An artificial reservoir was constructed for a project in the south-west region of India. The reservoir was constructed by cutting 9.8 m deep into the filled-up land with 1.5H:1V side slopes to generate a water body of 60000 m3 storage capacity. During the peak monsoon, a sudden failure of one of the side slopes of the reservoir was observed. The lab testing and field investigation using ground penetrating radar, seismic refraction test, infiltrometer test etc. and the other analytical study was carried out to analyse the root cause of the slope failure. Based on this study, it was observed that the slope failure was due to the peculiar soil properties in this region and the development of phreatic line and thus building up of excess pore water pressure below the embankment slope. The rectification measures were carried out by provision of non-return type pressure relief valves along the side slope to release the excess pore water pressure. The structural modifications were also carried out along and at the bottom of the slope to structurally strengthen the concrete lining of the slope. It has been observed that the rectification measures worked well to increase the geotechnical as well as structural stability of the slope to prevent any damages in future.