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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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