investigations and numerical modelling in the design and during the work-in-progress stages Guido Mazzà *, Giuseppe Donghi**, Aldo Carlo Marcello *** 76th International ICOLD Annual Meeting of the International Commission Large Dams Sofia (Bulgaria), 2-6 Giugno 2008 * CESI RICERCA **EDISON S.P.A. *** STUDIO MARCELLO S.R.L. Poglia dam, located in the Lombardia Region (North of Italy) is a hollow gravity buttress structure (Marcello type) owned by Edison; its aim is the hydroelectric power generation. Since the seventies the monitoring system started to show a slow drift in elevation of the different dam blocks and, less evident, in the upstream-downstream direction. For that reason the owner carried out investigations to reach a clear explanation of the causes which gave rise to the above said drift. The full reliability of measurements and the geotechnical survey excluded the presence of problems related to the stability of foundation and abutments. Hence, the presence of possible expansive phenomena in the concrete has been explored. To this aim a deep on site and laboratory campaign was carried out and the presence of alkali-aggregate reaction in the concrete has been ascertained. The phenomena are moderate but, due to the non-rectilinear longitudinal dam axis and to the peculiar geometry of the dam blocks, a relatively severe stress-strain state, additional to the one due to operational loads, has taken place in the dam body. A particularly advanced finite element model (three-dimensional, non linear, capable to simulate the expansion AAR phenomena), suitably calibrated on the basis of the measurements recorded on the dam, was used to analyse possible future scenarios, to investigate different hypothetical structural interventions and to give confidence during the work-in-progress phases. The final decision undertaken by the designer was the cut of the contraction joints in order to reduce the compression stresses in the blocks and in the right gravity shoulder. The works have been carried out during the spring 2005 in a very short stretch of time, in order to minimize the out-of- service of the plant. The dam monitoring system, suitably improved during the works, has allowed to reach a comprehensive knowledge of the actual dam safety conditions.