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CHAPTER 4

Soil Composition andEngineering Properties

4.1 INTRODUCTION

The engineering properties of a soil depend on thecomposite effects of several interacting factors. Thesefactors may be divided into two groups: compositionalfactors and environmental factors. Compositional fac-tors determine the potential range of values for anyproperty. They include:

1. Types of minerals2. Amount of each mineral3. Types of adsorbed cations4. Shapes and size distribution of particles5. Pore water composition6. Type and amount of other constituents, such as

organic matter, silica, alumina, and iron oxide

The influences of compositional factors on engineeringproperties can be studied using disturbed samples.

Environmental factors determine the actual value ofany property. They include:

1. Water content2. Density3. Confining pressure4. Temperature5. Fabric6. Availability of water

Undisturbed samples, or in situ measurements, are re-quired for the study of the effects of environmentalfactors on properties.

Soils are classified as coarse grained, granular, andcohesionless if the amount of gravel and sand exceeds50 percent by weight or fine grained and cohesive ifthe amount of fines (silt and clay-size material) ex-

ceeds 50 percent.1 The engineering properties of co-hesionless soil are often determined by appliedconfining pressure and looseness or denseness as in-dicated by the relation of the current void ratio to thelowest and highest possible values of void ratio for thesoil. The engineering properties of cohesive soil areoften characterized by stiffness and strength and byrelating the current water content and past consolida-tion history to the compositional characterization pro-vided by the plasticity index. Some engineeringcharacteristics of coarse-grained and fine-grained soilsare listed and compared in Fig. 4.1. Detailed discussionof the combined effects of compositional and environ-mental factors on the three most important propertyclasses for engineering problems, that is, conductivity,volume change, and deformation and strength, is givenin Chapters 9, 10, and 11.

Quantitative determination of soil behavior com-pletely in terms of compositional and environmentalfactors is impractical for several reasons:

1. Most natural soil compositions are complex, anddetermination of soil composition is difficult.

2. Physical and chemical interactions occur betweendifferent phases and constituents.

3. The determination and expression of soil fabricin quantitatively useful ways is difficult.

4. Past geologic history and present in situ environ-ment are difficult to simulate in the laboratory.

5. Physicochemical and mechanical theories for re-lating composition and environment to propertiesquantitatively are inadequate.

1 The terms cohesionless and cohesive must be used with care, aseven a few percent of clay mineral in a coarse-grained soil can impartplastic characteristics.

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

84 4 SOIL COMPOSITION AND ENGINEERING PROPERTIES

Gravel Sand Silt Clay

“Granular Soils” “Fines”

75 mm3 in.

5 mm0.2 in.

0.07 mm0.003 in.

0.002 mm0.00008 in.

Apples toEnglish peas

English peasto baking flour

Finer thanbaking flour

Much finer thanbaking flour

Particles visible without magnificationGrain size measurable with sieves

Particles not visible without magnificationGrain size not measurable with sievesGrain size measured by sedimentation rate

Grains do not form a coherent masseven when wet – cohesionless

Grains stick together when mixed with water dueto pore water suction and physicochemical porefluid-mineral interaction – cohesive

Nonplastic – there is no range of watercontent where the soil can be deformedwithout cracking or crumbling.

Plastic – deforms without cracking over a rangeof water content between the liquid limit and theplastic limit

Liquid (pancake batter)

Plastic (modeling clay)

Semisolid (chocolate bar)

Solid (chalk)

Liquid Limit (LL)

Plastic Limit (PL)

Shrinkage Limit

Permeability is moderate to high (10-6 to 10-1 m/s).Water flows easily through the voids.

Permeability is low to very low (<10-7 m/s).Water flows slowly through the voids.Drainage takes weeks to tens of years.

Drainage occurs rapidly except under dynamicloading; e.g., earthquakes.Only “drained” strength is important for conditionsother than earthquake loading or rapid landslides.

Both “drained” and “undrained” strengths are important.“Undrained” strength is low when preconsolidationpressure is low.

Most important indicators of mechanical behaviorare relative density, Dr, and applied confiningpressure

Dr = 0 to 20% Very looseDr = 20 to 40% LooseDr = 40 to 60% Med. denseDr = 60 to 80% DenseDr = 80 to 100% Very dense

Very loose _ CompressibleLiquefiable during earthquakesφ ~30°

Very dense _ Very low compressibilityStable during earthquakesφ ~45°

Behavior of siltsvaries from “sand-like” to “clay-like” asgrain size decreases

Most important mechanicalbehavior is “preconsolidationpressure pp” and appliedconfining pressure

Very soft – Very highly compressibleUndrained shear strength <12.5 kPa

Very dense _ Low compressibilityUndrained shear strength >100 kPa

pp = 0 to 50 kPa Very softpp = 50 to 100 kPa Softpp = 100 to 200 kPa Firmpp = 200 to 400 kPa Stiffpp = 400 to 800 kPa Very stiffpp = 0.8 to 1.6 MPa Hard

Figure 4.1 Compositional and environmental factors contributing to engineering properties(adapted from course notes by J. M. Duncan, 1994).

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ENGINEERING PROPERTIES OF GRANULAR SOILS 85

Nonetheless, compositional data are valuable for de-velopment of an understanding of properties and forestablishment of qualitative to semiquantitative guide-lines for how real soils behave. Accordingly, somerelationships between compositional factors and engi-neering properties are summarized in this chapter.

4.2 APPROACHES TO THE STUDY OFCOMPOSITION AND PROPERTYINTERRELATIONSHIPS

Study of soil composition in relation to soil propertiesmay be approached in two ways. In the first, naturalsoils are used, the composition and engineering prop-erties are determined, and correlations are made. Thismethod has the advantage that measured properties arethose of naturally occurring soils. Disadvantages, how-ever, are that compositional analyses are difficult andtime consuming, and that in soils containing severalminerals or other constituents such as organic matter,silica, alumina, and iron oxide the influence of any oneconstituent may be difficult to isolate.

In the second approach, the engineering propertiesof synthetic soils are determined. Soils of known com-position are prepared by blending different commer-cially available clay minerals of relatively high puritywith each other and with silts and sands. Although thisapproach is much easier, it has the disadvantages thatthe properties of the pure minerals may not be thesame as those of the minerals in the natural soil, andimportant interactions among constituents may bemissed. Whether the influences of constituents such asorganic matter, oxides and cementation, and otherchemical effects can be studied successfully using thisapproach is uncertain.

Regardless of the approach used, there are at leasttwo difficulties. One is that often the variability in bothcomposition and properties in any one soil deposit maybe great, making the selection of representative sam-ples difficult. Variations in composition and texture oc-cur in sediments within distances as small as a fewcentimeters. Residual soils, in particular, are likely tobe very nonhomogeneous.

A second difficulty is that the different constituentsof a soil may not influence properties in direct or evenpredictable proportion to the quantity present becauseof physical and physicochemical interactions. As anexample of physical interactions, blending of equalproportions of uniform sand and clay, each having acompacted unit weight of 17 kN/m3, would not nec-essarily yield a mixture also having a unit weight of17 kN/m3 after compaction. The resulting unit weightmight be as high as 20 kN/m3 because the clay canfill void spaces between sand particles.

Physicochemical interaction between clay mineralsis shown in Fig. 4.2. Mixtures of bentonite (sodiummontmorillonite) and kaolinite and of bentonite and acommercial illite containing about 40 percent illite claymineral, with the rest mostly silt-sized nonclay, wereprepared, and the liquid limits were determined. Thedashed line in Fig. 4.2 shows the liquid limit values tobe expected if each mineral contributed in proportionto the amount present. The data points and solid linesshow the actual measured values. Although the ben-tonite–kaolinite mixtures gave values close to theoret-ical, the liquid limit values for the bentonite–illitemixtures were much less than predicted. This resultedfrom excess salt in the illite that, when mixed with thebentonite, prevented full interlayer expansion of themontmorillonite particles in the presence of water.

4.3 ENGINEERING PROPERTIES OFGRANULAR SOILS

The mechanical behavior of granular materials is gov-erned primarily by their structure and the applied ef-fective stresses. Structure depends on the arrangementof particles, density, and anisotropy. Particle sizes,shapes, and distributions, along with the arrangementof grains and grain contacts comprise the soil fabric.The packing characteristics of granular materials arediscussed further in Chapter 5.

Particle Size and Distribution

Figure 4.3 illustrates the tremendous range in particlesizes that may be found in a soil, where different sizesare shown to the same scale. The largest size shownrepresents fine sand. It may be recalled that particlesfiner than about 0.06 mm cannot be seen by the nakedeye. The orders of magnitude difference in particlesizes found in any one soil is often better appreciatedfrom a representation such as that in Fig. 4.3 than bythe usual size distribution (or grading) curve whereparticle diameters are shown to a logarithmic scale.

The origin of a cohesionless soil can be reflected byits grading. Alluvial terrace deposits and aeolian de-posits tend to be poorly graded or sorted. Glacial de-posits such as Boulder clays and tills are often wellgraded, containing a wide variety of particle sizes.Small particles in a well-graded soil fit into the voidsbetween larger particles. Well-graded cohesionlesssoils are relatively easy to compact to a high densityby vibration. The loss of fine fraction by internal ero-sion can lead to large changes in engineering proper-ties. Uniformly graded soils are usually used forcontrolled drainage applications because they are notsusceptible to loss of fines by internal erosion and their

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Figure 4.2 Interactions between clay minerals as indicated by liquid limit (data from Seedet al., 1964).

hydraulic conductivity can be maintained within defin-able and narrow limits.

The slope of the grain size distribution curve is char-acterized by the coefficient of uniformity Cu:

d60C � (4.1)u d10

where d60 and d10 correspond to the sieve sizes that 60and 10 percent of the particles by weight pass through.A soil with Cu � 5 to 10 is considered well-graded.

The possible range of packing of soil particles isoften related to the maximum and minimum void ratios(or minimum and maximum densities) reflecting theloosest and densest states, respectively. Uniformlygraded soils tend to have a narrower range of possibledensities compared to well-graded soils. Soils contain-ing angular particles tend to be less dense than soilswith rounded particles, as discussed later in this sec-tion. However, angular and weak materials may crushsignificantly more during compression, compaction, ordeformation. Figure 4.4 shows how the maximum and

minimum void ratios change by mixing sand and siltin different proportions. At low silt contents, silt par-ticles fit into the voids between larger sand particles,so the void ratio of sand–silt mixtures decreases withincrease in silt content. However, at a certain silt con-tent, the silt fully occupies the voids, and the increasein silt content results in sand particles floating insidethe silt matrix. Then, the void ratios increase with fur-ther increase in silt content.

The relative density, DR, a measure of the currentvoid ratio in relation to the maximum and minimumvoid ratios, and applied effective stresses controls themechanical behavior of cohesionless soils. Relativedensity is defined by

e � emaxD � � 100% (4.2)R e � emax min

in which emax, emin, and e are the maximum, minimum,and actual void ratios.

The relative density correlates well with other prop-erties of granular soils. As different standard test meth-

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Figure 4.3 Different grain sizes in soil.

0 10 20 30 40 50 60 70 80 90 1000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Silt content (%)

Maximum void ratio

Minimum void ratio

Voi

d ra

tio

Figure 4.4 Maximum and minimum void ratios of Montereysand–silt mixtures (from Polito and Martin, 2001).

Morphology (large scale) Roundness Texture(intermediate scale)

Surface Texture(small scale)

Roundness Texture(intermediate scale)

Figure 4.5 Scale-dependent particle shape characterization.The solid line gives the particle outline. Morphology de-scribes overall shape of the particle as given by the heavydotted line. Texture reflects the smaller scale local featuresof the particles as identified by light dotted circles. The ex-amples are surface smoothness, roundness of edges and cor-ners, and asperities.

ods can give different limiting void ratios, the use ofthe relative density is sometimes criticized, especiallywhen considered in relation to the random in situ var-iations of the density of most sand and gravel deposits.Nonetheless, if properly interpreted, relative densitycan provide a very useful measure of cohesionless soilproperties.

Particle Shape

Particle shape is an inherent soil characteristic thatplays a major role in mechanical behavior of soils.Characterization of particle shape is scale dependent,as shown in Fig. 4.5. At larger scales, that is, that ofthe particle itself, the particle morphology might bedescribed as spherical, rounded, blocky, bulky, platy,

elliptical, elongated, and so forth. At smaller scales,the texture, which reflects the local roughness featuressuch as surface smoothness, roundness of edges andcorners, and asperities, is important.

With the exception of mica, most nonclay mineralsin soils occur as bulky particles.2 Most particles are

2 Quartz particles become flatter with decreasing size and may havea platy morphology when subdivided to a fineness approaching claysize (Krinsley and Smalley, 1973).

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Figure 4.6 Grain shape distribution of Monterey No. 0 sand. Results are based on study of277 particles, d50 � 0.43 mm, Cu � 1.4 (Mahmood, 1973).

not equidimensional, however, and are at least slightlyelongate or tabular. A frequency histogram of particlelength-to-width ratio (L /W) for Monterey No. 0 sandis shown in Fig. 4.6. This well-sorted beach sand iscomposed mainly of quartz with some feldspar. Themean of all the particle measurements is an L /W ratioof 1.39. This distribution is typical of that for manysands and silty sands.

Particle morphology in soil mechanics has histori-cally been described using standard charts againstwhich individual grains may be compared. A typicalchart and some examples are shown in Fig. 4.7 (Krum-bein, 1941; Krumbein and Sloss, 1963; Powers, 1953).Sphericity is defined as the ratio of the diameter of asphere of equal volume to the particle to the diameterof the circumscribing sphere. Roundness is defined asthe ratio of the average radius of curvature of the cor-ners and edges of the particle to the radius of the max-imum sphere that can be inscribed (Wadell, 1932).Sphericity and roundness are measures of two very dif-ferent morphological properties. Sphericity is most de-pendent on elongation, whereas roundness is largelydependent on the sharpness of angular protrusionsfrom the particle. Different definitions of sphericityand roundness are available, as shown in Table 4.1.Due to the variety of definitions available, the quanti-fication of particle shape requires accurate specifica-tion of their definition.

In recent years, techniques for computer analysis ofshape data by digital imaging have improved greatly,and standard software applications include determina-

tion of aspect ratio and roundness. A convenient wayto characterize particle shapes in more detail is by aFourier mathematical technique. For instance, the (R,�) Fourier method is in the following form:

N

R(�) � a � (a cos n� � b sin n�) (4.3)�0 n nn�1

where R(�) is the radius at angle �, N is the total num-ber of harmonics, n is the harmonic number, and a andb are coefficients giving the magnitude and phase foreach harmonic. The lower harmonic numbers give theoverall shape; for instance, the sphericity is expressedby the first and second harmonics. The coefficient val-ues for higher-order descriptors generally decay withincreasing descriptor or harmonic number, which ex-presses smaller features (i.e., texture) (Meloy, 1977).Other mathematical methods to curve-fit particleshapes are listed in Table 4.1. Further discussion onparticle shape characterization is given by Barrett(1980), Hawkins (1993), Santamarina et al. (2001), andBowman et al. (2001).

In an assembly of uniform size spherical particles,the loosest stable arrangement is the simple cubicpacking giving a void ratio of 0.91. The densest pack-ing is the tetrahedral arrangement giving a void ratioof 0.34. Particle shape affects minimum and maximumvoid ratios as shown in Fig. 4.8 (Youd, 1973). Thevalues increase as particles become more angular orthe roundness (defined as roundness 1 in Table 4.1)

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VeryAngular

Angular Rounded WellRounded

(b)

Roundness

(a)

Sph

eric

ity

High Sphericity

Low Sphericity

Subangular Subrounded

0.9

0.7

0.5

0.3

0.90.70.50.30.1

Figure 4.7 Particle shape characterization: (a) Chart for visual estimation of roundness andsphericity (from Krumbein and Sloss, 1963). (b) Examples of particle shape characterization(from Powers, 1953).

decreases. When R � 1, the particle is a sphere. Asparticles become more angular, R decreases to zero.Void ratios are also a function of particle size distri-bution; the values decrease as the range of particlesizes increases (increase in the coefficient of unifor-mity Cu).

The friction angle increases with increase in particleangularity, possibly as a result of an increase in coor-dination number. For example, values of the angle ofrepose3 are plotted against roundness in Fig. 4.9 and

3 Angle of repose can be determined by pouring soil in a graduatedcylinder filled with water. Tilt the cylinder more than 60� and bringit back slowly to the vertical position. The angle of the residual sandslope is the angle of repose. Further details of the method can befound in Santamarina and Cho (2004).

the following linear fit to the relationship is proposed(Santamarina and Cho, 2004);

� � 42 � 17R (4.4)repose

where R is the coefficient of roundness defined asroundness 1 in Table 4.1. Similar data relating frictionangle from drained triaxial tests and particle shape ispresented by Sukumaran and Ashmawy (2001).

Particle Stiffness

Soil mass deformation at very small strains originatesfrom the elastic deformations at points of contact be-tween particles. Contact mechanics shows that the elas-tic properties of particles control the deformations atparticle contacts (Johnson, 1985), and these deforma-

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Table 4.1 Methods for Particle Shape Characterization

Method Definition

Morphology—Sphere

Sphericity 1 Diameter of a sphere of equal volumeDiameter of circumscribing sphere

Sphericity 2 Particle volumeVolume of circumscribing sphere

Sphericity 3

Projection sphericity Area of particle outlineArea of a circle with diameter equal to the longest length of outline

Inscribed circle sphericity Diameter of the largest inscribed circleDiameter of the smallest inscribed circle

Morphology—Ellipse

Eccentricity �p /Rap, where the ellipse is characterized by Rp � �p cos 2� in polar coordinates

Elongation Smallest diameterDiameter perpendicular to the smallest diameter

Slenderness Maximum dimensionMinimum dimension

Texture—Roundness

Roundness 1 Average of radius of curvature of surface features, (�r ) /Ni

Radius of the maximum sphere that can be inscribed, rmax

Roundness 2 Radius of curvature of the most convex part0.5 (longest diameter through the most convex part)

Roundness 3 Radius of curveture of the most convex partMean radius

Morphology—Texture

Fourier method Eq. (4.3), first and second harmonics, characterize sphericity, whereas higher harmonics(around 10th) characterizes roundness. Surface texture is characterized by muchhigher harmonics.

Fourier descriptormethod

More flexible than the Fourier method by using the complex plane (Bowman et al.,2001). Lower harmonics give shape characteristics such as elongation, triangularity,squareness, and asymmetry. Higher harmonics (larger than 8th) give textural features.

Fractal analysis Use as a measure of texture (Vallejo, 1995; Santamarina, et al. 2001).

From Hawkins (1993), Santamarina et al. (2001), and Bowman et al. (2001).

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0.6

0.6

0.4

0.2

0.8

0.8

1.0

1.2

1.4

1 2 3 4 10 156

SubroundedRounded

Coefficient of Uniformity, Cu

0.350.490.70

0.300.25

0.20

R = 0.17

Subangular

Subangular

Angular

R = 0.20

Angular

R = 0.20

Angular

SubroundedRounded

0.70

0.49

0.35

0.30

0.25

0.20

Max

imum

Voi

d ra

tio, e

max

Min

imum

Voi

d R

atio

, em

in

Figure 4.8 Maximum and minimum void ratios of sands asa function of roundness and the coefficient of uniformity(from Youd, 1973).

50

40

30

20

100.0 0.2 0.4 0.6 0.8 1.0

Roundness R

φrepose = 42 – 17RAng

le o

f rep

ose

φ rep

ose

Figure 4.9 Angle of repose as a function of roundness (fromSantamarina and Cho, 2004).

Table 4.2 Elastic Properties of Geomaterials atRoom Temperature

MaterialYoung’s

Modulus (GPa)Shear Modulus

(GPa)Poisson’s

Ratio

Quartz 76 29 0.31Limestone 2–97 1.6–38 0.01–0.32Basalt 25–183 3–27 0.09–0.35Granite 10–86 7–70 0.00–0.30Hematite 67–200 27–78 —Magnetite 31 19 —Shale 0.4–68 5–30 0.01–0.34

After Santamarina et al. (2001).

tions in turn influence the stiffness of particle assem-blages. Elastic properties of different minerals androcks are listed in Table 4.2. The modulus of a singlegrain, which determines the particle contact stiffness,is at least an order of magnitude greater than that ofthe particle assembly. Further details on the relationbetween particle stiffness and particle assemblage stiff-ness are given in Chapter 11.

Particle Strength

The crushability of soil particles has large effects onthe mechanical behavior of granular materials. At highstresses, the compressibility of sand becomes large asa result of particle crushing, and the shape of an e–logp compression curve becomes similar to that of nor-mally consolidated clay (Miura et al., 1984; Coop,1990; Yasufuku et al., 1991). Under constant states ofstress, the amount of particle breakage increases withtime, contributing to creep of the soil (Lade et al.,1996). The amount of crushing in a soil mass dependsboth on the stiffness and strength of the individualgrains and how applied stresses are transmitted throughthe assemblage of soil particles.

Particle strength or hardness is characterized bycrushing at contacts or particle tensile splitting. Thereis a statistical variation in grain strength for particlesof a specified material and of a given size (Moroto andIshii, 1990; McDowell, 2001). Random variation ingrain strengths leads to distributions of particle sizeswhen large stress is applied to a soil assembly. Table4.3 lists the characteristic tensile strengths of some soilparticles. The values are smaller than the yield strengthof the material itself. The strength also depends on theparticle shape. For example, Hagerty et al. (1993) showthat angular glass beads were more susceptible tobreakage than round glass beads.

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Table 4.3 Strength of Soil Particles

Sand Name Size (mm)37% Tensilea

Strength (MPa)Mean Strengthb

(MPa) Reference

QuartzLeighton Buzzard silica sand 1.18 — 29.8 Lee (1992)

2.0 — 24.73.36 — 20.5

Toyoura sand 0.2 147.4 136.6 Nakata et al. (2001)Aio quartz sand 0.85 51.2 52.1 Nakata et al. (1999)

1.0 47.7 46.61.18 37.9 35.61.4 46.7 42.41.7 39.6 38.5

Silica sand 0.5 147.4 132.5 McDowell (2001)1 66.7 59.02 41.7 37.3

Silica sand 0.28 110.9 147.3 Nakata et al. (2001)0.66 72.9 73.11.55 31.0 29.7

FeldsparAio feldspar sand 0.85 20.9 24.6 Nakata et al. (1999)

1.0 24.3 22.81.18 18.1 18.21.4 23.1 21.41.7 18.9 18.3

Calcareous SandOolitic limestone particle 5 — 2.4 Lee (1992)

8 — 2.112 — 1.820 — 1.530 — 1.340 — 1.250 — 1.1

Carboniferous limestone 5 — 14.9particle 8 — 12.2 Lee (1992)

12 — 10.320 — 8.330 — 7.040 — 6.250 — 5.7

Quiou sand 1 109.3 96.19 McDowell and Amon (2000)2 41.4 36.204 4.2 3.878 0.73 0.63

16 0.61 0.54

OthersMasado decomposed granite

soil 1.55 24.2 22.1 Nakata et al. (2001)Glass beads 0.93 365.8 339.6 Nakata et al. (2001)Angular glass 0.93 62.1 60.0 Nakata et al. (2001)

aStress below which 37% of the particles do not fracture.bForce/d 2 at which particle of size d is crushed.

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Grain size (mm)

100

80

60

40

20

00.01 0.1 1.0

20.7 MPa41.4 MPa62.1 MPa103 MPa345 MPa517 MPa689 MPa

Maximum stress

UncrushedPer

cent

Fin

er b

y W

eigh

t

Figure 4.11 Evolution of particle size distribution curveupon crushing (from Hagerty et al., 1993).

1 5 10 50 1000.2

1.0

0.5

5.0

10

50

Average Particle Size (mm)

Leighton Buzzard Sand

Carboniferous Limestone

Angular River Gravel

Rounded River Gravel

Oolitic Limestone

Par

ticle

Str

engt

h (M

Pa)

Figure 4.10 Relationship between tensile strength and particle size (from Lee, 1992).

The breakage potential of a single soil particle in-creases with its size as illustrated in Table 4.3. This isbecause larger particles tend to contain more and largerinternal flaws and hence have lower tensile strength.Fig. 4.10 shows that oolitic limestone, carboniferouslimestone, and quartz sand exhibit near linear declinesin strength with increasing particle size on a log–logplot (Lee, 1992).

The amount of particle crushing in an assemblageof particles depends not only on particle strength, butalso on the distribution of contact forces and arrange-ment of different size particles. It can be argued thatlarger size particles are more likely to break becausethe normal contact forces in a soil element increasewith particle size and the probability of a defect in agiven particle increases with its size as shown in Fig4.10 (Hardin, 1985). However, if a larger particle hascontacts with neighboring particles (i.e., larger coor-dination number), the load on it is distributed, and theprobability of facture is less than for a condition withfewer contacts. Experimental evidences suggest thatfines increase as particles break by increase in appliedpressure. For example, the evolution of particle sizedistribution curves for Ottawa sand in one-dimensionalcompression is shown in Fig. 4.11 (Hagerty et al.,1993). Hence, the coordination number dominates oversize-dependent particle strength. Larger particles havehigher coordination numbers because they are in con-

tact with many smaller particles. The very smallestparticles have a lower coordination number becausethere are fewer smaller particles available for contact.Hence, the largest particles in the aggregate becomeprotected by the surrounding newly formed smallerparticles, and smaller particles are more likely to break

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Figure 4.12 Weight–volume relationships for a saturated clay-granular soil mixture.

or move. Further details on particle breakage effectson compression behavior of sands are given in Chap-ter 10.

4.4 DOMINATING INFLUENCE OF THE CLAYPHASE

In general, the more clay in a soil, the higher the plas-ticity, the greater the potential shrinkage and swell, thelower the hydraulic conductivity, the higher the com-pressibility, the higher the cohesion, and the lower theinternal angle of friction. Whereas surface forces andtheir range of influence are small relative to the weightand size of silt sand particles, the behavior of smalland flaky clay mineral particles is strongly influencedby surface forces, as discussed in Chapter 6. Water isstrongly attracted to clay particle surfaces, also dis-cussed in Chapter 6, and results in plasticity, whereasnonclay particles have much smaller specific surfaceand less affinity for water and do not develop signifi-cant plasticity, even when in finely ground form.

If it is assumed as a first approximation that all ofthe water in a soil is associated with the clay phase,the amount of clay required to fill the voids of thegranular phase and prevent direct contact betweengranular particles can be estimated for any water con-tent. The weight and volume relationships for the dif-ferent phases of a saturated soil are shown in Fig. 4.12.In this figure W represents weight, V is volume, C isthe percent clay by weight, GSC is the specific gravityof clay particles, w is the water content in percent, w

is the unit weight of water, and GSG is the specificgravity of the granular particles. The volume of voidsin the granular phase is eGVGS, where eG is the voidratio of the granular phase and VGS is the volume ofgranular solids, given by

C Wse V � 1 � e (4.5)� �G GS G100 G SG w

The volume of water plus volume of clay is given by

w W C WS SV � V � � (4.6)W C 100 100 G w SC w

If clay and water completely fill the voids in the gran-ular phase, then

w W C W C Wss � � 1 � e (4.7)� � G100 100 G 100 G w SC w SG w

which simplifies to

w C C eG� � 1 � (4.8)� �100 100G 100 GSC SG

The void ratio of a granular material composed ofbulky particles is of the order of 0.9 in its loosest pos-sible state. The specific gravity of the nonclay fractionin most soils is about 2.67, and that of the clay fractionis about 2.75. Inserting these values in Eq. (4.8) gives

C � 48.4 � 1.42w (4.9)

This relationship indicates that for water contents typ-ically encountered in practice, say 15 to 40 percent,only a maximum of about one-third of the soil solidsneed be clay in order to dominate the behavior by pre-venting direct interparticle contact of the granular par-ticles. In fact, since there is a tendency for clayparticles to coat granular particles, the clay can signif-icantly influence properties. For example, just 1 or 2percent of highly plastic clay present in gravel used asa fill or aggregate may be sufficient to clog handlingand batching equipment.

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ATTERBERG LIMITS 95

Figure 4.13 Plasticity chart.

4.5 ATTERBERG LIMITS

Atterberg limits are extensively used for identification,description, and classification of cohesive soils and asa basis for preliminary assessment of their mechanicalproperties. The potential usefulness of the Atterberglimits in soil mechanics was first indicated by Terzaghi(1925a) when he noted that ‘‘the results of the simpli-fied soil tests (Atterberg limits) depend precisely onthe same physical factors which determine the resis-tance and the permeability of soils (shape of particles,effective size, uniformity) only in a far more complexmanner.’’

Casagrande (1932b) developed a standard device fordetermination of the liquid limit and noted that thenonclay minerals quartz and feldspar did not developplastic mixtures with water, even when ground to sizessmaller than 2 �m. Further studies led to the formationof a soil classification system based on the Atterberglimits for identification of cohesive soils (Casagrande,1948). This system was adopted, with minor modifi-cations, as a part of the Unified Classification System.A plot of plasticity index as a function of liquid limitthat is divided into different zones, as shown in Fig.4.13, is termed the plasticity chart. This chart formsan essential part of the Unified Soil Classification Sys-tem.

Although both the liquid and plastic limits are easilydetermined, and their qualitative correlations with soilcomposition and physical properties are quite well es-tablished, fundamental interpretations of the limits andquantitative relationships between their values andcompositional factors are more complex.

Liquid Limit

The liquid limit test is a form of dynamic shear test.Casagrande (1932b) deduced that the liquid limit cor-responds approximately to the water content at whicha soil has an undrained shear strength of about 2.5 kPa.Subsequent studies have indicated that the liquid limitfor all fine-grained soils corresponds to shearing resis-tance of about 1.7 to 2.0 kPa and a pore water suctionof about 6 kPa (Russell and Mickle, 1970; Wroth andWood, 1978; Whyte, 1982).

Liquid limit values are determined using both theCasagrande liquid limit device and the fall cone device.Different standards adopt different devices and, there-fore, correlations based on liquid limit should be usedwith some caution. The variation of undrained shearstrength with water content can be obtained from aseries of fall cone tests and solutions are available us-ing the theory of plasticity for various geometries usedin fall cones (Houlsby, 1982; Koumoto and Houlsby,

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Table 4.4 Hydraulic Conductivity at Liquid Limit for Several Clays

Soil TypeLiquid Limit,

wL (%)

Void Ratio atLiquid Limit,

eL

HydraulicConductivity(10�7 cm/s)

Bentonite 330 9.240 1.28Bentonite � sand 215 5.910 2.65Natural marine soil 106 2.798 2.56Air-dried marine soil 84 2.234 2.42Oven-dried marine soil 60 1.644 2.63Brown soil 62 1.674 2.83

From Nagaraj et al. (1991).

2001). Furthermore, with the aid of critical state soilmechanics (see Chapter 11), some other engineeringproperties, such as compressibility, can be deduced(Wood, 1990).

Values of hydraulic conductivity at the liquid limitfor several clays are given in Table 4.4, from Nagarajet al. (1991). The striking aspect of these data is that,although the water contents and void ratios at the liq-uid limit for the different clays vary over a very widerange, the hydraulic conductivity is very nearly thesame for all of them. This means that the effective poresizes controlling fluid flow must be about the same forall the clays at their liquid limit. Such a microfabric isconsistent with the cluster model for hydraulic con-ductivity discussed in Chapter 9. In this model, theindividual clay particles associate into aggregates orflocs, as shown schematically in Fig. 9.11. The size ofvoids between the clusters or aggregates controls theflow rate according to either model.

The approximately equal strengths, pore water suc-tions, and hydraulic conductivities for all clays at theirliquid limit can be explained by the concepts that (1)the aggregates or clusters are the basic units that in-teract to develop the strength, that is, the aggregatesact somewhat like single particles, (2) the average ad-sorbed water layer thickness is about the same on allparticle surfaces, and (3) the average size of interclus-ter pores is the same for all clays. Concept 2 providesthe key to why different clays have different values ofliquid limit. All clays have essentially the same surfacestructures, that is, a layer of oxygen atoms in tetrahe-dral coordination with silicon, or a layer of hydroxylsin octahedral coordination with aluminum or magne-sium. The forces of interaction between these surfacesand adsorbed water should be about the same for thedifferent clay minerals. Thus, the amount of water ad-sorbed per unit area of surface that corresponds to apore water suction of 6 kPa should be about the same.This means that the greater the specific surface, the

greater the total amount of water required to reducethe strength to that at the liquid limit. The specificsurface areas of the different clay minerals (Table 3.6)are consistent with the liquid limit values of differentclay minerals in Table 4.5. Additional support for thisconcept is given by the following relationship foundfor 19 British clays:

LL � 19 � 0.56A (�20%) (4.10)s

where LL is the liquid limit and As is the specific sur-face in square meters per gram (Farrar and Coleman,1967).

The effects of electrolyte concentration, cation va-lence and size, and dielectric constant of the pore fluidon the liquid limit of kaolinite and montmorillonite areillustrated and discussed by Sridharan (2002). The ef-fects are generally consistent with the above interpre-tation and can be explained also through double-layer(see Chapter 6) influences on swelling, flocculationand deflocculation of clay particles, and shear strength.

Plastic Limit

The plastic limit has been interpreted as the water con-tent below which the physical properties of the waterno longer correspond to those of free water (Terzaghi,1925a) and as the lowest water content at which thecohesion between particles or groups of particles issufficiently low to allow movement, but sufficientlyhigh to allow particles to maintain the molded posi-tions (Yong and Warkentin, 1966). Whatever the struc-tural status of the water and the nature of theinterparticle forces, the plastic limit is the lowerboundary of the range of water contents within whichthe soil exhibits plastic behavior; that is, above theplastic limit the soil can be deformed without volumechange or cracking and will retain its deformed shape;below the plastic limit it cannot. Plastic limit values

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INFLUENCES OF EXCHANGEABLE CATIONS AND pH 97

Table 4.5 Atterberg Limit Values for the Clay Minerals

Minerala

LiquidLimit(%)

PlasticLimit(%)

ShrinkageLimit(%)

Montmorillonite (1) 100–900 50–100 8.5–15Nontronite (1)(2) 37–72 19–27Illite (3) 60–120 35–60 15–17Kaolinite (3) 30–110 25–40 25–29Hydrated halloysite (1) 50–70 47–60Dehydrated halloysite (3) 35–55 30–45Attapulgite (4) 160–230 100–120Chlorite (5) 44–47 36–40Allophane (undried) 200–250 130–140

a(1) Various ionic forms. Highest values are for monovalent; lowestare for di- and trivalent. (2) All samples 10% clay, 90% sand and silt.(3) Various ionic forms. Highest values are for di- and trivalent; lowestare for monovalent. (4) Various ionic forms. (5) Some chlorites arenonplastic.

Data Sources: Cornell University (1950), Samuels (1950), Lambeand Martin (1955), Warkentin (1961), and Grim (1962).

for different clay minerals are listed in Table 4.5. Theundrained shear strength at the plastic limit is reportedto be in the ranges of 100 to 300 kPa with an averagevalue of 170 kPa (Sharma and Bora, 2003).

Liquidity Index

The liquidity index (LI) is defined by

water content � plastic limitLI � (4.11)

plasticity index

wherein the plasticity index is given by PI � LL �PL. The liquidity index is useful for expressing andcomparing the consistencies of different clays. It nor-malizes the water content relative to the range of watercontent over which a soil is plastic. It correlates wellwith compressibility, strength, and sensitivity proper-ties of fine-grained soils as illustrated in later chaptersof this book.

4.6 ACTIVITY

Both the type and amount of clay influence a soil’sproperties, and the Atterberg limits reflect both of thesefactors. To separate them, the ratio of the plasticityindex to the clay size fraction (percentage by weightof particles finer than 2 �m), termed the activity, isvery useful (Skempton, 1953):

plasticity indexActivity � (4.12)

% � 2 �m

For many clays, a plot of plasticity index versus claycontent yields a straight line passing through the originas shown for four clays in Fig. 4.14. The slope of theline for each clay gives the activity. Approximate val-ues for the activities of different clay minerals arelisted in Table 4.6.

The greater the activity, the more important the in-fluence of the clay fraction on properties and the moresusceptible their values to changes in such factors astype of exchangeable cations and pore fluid composi-tion. For example, the activity of Belle Fourche mont-morillonite varies from 1.24 with magnesium as theexchangeable cation to 7.09 for sodium saturation ofthe exchange sites. On the other hand, the activity ofAnna kaolinite only varies from 0.30 to 0.41 for sixdifferent cation forms (White, 1955).

4.7 INFLUENCES OF EXCHANGEABLECATIONS AND pH

Cation type exerts a controlling influence on theamount of swelling of expansive clay minerals in thepresence of water. For example, sodium and lithiummontmorillonite may undergo almost unrestrictedinterlayer swelling provided water is available,the confining pressure is small, and the electrolyte

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Figure 4.14 Relationship between plasticity index and clay fraction (from Skempton, 1953).

Table 4.6 Activities of Various Clay Minerals

Mineral Activity

Smectites 1–7Illite 0.5–1Kaolinite 0.5Halloysite (2H2O) 0.5Halloysite (4H2O) 0.5Attapulgite 0.5–1.2Allophane 0.5–1.2

concentration is low. On the other hand, divalent andtrivalent forms of montmorillonite do not expand be-yond a basal spacing of about 17 A and form multi-particle clusters or aggregates, regardless of otherenvironmental factors.

In soils composed mainly of nonexpansive clay min-erals, adsorbed cation type is of the greatest impor-tance in influencing the behavior of the material insuspension and the nature of the fabric in sedimentsthat form. Monovalent cations, particularly sodium andlithium, promote deflocculation, whereas clay suspen-

sions ordinarily flocculate in the presence of divalentand trivalent cations.

pH influences interparticle repulsions because of itseffects on clay particle surface charge. Positive edgecharges can exist in low pH environments. These ef-fects are of greatest importance in kaolinite, lesserimportance in illite, and relatively unimportant insmectite. In kaolinite, the pH may be the single mostimportant factor controlling the fabric of sedimentsformed from suspension.

The influences of cations and pH are examined fur-ther in Chapter 6.

4.8 ENGINEERING PROPERTIES OF CLAYMINERALS

Different groups of clay minerals exhibit a wide rangeof engineering properties. Within any one group, therange of property values may also be great. It is afunction of particle size, degree of crystallinity, typeof adsorbed cations, pH, the presence of organic mat-ter, and the type and amount of free electrolyte in thepore water. In general, the importance of these factorsincreases in the order kaolin � hydrous mica (illite) �

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ENGINEERING PROPERTIES OF CLAY MINERALS 99

Table 4.7 Mineral Composition of Different Particle SizeRanges in Soils

ParticleSize(�m)

PredominatingConstituents

CommonConstituents

RareConstituents

0.1 Montmoril-loniteBeidellite

Mica inter-mediates

Illite (traces)

0.1–0.2 Mica inter-mediates

KaoliniteMontmo-rillonite

IlliteQuartz

(traces)0.2–2.0 Kaolinite Illite

Mica inter-mediates

MicasHalloysite

QuartzMontmo-rillonite

Feldspar

2.0–11.0 MicasIllites

QuartzKaolinite

Halloysite(traces)

Feldspars Montmo-rillonite(traces)

From Soveri (1950).

smectite. The chlorites exhibit characteristics in the ka-olin–hydrous mica range. Vermiculites and attapulgitehave properties that usually fall in the hydrous mica–smectite range.

Because of the influences of the above composi-tional factors, only typical ranges of property valuesare given in this section. Factors that determine theactual values in any case are analyzed in more detailin subsequent chapters.

Atterberg Limits

Plasticity values for different clay minerals are listedin Table 4.5 in terms of ranges in the liquid, plastic,and shrinkage limit values. Most of the values weredetermined using samples composed of particles finerthan 2 �m. Several general conclusions can be madeconcerning the Atterberg limits of the clay minerals.

1. The liquid and plastic limit values for any oneclay mineral species may vary over a wide range.

2. For any clay mineral, the range in liquid limitvalues is greater than the range in plastic limitvalues.

3. The variation in values of liquid limit among dif-ferent clay mineral groups is much greater thanthe variation in plastic limits.

4. The type of adsorbed cation has a much greaterinfluence on the high plasticity minerals (e.g.,montmorillonite) than on the low plasticity min-erals (e.g., kaolinite).

5. Increasing cation valence decreases the liquidlimit values of the expansive clays but tends toincrease the liquid limit of the nonexpansive min-erals.

6. Hydrated halloysite has an unusually high plasticlimit and low plasticity index.

7. The greater the plasticity the greater is the shrink-age on drying (the lower the shrinkage limit).

Particle Size and Shape

Different clay minerals occur in different size ranges(Table 3.6) because mineralogical composition is a ma-jor factor in determining particle size. There is someconcentration of different clay minerals in differentbands within the clay size range (less than 2 �m), asindicated in Table 4.7. The shapes of the most commonclay minerals are platy, except for halloysite, whichoccurs as tubes (Fig. 3.21). Particles of kaolinite arerelatively large, thick, and stiff (Fig. 3.13). Smectitesare composed of small, very thin, and filmy particles(Fig. 3.25). Illites are intermediate between kaoliniteand smectite (Fig. 3.29) and are often terraced and thin

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Figure 4.15 Ranges in effective stress failure envelopes forpure clay minerals and quartz (from Olson, 1974). Reprintedwith permission of ASCE.

Figure 4.16 Strength envelopes for a range of soil types(from Bishop, 1966).

at the edges. Attapulgite, owing to its double silicachain structure, occurs in lathlike particle shapes (Fig.3.31).

Hydraulic Conductivity (Permeability)

Mineralogical composition, particle size and size dis-tribution, void ratio, fabric, and pore fluid character-istics all influence the hydraulic conductivity. Thisproperty is considered in detail in Chapter 9. Over thenormal range of water contents (plastic limit to liquidlimit), the hydraulic conductivity of all the clay min-erals is less than about 1 � 10�7 m/s and may rangeto values less than 1 � 10�12 m/s for some of themonovalent ionic forms of smectite minerals at lowporosity. The usual measured range for natural claysoils is about 1 � 10�8 to 1 � 10�10 m/s. For clayminerals compared at the same water content, the hy-draulic conductivities are in the order smectite (mont-morillonite) � attapulgite � illite � kaolinite.

Shear Strength

There are many ways to measure and express the shearstrength of a soil, as described in most geotechnicalengineering textbooks. In most cases, a Mohr failureenvelope, where shear strength (usually peak, criticalstate, or residual) is plotted as a function of the directeffective stress on the failure plane, or a modified Mohrdiagram, in which maximum shear stress is plotted ver-sus the average of the major and minor principal ef-fective stresses at failure, is used. A straight line is fitto the resulting curve over the normal stress range ofinterest and the shear strength � is given by an equationof the form

� � c� � �� tan �� (4.13)n

where is the effective normal stress on the shear��nplane, c� is the intercept for equals zero, often called��nthe cohesion, and �� is the slope, usually called thefriction angle.

Effective stress strength envelopes are useful for re-lating strength to composition. Zones that encompassthe effective stress failure envelopes, based on peakstrength, for pure clay minerals and quartz are shownin Fig. 4.15. The increase in shear strength with in-crease in effective stress, that is, the friction angle, isgreatest for the nonclay mineral quartz, followed indescending order by kaolinite, illite, and montmoril-lonite. The ranges in the position of a failure envelopefor a given mineral result from differences in such fac-tors as fabric, adsorbed cation, pH, and overconsoli-dation ratio. A similar pattern of failure envelopes forsome natural soils is shown in Fig. 4.16. The finer

grained the soil and the greater the amount of clay, thesmaller the inclination of the failure envelope.

From a number of studies [e.g., Hvorslev (1937,1960), Gibson (1953), Trollope (1960), and Schmert-mann and Osterberg (1960), and Schmertmann(1976)], it has been believed that the total strength ofa clay is composed of two distinct parts: a cohesionthat depends only on void ratio (water content), and africtional contribution, dependent only on normal ef-fective stress. Evaluation of these two parts was doneby measurement of the strength of two samples bothat the same void ratio or water content, but at differentlevels of effective stress. This condition is obtained byusing one normally consolidated and one overconsol-idated sample. The strength parameters determined inthis way, often termed the Hvorslev parameters or true

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Figure 4.17 Residual friction angles for clay–quartz mixtures and natural soils (from Ken-ney, 1967).

cohesion and true friction, show increasing cohesionand decreasing friction with increasing plasticity andactivity of the clay.

However, two samples of the same clay at the samevoid ratio but different effective stresses are known tohave different structures, as discussed in Chapter 8.Thus, they are not equivalent, and the strength testsmeasure the effects of both effective stress and struc-ture differences. Furthermore, tests over large rangesof effective stress show that actual failure envelopesare curved in the manner of Fig. 4.16 and that thecohesion intercept is either zero or very small, exceptfor cemented soils. Thus, a significant true cohesion,if defined as strength in the absence of normal stresson the failure plane, does not exist in the absence ofchemical bonding. These considerations are discussedin more detail in Chapter 11.

Even the largest of the friction angle values for clayminerals is significantly less than the residual value forcohesionless soils, wherein values of drained frictionangle are generally in the range of 30� to 50�. Theresidual strengths of some quartz–clay mixtures areshown in Fig. 4.17. If each mineral were an equallyimportant contributor to strength, then the curve for agiven mixture should be symmetrical about the 50 per-cent point, as is the case for kaolinite and hydrous micawith no salt in the pore water. In the other mixtures,however, the clay phase begins to dominate at claycontents less than 50 percent. This is because with ex-pansive clay minerals (montmorillonite) or flocculatedfabrics (30 g salt / liter) the ratio of volume of wet clayto volume of quartz is greater than the ratio of dryvolumes. It is further illustration of the dominating in-fluence of the clay phase discussed earlier.

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Figure 4.18 Compression and unload–reload indices as a function of plasticity index (fromKulhawy and Mayne, 1990). Reprinted with permission from EPR1.

Compressibility

The compressibility of saturated specimens of clayminerals increases in the order kaolinite � illite �smectite. The compression index Cc, which is definedas the change in void ratio per 10-fold increase in con-solidation pressure, is in the range of 0.19 to 0.28 forkaolinite, 0.50 to 1.10 for illite, and 1.0 to 2.6 formontmorillonite, for different ionic forms (CornellUniversity, 1950). The more compressible the clay, themore pronounced the influences of cation type andelectrolyte concentration on compressibility.

Compression index values for a number of differentnatural clays are shown in Fig. 4.18 as a function ofplasticity index (Kulhawy and Mayne, 1990). The val-ues for pure clays plot generally within the definedranges in Fig. 4.18. The compression index for un-loading and reloading is about 20 percent of the valuefor virgin compression.

As both compressibility and hydraulic conductivityare strong functions of soil composition, the coefficientof consolidation cv is also related to composition be-cause cv is directly proportional to hydraulic conduc-tivity and inversely proportional to the coefficient ofcompressibility.4 Values of cv determined in one study

4 The coefficient of compressibility av is the negative of the rate ofchange of void ratio with effective stress.

(Cornell University, 1950) were in the ranges of 0.06� 10�8 to 0.3 � 10�8 m2/s for montmorillonite, 0.3 �10�8 to 2.4 � 10�8 m2/s for illite, and 12 � 10�8 to90 � 10�8 m2/s for kaolinite. Coefficients of consoli-dation for kaolinite, illite, montmorillonite, halloysite,and two-mineral mixtures of these clays ranged from1 � 10�8 m2/s for pure montmorillonite to 378 � 10�8

m2/s for pure halloysite in another study (Kondner andVendrell, 1964). Individual minerals did not influencethe coefficient of consolidation in direct proportion tothe amounts present.

Approximate ranges of the coefficient of consoli-dation for natural clays are given in Fig. 4.19. Theabove values for pure clays and clay mineral mixturesare within the same general ranges. One conclusionthat can be drawn from the comparability of compres-sion index and coefficient of consolidation values fornatural clays with those for pure clays is that the clayphase dominates the compression and consolidationbehavior, with the nonclay material playing a passiverole as relatively inert filler.

Swelling and Shrinkage

The actual amount of volume change of a soil in re-sponse to a change in applied stress depends on theenvironmental factors listed in Section 4.1 as well ason the cation type, electrolyte type and concentration,

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Figure 4.19 Coefficient of consolidation as a function ofliquid limit (from NAVFAC, 1982).

(Sur

char

geP

ress

ure

- 1p.

s.i.)

(Sur

char

geP

ress

ure

- 1p.

s.i.)

(Surc

harg

ePr

essu

re- 1

p.s.

i.)

(Surcharge Pressure - 6.94

p.s.i.)

Figure 4.20 Four correlations between swelling potentialand plasticity index (from Chen, 1975).

and pore fluid dielectric constant. However, the poten-tial total amount of swell or shrinkage is determinedby the type and amount of clay. From a considerationof the clay mineral structures and interlayer bonding(Chapter 3), it would be expected that smectite andvermiculite should undergo greater volume changes onwetting and drying than do kaolinite and hydrous mica.Experience indicates clearly that this is indeed thecase. In general, the swelling and shrinking propertiesof the clay minerals follow the same pattern as theirplasticity properties, that is, the more plastic the min-eral, the more potential swell and shrinkage. Illustra-tions of the influences of adsorbed cation type and porefluid composition are given in Chapter 10 and by Srid-haran (2002).

Because of the many problems encountered in theperformance of structures founded on high volumechange soils, numerous attempts have been made todevelop reliable methods for their identification. Themost successful of these are based on the determina-tion of some factor that is related directly to the claymineral composition, such as shrinkage limit, plasticityindex, activity, and percentage finer than 1 �m.

Simple, unique correlations between swell or swellpressure and these parameters that reflect only the typeand amount of clay are not possible because of thestrong dependence of the behavior on initial state(moisture content, density, and structure) and the otherenvironmental factors. This is illustrated by Fig. 4.20,which shows four different correlations between swell-ing potential and plasticity index (Chen, 1975). Thetwo curves showing the Chen correlations were ob-tained for different natural soils compacted to dry unitweights between 100 and 110 pounds per cubic foot(15.7 and 17.3 kN/m3) at water contents between 15and 20 percent. The large influence of surcharge pres-

sure during swelling is clearly shown. The tests bySeed et al. (1962b) were done using artificial mixturesof sand and clay minerals compacted at optimum watercontent using Standard AASHTO compactive effort al-lowed to swell under a surcharge pressure of 1 psi (7kPa). The measurements by Holtz and Gibbs (1956)were made using both undisturbed and remolded sam-ples allowed to swell from an air-dry state to saturationunder a surcharge of 1 psi (7 kPa).

The results of the tests on artificial sand–clay min-eral mixtures obtained by Seed et al. (1962b) correlatewell with compositional factors that reflect both thetype and amount of clay, that is, the activity A, definedas PI/ C, and the percent clay size C (% � 2 �m),according to

�5 2.44 3.44S � 3.6 � 10 A C (4.14)

where S is the percent swell for samples compactedand tested as indicated above. A chart based on thisrelationship is shown in Fig. 4.21. For compacted nat-ural soils the swelling potential could be related to theplasticity index with an accuracy of �35% accordingto

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Figure 4.21 Classification chart for swelling potential (mod-ified from Seed et al., 1962b).

Figure 4.22 Effect of amount and type of clay on ‘‘steady-state’’ creep rate (see Chapter 12).

�3 2.44S � 2.16 � 10 (PI) (4.15)

Somewhat different relationships have been found tobetter classify the swell potential of some soils, and nosingle relationship is suitable for all conditions. Thus,while the above relationships and plots such as Figs.4.20 and 4.21 illustrate the influences of compositionalfactors and provide preliminary guidance about the po-tential magnitude of swelling, reliable quantification ofswell and swell pressure in any case should be basedon the results of tests on representative undisturbedsamples tested under appropriate conditions of con-finement and water chemistry.

Time-Dependent Behavior

Different soil types undergo varying amounts of time-dependent deformations and stress variations withtime, as exhibited by secondary compression, creep,and stress relaxation. The potential for these phenom-ena depends on compositional factors, whereas the ac-tual amount in any case depends on environmentalfactors. For example, it is known that retaining wallswith wet clay backfills must be designed for at-restearth pressures because of stress relaxation along a po-tential failure plane that results in increased pressureon the wall. On the other hand, if dry clay is used, andif it is maintained dry, then designs based on activepressures are possible because time-dependent in-creases in pressure will be negligible.

In general, the greater the organic content and thewetter and more plastic the clay, the more pronouncedis the time-dependent behavior. Both the type andamount of clay are important, as indicated, for exam-ple, by the variation of creep rate with clay content forthree different clay mineral–sand mixtures, as shownin Fig. 4.22. In these tests, environmental factors wereheld constant by preparing all specimens to the sameinitial conditions (isotropic consolidation of saturatedsamples to 200 kPa) and application of a creep stressequal to 90 percent of the strength determined by anormal strength test. The variation in creep rate forthese specimens as a function of plasticity index isshown in Fig. 4.23. The correlation is reasonablyunique because the plasticity index reflects both thetype and amount of clay.

4.9 EFFECTS OF ORGANIC MATTER

Organic matter in soil may be responsible for highplasticity, high shrinkage, high compressibility, low

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CONCLUDING COMMENTS 105

Figure 4.23 Relationship between clay content, plasticity in-dex, and creep rate.

hydraulic conductivity, and low strength. Soil organicmatter is complex both chemically and physically, andmany reactions and interactions between the soil andthe organic matter are possible (Oades, 1989). It mayoccur in any of five groups: carbohydrates; proteins;fats, resins, and waxes; hydrocarbons; and carbon. Cel-lulose (C6H10O5) is the main organic constituent ofsoil. In residual soils organic matter is most abundantin the surface horizons. Organic particles may rangedown to 0.1 �m in size. The specific properties of thecolloidal particles vary greatly depending upon parentmaterial, climate, and stage of decomposition.

The humic fraction is gel-like in properties and neg-atively charged (Marshall, 1964). Organic particles canstrongly adsorb on mineral surfaces, and this adsorp-tion modifies both the properties of the minerals andthe organic material itself. Soils containing significantamounts of decomposed organic matter are usuallycharacterized by a dark gray to black color and an odorof decomposition. At high moisture contents, decom-

posed organic matter may behave as a reversible swell-ing system. At some critical stage during drying,however, this reversibility ceases, and this is oftenmanifested by a large decrease in the Atterberg limits.This is recognized by the Unified Soil ClassificationSystem, which defines an organic clay as a soil thatwould classify as a clay (the Atterberg limits plotabove the A line shown in Fig. 4.13) except that theliquid limit value after oven drying is less than 75 per-cent of the liquid limit value before drying (ASTM,1989).

Increasing the organic carbon content by only 1 or2 percent may increase the limits by as much as anincrease of 10 to 20 percent in the amount of materialfiner than 2 �m or in the amount of montmorillonite(Odell et al., 1960). The influences of organic mattercontent on the classification properties of a soft clayfrom Brazil are shown in Fig. 4.24.

The maximum compacted densities and compressivestrength as a function of organic content of both nat-ural samples and mechanical mixtures of inorganicsoils and peat are shown in Figs. 4.25 and 4.26, re-spectively. Both the compacted density and strengthdecrease significantly with increased organic contentand the relationships for natural samples and themixtures are about the same. Increased organic contentalso causes an increase in the optimum water contentfor compaction.

The large increase in compressibility as a result ofhigh organic content in clay is illustrated by the datain Fig. 4.27 for the clay whose classification propertiesare shown in Fig. 4.24. In Fig. 4.27 CR is the com-pression ratio, defined as CC / (1 � e0) expressed as apercentage, and C� is the secondary compression ratio,defined as the change in void ratio per 10-fold increasein time after the end of primary consolidation.

The effect of organic matter on the strength andstiffness of soils depends largely on whether the or-ganic matter is decomposed or consists of fibers thatcan act as reinforcement. In the former case, both theundrained strength and the stiffness, or modulus, areusually reduced as a result of the higher water contentand plasticity contributed by the organic matter. In thelatter, the fibers can act as reinforcements, thereby in-creasing the strength.

4.10 CONCLUDING COMMENTS

Knowledge of soil composition is a useful indicator ofthe probable ranges of geotechnical properties andtheir variability and sensitivity to changes in environ-mental conditions. Although quantitative values ofproperties for analysis and design cannot be derivedfrom compositional data alone, information on com-

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Figure 4.24 Influence of organic content on classification properties of Juturnaiba organicclay, Brazil (from Coutinho and Lacerda, 1987).

position can be helpful for explaining unusual behav-ior, identification of expansive soils, selection ofsampling and sample handling procedures, choice ofsoil stabilization methods, and prediction of probablefuture behavior.

For example, if it is known that a soil to be used inearthwork construction contains either hydrated halloy-site, organic matter, or expansive minerals, then air-drying laboratory samples prior to testing is likely toresult in erroneous data on mechanical properties andmust be avoided. If a soil contains a large amount ofactive clay minerals, then it can be anticipated thatproperties will be sensitive to changes in chemical en-vironment. Compositional data on the soil and porewater are useful to estimate the dispersion and erosionpotential of a soil (Chapter 8) and the risk of instabilityas a result of leaching and solutioning processes.

In many cases, the effects of composition on behav-ior are reflected by information on particle size, shape,and size distribution of the coarse fraction, and theAtterberg limits of the fine fraction. On large projectsand whenever unusual behavior is encountered, how-ever, compositional data are valuable aids for interpre-tation of observations. Furthermore, the influences ofcompositional and structural factors are not always ad-equately reflected by the usual classification properties,

and more direct evaluation of their significance isneeded. Examples of some soil types in which thesefactors may be especially important are decomposedgranite, tropical residual soils, volcanic ash soils, col-lapsing soils, loess, and carbonate sand, as discussedin more detail by Mitchell and Coutinho (1991).

QUESTIONS AND PROBLEMS

1. Show that the loosest and densest packings of uni-form size particles give void ratios of 0.91 and 0.34,respectively. What is the coordination numbers(number of particle contacts for each particle) foreach packing?

2. Explain why smaller particles are stronger thanlarger particles and why angular particles are moresusceptible to breakage than round particles.

3. Using Figs. 4.8 and 4.11, show how the maximumand minimum void ratio changes with applied loadas particles progressively break and the coefficientof uniformity Cu increases. Plot the data in e–log�v space and discuss the result.

4. Using Eq. (4.8), derive a relationship between C(the percentage of clay) versus w (water content)

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QUESTIONS AND PROBLEMS 107

Figure 4.25 Maximum dry density as a function of organiccontent for a natural soil and soil–peat mixtures (from Frank-lin et al., 1973). Reprinted with permission of ASCE.

Figure 4.27 Effect of organic content on the compressibilityproperties of Juturnaiba organic clay, Brazil (from Coutinhoand Lacerda, 1987).

Figure 4.26 Unconfined compressive strength as a functionof organic content for a natural soil and soil–peat mixtures(from Franklin et al., 1973). Reprinted with permission ofASCE.

for different values of eG (the void ratio of the gran-ular phase). Discuss the sensitivity of eG on sand–clay mixture packing. What happens if silt is mixedinstead of clay?

5. Using the reported undrained shear strengths at liq-uid limit and plastic limit, derive a relationship be-tween the compression index Cc and plasticity indexPI. Assume that the ratio of undrained shearstrength to vertical effective stress, su / , is 0.3.��v

Compare the result with the data presented in Fig.4.18.

6. Assuming the thickness of adsorbed water layer is100 A, estimate the amount of free water per gramof clay for the following conditions and discuss theresults:a. Montmorillonite at its liquid limit with mono-

valent adsorbed cations (specific surface � 840m2/g of dry clay), liquid limit � 900 percent

b. Montmorillonite at its plastic limit with mono-valent adsorbed cations (specific surface � 840m2/g of dry clay), plastic limit � 100 percent

c. Montmorillonite at its liquid limit with divalentadsorbed cations (specific surface � 50 m2/g ofdry clay), liquid limit � 100 percent

d. Montmorillonite at its plastic limit with divalentadsorbed cations (specific surface � 50 m2/g ofdry clay), plastic limit � 50 percent

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e. Kaolinite at its liquid limit (specific surface �15 m2/g of dry clay), liquid limit � 70 percent

f. Kaolinite at its plastic limit (specific surface �15 m2/g of dry clay), plastic limit � 30 percent

7. By examining the data presented in Figs. 4.24 and4.29, discuss why organic clays exhibit larger com-pressibility compared to inorganic clays (see Fig.4.18).

8. Assume that you are able to determine accurate,reliable quantitative values for all details of the min-eralogical, chemical, and biological constituents ofa given soil. All particle sizes, shapes, and distri-butions are also known. Speculate on your abilityto predict the volume change, strength, and per-meability properties of this soil over a range of wa-ter contents. Give reasons for why you would havelow or high confidence in your predictions.

9. In light of what is known about the dependence ofengineering properties on soil composition—bothof the particles and of the other phases present in a

soil—discuss the strengths and weaknesses of theUnified Soil Classification System (USCS) in pro-viding a clear and unambiguous picture of the prob-able behavior of the following soil types. Indeveloping your answer, be specific concerningwhat is measured and the terms of reference usedin the USCS and what is most important in deter-mining any property being discussed. (Note: Someof the information in Chapter 8 may be useful indeveloping your answer to this question.)a. Clean sandb. Decomposed granitec. Calcareous sandd. Organic silte. Expansive clayf. Glacial tillg. Loessh. Dispersive clayi. Volcanic ashj. Estuarine mud

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