Pore Water Pressure Changes during Compression and Its Impact on Soil Strength

Satish GuptaDepartment of Soil, Water, & Climate

University of MinnesotaSt. Paul, MN

Capillarity

Pressure across a membrane

P1 P1P1

P2 P2P2

P1=P2 P1>P2 P1<P2

Water Mercury

Unsaturated SoilMore negative pressureinside than outside. It is this pressure that is pulling particles together and giving additional strength.

Water distribution in unsaturated soil-large pore are empty

Triaxial set-up: Shear strength

Stress Distribution on a Soil Core in a Triaxial Set-up

Major Stress

Shear Strengthτf = c + σntan φ

Minor Stress

Normal Stress

Friction Angle

Mohr Diagram

McKyes (1989)

Shear Failure Law-Saturated Soils

τf = c + (σn-uw) tan φ =c + σ' tan φwhere τf is the shear strength, σ is the normal stress, uw is the pore water pressure, φ is the angle of friction and σ’ is the effective normal stressEffective normal stress σ’ is the stress carried by soil particles.For saturated soils, uw is zero.

Effective stress

McKyes (1989)

Shear Failure Law-Unsaturated soils

τf = c + (σn -χuw) tan φ

where χ is an empirical parameter.It relates to the wetted cross-sectional area. It could also be defined as the area over which pore water pressure is effective.

Chi (χ ) vs. degree of saturation

Yong & Warkentin (1966)

Summary of Stresses

Shear strength depends uponCohesionEffective normal stressFriction properties of the material

Shear strength is higher in unsaturated soils than saturated soils due negative pore water pressure or soil suction

Wetting TableSoil sample

Cruse & Larson (1977)

Mohr diagram

Cruse & Larson (1977)

Shear stress vs. negative pore water pressure

τf = c + (σn-uw) tan φ

Cruse & Larson (1977)

Shear stress vs. negative pore water pressure

τf = c + (σn-uw) tan φ

Cruse & Larson (1977)

Soil detachment vs. shear stress

Cruse & Larson (1977)

Compression curve

Larson et al. (1980)

Uniaxial Compression

Larson et al. (1980)

Compression set-up

Larson et al. (1980)

Compression set-up

Compression of Unsaturated Soils

Density changes

Pore water pressurechanges

Larson et al. (1980)

Variation in Compressibility

Larson et al. (1980)

Compressibility index vs. clay content

Mollisols, Spodosols, Entisosl, Inceptisoils,Vertisols

Larson et al. (1980)

Compressibility index vs. clay content

Alfisols, Ultisols, and Oxisols

Larson et al. (1980)

Compression of Unsaturated Soils

Density changes

Pore water pressurechanges

Larson et al. (1980)

Effect of Stress on Pore Water Pressure

Larson & Gupta (1980)

Pore Water Pressure vs. degree of saturation

Larson & Gupta (1980)

Normalized stress vs. degree of saturation

Larson & Gupta (1980)

Normalized Pore Water Pressure

Larson & Gupta (1980)

Normalized Stress at Minimum Pore Water Pressure

Larson & Gupta (1980)

Stress distribution in soil

Gupta & Allmaras (1987)

Boussinesq’s circles

ν =Concentration factor

Gupta & Allmaras (1987)

Tire Impression

Gupta & Allmaras (1987)

Impact of Point Load on stress in a soil element

Boussinesq’s equation

Gupta & Allmaras (1987)

Stress distribution in a soil

Gupta & Allmaras (1987)

Applications to Pavement Design?

How does subgrade strength changes under traffic when soils are nearly saturation?What is the impact of these changes on soil failure and other phenomenon such as freezing and thawing?What minimum level of unsaturation is needed in subgrade so that saturation is not reached when under load?