Pore Water Pressure Changes During Compression and · PDF filePore Water Pressure Changes...
Transcript of Pore Water Pressure Changes During Compression and · PDF filePore Water Pressure Changes...
Pore Water Pressure Changes during Compression and Its Impact on Soil Strength
Satish GuptaDepartment of Soil, Water, & Climate
University of MinnesotaSt. Paul, MN
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
Stress Distribution on a Soil Core in a Triaxial Set-up
Major Stress
Shear Strengthτf = c + σntan φ
Minor Stress
Normal Stress
Friction Angle
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.
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.
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
Compressibility index vs. clay content
Mollisols, Spodosols, Entisosl, Inceptisoils,Vertisols
Larson et al. (1980)
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?