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Unsaturated soil mechanics is the branch of geotechnical engineering concerned with the stress-strain, strength, volume-change, and flow behavior of soils in which the void space contains both water and air, so that the pore-fluid pressure is negative relative to atmospheric pressure ^[1]. The state of such soils is characterized by matric suction, defined as the difference between pore-air pressure and pore-water pressure, and by the soil-water characteristic curve that relates suction to volumetric water content ^[2]. Because effective stress in unsaturated soils depends on two independent stress variables—the net normal stress and the matric suction—the constitutive relations for shear strength, volume change, and hydraulic conductivity are expressed as functions of both variables rather than the single effective-stress variable used in saturated soil mechanics ^[1]. The framework also governs the one-dimensional consolidation process, in which water drains from the soil skeleton while suction gradients evolve, and it underpins the analysis of slopes, earthen covers, and shallow foundations located above the water table ^[3].