Stick-slip behavior was observed while characterizing the shear strength of fly ash collected from a thermal power plant at Delhi, India. Stick-slip oscillations of the shear stress were observed in the shear-deformation curve of fly ash. Pullout tests were carried out on geogrid embedded in compacted dry fly ash to determine the soil-geosynthetic interaction parameters. The pullout test results also exhibited stick-slip oscillations. Subsequently, triaxial tests were conducted to study the influence of strain rate and confining pressure on the stick-slip motion of shear stress. The amplitude of stick-slip oscillations increased with increasing confining pressure whereas decreased with increasing strain rate. In addition to fly ash, the tests were also conducted on a locally available Delhi silt soil to study the influence of particle morphology on stick-slip oscillation of shear stress. These results indicated that due to stick-slip motion, it is not possible to identify a true peak value of shear strength of fly ash. This should be considered while designing a mechanically stabilized earth wall and slope with fly ash as a backfill material. © 2019, © 2019 Taylor & Francis Group, LLC.