How does unsaturated soil behavior differ and how is it analyzed?

Unsaturated soils have negative pore water pressure (suction) that increases effective stress and strength. Key concepts: soil-water characteristic curve (SWCC) relating suction to water content, extended Mohr-Coulomb with suction term (tau = c' + sigma'*tan(phi') + ua-uw)*tan(phi_b)), and volume change due to wetting (collapse) or drying (shrinkage). Applications: expansive clay behavior, slope stability with infiltration, compaction control, and arid climate foundations. Suction is measured using tensiometers (low suction) or filter paper method (high suction).