soil Composition Structure Classification
Transcript of 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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Regional Soil Deposits of India
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:
Regional Soil Deposits of India
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:
Regional Soil Deposits of India
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
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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
Thank You