The structural stability analysis of the Mullaperiyar dam by the Department of Earthquake Engineering of the Indian Institute of Technology, Roorkie, has concluded that the main Mullaperiyar dam and baby dam are likely to undergo damage in the event of an earthquake. Such damage may lead to failure of the dam.
In the case of the main dam, damage is predicted in the event of an earthquake of a magnitude of 6.5 on the Richter scale in the vicinity of dam (within 16 km) when the reservoir level is at 136 feet. The baby dam is found to be safe for a reservoir level of 155 feet.
However, it would also be unsafe under probable maximum flood condition when the reservoir level would rise up to 160.22 feet.
The Kerala government will present the (final) report of the IIT team before the Supreme Court during the hearing of the case filed by Tamil Nadu challenging the storage limit of 136 feet for Mullaperiyar reservoir fixed by Kerala through an enactment.
The report said that at the reservoir level of 136 feet, the tensile stresses induced by the earthquake at the dam heel would be more than double the permissible value.
The value was evaluated on the basis of data supplied by the Kerala Irrigation Department on average ultimate tensile strength of random rubble masonry employed in the construction of the dam. Most of the values adopted for material properties were based on the test conducted some 20 to 25 years back.
The report noted that the dam would have undergone considerable deterioration during the intervening period because of ageing and weathering. As such, the assumed parameters might be naturally higher than the in-situ condition.
Proper assessment of existing material properties is very important for safety assessment. So, further testing on the dam and foundation materials are recommended, it said.
The IIT team remarked that the Mullaperiyar dam was a high hazard dam as per the criteria fixed by the Central Water Commission. However, neither Commission nor the expert committee appointed earlier had carried out dynamic analysis, taking into consideration the present actual in situ properties of structure, its foundation, geology and the site specific seismic parameters.
They had concluded that the dam was safe, following simple pseudo statistical method of analysis.
The earthquake coefficient taken by the CWC for the stability analysis of the Mullaperiyar dam was only 0.1g while the Indian standard recommended 0.18g.
However, the team found that earthquakes in the region could produce possible peak ground acceleration of 0.21g at the dam site (unit g is the acceleration due to earth’s gravity).