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
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?