Expansive soils undergo a considerable amount of volume change because of moisture fluctuations and cause severe damage to lightweight structures. In this study, an attempt is made to stabilize the expansive soils to improve their swelling, stiffness, and damping properties with class C fly ash. The percentage of fly ash was varied between 5 and 20% by dry weight of the soil. A series of fixed-free type resonant column tests were performed to determine the dynamic properties, including shear modulus (G) and damping ratio (D); and Poisson's ratio (ν) of untreated and fly ash treated expansive soils. Prior to the testing, the specimens were cured for 1, 7, and 28 days in a humidity chamber. It is observed that the shear modulus of expansive clay has increased and corresponding damping ratio has decreased with increase in the fly ash content. The curing time has practically negligible influence on the normalized shear modulus and damping ratio of the treated specimens. A generalized equation is proposed for the upper and lower bound normalized shear modulus degradation curves for stabilized clays. An economical design alternative for a vertically vibrating machine is suggested based on the improved dynamic soil properties of the clay. © 2016 American Society of Civil Engineers.