Monitoring of the natural frequencies of Chirkey arch dam

The article presents a method for monitoring the natural frequencies of HPP dams according to continuous seismic observations. The object of the research is the largest arched dam in Russia, the Chirkey HPP located in the Caucasus. If damaged, it could cause great loss of property and human life, but disasters can be minimized by using effective dam structural health monitoring. The study for changes in the natural frequencies of engineering structures is one of the most common methods of remote control over their structural health. However, the determination of values of natural frequencies of huge concrete dams is a very difficult procedure due to their have complex construction. Moreover, interpretation of changes in the natural frequencies values is difficult due to the significant influence of the water level in the reservoir.  Consequently, at the initial stage, we performed a detailed study of the natural oscillations of the dam using the method of coherent restoration of the standing wave fields with the definition of both the natural frequencies of the structure and their modes. They were conducted twice at the minimum and maximum upstream level and for the first time highlighted the features of seasonal changes in the full field of standing waves. The normal modes were determined that are present in oscillations at different upstream levels and which frequencies can be detected continuously from the records of seismic equipment. The series of frequency changes during the year are calculated. For the first time we established that, frequency changes are by 5 to 11 days behind reservoir level changes and assumed that relaxation processes of the dam body and / or its base cause the delay after the upstream level changes. We calculated dependencies for predicting the frequency values from the reservoir level, taking into account the delay time. As a result, we proposed an approach for monitoring of the dam structural health based on a comparison of the observed natural frequencies with the predicted ones. The developed method can be applied to monitor the structural health of concrete dams of other HPPs.