Analysis of dam behaviour after eighty years of service

The paper reports on the behaviour of a masonry dam built in the Italian Alps in the early twenties of the last century and still in operation for electric power supply. The dam is 31 m high and its waterproofing is ensured by a multi-layered impervious facing, a concrete cut-off and a grout curtain protruding into the foundation soils. The geological location of the dam is rather complex, because its left abutment is located on a rock formation while the right one rests on a thick moraine deposit. Since its first impounding, large downstream water flows and significant movements of the dam and of its moraine abutment were observed. A comprehensive investigation has thus been conducted in order to understand the overall behaviour of the reservoir, after eighty years of service. For this purpose, historical documents have been first reviewed in order to reconstruct the design assumptions, construction operations and early observations on site. Experimental investigations of the moraine deposit have then been conducted, in order to estimate the subsoil properties. Seepage flow rates and reservoir impoundment levels, recorded for more then twenty years, have also been analysed, showing the correlation existing between these two variables. More recently, a system for monitoring the displacements of the dam and the moraine has been implemented and the recorded data have been examined. All the available observations have been evaluated and a reasonable interpretation of the coupled hydraulicalmechanical behaviour of the dam and of its moraine abutment has been inferred. Numerical calculations have finally been conducted, in order to verify such explanation.