AbstractThe mechanical behavior (strength, stiffness, and volume change) of soils depends on the stress state, which for unsaturated soils is controlled by interparticle forces in addition to the skeletal forces. The interparticle forces for an uncemented soil (i.e., adsorptive and capillary forces) are functions of saturation that are unique for each soil. Previous studies have quantified the contribution of interparticle forces to soil shear strength using the suction stress characteristic curve (SSCC) and demonstrated the initial evidence of transitions between the interparticle force components. This study quantifies the stiffness of uncemented kaolinite over a wide range of saturation in dry of optimum conditions using resilient modulus and shear modulus tests. The stiffness-saturation curves show two unique inflection points that mark (1) the transition between an adsorption-dominated water uptake regime to a capillarity-dominated water uptake regime at ∼0.04–0.1 saturation; and (2) the onset of a reduction in capillary forces at ∼0.45–0.55 saturation. The shape of stiffness-saturation curves is compared with the adsorptive and capillary components of SSCC and SWRC. The results provide further evidence on water uptake mechanisms and corresponding evolution in interparticle force components in compacted kaolinite, enhancing our understanding of the stiffness behavior of unsaturated soils.