http://JUDIS.NIC.IN SUPREME COURT OF INDIA Safety Aspect Tehri Dam is being constructed at the confluence of the Bhagirathi and Bhilangana rivers in the neighborhood of the Garhwal town in the State of Uttaranchal. According to the petitioners the structure of the dam itself is not safe and also alleged that its existence increases the seismic vulnerability of the entire Himalayan region. By way of abundant caution they want the concerned authorities to conduct Three-Dimensional (3D) NonLinear Test to evaluate the earthquake susceptibility of the dam against the Maximum Credible Earthquake. They also insist upon the computer simulated Dam Break Analysis to estimate the magnitude of damages in the unexpected eventuality of any dam failure. The respondents submitted that they explored the possibility of such tests and arrived at a conclusion that such tests are neither practical nor necessary in the present case. The respondents added that the studies related to site specific assessment of seismicity, testing of fill materials for determining dynamic properties and detailed dynamic analysis, were carried out independently by two agencies; that they are the Department of Earthquake Engineering, University of Roorkee, and Hydro Project Institute (HPI), Moscow; that the result of their studies indicated that earth and rockfill type dams as chosen for Tehri are the safest man-made structures in earthquake prone zones due to their large inertia, high damping and high flexibility; that the dam fill material is being compacted to concrete like density, which provides high strength while retaining the flexibility; that relatively flat slopes have been adopted for up-stream and down stream of the dam, which are flatter than the slopes provided in some recent high dams built/ planned in regions of similar high seismicity; that, therefore, the Tehri Dam is a fail-safe structure and the design has been found safe against the worst earthquake scenario of the area, even when very conservative and severe seismic parameters were assumed for these tests. A high level Committee under the Chairmanship of Director General, Geological Survey of India, which consists members from Central Water Commission; Director, National Geophysical Research Institute; Head of Department of Earthquake Engineering, University of Roorkee, and Dr. V.K. Gaur, the then Secretary, Department of Ocean Development was also constituted to examine issues concerning safety of the dam. The Committee in its report established that even in the worst scenario of possible occurrence of a large magnitude earthquake of 8+ in richter scale with the probable location at a depth of 15 km below the dam site, the same would be safe. They concluded that dam design is safe and added that all danger arising out of the seismicity have been taken note of and taken care of in the planning of Tehri Dam project. From the documents before us, it could be gathered that the Government also referred the matter to Seismic Expert, Prof. Jai Krishna who was the former President of International Academy of Earthquake Engineering and also ViceChancellor of the University of Roorkee. He examined the matter and opined that "the proposed dam section for the Tehri Project is safe from the point of view of seismicity of the region" and "since conservatism has entered at almost every step of decision making, the overall factor of safety of the dam is high enough to eliminate any risk from earthquake of the future." The next Expert Group that consisted of the Director General, Geological Survey of India, made a report on 25.7.1991 and recommended that the reports of the High Level Committee and the review given by Prof. Jai Krishna should be accepted. The seismic stability of the dam was further got tested through the Hydro Project Institute, Moscow, against actual acceleogram of the Gazli earthquake for the maximum vertical acceleration of 1.36g and horizontal acceleration of 0.72g both acting simultaneously and the dam design was again found to be safe. Even though the design of the dam had been earlier independently checked by two separate agencies, it was again examined and it was found that the project appears to be safe. Subsequently, during the pendency of this petition, in order to allay fears expressed in certain quarters, the Government of India in June 1996 appointed a Group of Five Experts to examine the relevant scientific and Page 4 of 37