After the event of December 1967 earthquake at Koyna with, the epicentre near the dam, some of the non-overflow blocks showed considerable distress in the form of major cracks at the level of a kink both in the downstream and upstream faces. It was necessary to undertake immediate repairs to the dam. These were accomplished through immediate use of prestressing cables to stitch across the cracks developed on NoF faces.
Subsequently strengthening of NOF by concrete backing and buttressing was carried out in 1972 as a permanent measure. After a 6.3 magnitude earthquake in 1993 in the southern part of the state at Killari, the Government of Maharashtra set up a committee for deciding modifications to be carried out in design standards and ascertaining the safety of major and medium completed dams in earthquake prone regions of Maharashtra State. Accordingly, design standards were set and stability for 27 dams was received and it was decided to strengthen 11 dams, which included Koyna Dam spillway. Having an astounding generation of 3300 million units of power every year and catering to the irrigation need of about 50, 000 Ha of land, Koyna catchment has a very high precipitation with a reservoir filling success rate of more than 80% over the last 40 years. However, known to be a silent seismic area of more than 200 years, the spurt of earthquakes started from 1963 and so far 1, 11, 098 shocks of various magnitudes upto a magnitude of 6.5 on Richter scale have been recorded. Therefore, in view of the importance of this project, the energy and irrigation needs of the State and in view of the continued seismic disturbances in the region, protecting this dam against any severe earthquake has become mandatory. At the same time storage augmentation to the tune of 184 Mm3 was accomplished by providing 1.52 m high movable flaps over the existing radial gates. All these factors earnestly warranted the strengthening of Koyna spillway. It was decided to strengthen Koyna Dam spillway for revised seismic parameters and raised FRL.
The task involved designing the section through a series of analyses including dynamic analysis, and judging the efficacy of hydraulic profile through competent model experiments. It also required meticulous planning of construction activities as the entire strengthening work was to be completed within a span of 2 years, the net available time duration being 400 days.
This paper takes a brief overview of the impact of 1967 earthquake, strengthening measures adopted for the NOF portion thereafter and the necessity of strengthening of spillway portion. It also discusses the design of strengthening section for dam spillway and the construction planning, which is based on the thermal studies and blasting experiments conducted by CW&PRS and physical model studies conducted by Maharashtra Engineering Research Institute. The Construction Methodologies adopted and problems faced during construction and remedial measures adopted are also elaborately discussed in this paper.
