The Mullaperiyar dam is a composite gravity dam built during the period 1887-1895. The front and rear faces of the dam are built with uncoursed rubble masonry in lime surki mortar. The hearting, which accounts for 62% of the volume of dam, is constructed of lime surkhi concrete. It lies on the Western Coast of India in the State of Kerala and lies in Seismic Zone III as per the seismic zoning map of India. The 53.46 m (176 ft) high composite gravity dam is now more than 114 years old.

Lime surkhi has a tendency of leaching when it comes in contact with water. This process reduces the strength of lime surkhi mortar which reduces the strength of the masonry. In 1930s and 1960s grouting and guniting were done to check the seepage of water from the body of the dam. Concrete backing on the downstream face as strengthening measure was undertaken in 1980s. It is anticipated that ageing effect may have deteriorated the dam material and may have become vulnerable under a future strong motion earthquake and in the eventuality of dam failure human and economical losses may result. It is therefore considered appropriate to assess the safety of the dam under future seismic threat. The complete seismic safety study on structural stability of Mullaperiyar dam has been carried out in two parts. Part-I deals with the Seismic Hazard Assessment and Part-II deals with the Seismic Stability Analysis.

Accordingly, seismic hazard assessment studies at dam site have been undertaken considering the recent increased seismic activities using the current methodology. The details of Part-I study can be found in Ref. 2. The Peak Ground Acceleration (PGA) value under Maximum Credible Earthquake (MCE) condition is estimated as 0.21g for 2 % exceedence in 50 years.

Here, the details of Part-II studies are presented. The stress analyses of the Main and Baby dams have been carried out by Finite Element Analysis under MCE condition. The safety of dam under various reservoir levels has been studied. Based on the study, recommendations have been made. Both the main and baby dams are found to be unsafe under the Static plus Earthquake (MCE) condition and warrants immediate attention. It is suggested that safety analysis should be ascertained by taking into account the current material properties affected by ageing etc.