To begin with, monitoring the evolution of the natural frequencies of a structure is a way of checking its general state of health and thus providing an objective aid to the decision to carry out a "detailed inspection of the structure". This is what AD-SIGNUM did during a visit to the Ribou dam. Based on this principle AD-SIGNUM developed PROBE-2. The monitoring system allows the health status of civil engineering structures to be diagnosed continuously, without interrupting their operation. While overcoming the limitations of conventional means, it offers solar power supply, reduced data transmission, no need for sensor synchronisation or cabling.
Monitoring
In this context, the Ribou dam (Fig1), a class B drinking water production dam owned by the Choletais agglomeration community, is already equipped with piezometers, pendulums, water level sensors, thermometers, fissurometers... It is an ideal framework to validate our solution based on daily monitoring of natural frequencies, in collaboration with the operator.
AD-SIGNUM has been monitoring the dam since March 2018, using a conventional seismology-type monitoring system, the deployment of which is complex. This system is composed of: a triaxial sensor, an acquisition system, a battery, a regulator, a solar panel, a GPS and cables. This installation allows a continuous recording of the data as well as a continuous monitoring of the natural frequency (Fig2).
The new monitoring system has been installed since September 2020 and provides the same quality of information as the seismological system. (Fig3)
Moreover, the natural frequencies identified and monitored have shown, since the first months, a rather particular evolution (sinusoidal trend of annual period). Without exceeding the alert threshold, this trend is essentially linked to the internal temperature of the concrete. After the implementation of an HST modelling, whose aim is to link the observed evolutions to reversible phenomena (Hydrostatic and Season) and to irreversible phenomena (Time: ageing & climate change), we observed a remarkable correlation between the model prediction and the real measurements (Fig4)
Conclusion
In the longer term, we expect a very slight drift in the natural frequency compared to the HST model. The challenge would be to distinguish between the intrinsic phenomenon of the natural ageing of the dam and the extrinsic phenomenon of climate change.
