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Pore Water Pressure Changesduring Compression and Its

Impact on Soil Strength

Satish Gupta

Department of Soil, Water, & Climate

University of Minnesota

St. Paul, MN

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Capillarity

Pressure across a membraneP1

P1P1

P2 P2P2

P1=P2 P1>P2 P1

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Unsaturated Soil

More negativepressureinside thanoutside. It is

this pressurethat is pullingparticles

together andgivingadditional

strength.Water distribution in unsaturated soil-large pore are empty

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Triaxial set-up: Shear strength

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Stress Distribution on a Soil

Core in a Triaxial Set-upMajor Stress

Shear Strength

f= c + ntan

Minor Stress

Normal Stress

Friction Angle

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Mohr Diagram

McKyes (1989)

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Effective stress

McKyes (1989)

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Shear Failure Law-Unsaturated

soils

f= c + (n -uw) tan

where is an empirical parameter.

It relates to the wetted cross-sectionalarea. It could also be defined as the

area over which pore water pressure iseffective.

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Chi ( ) vs. degree of

saturation

Yong & Warkentin (1966)

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Summary of Stresses

Shear strength depends upon Cohesion

Effective normal stress

Friction properties of the material

Shear strength is higher in unsaturated

soils than saturated soils due negativepore water pressure or soil suction

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Wetting TableSoil sample

Cruse & Larson (1977)

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Mohr diagram

Cruse & Larson (1977)

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Shear stress vs. negative pore

water pressure

f= c + (n-uw) tan

Cruse & Larson (1977)

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Shear stress vs. negative pore

water pressure

f= c + (n-uw) tan

Cruse & Larson (1977)

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Soil detachment vs. shear

stress

Cruse & Larson (1977)

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Compression curve

Larson et al. (1980)

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Uniaxial Compression

Larson et al. (1980)

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Compression set-up

Larson et al. (1980)

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Compression set-up

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Compressibility index vs. clay

content

Mollisols, Spodosols,Entisosl, Inceptisoils,Vertisols

Larson et al. (1980)

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Compressibility index vs. clay

content

Alfisols, Ultisols, and Oxisols

Larson et al. (1980)

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Compression of Unsaturated

Soils

Density changes

Pore water pressurechanges

Larson et al. (1980)

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Effect of Stress on Pore Water

Pressure

Larson & Gupta (1980)

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Pore Water Pressure vs.

degree of saturation

Larson & Gupta (1980)

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Normalized stress vs. degree

of saturation

Larson & Gupta (1980)

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Normalized Pore Water

Pressure

Larson & Gupta (1980)

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Normalized Stress at Minimum

Pore Water Pressure

Larson & Gupta (1980)

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Stress distribution in soil

Gupta & Allmaras (1987)

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Boussinesqs circles

=Concentration factor

Gupta & Allmaras (1987)

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Tire Impression

Gupta & Allmaras (1987)

I t f P i t L d t

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Impact of Point Load on stress

in a soil element

Boussinesqs equation

Gupta & Allmaras (1987)

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Stress distribution in a soil

Gupta & Allmaras (1987)

A li ti t P t

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Applications to Pavement

Design?

How does subgrade strength changesunder traffic when soils are nearlysaturation?

What is the impact of these changes onsoil failure and other phenomenon suchas freezing and thawing?

What minimum level of unsaturation isneeded in subgrade so that saturation is

not reached when under load?