KOCHI: Fatigue is a natural phenomenon for both living & inanimate things. The structures also get fatigued due to aging. Normally, structures have a designed useful life. Beyond the designed life of any structure, constant vigil is to be exercised to check any deterioration in the ability of the structure to withstand the designed job of that structure. In the case of the Mullaperiyar dam, it is an old dam constructed with surki mortar in the 19th century during the British rule. This ‘gravity dam’ is supported on its own weight against the pressure imposed on it by the impounding water in the reservoir. The pressure of water is concentrated on the dam at its two-thirds height above floor level. The principle is that the overturning effect (upsetting couple) of the dam against the water pressure must be counterbalanced by the restoring couple acting at the centre of gravity of the dam.If the upsetting moment is higher than the restoring moment, then the dam is exposed to danger by overturning. Therefore, the restoring couple or moment must be greater than the upsetting couple or moment. To attain this, the design is taken care of to see that the resultant thrust of water & the weight of dam are kept always within the middle third rule. That is, the resultant line of force should be within the middle third of the base of the dam. The authorities must immediately check the stability of dam and ensure that the gravity force of the dam is greater than the impounding pressure of the water in the reservoir.If the restoring moment is not sufficient, the dam must be strengthened by additional masonry structure at the tail race by widening the base width and masonry stem up to the 2/3 height. Another aspect to be checked is the strength of the dam against sheer force acting at the two-thirds height. This strength may be low as the dam is very old.To increase the sheer strength of the structure, pressure grouting of the dam with high grade cement must be done at suitable spacing with modern equipment under close supervision by experts specialised in such critical jobs.But there is no need to get panicky about the life of the people around if the present water level is brought down to a safe level as suggested by the engineers.(The writer (IRSE, Retd) is a chartered engineer & registered valuer)
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