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Essential Soil MechanicsEssential Soil Mechanics

J S Muhammathu FowzeBSc Eng (Hons), MPhil (Geotech)

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Essential Soil MechanicsEssential Soil Mechanics

Outline of PresentationPart 1 : Description, Classification, & Phy. Properties

Part 2 : Soil Water 

Part 3 : Stresses in the Ground 

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Part 1Part 1 : Description, Classification, &: Description, Classification, & PhyPhy. Properties. Properties

Soil: any uncemented/weakly cemented accun of mineral particles

Weathering of Rock 

, void space in b/w the particles containing water and/or air

Residual: Tropical Zones

 products of weathering remain at their original location

Transported/Sedimentary: Temperate Zones

transporting agents : gravity, wind, water, & glaciers

Peaty/Organic SoilsSoil in combination of partially decayed vegetation matter 

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Types of SoilTypes of Soil

ResidualResidual

SedimentarySedimentary

Peat/OrganicPeat/Organic

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Found in abundance in the Region

RESIDUAL SOILS : WOMENRESIDUAL SOILS : WOMEN

 precise description

classification

 predicted behaviour 

Continue to defy

heterogeneous

anisotropic

difficulties in sampling and testing

have received little attention

Although they are found in abundance

temperate zone soils of Europe and North America!!! Soil Mechanics:

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Soil : A particulate materialSoil : A particulate material

Particles occur in a wide range of sizes

BSCS

Field IdentificationField Identification

Sand (& coarser) : visible to the naked eye.

Silt : dusty when dry - easily be brushed off hands and boots.

Clay : greasy and sticky when wet and hard when dry

scraped or washed off hands and boots.

Shape?

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Clay : Shape CharacteristicsClay : Shape Characteristics

Clay particles are FLAKY

Thickness

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Clay MineralsClay Minerals

Weathering of Feldspars ( Orthoclase and plagioclase, and some micas)

( a) Kaolinites

the principal constituent in china clay & ball clay

(b) Illites

 predominant in marine clays and shales(e.g. London clay, Oxford clay)

(c) Montmorillonites

have high swelling/shrinking potential

3 Main Groups of Clay Minerals

Silica Tetrahedron

Alumina OctahedronMg

2+

Al3+

Mg2+

Cations other than K+

Partial Substitution

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Clay Minerals, A Microscopic ViewClay Minerals, A Microscopic View

Mineral/SoilSpecific Surface

m²/N

Quartz grain 0.0023

Quartz sand 0.0001 - 0.004

Kaolinite 2Illite 8

Montmorillonite 80

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ClassificationClassification

BSCS (Grain wise)

Very CoarseCoarseFine

after wetting &

subsequent

drying

“adhere”

Cohesive

Materials

Frictional

Materials

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OccurrenceOccurrence

Most type of soils consists of a graded mixture

75%

100%

50%

25%

FINE SOILS: > 35% finer than 0.06 mm

“Plasticity Characteristics”

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Typical Gradation CurvesTypical Gradation Curves

A - a poorly-graded medium SAND (probably estuarine or flood-plain alluvium)

B - a well-graded GRAVEL-SAND (i.e. equal amounts of gravel and sand)C - a gap-graded COBBLES-SAND

D - a Sandy SILT

E - a typical Silty CLAY (e.g. London clay, Oxford clay)

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Phase RelationshipsPhase Relationships

Soils can be of either 2-phase or 3-phase composition

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Phase RelationshipsPhase Relationships……

Soil Density

ewGS    S r    =

wS S 

ewGG  ρ  ρ +

+

=1

)(

wr S 

e

eS G ρ  ρ 

+

+=

1

)(

wS 

wet sat e

eG ρ  ρ  ρ 

+

+==

1

)(w

S dry

e

G ρ  ρ 

+=

1

drym   ρ  ρ  )1(   +=

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Water to Soil Density Ratio, rWater to Soil Density Ratio, r

3

2

381.981.91000

m

kN ms

m

kggw   =×=

  − ρ Unit Weight of Water 

Generally, for soils

317

m

kN 3

)2014(~m

kN gdry   − ρ  AverageDry Unit Weight of Soils

320

m

kN 

3)2318(~m

kN gwet    − ρ  AverageWet Unit Weight of Soils

5.0≈=wet 

wr  ρ 

 ρ 

End of Part 1End of Part 1

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Part 2Part 2 : Soil Water :: Soil Water : Pore Water :Pore Water : held under pressureheld under pressure

Pore water pressure (u) : measured rel. to the patm (zero)

Level : p = patm Water Table/ Phreatic Surface

%100=r S 

Above WT water could be held at –ve p ; Surface Tension

attractive forcesattractive forces( SUCTION)( SUCTION)

Below WT Probably

 S e e p i n g 

Stationary

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Pore water pressure and PotentialPore water pressure and Potential

Still Water h

 z

O

Datum

Q

ghU  w X    ρ =Pore water pressure at point X

POTENTIAL has two componentsPressure head Pore water pressure

Ht. above an arbitrary datum Elevation head  

 zh zg

U P   X  X    +=+= ρ 

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Potential Difference & Hydraulic GradientPotential Difference & Hydraulic Gradient

hPPP  X Y    ∆−=−=∆Potential difference

YFlow X

Spaced at a dist ‘L’ measured in the dir of flow

0

iV  ∝

 X 

Y h

Y 

 z

YX

 L

h

 L

Pi

  ∆=

∆−=

ik V S 

= X  z

 X h

Darcy, 1856

Permeability/Hydraulic Conductivity

(m/hr)

Groundwater is driven along the max. gradient of potential

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Hydraulic ConductivityHydraulic Conductivity

The vertical integral of hydraulic conductivity : TRANSMISSIVITY

S k 

t 

S bk T   =

(m2

/hr)θ 

θ 

θ TSin

T 

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A simple problemA simple problem

W  D

 D   X 

?=W U 

θ 

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SolutionSolution

g

U 

W 

 X 

 ρ 

 X 

?= X U 

θ  ZERO

gCos DU  W W  X    θρ 2

=

W  D

  F  l o w  f o  l  l

 o w s  T o

 p o G r a d

 D

End of Part 2End of Part 2

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Stresses in the Ground Stresses in the Ground 

g Dg D D sat W d W V  X   ρ  ρ σ    +−= )(

 D

d  ρ 

?= X V σ 

sat  ρ W  D

Due to whatever lying abovethe pt of interest

Total Vertical Stress

Pore Water Pressureg Du W W  X    ρ =

 X 

Hydrostatic State of Stress/Neutral Stress

 X V V    u X  X  −=′   σ σ 

Component of total vertical stress in excess of neutral stress

Has its seat exclusively in the solid phase of the soil

Effective Vertical Stress

Th P i i l f Eff ti StTh P i i l f Eff ti St

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The Principle of Effective StressThe Principle of Effective Stress

u−=′   σ σ 

All measurable effects of a change of stress, such as change of

strength, etc are exclusively due to change of effective stresses

Karl Terzaghi, 1936

Single Most Important Concept in Soil Mechanics and its

importance cannot be overstated!!!

Terzaghi (1883-1963) : Father of Soil Mechanics

St th f S ilSt th f S il

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Strength of SoilsStrength of Soils

Could resist a certain amount Shear Stress

Max. Shear Stress a soil could resist : SHEAR STRENGHT

S h C ib i A

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Strength : Contributing AgentsStrength : Contributing Agents

SHEAR STRENGTH

 basically

COHESION FRICTION

sc′

r c′

ϕ ′Intermolecular Bonds

Roots if present

Internal Friction Angle

)(r s

  cc   ′+′

 RF    µ =

φ σ    ′− tan)(   u+ kPa= f 

τ 

Mohr-Coulomb Model

kPa

Reasonably described 

MohrMohr Coulomb ModelCoulomb Model

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Mohr Mohr --Coulomb ModelCoulomb Model

)( r s   cc   ′+′

φ ′

τ 

)(   u−=′   σ σ 

 f mob

  τ τ    If 

mob

 f 

FS  τ 

τ 

=

End of Part 3End of Part 3

Factor of Safety against failure

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