SEISMIC RESISTANCE OF POCHAMPAD MASONRY DAM

Abstract

The peninsular shield of India has been generally regarded as one of the non seismic precambrian blocks existing on the earth. Due to the recent seismic activity in this part of the country, the seismic zoning has been revised(1) taking into account the tectonic features and also the three very important recent earthquakes in the area viz. Koyna earthquake of December 11,.1967, Kothagudem earthquake of April 13,1969, and Broach earthquake ofNarch 23, 1970, which have been extensively investigated. Consequent on this revision, it is necessary to estimate the seismic resistance of all the dams existing and under construction in the peninsular India which were not designed for resisting earthquake forces; Pochampad dam in Andhra Pradesh is one of them. This is a masonry straight gravity dam with earthen flanking bunds and is now in the advance stage of construction and its dam site was considered to be non-seismic at the time of planning and design. The only significant tectonic feature is a fault oriented in NNW-SSE, direction and lies about 150 to 300 Km. to the east of the dam site. The recent Kothagudem earthquake (1969) can be attributed to thins important tectonic feature. The Indian Meteorological Department, Government of India also opined that crustal readjustments reported occasionally from different parts of the Deccan Plateau are not unlikely in the area of Pochampad dam site. In the present study covered by this dissertation, an attempt has been made to estimate the seismic resistance of the Pochampad dam allowing a nominal tension of 10.5 tonnes/sq.m.(or 15 psi). Also by allowing an ultimate tensile stress of 105 tonnes/ sq.m( or 150 psi), the seismic strength of Pochampad masonry dam section is estimated. Gravity analysis of the highest monoliths of non over flow and spillway sections has been carried out for determining the seismic resistance. From these studies carried out it can be inferred that the Pochampad masonry dam section as constructed is marginally safe allowing a nominal tension of 10.5 tonnes/sq.m. (or 15 psi). However, under worst earthquake condition by permitting ultimate tensile stress the masonry dam can resist about 25 percent of Koyna type shock (1967) or a ground motion of 0.161. This ground motion of 0.16 g will be expected at dam site with earthquake of magnitude 7.65 originated at a distance of 50 km, which is the nearest distance of the dam site from the rift, having a focal depth of 33 km.