In this paper efforts have been made to understand the influence of unit weight of slurry-deposited coal ash on the slope stability of ash dykes raised by upstream method. Using the geotechnical properties of compacted ash as well as slurry-deposited ash, a series of stability analysis have been carried out to identify the critical factor of safety for incrementally raised ash dykes. A 19 m high ash pond in Eastern India had been incrementally raised to its current elevation, by upstream method in four raisings. The ash pond was to be further raised by providing two raisings (5th and 6th) of 3 m each making the ash pond 25 m high. Slope stability analysis of downstream slope of the dykes was carried out for steady state seepage condition using effective stress parameters for the ash as well as foundation soil. For the 5th and 6th raising the critical failure surface was observed to pass predominantly through slurry deposited ash and the foundation soil beneath it. The factor of safety (FOS) was observed to be low on account of the low unit weight as well as the low shear strength of ash. It was observed that for saturated unit weight (γsat) of 11.7 kN/m3 the FOS was lower by 12.5% in comparison to γsat of 16 kN/m3 for 5th raising and by 14% for 6th raising in static condition. Similarly, in seismic condition, the FOS at γsat of 11.7 kN/m3 was observed to be low by 14% than that at γsat of 16 kN/m3 for 5th raising and by 15% for 6th raising. The role of unit weight in influencing slope stability of incrementally raised dykes of slurry ponds has been reported for the first time in this study and it shows that light-weight deposits such as coal ash can lead to low FOS when the failure surface passes through the slurry-deposited material. © 2019, Springer International Publishing AG, part of Springer Nature.