soil Composition Structure Classification

7/24/2019 soil Composition Structure Classification

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CE 601: Soil Composition,Structure & Classification

Earth and its Interior

8-35 km crust% b wei ht in crustC

O = 49.2Si = 25.7

Al = 7.5

Fe = 4.7Ca = 3.4

Na = 2.6

82.4%

IC OC

M

K = 2.4Mg = 1.9

other = 2.6

12500 km dia

IC = Inner CoreOC = Outer CoreM = MantleC = Crust


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Physical Properties atInner Core & Crust of the Earth

Inner Core Crust

Temperature ~ 25000C ~ 250C

~ 4 million

atmospheres

Density ~13.5 g/cc ~1.5 g/cc

Soil Formation: Rock Cycles

(http://www.uen.org/utahlink/activities/uploads/104

74_a_cycle.gif)


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TypesofRock

I neous Sedimentary Metamor hic

Formedbycoolingofmolten

magma(lava)

Formedbygradualdeposition,andin

layersFormedbyalterationofigneous&

sedimentaryrocksby

pressure/temperature

e.g.,Limestone,Shale

e.g.,Marble

e.g.,Granite

Residualsoil Transportedsoil

~insituweathering

(byphysical&chemical

agents)ofparentrock(bywind,waterandice)

~weatheredandtransported

faraway

Soil Formation: Bowens Reaction

Series

More stable

Higher weathering

resistance

Main mineralconstituent in Sands


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Stages: Formation of Soil from Rock

Weathering

Physical weathering Chemical weathering

Unloading

e.g. uplift, erosion, or change influid pressure.

Thermal expansion andcontraction

Alternate wetting and drying

y ro ysis

is the reaction with water

will not continue in the staticwater.

involves solubility of silica andalumina

Chelation

Involves the com lexin and

rys a grow , nc u ng rosaction

Organic activity

e.g. the growth of plant roots.

removal of metal ions .

Cation exchange

Oxidation and reduction.

Carbonation

is the combination of carbonate ionssuch as the reaction with CO2


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Weathering: Effects of Climate, Topography, ParentMaterial, Time & Biotic Factors

Wet climate and good drainage; both accelerate weathering

For a given amount of rainfall, chemical weathering rate is higher inwarmer climates

Water table influences weathering by determining the depth towhich air is available

Type of rainfall: short, intense rainfall erosion;

light, prolonged rainfall leaching

Topograpghy: important factor in determining rates of erosion, ratesof soil accumulation

Steep topography: encourages mechanical weathering Vegetation affects rate of erosion

Organic compound aid weathering

Residual Soils

Soil formed by in-situweatherin

The top layer of rock isdecomposed into residual soilsdue to the warm climate andabundant rainfall .

Depth of profile variesdepending on climate, parent

ma er a , ra nage con ons,water table

Engineering properties ofresidual soils are different withthose of transported soils


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Transported Soils

Trans orted b : Soil De osit:

River (running water) Alluvial

Lake (fresh water) Lacustrine

Sea (salt water) Marine

Wind Aeolian

Ice Glacial

Effects of Method of Transportationon Soil formation

Water Air Ice Gravity Organisms

Size Ma or reduction Considerable Considerable Considerable Minor

through solution,little abrasion in

suspended load

reduction grinding andimpact

impact abrasion fromdirect organic

transportation

Shape and

roundness

Rounding of sand

and gravel

High degree

of rounding

Angular

particles

Angular

non-spherical

Surface

texture

Smooth polished,

shiny particles of

sand

Impact

produces

frosted

surfaces

Striated

surfaces

Striated

surfaces

Sorting Considerable

sorting

Considerable

sorting

(progressive)

Very little

sorting

No sorting Limited

sorting


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Different Soils Formed by theMethod of Transportation

1) Loess: Loose deposit of wind-blown silt

2) Tuff: Fine grained Slightly cemented volcanic ash

3) Bentonite: Chemically weathered volcanic ash

4) Glacial Till: Mixture of boulders, gravel, sand, silt and clay(usually called as boulder clay)

arve ay: Alternate thin layers of silt and clay

6) Marl: Fine grained marine soil

7) Gumbo: Sticky, Plastic, dark colored clay

Different Soils Formed by theMethod of Transportation

8) Peat: Highly Organic soil, good for vegetation

9) Muck: Mixture of fine grained inorganic soil and decomposedorganic matter (imperfect drainage)

10) Humus: Organic amorphous soil (consisting of partlydecomposed vegetative matter)

11) Hard Pan: Extremely hard cohesive soil

ccumu a on o roc e r s a e ase o roc .Its position results mainly from the effect of gravity force acting onthe rock fragments

13) Mine Tailings: Silt sized material (waste from extraction ofminerals from natural rock)


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Regional Soil Deposits of India

a) Marine deposits

b) Lateritic soils

c) Black cotton soils

following groups:

d) Alluvial soils

e) Desert soils

f) Boulder deposits

Soil Map of India


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a) Marine deposits:ery so c ay, may con a n organ c ma ers

Low shear strength and low compressibilityFound all along the coast in tidal plains of India

b) Lateritic soils:Decomposition of rock, removal of bases & silicaShear strength depends on the stage of weatheringKerala, Karnataka, Maharash., Orissa & Bengal(Total area covers around 1,00,000 sq. km)

c) Black cotton soils:


Contains Montmori onite c ay, responsi e orexcessive swelling and shrinking

Shear strength depends on volume change in soilMaharashtra, MP, UP, AP, Karnataka, & TN(Total area covers around 3,00,000 sq. km)

d) Alluvial soils:Contains alternating layers of sand, silt and clayProne to liquefaction under earthquake shocksExtends from Assam (East) to Punjab (West)


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e) Desert soils:


Non plastic uniformly graded fine sand

Strength depends upon the permeability of soilLarge Part of Rajasthan (covers 5,00,000 sq. km)

f) Boulder deposits:ontains a ternating ayers o san , si t an c ay

Strength cant be measured in the lab due to its big

size soil particles, shear box tests are performed inthe field for obtaining its strengthSub-Himalayan region of HP and Uttaranchal

Soil Groups Based on its Particle Size

-

Clay minerals

0.002 300804.750.075

BoulderClay Silt Sand Gravel Cobble

Granular soils orCohesion less soils

Cohesivesoils

0.425 2.0

Fine Medium Coarse Fine Coarse

20

20

Fine grainsoils

Coarse grainsoils

Grain size (mm) (IS code)


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General Characteristics of SoilsSoil Characteristics Gravel, Sand Silt Clay

Grain size Granular, Coarse-grained,

articles can be seen

Fine-grained, can

not see individual

Fine-grained, can

not see individual

through naked eyes particles particles

Plasticity and Cohesion Non-plastic, Cohesion less Slightly or no

plasticity, Cohesion

Plastic, Cohesive

Effect of grain size

distribution (Sieve analysis)

Important Less important Unimportant

Effect of water (Atterberg

limits)

Unimportant (except for

loose saturated soils under

dynamic loadings)

Important Very important

Permeability and Drainage Pervious, Freely draining Less pervious Almost impervious

Compressibility Low Medium High

Shear Strength Depends on relative

density (generally high)

Intermediate Depends on

consistency

(generally poor)

Grain Size Distribution

60u

DC

Coefficient of Uniformity

Poorly Graded

Well Graded

GapGraded

30

For Gravel:Cu < 4 Poorly gradedCu > 4 Well graded

or Gap graded

For Sand:Cu < 6 Poorly gradedCu > 6 Well graded

or Gap graded

2

30

60 10

c

DC

D D

Coefficient of Curvature

1 < Cc


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Grain Size Distribution Curve

Gravel: Sand:

Soil Texture

Particle size, shape and size distribution- ,

Fine-textured (Silt, Clay) Visibility by the naked eye (0.05mm is the approx

limit)

Particle size distribution Sieve/Mechanical analysis or Gradation Test H drometer anal sis for smaller than .05 to .075 mm

(#200 US Standard sieve) Particle size distribution curves

Well graded Poorly graded 60

10

u

DC

D

2

30

60 10

c

DC

D D


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Effect of Particle size

Particle Assemblage: Void RatioTypical values

Simple cubic (SC), e = 0.91, Contract

Cubic-tetrahedral (CT), e = 0.65,

Dilate

Volume change tendencyStrength

(Lambe and Whitman, 1979)


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Relative Density

1.0

max

max min

re eD

e e

Voidratio(e)

0.8

0.6

0.4

emaxDr = 0%

e0%


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Typical Values of Atterberg Limits

(Mitchell, 1993)

Indices

Plasticity index PI

For describin the ran e of

Liquidity index LI

For scalin the natural waterwater content over which a soilwas plastic

content of a soil sample to theLimits.

contentwatertheisw

PLLL

PLw

PI

PLwLI

LI >1 (C), viscous liquid if sheared

PI LL PL

Li uid Limit, LL

Liquid State C

0


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Plasticity ChartHL

Sensitivity

disturbedStren th

)dundisturbe(StrengthSt

strengthshearUnconfined

w > LL

Clayparticle

Water


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Activity

wei htfractioncla%

PIA

Both the type and amount ofclay in soils will affect theAtterberg limits. This index is

mm002.0:fractionclay

Normal clays: 0.75 < A < 1.25

Inactive clays: A 1.25

a me o separa e em.

g ac v y:

large volume change when

wetted

Large shrinkage when dried

Very reactive (chemically)Mitchell, 1993

Thixotropy Loses strength when remolded; Gains strength while at rest

Remolding produces a structure that is compatible with themec an ca process; a s ruc ure s no necessar y compa ewith environment (composition of pore solution)

Structure re-adjusts when left undisturbed

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Relationship Between Soil Compositionand Engineering Properties

Mineralogy does strongly affect the size and shape of particles insoil. For cohesive soils, knowledge of composition is helpful inpredicting and/or explaining unusual or adverse behavior.

Halloysite very low dry density

Montmorillonitehighly expansive

Illite quick clays

However, composition alone can not predict the engineeringproperties of most cohesive soils because of the followingcomplicating factors

Variation in particle size of the same mineral (e.g. quartz can be stone size to siltsize)

Cementing agents (e.g. CaCO3, Al/Fe oxides, organic matter) Soils are usually mixture of several minerals

Effect of pore fluid composition and its interaction with the minerals.


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An Atom

Nucleus: contains protons,

Electron Shells

about 1 A0

Clay: Basic Structural Unit

Clay minerals are made of two distinct structural units.

oxygen

silicon

aluminium or

magnesium

hydroxyl or

oxygen

0.26 nm0.29 nm

Silicon tetrahedron Aluminium Octahedron


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Different Clay Minerals

Different combinations of tetrahedral andoc a e ra s ee s orm eren c ay m nera s:

1:1 Clay Mineral (e.g., kaolinite, halloysite):

Different Clay Minerals

Different combinations of tetrahedral andoc a e ra s ee s orm eren c ay m nera s:

2:1 Clay Mineral (e.g., montmorillonite, illite)


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Tetrahedral & Octahedral Sheets

For simplicity, lets represent silica tetrahedral sheet by:

Si

and alumina octahedral sheet by:

Al

Kaolinite

Al

Si

Al

Si

Al

joined by strong H-bond

no easy separation

7.2 A

Typically 70-100 layers

Si

Al

(OH)8Al4Si4O10


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Illite

Al

Si

Si

Al

Si

9.6 A

joined by K+ ions

fit into the hexagonal

holes in Si-sheet

Si

Al

Montmorillonite

Si

also called smectite; expands on contact with water

Si

Al

Si

Si

Al

easily separated

by water9.6 A

Si

AlSijoined by weak

van der Waals bond

A highly reactive (expansive) clay

(OH)4Al4Si8O20.nH2O


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Others

Chlorite

A 2:1:1 (???) mineral.

montmorillonite famil ; 2 interla ers of water

Vermiculite

Si Al Al or Mg

chain structure (no sheets); needle-like appearance

Attapulgite

Clays

The size of clay particles are approx 2 m.

Clay particles are like plates or needles.

Plate-like or Flaky Shape

Clays are plastic; However, Silts, sands andgravels are non-plastic.


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Clay Microfabric

edge-to-face contact face-to-face contact

Flocculated Dispersed

Clay Microfabric

Electrochemical environment (i.e., pH, acidity,temperature, cations present in t e water uring t etime of sedimentation influence clay fabricsignificantly.

Clay particles tend to align perpendicular to theload applied on them.


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Clay fraction, clay size particlesParticle size < 2 m (.002 mm)

Clay Mineralogy

Clay minerals

Kaolinite, Illite, Montmorillonite (Smectite)

- negatively charged, large surface areas

Non-clay minerals

- e.g. finely ground quartz, feldspar or mica of "clay" size

Implication of the clay particle surface being negativelycharged double layer

Exchangeable ions- Li+


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Clay Mineralogy

Soils containing clay minerals tend to be cohesive and plastic.

Given the existence of a double layer, clay minerals have an affinityfor water and hence has a potential for swelling (e.g. during wetseason) and shrinking (e.g. during dry season). Smectites such asMontmorillonite have the highest potential, Kaolinite has thelowest.

Generally, a flocculated soil has higher strength, lowercompressibility and higher permeability compared to a non-flocculated soil.

Sands and gravels (cohesionless ) :Relative density can be used to compare the same soil. However, thefabric may be different for a given relative density and hence thebehaviour.

Identification of Clay Mineral

Scanning Electron Microscope (SEM)

X-Ray Diffraction (XRD)

to identify the molecular structure and mineralspresent

to identify the geometric arrangement of particles

Differential Thermal Analysis (DTA)

to identify the minerals present


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Scanning Electron Microscope (SEM)

p a e- estructure

XX--ray Diffraction Techniqueray Diffraction Technique Braggs law:n

= 2d.sin

= wave length of X-rays (1.5406 A0)

n = whole numbercorresponds to theorder of reflection(for first order ofreflection, n=1)

d = spacing betweenatomic planes (for

e.g. spacing between001 planes = 7.13 A0)X-rays penetrate to a depth of several million of

atomic layers (depth up to 30-50 m), and themethod can tell the microfabric of the sampleup to a certain depth below topmost layer of thesample.


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Atomic planes in unit cell of clay crystalAtomic planes in unit cell of clay crystal(a) Basal planes

(001) plane

(002) plane

(b) Prism planes

(010)

plane

(020)

plane

(110) plane

Arrangement of atoms in a unit cell ofArrangement of atoms in a unit cell ofKaolin clayKaolin clay

(001) plane

(001) plane

Face

(001) plane

Face

(001) plane

(001) plane

(010)

plane

(010) plane Edge

Unit cell of Kaolin Clay and itsposition in clays platelet(Carroll 1970)

Stacking of unit layers of Kaolin clayalong the a and b axes(Brindley 1951)

(001) plane


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XRD pattern of Kaolin clayXRD pattern of Kaolin clay

Basal Peaks

(001, 002)

Prism Peaks

(130, 202)Prism Peaks

(020, 110)

2 (degrees)

asa ea s

(003, 004)

Grou s mbols:

Soil Classification Systems

G - gravelS - sandM - siltC - clayO - organic silts and clayPt - peat and highly

or anic soilsH - high plasticityL - low plasticityW - well gradedP - poorly graded

Plasticity Chart


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Casagrandes PI-LL Chart

60U-line

10

20

30

40

50

Plasticity

Index

A-line

illite

kaolinite

halloysite

0

0 10 20 30 40 50 60 70 80 90 100

Liquid Limit

chlorite


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Specific Surface

surface area per unit mass (m2/g)

smaller the grain, higher the specific surface

e.g., soil grain with specific gravity of 2.7

10 mm cube1 mm cube

spec. surface = 222.2 mm2/g spec. surface = 2222.2 mm2/g

Isomorphous Substitution

substitution of Si4+ and Al3+ by other lower valence(e. ., M 2+) cations

results in charge imbalance (net negative)

+

+

++ ++

____

_ _

positively charged edges

negatively charged faces

+

_

__

_

_

__

_

_

_

_ _

_

__

__

Clay Particle with Net negative Charge

The clay particle derives

its net negative charge

from the isomorphous

substitution and broken

bonds at the boundaries.


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Cation Exchange Capacity (CEC)

known as exchangeable cations

capacity to attract cations from the water (i.e., measure ofthe net negative charge of the clay particle)

measured in meq/100g (net negative charge per 100 g of clay)

millie uivalents

The replacement power is greater for higher valence andlarger cations.

Al3+ > Ca2+ > Mg2+ >> NH4+ > K+ > H+ > Na+ > Li+

Cation Exchange Capacity (CEC)

a on xc ange apac y :

The negatively charged clay particles can attract cations from the water.

These cations can be freely exchanged with other cations present in the

water. For example Al3+ can replace Ca2+ and Ca2+ can replace Mg2+.


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Conceptual aspects behind the variation inConceptual aspects behind the variation inmicrofabric of clay using Double layer theorymicrofabric of clay using Double layer theory

Diffuse layer

Cation Monovalent

+

DispersedMicrofabric

Clayparticlewith (-)chargeon face

Clay particle inaqueous medium

DiffuseDoublelayer

L

++DivalentCation(Ca++)

FlocculatedMicrofabric

++ ++

Flocculated Microfabric:

Dispersed Microfabric:++ ++

Clay + Ca++ L ( ) Electric potential ( )

Clay + Na+ L ( ) Electric potential ( )

The presence of a surface charge and a diffused layer of adsorbed cations arounda particle results in an electrical potential, which varies with distance from theparticle surface. Electrostatic repulsion occurs when the electrical double layersof the particles overlap, achieving stability. Thickness of double layer (L) is the

Electric potential and microfabric of clayElectric potential and microfabric of clay

distance between particle surface (x =0) and the double layer surface (x = L). Zetapotential (Z) is the electric potential at x=L.

0Surface potential ( )

0 Surface charge density ( )q

Concentration and valency of Cationq

X = 0

X = L

e a po en a or sperse m cro a r c o ao n c ay = - . mZeta potential for flocculated microfabric of Kaolin clay = - 44.4 mV++

Flocculated microfabric:

Flocculated orientationZ ( ) Inter-particle repulsion ( )Clay + Ca++

Dispersed microfabric:

Z ( )Clay + Na+ Inter-particle repulsion ( ) Dispersed orientation


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Adsorbed Water

A thin layer of water tightly held to particle; like a skin

- -- -

1-4 molecules of water (1 nm) thick

more viscous than free water

adsorbed water

- -- -

- -- -- -


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Particle Size210 m

Sand

75 m

160m

S

ilt

Clay

2m2 m

10 m

Particle Shapes

Angular

Subangular

Subrounded

Rounded

Wellrounded

Isitsufficient

?


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Particle Morphology and Texture

Sphericity and Roundedness

Sphericity =

Roundedness

Diameter of a sphere of equal volume of particle

Diameter of a sphere circumscribing the particle

Avg radius of curvature of corners and edges

Radius of maximum inscribing sphere


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Sphericity andRoundedness

Sphericity going downSphericity going down

Effect ofRoundedness Yond (1973)Yond (1973)

Santamarina and ChoSantamarina and Cho(1973)(1973)


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Crushing of Particles Under Stress

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soil Composition Structure Classification