Cahora Bassa vibrations monitoring between 2010-2022. Analysis of frequencies using HST statistical models for effects separation

Abstract

Cahora Bassa dam is a 170m-high arch dam located in western Mozambique. In operation for almost 50 years, since 1974, the dam is affected by a swelling process of small/medium amplitude. Aiming to control its structural health over time, in 2010 the dam was instrumented with a continuous vibrations monitoring system, designed for Seismic and Structural Health Monitoring (SSHM), which includes 10 uniaxial accelerometers in the crest gallery and 3 triaxial accelerometers in the dam-foundation interface. The latest results on the dynamic behaviour monitoring of the dam are presented, namely the evolution of the main natural frequencies over time and the corresponding mode shapes. Then, HST statistical models for separation of effects, considering the hydrostatic pressure (H), seasonal variations (S), and time (T) dependent factors, are used to analyse the evolution of the experimental natural frequencies over the last 12 years. The achieved results show that the dynamic behaviour of the dam is mostly influenced by the water level variations. However, a very small decreasing tendency of the natural frequency values was detected, which may indicate a slight decrease of the global stiffness of the dam, possibly due to the evolution of the concrete swelling pathological process.