Course3_SoilMechanics_EngineringGeology
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Transcript of Course3_SoilMechanics_EngineringGeology
Geological Engineering DepartmentFaculty of Engineering
Introduction to Soil Mechanics
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Geological Engineering DepartmentFaculty of Engineering
Rock Cycles
Soils
(Das, 1998)
The final products due to weathering are soils
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Geological Engineering DepartmentFaculty of Engineering
Bowen’s Reaction Series– The reaction series are similar to the weathering stability
series.
•More stable
•Higher weathering resistance
(Das, 1998)
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Geological Engineering DepartmentFaculty of Engineering
Soils• Glacial soils: formed by transportation and deposition of glaciers.• Alluvial soils: transported by running water and deposited along
streams.• Lacustrine soils: formed by deposition in quiet lakes (e.g. soils in
Taipei basin).• Marine soils: formed by deposition in the seas • Aeolian soils: transported and deposited by the wind (e.g. soils in the
loess plateau, China).• Colluvial soils: formed by movement of soil from its original place by
gravity, such as during landslide
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Geological Engineering DepartmentFaculty of Engineering
Three Phases in SoilsS : Solid Soil particle
W: Liquid Water (electrolytes) A: Air Air
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Geological Engineering DepartmentFaculty of Engineering
Three Volumetric Ratios• (1) Void ratio e (given in decimal, 0.65)
• (2) Porosity n (given in percent 100%, 65%)
• (3) Degree of Saturation S (given in percent 100%, 65%)
)V(solidsofVolume)V(voidsofVolumee
s
v
)V(samplesoilofvolumeTotal)V(voidsofVolumen
t
v
%100)V(voidsofvolumeTotal
)V(watercontainsvoidsofvolumeTotalSv
w
e1e
)e1(VeVn
s
s
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Geological Engineering DepartmentFaculty of Engineering
• Completely dry soil S = 0 %• Completely saturated soil S = 100%• Unsaturated soil (partially saturated soil) 0% < S <
100%
• Demonstration:• Effects of capillary forces
• Engineering implications:– Slope stability– Underground excavation
%100)V(voidsofvolumeTotal
)V(watercontainsvoidsofvolumeTotalSv
w
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Geological Engineering DepartmentFaculty of Engineering
Engineering Applications• 80 % of landslides are due to
erosion and “loss in suction” in Hong Kong.
• The slope stability is significantly affected by the surface water.
(Au, 2001)
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Geological Engineering DepartmentFaculty of Engineering
Density and Unit Weight• Mass is a measure of a
body's inertia, or its "quantity of matter". Mass is not changed at different places.
• Weight is force, the force of gravity acting on a body. The value is different at various places (Newton's second law F = ma) (Giancoli, 1998)
• The unit weight is frequently used than the density is (e.g. in calculating the overburden pressure). w
s
w
s
w
ss
3
2
ggG
mkN8.9,Water
secm8.9g
gravitytodueonaccelerati:g
VolumegMass
VolumeWeight,weightUnit
VolumeMass,Density
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Geological Engineering DepartmentFaculty of Engineering
Weight Relationships
• Water Content w (100%)
• For some organic soils w>100%, up
to 500 %• For quick clays, w>100%
• Density of water (slightly varied with temperatures)
Density of soila. Dry density
b. Total, Wet, or Moist density (0%<S<100%, Unsaturated)
c. Saturated density (S=100%, Va =0)
d. Submerged density (Buoyant density)
%100)(
)(
s
w
MsolidssoilofMassMwaterofMassw )V(samplesoilofvolumeTotal
)M(solidssoilofMass
t
sd
)V(samplesoilofvolumeTotal)MM(samplesoilofMass
t
ws
)V(samplesoilofvolumeTotal)MM(watersolidssoilofMass
t
wssat
wsat'
333w m/Mg1m/kg1000cm/g1
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Geological Engineering DepartmentFaculty of Engineering
Weight Relationships (Cont.)• Submerged unit weight:
• Consider the buoyant force acting on the soil solids:
• Archimede’s principle:• The buoyant force on a body
immersed in a fluid is equal to the weight of the fluid displaced by that object.
wsat'
wsat
t
wtws
t
wwts
t
wwts
t
wss
VVWW
VWVW
%)100S(V
)VV(WVVW
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Geological Engineering DepartmentFaculty of Engineering
Engineering Applications (w)• For fine-grained soils, water
plays a critical role to their engineering properties (discussed in the next topic).
• For example,• The quick clay usually has a
water content w greater than 100 % and a card house structure. It will behave like a viscous fluid after it is fully disturbed.
Clay particle
Water
(Mitchell, 1993)
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Geological Engineering DepartmentFaculty of Engineering
Other Relationships
(1) Specific gravity
(2)
• Proof:
w
s
w
ssG
s
sw
GweSweS
s
w
w
w
s
s
s
w
w
s
s
ws
s
w
s
v
v
w
s
VV
VM
VM
MM
MMGw
VV
VV
VVeS
GweS
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Geological Engineering DepartmentFaculty of Engineering
Typical Values of Specific Gravity
(Lambe and Whitman, 1979)(Goodman, 1989)
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Geological Engineering DepartmentFaculty of Engineering
Solution of Phase Problems
• Remember the following simple rules (Holtz and Kovacs,
1981):
• Remember the basic definitions of w, e, s, S, etc.
• Draw a phase diagram.• Assume either Vs=1 or Vt=1, if not given.
• Often use wSe=ws, Se = wGs
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Geological Engineering DepartmentFaculty of Engineering
Problem:
1. Suatu sampel lempung jenuh mempunyai kadar air 56%, jika Gs = 2.72, hitunglah e dan n?
2. Suatu sampel pasir seragam mempunyai porositas sebesar 43% dan kadar air 12%, anggap Gs = 2.65, hitung angka pori dan tingkat kejenuhan!
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Geological Engineering DepartmentFaculty of Engineering
SOIL TEXTURE Particle Sizes
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Geological Engineering DepartmentFaculty of Engineering
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Geological Engineering DepartmentFaculty of Engineering
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Geological Engineering DepartmentFaculty of Engineering
Coarse-Grained Soil Texture
• Typical values• Engineering
applications:
– Volume change tendency– Strength
(Lambe and Whitman, 1979)
Simple cubic (SC), e = 0.91, Contract
Cubic-tetrahedral (CT), e = 0.65, Dilate
Link: the strength of rock joint
)itan(strengthShear n
i
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Geological Engineering DepartmentFaculty of Engineering
Engineering Implications (e)(Cont.)
– Hydraulic conductivity• Which packing (SC or
CT) has higher hydraulic conductivity?
SC
e = 0.91
CT
e = 0.65
The fluid (water) can flow more easily through the soil with higher hydraulic conductivity
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Geological Engineering DepartmentFaculty of Engineering
Engineering Applications (e)(Cont.)
SC
e = 0.91
CT
e = 0.65
The finer particle cannot pass through the void
•Clogging
Critical state soil mechanics
Filter
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Geological Engineering DepartmentFaculty of Engineering
Soil Texture: Grain Shape
Important for coarse granular soils Angular soil particle higher friction Round soil particle lower friction
Rounded Subrounded
Subangular Angular
(Holtz and Kovacs, 1981)
Coarse-grained soils
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Geological Engineering DepartmentFaculty of Engineering
Coefficient of Friction, = 0.4
Weight, N = 4T
Force, T = 1.6T
T = N.
Friction
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Geological Engineering DepartmentFaculty of Engineering
Friction
T = N.
T = N. tan ()
= angle of internal friction
EXTERNAL
INTERNAL
SOIL
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Geological Engineering DepartmentFaculty of Engineering
Grain Shape and
Rounded 30 - 35Rounded 30 - 35oo Sub-rounded 32 - 37Sub-rounded 32 - 37oo
Sub-angular 34-39Sub-angular 34-39oo Angular 36-41Angular 36-41oo
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Geological Engineering DepartmentFaculty of Engineering
Mineral type and
• QuartzQuartz 3030oo
• CalciteCalcite 3838oo
• KaoliniteKaolinite 1515oo
• IlliteIllite 1010oo
• SmectiteSmectite 5 5oo
Sands
Clays
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Geological Engineering DepartmentFaculty of Engineering
Cohesion
Some soils, and all rocksdisplay some interparticlebonding, which gives them
strength even when the normal stress is zero –
we call this COHESION
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Geological Engineering DepartmentFaculty of Engineering
Fine Grained Size: Atterberg Limits• The presence of water in fine-grained soils can significantly affect
associated engineering behavior, so we need a reference index to clarify the effects.
(Holtz and Kovacs, 1981)
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Geological Engineering DepartmentFaculty of Engineering
Atterberg Limits (Cont.)
Liquid Limit, LL
Liquid State
Plastic Limit, PL
Plastic State
Shrinkage Limit, SL
Semisolid State
Solid StateDry Soil
Fluid soil-water mixture
Incr
easi
ng w
ater
con
tent
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Geological Engineering DepartmentFaculty of Engineering
Typical Values of Atterberg Limits
(Mitchell, 1993)
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Geological Engineering DepartmentFaculty of Engineering
Indices•Plasticity index PI •For describing the range of water content over which a soil was plastic•PI = LL – PL
• Liquidity index LI • For scaling the natural
water content of a soil sample to the Limits.
contentwatertheiswPLLL
PLwPI
PLwLI
LI <0 (A), brittle fracture if sheared0<LI<1 (B), plastic solid if sheared LI >1 (C), viscous liquid if sheared
Liquid Limit, LL
Liquid State
Plastic Limit, PL
Plastic State
Shrinkage Limit, SL
Semisolid State
Solid State
PI
A
B
C
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Geological Engineering DepartmentFaculty of Engineering
• The Atterberg limits are usually correlated with some engineering properties such as the permeability, compressibility, shear strength, and others. In general, clays with high plasticity have lower permeability,
and they are difficult to be compacted. The values of SL can be used as a criterion to assess and
prevent the excessive cracking of clay liners in the reservoir embankment or canal.
Engineering Applications
The Atterberg limit enable clay soils to be classified.
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Geological Engineering DepartmentFaculty of Engineering
(Holtz and Kovacs, 1981)
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Geological Engineering DepartmentFaculty of Engineering
OTHER SOIL DESCRIPTORSMOISTURE CONTENTDry - Absence of moistureMoist - Damp, but no visible waterWet - Visible water
CEMENTATIONWeakly - Crumbles or breaks easilyModerately - Crumbles or breaks with considerable finger pressureStrongly - Will not crumble or break with finger pressure
COLOR (Soil Color Chart)ODORADDITIONAL COMMENTS
COARSE-GRAINED SOIL
FINE-GRAINED SOIL
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Geological Engineering DepartmentFaculty of Engineering