essential soil mechanic - gic
TRANSCRIPT
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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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