ani-sotropic, in which direction would the hydraulicconductivity be greatest? Why?

4. Could X-ray diffraction alone be used to distinguishamong the fabrics shown in Exhibit 5.1? Explainyour answer. Pertinent geometrical parameters oftypical X-ray diffractometers are: distance from X-ray source to sample � 17 cm, divergence of X-raybeam � 1�, angle of incidence of X-ray beam tothe sample surface in the range of 10� to 35�.

5. You are analyzing a new type of laboratory strengthtest that imposes unusual boundary conditions onthe sample being tested. What methods of fabricstudy would you use to examine the location, di-rection, thickness, and fabric of shear zones withinspecimens? What would these methods tell you?

6. Several methods for study and characterization ofsoil fabric are listed in Table 5.3. Indicate some

specific soil types and states for which each of thesemethods might be useful for gaining insights andunderstanding of the macro- and microfabrics andtheir influences on volume change, strength, andpermeability properties.

7. To obtain an essentially undisturbed sample of co-hesionless soil from the field that preserves the insitu fabric is usually impossible without resortingto expensive and time-consuming procedures suchas ground freezing or injection followed by settingof a grout or resin. Suppose that you do not havethe time or budget that will allow this, but wish toreconstitute disturbed specimens of the soil in thelaboratory by forming them in such a way that theywill have fabrics that reasonably duplicate the un-disturbed condition in the field. Suggest practicallaboratory procedures that might be used, startingwith dry and disturbed soil of the type indicated, toreproduce specimens that could then be used forfabric studies and measurements of mechanicalproperties:a. Beach sandb. Alluvial depositc. Wind-blown dune sandd. Uniform sand placed as a hydraulic fille. Uniform sand placed as a hydraulic fill and then

densified using vibratory probesf. Sand fill placed as a pavement base and densified

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