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soil composition lecture
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8/1/2012
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IIT Kanpur, 2012‐13 (1st Semester)
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Dr. Prishati RaychowdhuryOffice: FB 336, Tel: #6692, Email: [email protected]
Lecture ‐ 2
Most common clay minerals:y
• Kaolinite
• Montmorillonite
• Illite
Other clay minerals:• Chlorite
• Halloysite
• Vermiculite etc.
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8-35 km crust % by weight in crust% by weight in crust
O = 49.2Si = 25.7Al = 7.5Fe = 4.7Ca = 3.4
82.4%
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12500 km dia
Na = 2.6K = 2.4Mg = 1.9other = 2.6
Clay minerals are made of two distinct structural units.
oxygen
siliconaluminium or magnesium
hydroxyl or oxygen
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0.26 nm0.29 nm
Silicon tetrahedron Aluminium Octahedron
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Several tetrahedrons joined together form a t t h d l h ttetrahedral sheet.
tetrahedron
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hexagonal hole
For simplicity, let’s represent silica tetrahedral sheet by:
Si
and alumina octahedral sheet by:
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Al
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Si
Al
Si
Al
0.72 nm
Typically 70-100 layers
Si
Al
Si
Aljoined by strong H-bondno easy separation
joined by oxygensharing
used in paints, paper and in pottery and pharmaceutical industries
Halloysite
(OH)8Al4Si4O10
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kaolinite family; hydrated and tubular structure
(OH)8Al4Si4O10.4H2O
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Si
also called smectite; expands on contact with water
Al
Si
Si
Al
Si
0.96 nm
9Si
Al
Si
Si
joined by weakvan der Waal’s bond
easily separated by water
A highly reactive (expansive) clay
(OH)4Al4Si8O20.nH2O
high affinity to waterBentonite
swells on contact with water
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montmorillonite family
used as drilling mud, in slurry trench walls, stopping leaks
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Al
Si
Si
Al
Si
Si
Al
0.96 nm
joined by K+ ions
fit into the hexagonal holes in Si-sheet
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Si
Al
Si
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Plate-like or Flaky Shape
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edge-to-face contactface-to-face contact
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Flocculated Dispersed
common technique to see clay particles
plate-like structure
qualitative
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structure
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X-Ray Diffraction (XRD)
Differential Thermal Analysis (DTA)
to identify the molecular structure and minerals present
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Differential Thermal Analysis (DTA) to identify the minerals present
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A thin layer of water tightly held to particle; like a skin
1-4 molecules of water (1 nm) thick1-4 molecules of water (1 nm) thick
more viscous than free water
‐ ‐‐ ‐‐ ‐
adsorbed water
1nm
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‐ ‐‐ ‐‐ ‐‐ ‐
free water
double layerwater
50 nm
Clay particles are generally more platy in shape (sand more equi‐dimensional)
Clay particles carry surface charge
Amount of surface charge depends on type of clay minerals
Surface charges that exist on clay particles have major influence on their behavior (for e g plasticity)influence on their behavior (for e.g. plasticity)
Clays are plastic; Silts, sands and gravels are non‐plastic
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0.002 300804.750.075
BoulderClay Silt Sand Gravel Cobble
Granular soils or Cohesionless soils
Cohesivesoils
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Grain size (mm)
Fine grain soils
Coarse grain soils
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ASTM Standard Sieves
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This procedure is suitable for coarse grained soils No.10 sieve …. Has 10 apertures per linear inch
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Sieve AnalysisHydrometer Analysis
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0.075 mm
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Also called Sedimentation AnalysisAnalysis
Stoke’s Law
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)(2Lsw GGD
v
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Assumptions: All particles are spherical Falling in an infinite extent of liquid Terminal velocity same Particles have same unit weight
Grain Size Distribution Curves
Particle‐size distribution curve – sieve analysis and hydrometer analysis
Source: B. M. Das
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Grain Size Distribution Curves
Different types of particle‐size distribution curves
Grain Size Distribution Curves
Coefficient of Uniformity, Cu= D60/D10
Coefficient of Curvature, Cc = (D30)2/(D10)(D60)
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Well Graded Soils Poorly Graded Soils
Wide range of grain sizes present
Gravels: Cc = 1‐3 & Cu >4
Sands: Cc = 1‐3 & Cu >6
Others, including two special cases:
(a) Uniform soils – grains of same size
(b) Gap graded soils – no grains in a specific size range
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)
Part c
Cu= D60/D10
=4.56
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Cc = (D30)2/(D10)(D60)
=0.928
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Particle Shapes
Clay minerals photographed in an electron microscope (Note: they are plate or flake like and are stacked on top
Electron micrograph of some fine subangular and subroundedquartz sand
