The Vagli arc-gravity dam (95.50m in high), in the Northern Apennines (Italy), was built in the early '50s of the last century. Its basin contains about 34Mm3 and is one of the largest hydroelectric basins in the peninsular, Italy. Early after the water in-filling, a large landslide developed on a slope of the basin. That resulted in a restriction, with the water level authorized only up 2/3 of the basin capacity. Recently, due to the increase of the petroleum costs and energy demand, a joint research program Universities-ENEL was established to overcome that restriction by verifying the real state of stability of that slope. The first step of the research was a detailed field geological, geostructural and geomechanical survey finalized to define the geological setting and the geomechanical model of the whole slope: the results showing a sub-horizontal setting of thin to thick bedded limestone and limestone with marls, with a thick debris cover. Seismic survey evidenced that near the surface rock-mass appear to be loosened for about a few tens of a meter. Critical analysis of the data from the monitoring performed in the last thirty years outlined movements in the order of about a few mm/y, which is in the range of a natural slope-dynamic. Critical field analysis and document reconstruction of the historical landslide points in favor of a large debris collapse due to the cut of a new road above the full-water-level. A decision was made to perform a full stability analysis of the whole slope according to the most recent geomechanical methods. The slope was analyzed by using a self-developed GIS tool suitable to apply the SMR and the Critical Equilibrium methods to each TIN-grid of the slope: it resulted into a generally stable but raveling slope, thus justifying the thick debris cover. The slope stability was also verified by means of numerical models, namely PHASE2 (© Rocscience Inc.): the slope resulted to be generically stable, but debris cover and loosened rock-mass appear to have a Stability Factor close to 1.3 in the slope segment between 500m and 600m above sea level. Here, some criticalities could grow up in the event of strong rainfall with water saturation of the slope or of a large earthquake.


The Vagli arc-gravity dam is 95.50m in height and was built in the early '50s of the XX century in the Edron creek Valley,Tuscany(Italy). Its basin contains about 34Mm3 and is one of the largest hydroelectric basins in the peninsularItaly. A first dam, 65.50m high, was built during theII World War(1941–1946)and wasenlarged intheyear1952–1953uptotheheightof95.50m. The Vagli Basin allowed a yearly electric power production of about 120GWh.

Because the new basin top water level (560m a.s.l) resulted higher than the existing road, a new road was cut in the slope above (180m a.s.l) the top water level.

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