UNSATUR UNSA TURA A TE TED D SOIL SOIL ME MECHA CHANI NICS CS INTR IN TROD ODUC UCTI TION ON & A PP PPL L IC ICA A TI TION ON BY Tar i q B . Ham i d
December 2006
OUTL OUT L INE OF PRES PRESENT ENTA A TIO TION N Effective
Stress Principle
Saturated Shear
Soils vs. Unsaturated Soils
Strength of Unsaturated Soils
Unsaturated Application
Soil Testing
of Unsaturated Unsaturated Soil Mechanics Mechanics
OUTL OUT L INE OF PRES PRESENT ENTA A TIO TION N Effective
Stress Principle
Saturated Shear
Soils vs. Unsaturated Soils
Strength of Unsaturated Soils
Unsaturated Application
Soil Testing
of Unsaturated Unsaturated Soil Mechanics Mechanics
TERZAGHI TERZA GHI’’ S EFFECTI EFFECTIVE VE STRESS PRINCIPLE • Changes in volume and shearing strength of a soil are due to changes in effective stress. • The effective stress is defined as the excess of the total applied stress over the pore pressure (σ − u w ).
GENERALIZED WORLD OF SOIL MECHANICS Negative pore-water pressure Net normal stress Matric suction
(σ − u a )
(u a
Effective stress (σ − u w ) Positive pore-water pressure
− uw )
SOIL COLLAPSE
SATURATED VS. UNSATURATED SOIL SOIL CONDITION
PORE MEDIUM
uw
STRESS VARIABLES
SATURATED WATER
WATER
≥
(σ
AIR & WATER
<0
(σ − u a )
0
−
uw )
SOLID
UNSATURATED AIR WATER SOLID
(u a
− uw )
CHATEGORIZATION BASED ON GEOLOGIC ORIGINS Each soil type can be unsaturated in its natural or its compacted condition.
Lacustrine Aeoline
Natural or Alluvial remolded states Residual Others
Unsaturated soil behavior does not favor a particular geologic genesis.
NEED FOR UNSATURATED SOIL MECHANICS In the USA alone “Each year, shrinking and swelling soils inflict at least $2.3 billion in damages to houses , buildings, roads, and pipelines-more than twice the damage from floods, hurricanes, tornadoes, and earthquakes!” (Jones and Holtz,1973)
NEED FOR UNSATURATED SOIL MECHANICS Krohn and Slosson (1980) “$7 billion are spent every year in the USA as a result of damage to all type of structures built on expansive soils”. Snethen (1986): “Expansive soils “hidden disaster”: economically, one of the USA costliest natural hazards. More than one fifth of American families live on such soils.”
Near Ground Surface Structures
Spread footing foundation
(u a-u w )>0
Sr <100%
Retaining Wall
Unsaturated soil Saturated soil (u a-u w )= 0
Sr = 100%
Roadway
EFFECT OF MATRIC SUCTION (ua-uw)
Meniscus
Nc
Interparticle force due to capillarity
Unstable
Stable
(after Burland and Ridley 1996)
(after Burland and Ridley 1996)
SOIL WATER CHARACTERISTIC CURVE • It defines the relationship between the amount of water in the soil and the suction. Secondary
Boundary effect zone Primary transition zone
transition
Residual
zone
zone of unsaturation
SOIL WATER CHARACTERISTIC CURVE & SHEAR STRENGTH
SHEAR STRENGTH SATURATED
UNSATURATED
SOIL
SOIL
τ ff = c'+(σ f − uw ) tanφ ' τ = c'+(σ n
b
− u a ) tan φ '+ (u a − u w ) tan φ
EXTENDED MOHR-COULOMB FAILURE ENVELOPE FOR UNSATURATED SOIL τ
φ b
ua-uw
φ '
c′ σ−
ua
INCREASE IN SHEAR STRENGTH WITH SUCTION
Gan et al. (1988)
NULL TEST RESULTS Shearing Phase
Equalization Phase 16.4
first null test, increase σ,ua, uw
16.0 )
Null Test Data for DH/D=0-.101: σ=195, u a=91, u w=41 (kPa) for DH/D>0.101: σ=216, u a=112, u w=62 (kPa)
(a) 120.0 100.0
% ( w
Comparison Test Data for all DH/D: σ=175, u a=71, u w=21 (kPa)
15.6
equalization complete
) a P k (
80.0
τ
40.0
15.2
60.0 20.0 0.0 -0.002
(b)
12.25
0
H / v
) 12.30 % ( 0 H / 12.35 v
0.000 0.002 0.004 0.006
12.40 12.45 0
500
1000 Time (min.)
1500
2000
) % ( w
16.0 15.8 15.6 15.4 15.2 15.0 14.8 14.6
UNSATURATED SOIL TESTING STRENGTH & COMPRESSIBILITY TESTS
TRIAXIAL DIRECT
TEST
SHEAR TEST
OEDOMETER
TEST
UNSATURATED SOIL TESTING MEASUREMENT OF MATRIC SUCTION TENSIOMETER FILTER
PAPER METHOD
PRESSURE AXIS
PLATE
TRANSLATION TECHNIQUE
BEARING CAPACITY • Footings are placed well above the groundwater table • Water table may rise due to excessive watering of the vegetation surrounding the building • Measurement of in-situ suction may be valuable
BEARING CAPACITY • Extension of Saturated Soil Mechanics
q u = cN c
D f N q + 0.5γ BN γ + γ
Where: c = c '+(u a
b
− u w ) tan φ
BEARING CAPACITY OF A STRIP FOOTING FOR VARIOUS MATRIC SUCTION VALUES 2000 ) a P k ( 1600 e r u s s e r 1200 P g n i r a 800 e B e t a 400 m i t l U
0
φ ' = 20 b
φ = 15
0
c' = 5 kPa 2
γ = 18 kN/m Df = 0.5 m
m 0 . 5 B =
0 0
50
100
150
200
Matric Suc tion (kPa)
250
300
EXCAVATION SUPPORT SYSTEM IN UNSATURATED SOIL
H
Unsaturated Retained Soil
H
Bottom of Excavation C
Unsaturated Retained Soil
Depth of penetration
b
EXCAVATION SUPPORT SYSTEM IN UNSATURATED SOIL 12.00
) m ( , 10.00 D , h t 8.00 p e D 6.00 n o i t a 4.00 r t e n 2.00 e P
b
0
φ = 5
5
b
0
10
b
0
15
b
0
20
b
0
25
φ = 10 φ = 15 φ = 20
0.00 0
100
200
300
400
Matr ic Suctio n, u a-u w, (kPa)
500
600
φ = 25
Variation in Depth of Penetration (D) With Matric Suction
NATURAL UNSATURATED SOILSLOPE STABILITY • Shear strength equation for unsaturated soil conveniently separates environmental boundary conditions from stress related loading. • Major problem - determine the reduction in suction and positive pore pressure increase as a function of precipitation history.
TEMPORARY EXCAVATION Anchor for membrane
Surface drain
Plastic membrane Runoff Collection system for runoff
Footing
Residual soil l i o s d e t r a u t s a n l i U o s d e t a r u t S a
Bedrock
SEASONAL DEPENDANCE OF IN-SITU TEST PARAMETERS
EXAMPLES OF UNSATURATED INTERFACES PILES EMBEDDED IN UNSATURATED SOIL
RETAINING WALLS WITH UNSAT. SOIL BACK FILL
BURIED PIPE
EXTENDED MOHR-COULOMB FAILURE CRITERION INTERFACE STRENGTH IN UNSATURATED SOIL:
' ' b τ s = ca + (σ n − ua ) tanδ + (ua − uw ) tanδ
LABORATORY TESTING FOR INTERFACE STRENGTH PARAMETERS ca, δ, δ b
UNSATURATED INTERFACE DIRECT SHEAR APPARATUS
Air Pressure Control Panel
Vertical LVDT
Vertical Load Cell
Air Chamber
Air Pressure Line Diffused Air Volume Indicator
Horizontal Load Cell
Computer Pore Water Pressure And Volume Controller Pressure Transducer
Direst Shear Device Base
INCREASE IN INTERFACE SHEAR STRENGTH WITH SUCTION ua-uw=20 kPa ua-uw=50 kPa ua-uw=100 kpa
250 200 ) a 150 P k ( τ
100 50 0
-0.015 -0.010 -0.005 0 H 0.000 / v 0.005 0.010 0.015
0.000 0
V /
-0.005
w V -0.010
-0.015 -0.020 0
2
4
6
u (mm)
8
10
