ASCE expansive soils presentation

7/23/2019 ASCE expansive soils presentation

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UNSATURATED SOIL MECHANICS -INTRODUCTION & APPLICATION

BY

Tariq B. Hamid

December 2006



OUTLINE OF PRESENTATION

Effective Stress Principle

Saturated Soils vs. Unsaturated Soils

Shear Strength of Unsaturated Soils

Unsaturated Soil Testing

Application of Unsaturated Soil Mechanics



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



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



SOIL COLLAPSE



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



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



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)



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.”



Near Ground Surface Structures

Unsaturated soil

Saturated soil

Retaining Wall Roadway

Spread footing foundation

Sr <100%(ua-uw)>0

(ua-uw)= 0 Sr = 100%



Meniscus

Nc

Interparticle forcedue to capillarity

Unstable(after Burland

and Ridley 1996)

Stable(after Burland

and Ridley 1996)

EFFECT OF MATRIC SUCTION (ua

-uw)



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



SOIL WATER CHARACTERISTIC

CURVE & SHEAR STRENGTH



SHEAR STRENGTH

SATURATED

SOIL

UNSATURATED

SOIL

'tan)(' φ σ τ w f ff uc −+= bwaan uuuc φ φ σ τ tan)('tan)(' −+−+=



EXTENDED MOHR-COULOMB

FAILURE ENVELOPE FORUNSATURATED SOIL

'φ

b

φ

ua-uwτ

σ −

ua

c′



INCREASE IN SHEAR

STRENGTH WITH SUCTION

Gan

et al. (1988)



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)



UNSATURATED SOIL TESTING

STRENGTH & COMPRESSIBILITY TESTS

TRIAXIAL TEST

DIRECT SHEAR TEST

OEDOMETER TEST



UNSATURATED SOIL TESTING

MEASUREMENT OF MATRIC

SUCTIONTENSIOMETER

FILTER PAPER METHOD

PRESSURE PLATE

AXIS TRANSLATION TECHNIQUE



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



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++=



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



EXCAVATION SUPPORT SYSTEM IN

UNSATURATED SOIL

UnsaturatedRetained Soil

H

Bottom of Excavation

Unsaturated Retained Soil

Depth of

penetration

H

C

b



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)



•

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



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



SEASONAL DEPENDANCE OF

IN-SITU TEST PARAMETERS

Lehane et al. 2004



EXAMPLES OF UNSATURATED

INTERFACES

PILES EMBEDDED IN

UNSATURATED SOIL

RETAINING WALLS WITH

UNSAT. SOIL BACK FILL

BURIED PIPE



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)(

''

−+−+=



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)



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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ASCE expansive soils presentation

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