Geotechnical analysis of an ancient dam built in unfavourable geological conditions.

The paper reports on the behaviour of the Alpe Cavalli dam, built in the Italian Alps between 1922 and 1926 and still in operation for electric power supply. The dam, which is 31 m high, made by dry masonry, is waterproofed by a multi-layered impervious facing, a concrete cut-off and a grout curtain in the foundation soils. The geological conditions of the dam site are quite peculiar since the reservoir location coincides with an ancient natural lake of glacial origin which was emptied, probably after the last glaciation, when the front moraine was eroded by the emissary river and a deep narrow cut was opened on the left side of the valley. For such reason the right shoulder of the dam rests on the moraine deposit, its central body is placed on the river bed alluvium while its left shoulder stands on a metamorphic formation composed of highly fractured rocks. A comprehensive investigation has been conducted in order to understand the overall behaviour of the reservoir, after about ninety years of service. For this purpose, historical documents have first been reviewed for reconstructing the design assumptions and the building operations. Experimental investigations of the moraine deposit have then been conducted, in order to estimate the subsoil properties. Seepage flow rates and reservoir impoundment levels have been thoroughly analyzed, showing a strong correlation between these two variables. The displacements of the dam body and of the moraine deposit have also been examined in detail. The available observations have been evaluated in order to gain an over all interpretation of the reservoir behaviour, with particular reference to the moraine deposit. Stability analyses have finally been performed to check the safety level of the reservoir.