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UNSATURATED SOIL MECHANICS -INTRODUCTION & APPLICATION
BY
Tariq B. Hamid
December 2006
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OUTLINE OF PRESENTATION
Effective Stress Principle
Saturated Soils vs. Unsaturated Soils
Shear Strength of Unsaturated Soils
Unsaturated Soil Testing
Application of Unsaturated Soil Mechanics
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TERZAGHI’S EFFECTIVE
STRESS PRINCIPLE
•
Changes in volume and shearing strength of asoil are due to changes in effective stress.
•
The effective stress is defined as the excess ofthe total applied stress over the pore pressure
).( wu−σ
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GENERALIZED WORLD OF SOIL
MECHANICS
)( au−σ )( wa uu −
Net normal stress Matric suction
Effective stress
)( wu−σ
Negative pore-waterpressure
Positive pore-water
pressure
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SOIL COLLAPSE
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SATURATED VS.
UNSATURATED SOILSOIL
CONDITION
PORE
MEDIUM
uw STRESS
VARIABLES
SATURATED
WATER ≥
0
UNSATURATED
AIR &WATER
<0
WATER
SOLID
WATER
SOLID
AIR
)( wa uu −
)( au−σ
)(w
u−σ
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CHATEGORIZATION BASED
ON GEOLOGIC ORIGINSEach soil type can be unsaturated in its natural or its compacted condition.
Unsaturated soil behavior does not favor a particular geologic genesis.
Aeoline
Residual
Lacustrine
Alluvial
Others
Natural orremolded
states
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NEED FOR UNSATURATED SOIL
MECHANICS
In the USA alone “Each year, shrinking andswelling soils inflict at least $2.3 billion in
damages to houses , buildings, roads, andpipelines-more than twice the damage fromfloods, hurricanes, tornadoes, and
earthquakes!”(Jones and Holtz,1973)
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NEED FOR UNSATURATED SOIL
MECHANICSKrohn
and Slosson
(1980)
“$7 billion are spent every year in the USAas a result of damage to all type ofstructures built on expansive soils”.
Snethen
(1986):
“Expansive soils “hidden disaster”:
economically, one of the USA costliestnatural hazards. More than one fifth of American families live on such soils.”
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Near Ground Surface Structures
Unsaturated soil
Saturated soil
Retaining Wall Roadway
Spread footing foundation
Sr <100%(ua-uw)>0
(ua-uw)= 0 Sr = 100%
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Meniscus
Nc
Interparticle forcedue to capillarity
Unstable(after Burland
and Ridley 1996)
Stable(after Burland
and Ridley 1996)
EFFECT OF MATRIC SUCTION (ua
-uw)
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SOIL WATER CHARACTERISTIC
CURVE•
It defines the relationship between the
amount of water in the soil and thesuction.
Vanapalli et al. 1996
Primary
transition
zone
Boundary
effect zone
Secondary
transition
zone
Residual
zone of unsaturation
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SOIL WATER CHARACTERISTIC
CURVE & SHEAR STRENGTH
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SHEAR STRENGTH
SATURATED
SOIL
UNSATURATED
SOIL
'tan)(' φ σ τ w f ff uc −+= bwaan uuuc φ φ σ τ tan)('tan)(' −+−+=
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EXTENDED MOHR-COULOMB
FAILURE ENVELOPE FORUNSATURATED SOIL
'φ
b
φ
ua-uwτ
σ −
ua
c′
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INCREASE IN SHEAR
STRENGTH WITH SUCTION
Gan
et al. (1988)
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NULL TEST RESULTS
w ( %
)
15.2
15.6
16.0
16.4first null test,increase σ,ua, uw
Time (min.)
0 500 1000 1500 2000
v / H 0
( % )
12.25
12.30
12.35
12.40
12.45
equalization complete
(a)
(b)
τ ( k
P a )
0.0
20.0
40.0
60.0
80.0
100.0
120.0
v / H 0
-0.002
0.000
0.002
0.004
0.006
Null Test Datafor DH/D=0-.101: σ=195, ua=91, uw=41 (kPa)
for DH/D>0.101: σ=216, ua=112, uw=62 (kPa)
Comparison Test Datafor all DH/D: σ=175, ua=71, uw=21 (kPa)
ΔH/D (mm/mm)
0.00 0.04 0.08 0.12 0.16
w ( %
)
14.6
14.815.0
15.2
15.4
15.6
15.8
16.0
Equalization PhaseShearing Phase
(Hamid, 2005)
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UNSATURATED SOIL TESTING
STRENGTH & COMPRESSIBILITY TESTS
TRIAXIAL TEST
DIRECT SHEAR TEST
OEDOMETER TEST
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UNSATURATED SOIL TESTING
MEASUREMENT OF MATRIC
SUCTIONTENSIOMETER
FILTER PAPER METHOD
PRESSURE PLATE
AXIS TRANSLATION TECHNIQUE
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BEARING CAPACITY
•
Footings are placed well above the
groundwater table•
Water table may rise due to excessive
watering of the vegetation surrounding thebuilding
•
Measurement of in-situ suction may bevaluable
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BEARING CAPACITY
•
Extension of Saturated Soil Mechanics
Where:b
wa uucc φ tan)(' −+=
γ γ γ BN N DcN q q f cu 5.0++=
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BEARING CAPACITY OF A STRIP
FOOTING FOR VARIOUS MATRICSUCTION VALUES
0
400
800
1200
1600
2000
0 50 100 150 200 250 300
Matric Suction (kPa)
U l t i m a
t e B e a r i n g P r e s s
u r e ( k P a ) φ' = 20
0
φb = 15
0
c' = 5 kPa
γ = 18 kN/m
2
Df = 0.5 m
(Fredlund and Rahardjo, 1998)
B = 0. 5
m
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EXCAVATION SUPPORT SYSTEM IN
UNSATURATED SOIL
UnsaturatedRetained Soil
H
Bottom of Excavation
Unsaturated Retained Soil
Depth of
penetration
H
C
b
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EXCAVATION SUPPORT SYSTEM IN
UNSATURATED SOIL
0.00
2.00
4.00
6.00
8.00
10.00
12.00
0 100 200 300 400 500 600
Matr ic Suction, ua-u
w, (kPa)
P e n e t r a t i o
n D e p t h ,
D , ( m
)
5
10
15
20
25
φb = 5
0
φb = 100
φb = 15
0
φb = 20
0
φb = 25
0
Variation in Depth of Penetration (D) With Matric Suction
(Hamid & Tawfik 2005)
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•
Shear strength equation for unsaturated soilconveniently separates environmentalboundary conditions from stress relatedloading.
•
Major problem -
determine the reduction insuction and positive pore pressure increaseas a function of precipitation history.
NATURAL UNSATURATED SOIL-
SLOPE STABILITY
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TEMPORARY EXCAVATION
Bedrock
Residual soil
U n s
a t u r a t
e d s o i l
S a t u r
a t e d s o i l
Collection system for runoff
Runoff
Plastic membrane
Anchor for membrane Surface drain
Footing
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SEASONAL DEPENDANCE OF
IN-SITU TEST PARAMETERS
Lehane et al. 2004
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EXAMPLES OF UNSATURATED
INTERFACES
PILES EMBEDDED IN
UNSATURATED SOIL
RETAINING WALLS WITH
UNSAT. SOIL BACK FILL
BURIED PIPE
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EXTENDED MOHR-COULOMB
FAILURE CRITERION
LABORATORY TESTING FOR INTERFACE
STRENGTH PARAMETERS ca, δ, δ b
INTERFACE STRENGTH IN UNSATURATED SOIL:
bwaanas uuuc δ δ σ τ tan)(tan)(
''
−+−+=
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Air Chamber
Pore WaterPressure AndVolume Controller
Diffused AirVolume Indicator
Air Pressure Line
HorizontalLoad Cell
Computer
Horizontal LVDTPressure Transducer
Vertical LVDT Air PressureControl Panel
Direst ShearDevice Base
Vertical Load Cell
UNSATURATED INTERFACE DIRECT SHEAR APPARATUS
(Miller & Hamid, 2007)
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INCREASE IN INTERFACE
SHEAR STRENGTH WITH SUCTION
τ
( k P a )
0
50
100
150
200
250
ua-uw=20 kPa
ua-uw=50 kPa
ua-uw=100 kpa
v / H 0
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
u (mm)
0 2 4 6 8 10
V w
/ V 0
-0.020
-0.015
-0.010
-0.005
0.000
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