Analisi geotecnica di una diga di antica costruzione

GEOTECHNICAL ANALYSIS OF AN OLD DAM 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 very peculiar, 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 more then 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 For this purpose, historical documents have been first reviewed in order to reconstruct the design assumptions, construction operations and early observations on site.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 performed, 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 hydraulical-mechanical behaviour of the dam and of its moraine abutment has been inferred. Numerical calculations have finally been conducted, in order to verify such explanation. The most peculiar aspect observed in this case is the complex interaction between the reservoir, the dam body and the different subsoil formations. In particular, a predominant role is played by the moraine deposit, forming a natural prolongation of the dam. Geological studies and geotechnical investigations have provided basic information on this deposit but the grain size composition of the moraine, consisting mainly of coarse grained material, has greatly reduced the possibility of performing laboratory and site investigations. The gap on experimental data has then been filled by direct observations and monitoring and by complementary numerical analyses. The former have provided a logical correlation between reservoir level excursions, water seepage and movements of the moraine. The results of numerical back analysis have also shown that the soil response is much stiffer than estimated by pressumeter tests and similar to a non linear elastic medium with increasing stiffness. Such response is consistent with previous laboratory observation on gravelly soils subjected to a large number of loading cycles.