MIGUEL RODRIGUES, SERGIO OLIVEIRA, JOSE NUNO LIMA & JORGE PROENCA 7. Conclusions
In this paper the displacement history measured by a Global Navigation Satellite System (GNSS), from an antenna located at the top of the central section of Cabril dam in Portugal, was analyzed. The main objective was to verify the reliability of the GNSS system. Taking into account that at the crest centre it is not possible to measure displacements using the plumbline method, only by triangulation campaigns carried out once or twice a year, it was decided that the best strategy to verify the reliability of the displacements measured by GNSS would be to validate a Finite Element Model (FEM) using the abundant plumbline observations and then, with the FEM validated, verify whether or not the GNSS observations were coherent. The good fit between the HSCT-FEM hybrid model and the radial observations measured by the plumbline, triangulation or even the GNSS method were all coherent with the FEM applied in this study. Not only did the overall adjustments present a good fit, but the HP elastic effect and the temperature effect also presented a good adjustment to the applied FEM. The adopted FEM considered a crack fully open from the downstream face to the upstream face, which proved to be a better solution than one adopted in a previous study[13] which considered the same crack, opened to 3/4 of the total length (the total length is equal to the dam thickness at that location), because the previous study considered higher displacements for the HP influence line, calculated by the FEM, which were incompatible with what now results from the GNSS separation of effects, considering nearly three years of monitoring, opposed to only 6 months of monitoring available in the last study. Hence, the good adjustment between the adopted FEM and the HSCT-FEM results for the coordinometer bases and triangulation marks analyzed not only proves good calibration of the applied FEM, but actually the good quality of the separation of effects results already obtained from the GNSS, with corresponding high coefficient of determination, helped towards adopting a better FEM.
Naturally, the redundancy of instruments used in the FEM calibration granted its validity. Therefore, when comparing its results with those obtained by the HSCT-FEM separation of effects for the GNSS radial displacement measurements, the agreement was also very good; validating, as expected, the GNSS results.
The significant time effect component detected in every data instrument analyzed (with the exception of the GNSS because of its limited monitoring period), probably mostly due to the swelling reactions effect, deserves further study due to its pathological nature. Ultimately, the quality of the GNSS data analyzed enabled the initial goal of this study to be surpassed because the GNSS data was not simply used for comparison with the FEM results for its validation but, due to its quality, was also used to validate the FEM itself and contribute to the development of a better FEM. It is now more evident that the horizontal cracking at Cabril dam can be better simulated by the FEM when it considers a crack fully open. GNSS clearly presents itself as an accurate, reliable and high frequency remote instrument to monitor dam displacements.
164 DAM ENGINEERING
Vol XXXI Issue 3
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