Dam not quake-proof

A structural stability analysis of the Mullaperiyar dam conducted by the Department of Earthquake Engineering of the Indian Institute of Technology, Roorkie, has concluded that the main Mullaperiyar dam and the baby dam are likely to face 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 the 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 conditions 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 on a case filed by Tamil Nadu challenging the storage limit of 136 feet for the Mullaperiyar reservoir fixed by Kerala through an enactment.

The report said that at a reservoir level of 136 feet, the tensile stresses induced by the earthquake on the dam heel would be more than double the permissible value. The value was evaluated on the basis of data supplied by the Kerala Water Resources 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 a test conducted some 20 to 25 years ago.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 its foundation materials is 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 (CWC). However, neither the Commission nor an expert committee appointed earlier had carried out any dynamic analysis, taking into consideration the present actual in-situ properties of the structure, its foundation, geology and the site-specific seismic parameters. They had concluded that the dam was safe, following a 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.

The Hindu, 28th October 2009