Continuous ambient vibration measurements: experimental setup and first results on an arch dam


Figure 14. Display of the automatic detection of modes: in orange, focus on the poles eliminated by the damping filter and the modal shape filter


This additional mode follows a very different variation than the other mode perfectly correlated with


the water level of the reservoir and shows low damping, suggesting it may be driven by forced excitation (characteristic, for example, of the presence of machinery). It is likely associated with the first mode of the reservoir, whose frequency can be approximated by 1440/4H, where H is the height of the reservoir. This hypothesis could be further supported, for instance, through the use of hydrophones placed in the reservoir at the base of the structure.


5. Synthesis of the First Results and Feedbacks on the Experiment


5.1. Regarding the experimental setup The experimental installation setup on the dam has been operating correctly for one year and continues to function. The choice of a setup with independent sensors, synchronized by GNSS, has proven to be relevant as it is simpler to implement on site while ensuring the ability to analyse data retrospectively. However, this implementation was facilitated by the existence of protected rooms at the crest of the dam, allowing us to avoid issues related to the exposure of sensors to external conditions with power supply. Today, automatic data retrieval has not been studied. Preliminary work will need to be done on data sampling at the appropriate level to ensure an acceptable volume for transmission. On that topic: data storage can quickly be a challenging task and the solution described in[1]


on site might prove relevant in the long term. A preliminary analysis, as shown by the results in Section 4.3, with a higher density of sensors during a short time measurement (~1h) proves to be useful to define the position of the sensors. This allows for limiting the number of deployed sensors while maintaining a good capacity for modal analysis.


Vol XXXIII Issue 3 | Dam Engineering | 153


to directly identify the frequencies


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