soil classification - university of · pdf filepermeability (hydraulic conductivity) ... soil...

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CIVE 554/650Soil Classification

Typical Properties

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Soil Phase Relationships

Soil Phase Relationships

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Soil Phase Relationships

Soil Phase Relationships

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Soil StructureAssemblage of individual particles

Bulky particles

Plate particles (Includes flaky or needle particles)Card house structure

Loose Bulky SoilVoid (VV) Bulky particle (VS)

Container (VT)

VT = VV +VS

e = VV/VS

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Dense Bulky SoilVoid (VV) Bulky particle (VS)

Container (VT)

VT = VV +VS

e = VV/VS

Vv = e

Vs = e

Bulky Soil PackingMost soils contain a variety of particle sizes

In general the greater the range of soil particles the lower the void ratio

Fill voids with smaller particles

Soil strength also depends on particle interlocGreater interloc higher shear strength

Interloc function of: particle shape and amount of inter particle contact

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Shape of Granular Soil Particles

Angular short transport distance

Round large transport

Bulky Soil Engineering PropertiesCompressibility

Relatively small in loose and dense stateLoose greater than dense

StrengthRelatively high in loose and dense stateDense greater than looseHigh for angular particlesLow round particles

Permeability (hydraulic conductivity)Relatively high in loose and dense stateLoose greater than dense

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Bulky Soil ConsistencyVery looseLooseCompactDenseVery dense Low void ratio (e)

High strength, incompressible

high void ratio (e)

Low strength, compressible

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Bulky Loose SoilHoneycomb soil structure in granular soil

Loess (wind) deposit

Good compressive strength

Unstable if loaded sideways (shear)

Bulky Dense SoilDense soil

Low Vv, high Vs therefore low eLower the soil void ratio the more dense,

less settlement and higher strength

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Bulky Soil PropertiesMost soils have some water in voids

Voids filled then soil is 100 percent saturatedPortion of voids filled then soil is partially saturated

Due to varying amount of water in a soil it density and void ratio will vary

Other Bulky ParticlesOrganics

PeatDecaying plant fragments

Municipal solid wastePaper, plastic, metal, etcNot particulate materials

Soil mechanic principles apply only to particulate materials

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Soil Particle Size

Soil Classification Systems

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UNIFIED Soil Classification System (USCS)

Boulder > 300 Cobble 76.2 - 300 Gravel 4.75 – 76.2 Sand 0.075 – 4.75 Silt .005 – 0.075 Clay 0.002 - .005 Colloid < 0.002

Particle diameter (mm)

Bulky Particles

Plate/Needle Particles

Sieve Analysis

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Fine sand, silt and clay

Hydrometer:

Soil fraction passing

No. 40 sieve (0.425mm)

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Plate Like ParticlesFlat thin particles

Clays and colloids

Fine grained and cohesive soils

Large surface area with small mass

Plate Like ParticlesSpecific surface = surface area/mass

Sand = 0.001 to 0.4 m2/gramSilt = 0.4 to 1.0 m2/gramClay = 5 to 800 m2/gram

Electrical forces dominate behaviourNo mass for gravity forces

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Plate Like Mineral FormationSecondary minerals formed by weathering or oxidation of aluminous minerals or volcanic glass

Clay Mineral FormationMicas, feldspar& feldspathoids kaolinite

Siltstone/claystone/mudstone/shale illite

Basalt, other mafic rocks montmorillonite

Tuffs & volcanic ash bentonite

Biotite vermiculite (hydro-thermal alteration)

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Clay Mineral behaviourCharge particlesStrong electrical forces on the particle surfaces

Negative charge on flat surface

-

+ +

----

+ +-----

Negative charge attracts positive cations:

•H+ from water

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Clay Mineral behaviourCations adsorb to clay mineral surface

Absorb - within (sponge)Adsorb - on surface

Ability of clay mineral to adsorb cations is known as cation exchangeDiffers widely for different types of clays

Cation ExchangeCation exchange = # positive ions adsorbed per 100 gm of clayDepends on strength of negative charge

Montmorillonite = 360 to 500 x10-20

Illite = 120 to 240 x10-20

Kaolinite = 20 to 90 x10-20

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Clays and WaterWater adsorbed to clay mineral is called double layer water - strongly held

Some weakly held

Water outside double layer called free water

Adsorbed water gives clay plasticityAbility to roll out into 1/8th inch diameter thread

Clays and Water

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ClaysPresence of clay minerals in soil will greatly impact engineering behavior

For practical purposes when clay content in soil is >50% soil particles will float in clay

High void ratio (>1 is typical)

Water sensitive

Water adsorb to surface - tightly held low hydraulic conductivity

Clay Consistency StateVery softSoftFirmCompactStiff Very stiff

Low void ratio (e)

Low water content

high void ratio (e)

high water content

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Clay Engineering PropertiesCompressibility

Soft high and time dependantStiff low and time dependant

Strengthlow in soft stateHigh compact to stiff state

PermeabilityVery low in soft and stiff state (10-5 to 10-9cm/s)

Clay StructureDue to electrical forces and deposition in very low energy environments develop loose structures (high e)

Flocculated clay - particles attract each other then settle out due to gravity

Dispersed - particles repel each other (negative to negative charge)

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Clay Structure

Clay Structure

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Special Types of ClaysSensitive clay

Structure changes substantially due to disturbance (kneading or re-working)Therefore engineering properties change

Quick clayAfter disturbance clay changes from solid to flowable fluid Marine clays - leached out Na++ due to fresh water

Canada Quick ClaysLeda Clay – Ottawa river valleyQuebec – St Lawrence River valley

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Shrinkage Limit (SL)

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Liquid Limit (LL)

Atterberg Device

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Plastic Limit (PL)

Roll clay until it breaks at 1/8 inch thick

Plastic Index (PI)PI = LL-PL

Measure of how close the LL and PL areNote clay sensitive if PI ~ 0

Normal consolidated clay

If PI high then clay most likely over consolidated

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Liquidity Index (LI)

Compares field water content with LL and PL.

PL - LLPL - w =LI

Activity (A)

by weight)fraction size-clay(percent PI =A

( )weightbyfractionsizeclaypercentPIA

−=

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Plasticity Index and Clay Content

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Atterberg Limits and Clay Minerals

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