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SOIL STRUCTURE AND FABRIC

The structure of a soil is taken to meanboth the geometric arrangement of theparticles or mineral grains as well as theinterparticle forces which may act betweenthem.

Soil fabric refers only to the geometricarrangement of particles (from Holtz andKovacs, 1981).

*Fabric and structure are used interchangeably sometimes.

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The interparticle forces (or surface forces)are relatively important for fine-grainedsoils at low confinement (low state ofstress).

Although the behavior of a coarse-grainedsoil can often be related to particle sizedistribution, the behavior of a fined-grainedsoil usually depends much more on:

geological history and

structure

than on particle size.

SOIL FABRIC AND STRUCTURE

Fabric is the arrangement of particles, particle group and pore spaces in a soil.

Structure is the combined effects of fabric, composition and interparticle forces.

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Microfabric → at least an optical

microscope is needed.

Macrofabric → stratification, fissuring, voids and

large scale inhomogeneties (by naked eye or a hand lense).

NET ENERGY AND FORCE OF INTERACTION

Dispersion or flocculation →

Fabric of soil →

Determines the engineering properties.

If repulsion → dispersion

If attraction → flocculation

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Very small particles – provide very large surface area.

Negatively charged surface –provide very active surface for chemical interaction.

(From Bennett and Hulbert, 1986)

DISPERSION AND FLOCCULATION OF CLAY

Colloidal clay

Clay is a colloid. Colloidal particles have special properties due to their very small size.

Firstly, their large surface area in relation to their mass makes them very reactive;

in clays, this reactivity is shown as an electrostatic attraction of cations.

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Secondly,

colloids can exist in water as either:

o suspensions (dispersed) or

o as gels (flocculated).

The tendency of a colloid to

o flocculate or

o disperse

depends on three things:

the nature of the colloidal particles;

the total salt concentration;

the nature of the adsorbed ions.

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The type and amount of different cations in a clay-water-electrolyte system have a major influence on double layer interaction.

Flocculation to describe particles that are connected

edge–to–edge or edge–to–face,

Aggregation to

describe particles that are connected

face–to–face.

TERMINOLOGY

Dispersed: No face-to-face association of clay particles

Aggregated: Face-to-face association (FF) of several clay particles.

Flocculated: Edge-to-Edge (EE) or edge-to-face (EF) association

Deflocculated: No association between aggregates or particles.

Face (F)

Edge (E)

Clay Particle

van Olphen, 1991 (from Mitchell, 1993)

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Flocculated fabric Dispersed fabric

Edge-to-face (EF):positively charged edges and negatively charged surfaces (more common)Edge-to-edge (EE)

The net interparticle force between surfaces is repulsive

Aggregated fabric

Face-to-Face (FF)

Shifted

Face-to-Face (FF)

CLAY FABRIC

Flocculated Aggregated

edge-to-face contactface-to-face contact

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ENVIRONMENT EFFECT ON CLAY FABRIC

Electrochemical environment i.e.:

pH,

acidity,

temperature,

cations present in the water

during the time of sedimentation influence clay fabric significantly.

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Flocculation is the first step in aggregate formation.Examples of flocculated and dispersed organic molecules.

Thickness of the diffuse double layer will depend on:

Concentration of soil solution:

High concentration of soil solution yields a thin DDL.

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Valence of exchange ions: Monovalent ions yield a thick DDL

Size of an ion (or hydration radius): Strongly hydrated ions yield a thick DDL.

Particles with thick DDL tend to

DISPERSE

Particles with thin DDL tend to

FLOCCULATE

Colloidal particles are either:

hydrophilic (water-loving) or

hydrophobic (water-hating).

Hydrophilic colloids form

stable suspensions and do not readily flocculate.

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Hydrophobic colloids form

unstable suspensions and flocculate easily.

The nature of the colloidal clay particle (hydrophobic) means that clay

will flocculate if allowed to.

This is good for soil structure!

The more concentrated the salts (electrolytes) in the soil solution,

the more likely it is that clay will flocculate.

This is the 'electrolyte effect'.

The salt is not necessarily common salt, sodium chloride.

Any soluble salt, such as gypsum, will have this effect.

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An 'electrolyte' is any salt.

It is not necessarily common salt (sodium chloride).

It could be any combination of cation and anion.

Salts in soil can come

from the weathering of soil minerals.

Weathering releases cations such as

sodium,

potassium,

calcium,

iron and magnesium.

Anions produced by weathering include:

sulphate,

chloride,

carbonate and phosphate.

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Calcium adsorbed onto the clay surface allows the clay to flocculate

when the total salt concentration is fairly low.

However,

Sodium adsorbed onto the clay surface

will not allow the clay to flocculate

until the total salt concentration is much higher.

Changes in the double layer thickness

modifies the soil properties like:

the shear strength,

compressibility and

plasticity.

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MODES OF PARTICLE ASSOCIATIONS IN CLAY SUSPENSION

1. Dispersed → no face to face association of clay particles.

2. Aggregated → face to face association of several clay particles.

3. Flocculated → edge to edge or edge to face association of particles or aggregates.

4. Deflocculated → no association of particles or aggregates.

PARTICLE ASSOCIATIONS

Dispersed and deflocculated Aggregated but deflocculated

Edge-to-face flocculated and aggregated

Edge-to-edge flocculated and aggregated

Edge-to-face and edge to edge flocculated and aggregated

Edge-to-edge flocculated but dispersed

Edge-to-face flocculated but dispersed

van Olphen, 1991

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FABRIC IN COHESIVE SOILS

Dispersed fabric: formed by settlement of individual clay particles. More or less parallel orientation.

Flocculant fabric: formed by settlement of flocs of clay particles.

Domain: aggregated or flocculated sub-microscopic units of clay particles.

Cluster: domains group to form clusters, can be seen under light microscope.

Peds: they are clusters group to form peds, can be seen without microscope.

DOMAIN CLUSTER PEDThe individual clay particles seem to always beaggregated or flocculated together in submicroscopicfabric units called domains.

Domains then in turn group together to form clusters,which are large enough to be seen with a visible lightmicroscope.

Clusters group together to form peds and even groupsof peds.

Peds can be seen without a microscope, and they andother macrostructural features such as joints andfissures constitute the macrofabric system.

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FABRIC OF NATURAL CLAY SOILS

Enlargement

Domains and clusters with micropores

1.Domain

2.Cluster

3.Ped

4.Silt grain

5.Micropore

6.Macropore

Yong and Sheeran (1973) (from Holtz and Kovacs, 1981)

Diagram of the fundamental particle units called domains that comprise the “building blocks” of clay fabric in sediments and rocks. (From Bennett et al., 1991)

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MICROFABRIC FEATURES IN NATURAL SOILS

1.Elementary particle arrangements, which consistof single forms of particle interaction at the level ofindividual clay, silt, or sand particles or interactionbetween small groups of clay platelets or clothedsilt and sand particles.

2.Particle assemblages, which are units of particleorganization having definable physical boundariesand a specific mechanical function. Particleassemblages consist of one or more forms ofelementary particle arrangements or smallerparticle assemblages.

3.Pore spaces within and between elementaryparticles arrangements and particle assemblages.

Collins and McGown, 1974 (from Holtz and Kovacs, 1981)

ELEMENTARY PARTICLES

Individual clay platelet interaction

Individual silt or sand particle interaction

Clay platelet group interaction

Clothed silt or sand particle interaction

Particle discernible

Collins and McGown, 1974 (from Holtz and Kovacs, 1981)

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PARTICLE ASSEMBLAGES

Collins and McGown, 1974 (from Holtz and Kovacs, 1981)

PARTICLE ASSOCIATIONS IN SOILS

Those main groupings can be identified:

1. Elementary particle arrangements, particle interaction of individual clay, silt or sand particles

2. Particle assemblages

3. Pore spaces

4. Intrapedal pores→ pore within the ped

5. Interpedal pores → pores between the ped

6. Transpedal pores → the pores that transverse the soil beyond the limits of a single ped.

Ped: it is an individual soil aggregate consisting of a cluster of primary particles and separated from adjoining peds by surfaces of weaknesses.

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PORE SPACE TYPES

Collins and McGown, 1974 (from Mitchell, 1993)

EARLY CONCEPTS OF CLAY FABRIC

Minerals of chemically sensitive clays:

in a flocculated system, “cardhouse structure”(flocculated ).

Lambe (1953), particle orientation in a dispersed system is a parallel arrangement (oriented fabric).

Mitchell (1956) pointed out important differences between dispersed and flocculated clays in relation to their geotechnical properties.

Cardhouse, of saltwater Cardhouse of freshwater

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Van olphen proposed various modes of particles association when clay particles flocculate: FF, EF, and EE.

EE and EF produce agglomerates (called “floc”).

FF association is termed “aggregation”.

Flocculation and aggregation have major effects on engineering properties.

Flocculation affects flow behavior.

It influences permeability,

the ease with which a liquid moves through the soil.

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Particles that are dispersed would have less permeability.

Flocculation also affects shear strength and compressibility.

Soils that have an edge-to-face contact of clay particles (flocculated) are much stronger than soils

with a parallel alignment (dispersed).

One effect of the double layer is to cause two

clay particles to repel each other when they approach so closely.

Repulsive forces caused by overlapping double layers have been used to describe the compression and swelling behavior of clays.

Dispersion phenomena is used to explain

erosion of clays and tunneling failures in dams.

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EROSION AND PIPING IN CLAYS

In the past, clay soils were considered to be highly resistant to erosion by flowing water;

however, in the recent years it is recognized that highly erodible clay soils exist in nature.

Some natural clay soils disperse or deflocculate in the presence of relatively pure water and are, therefore, highly susceptible to erosion and piping.

The importance of the subject in civil engineering practice was not recognized

until the early 1960's when research on piping failure in earth dams

due to dispersive clay behavior was initiated

in Australia because of many failures of small clay dams (Aitchison and Wood, 1965).

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The tendency for dispersive erosion in a given soil depends on variables such as:

• mineralogy and chemistry of the clay,

• dissolved salts in the water in soil pores

and in the eroding water.

Such clays are eroded rapidly by slow-moving water, even when compared to cohesionless fine sands and silts.

When dispersive clay soil is immersed in water, the clay fraction behaves like single-grained particles;

that is, the clay particles have a minimum of electrochemicalattraction and

fail to closely

adhere to, or bond with other soil particles.

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Thus, dispersive clay soil erodes in the presence of flowing water when

individual clay platelets are split off and carried away.

Such erosion may start in a drying crack, settlement crack, hydraulic fracture crack, or other channel of high permeability in a soil mass.

SUSCEPTIBILITY TO DISPERSION PIPING

One of the properties controlling the susceptibility to dispersion piping is

the percentage of adsorbed sodium cations within the clay particles relative to the quantities of

other polyvalent cations (calcium, magnesium, and potassium).

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A second factor controlling susceptibility of a clay mass to dispersion piping is the

total content of dissolved salts

in the reservoir or canal water.

The lower the content of dissolved salts in the reservoir or canal water,

the greater the susceptibility of sodium saturated clay to dispersion.

SWELL

Any change in the pore solution chemistry that

depresses or reduces the double layer leads to a

reduction in swell.

Calcium ions in the interlayer region

compress the double layer,

so the sheets are closer together and do not adsorb water and swell as easily.

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If DDL Thickness is small

swell is small.

With sodium ions,

the clay swells more easily.

Thus the clay mineralogy has a direct

effect on

its surface chemistry.

Through its effect on surface chemistry,

clay mineralogy controls

microstructure.

The result is the: engineering behavior of soil, its cohesive strength, flow behavior, permeability, and swelling potential.

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Dispersed fabrics are more common in clays deposited in fresh water,

while flocculated fabrics are typical of seawater deposition.

Remolding (disturbance) of soils alters flocculated fabrics

to dispersed fabrics.

Chemical factors favoring flocculation (favor structure):

High salt concentration Polyvalent cations Low pH

Chemical/physical factors favoring dispersion(unfavorable for structure)

Low salt concentration Sodium is dominant cation High pH Mechanical disturbance

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Saline water applied to soil will allow the clay to flocculate.

If the water is saline due to high levels of soluble calcium, the flocculation will persist.

If, however, the water is saline due to high levels of sodium,

the flocculation will last only as long as the soil solutionremains concentrated.

When rain washes excess salts from the soil,

the soil solution becomes dilute and the clay disperses.

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SALINE WATER

Saline water is a general term for water that contains a significant concentration of dissolved salts (NaCl).

According to United States Geological Surveythree categories of saline water:

Slightly saline water contains around 1,000 to 3,000 ppm,

Moderately saline water contains roughly 3,000 to 10,000 ppm.

Highly saline water has around 10,000 to 35,000 ppm of salt. Seawater has a salinity of roughly 35,000 ppm, equivalent to 35 g/L.

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Gypsum acts on clay in two ways.

Firstly, by raising the level of soluble salts in the soil solution,

gypsum allows the clay to flocculate even if the clay has a high percent of exchangeable sodium (this isthe electrolyte effect).

Secondly, soluble calcium in the gypsum replaces sodium on the cationexchange sites.

The calcium dominated clay will remain flocculated after the free sodium iswashed from the soil and the total salt concentration falls.

In practice, however, several follow-up applications of gypsum are necessary to maintain the electrolyte effect.

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PACKING IN COHESIONLESS SOILS

Dense packingLoose packing

Honeycombed fabric•Meta-stable structure

•Loose fabric

•Liquefaction

•Sand boil

Holtz and Kovacs, 1981

HONEYCOMEDRelatively fine sand and silt form small arches with chains of particles.

Such soils have large void ratio, e and they can carry ordinary static loads.

However under heavy loads or when subjected to dynamic loading, the fabric breaks down causing large settlements.

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PACKING -SAND BOIL

Loose sand

Kramer, 1996

THE RELATIVE DENSITY (DR)The relative density Dr is used to characterize the density of natural granular soil.

%100

%100ee

eeD

mindmaxd

mindd

d

maxd

minmax

maxr

(Lambe and Whitman, 1979)The relative density of a natural soil deposit very stronglyaffects its engineering behavior. Consequently, it isimportant to conduct laboratory tests on samples of thesand at the same relative density as in the field ( from Holtzand Kovacs, 1981). (compaction)

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THE RELATIVE DENSITY (DR) “The relative density (or void ratio)alone is not sufficient tocharacterize the engineeringproperties of granular soils” (Holtz and

Kovacs, 1981). Two soils with the samerelative density (or void ratio) maycontain very different pore sizes.That is, the pore size distributionprobably is a better parameter tocorrelate with the engineeringproperties (Santamarina et al., 2001).

2 1:Holtz and Kovacs, 1981

FABRIC IN COHESIONLESS SOILS

Single grained

Honey combed

Single grained: properties can be studied by uniformly sized spheres.

Type of packing

Coordination number

Porosity

(%)

Void ratio

Single cubic 6 47.64 0.91

Cubical tetrahedral

8 39.54 0.61

Teragonal &

Sphenoidal

10 30.19 0.43

Pyramidal 12 25.95 0.34

Tetrahedral 12 25.95 0.34