A typical example of dam foundation on Neogenic conglomerate deposits is presented in this paper. The case of Agiokampos dam in east Thessaly in central Greece. The level of natural compactness and the nature of the cementing material are crucial elements for the strength, the bearing capacity and the permeability of these formations. These conglomerates of medium consistency, do not allow relying on traditional sampling and tests in the boreholes. Lugeon tests are meaningless and falling head tests are recommended. Back analysis on slopes is certainly a useful tool for approaching the strength parameters of the global rock mass. Thus it is understood that the engineering geological assessment before any testing and analysis is of prime importance.Due to the fine nature of the cement, permeability is generally low and no leakages are expected. However, an impermeable grout curtain must be created due to some higher values. Conglomerates are not easily groutable and a combination of a diaphragm wall is suggested. Finally, a carpet grouting is designed since the surficial zone is susceptible to internal erosion.