2_Clay_Mineralogy.pptx

CE 831 Advance Soil Mechanics

Dr. Muhammad Kashif - TVFPhD Geotechnical Engineering (USA)Email: [email protected]; Ph.:0304-5393438

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

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

Clay Mineralogy

(a) Tetrahedral layer, (b) Octahedral layer

Clay Mineralogy

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

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

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

Mineralogical Composition of Soft Clays

Index Prperties of Soft Clays

Index Properties of Soil

Undisturbed Fabric of Boston Blue Clay

Undisturbed Fabric of St. Hilaire Clay

Index Properties of Soil

Undisturbed Fabric of Berthierville Clay

Undisturbed Fabric of Vasby Clay

Index Properties of Soil

Undisturbed Fabric of LaGrande Clay

Undisturbed Fabric of Pancone Clay

Index Properties of Soil

Fibrous Middleton Peat Poriferous Cellular Peat

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

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

Suspended clay particle in electrical fluid will move from cathode to anode - electrophoresis

Electrical Nature of Clay Particles

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

Surface Charge Density (σ∘)

σ∘ = No. of Charges / Unit Area = Cation Exchange Capacity / Specific Surface Area = CEC / SSA

Adsorbed Water Relative Humidity (RH%) = Pw/Ps x100 Pw = vapor pr. of water Ps = saturation vapor pr. at same temperature

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

Diffuse Double Layer (DL)

Double Layer Repulsion

Double Layer Repulsion

Double Layer Repulsion

Effect of Application of Stress