High resolution monitoring of expansion joints of a concrete arch dam using fiber optic sensors

In many countries existing hydro-electric power plants are currently modified to operate in pump-storage mode. This type of operation significantly increases the number of load cycles of the dams and induces high frequent deformations with low amplitudes. Current monitoring systems are not sensitive enough to measure these small scale deformations. In this paper we report about the development of an autonomous monitoring system for block joint movements based on fiber optic strain measurements. The monitoring system is based on FBG (fiber Bragg grating) sensors. One key factor of such a system is the optimal sensor configuration to ensure the measurement of the whole displacement range without overlapping signals from different sensors. Another important aspect of the system design is that the FBG sensors have to withstand also movements during extreme events. Such an event occurs every five to ten years when the water reservoir is entirely emptied for inspection purposes. Due to the missing water load the joints experience maximum expansion during this event. The developed system was installed in autumn 2013 in a 122m high concrete arch dam in Austria. Fifteen expansion joints of one maintenance corridor were equipped with FBG sensors. Continuous long term measurements started right after the installation of the measurement systems. We report the results of the first two months of operation. We demonstrate that the impact of small water level changes which typically occur in pump-storage operation can be measured reliably with the developed monitoring system. A water level change of 1m results in length changes of the expansion joints of up to 6μm. Temperature induced movements can also be measured due to the high precision of 0.4μm of the monitoring system. Daily temperature variations result in cyclic expansion joint movements with amplitudes up to 4.5μm. The developed monitoring system is the basis for modern structural monitoring of concrete dams with pump-storage operation. The high measurement resolution gives new insights into the structural behavior of dams and can potentially improve the design of new dams.