2_clay_mineralogy.pptx
TRANSCRIPT
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CE 831 Advance Soil Mechanics
Dr. Muhammad Kashif - TVFPhD Geotechnical Engineering (USA)Email: [email protected]; Ph.:0304-5393438
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The most important grain property of fine-grained soil materials is the mineralogical composition
For soil particles are < 0.002 mm electrical forces are more significant
If influence of surface charges is predominant in a material it is said to be in colloidal state
The colloidal particles of soil consists of Clay minerals
Clay Mineralogy
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All Clay minerals are crystalline hydrous alumino-silicates
Arranged in layers like pages of book The arrangement of layers determine the
type of clay mineral
Clay Mineralogy
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Clay Mineralogy
(a) Tetrahedral layer, (b) Octahedral layer
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Clay Mineralogy
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Characteristics of Principal Clay Minerals Kaolinite: Most common clay mineral in
sedimentary and residual soils Unit Sheet Thickness = 0.7 nm (nm = 10-
9m) Composed of 1 aluminum octahedral layer
and 1 silicon tetrahedral layer joined by shared oxygen
Photomicrograph of kaolinite
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Characteristics of Principal Clay Minerals Illite (fine-grained or weathered mica): Most
common clay mineral in stiff clays, shales and postglacial marine and lacustrine soft clay and silt deposits
Unit Sheet Thickness=10-30nm (nm=10-9m)
Photomicrograph of illite
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Characteristics of Principal Clay Minerals Montmorillonite (smectites): Dominant clay
mineral in clays and shales and in refusal soils derived from volcanic ash
Unit Sheet Thickness=1nm (nm=10-9m)
Photomicrograph of Montmorillonite
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Mineralogical Composition of Soft Clays
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Index Prperties of Soft Clays
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Index Properties of Soil
Undisturbed Fabric of Boston Blue Clay
Undisturbed Fabric of St. Hilaire Clay
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Index Properties of Soil
Undisturbed Fabric of Berthierville Clay
Undisturbed Fabric of Vasby Clay
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Index Properties of Soil
Undisturbed Fabric of LaGrande Clay
Undisturbed Fabric of Pancone Clay
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Index Properties of Soil
Fibrous Middleton Peat Poriferous Cellular Peat
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Soil Structure Soil Structure = Inter particle forces + Fabric Inter particle forces: How shear stresses (τ) and
normal stresses are transmitted between soil particles
Fabric: Distinguish between macro level vs micro level
σ׳v = σ – u The eff stress is transmitted by forces acting
between soil particles σ׳v = short range stresses due to contact area +
long range stresses due to surface (double layer) forces
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Electrical Nature of Clay Particles
Two faces of all platy particles have a negative charge The edge of clay particle usually have positive charge Net charge of clay particle is always negative
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Suspended clay particle in electrical fluid will move from cathode to anode - electrophoresis
Electrical Nature of Clay Particles
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Since the soil must be electrically neutral:◦ The negative forces attract exchangeable cations
(Na++, Ca++, Mg++ etc◦ Positive edge attract exchangeable anions (or
cations if negatively charged)
Electrical Nature of Clay Particles
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Surface Charge Density (σ∘)
σ∘ = No. of Charges / Unit Area = Cation Exchange Capacity / Specific Surface Area = CEC / SSA
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Adsorbed Water Relative Humidity (RH%) = Pw/Ps x100 Pw = vapor pr. of water Ps = saturation vapor pr. at same temperature
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Adsorbed Water Soil Suction (s) = (ϱw*Rg*T/M)*ln(100/RH)
◦ ϱw = density of water (998 kg/m3)◦ Rg = gas constant (8.314 J/mol K)◦ T = absolute temperature (273+ tem °C)◦ M = molecular mass of water (18 gm/mol)
S (bar) =1350*ln(100/RH) at 20 °C
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Diffuse Double Layer (DL)
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Double Layer Repulsion
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Double Layer Repulsion
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Double Layer Repulsion
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Effect of Application of Stress