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The Koyna earthquake of December 11, 1967 and the performance of Koyna Dam

Chopra, Anil K.; Chakrabarti, Parthasarathi

UCB/EERC-71/01, Earthquake Engineering Research Center, University of California, Berkeley, 1971-04, 51 pages (710.4/C49/1971)

This work is aimed at evaluating the concepts currently employed in the design of gravity dams in light of the performance of Koyna Dam during the Koyna earthquake. It includes a brief description of the geology and the seismic history of the area, a discussion of the relationship between the filling of the reservoir and the increase in seismic activity in the region, the properties of the Koyna ground motion and how they compare with those of California earthquakes, and a description of the damage caused to various structures of the Koyna project, buildings and bridges. The study is not limited only to the Koyna Dam and earthquake; it also considers a dam with a typical gravity section and another earthquake motion having similar intensity but different peak acceleration and frequency characteristics. The earthquake response in a number of cases was analyzed by the finite element method, with the dynamic effects of the reservoir included in some analyses, and results are presented. The following conclusions were made: The cracking anticipated in the monoliths of Koyna Dam on the basis of analytically obtained stress responses and strength of concrete in the dam is consistent with the actual dam. The strengthening buttress designed for Koyna Dam leads to a significant improvement in the earthquake behavior. The analyses demonstrate that typical gravity dams of height and concrete properties similar to that of Koyna Dam would have suffered comparable damage. Higher concrete strength is required in the upper parts of gravity dams. The Koyna ground motion is relatively more severe, compared to California earthquakes of similar intensity, on concrete gravity dams of height 400 ft and less. Present design criteria for gravity dams need to be improved to recognize that rather large tensile stresses can occur in various parts of the dam--in the upper part near the faces and the heel--during earthquakes, and to provide for the consequences of these tensile stresses. The extra concrete mass near the crest of a gravity dam to support the roadway, etc. is responsible for causing a significant part of the critical tensile stresses; attention should be given to developing light-weight supporting systems.

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